Contaminant generation and transport from mine pit lake to perennial stream system: Multidisciplinary investigations at the Big Ledge Mine, Nevada, USA

Journal Pre-proof Contaminant generation and transport from mine pit lake to perennial stream system: Multidisciplinary investigations at the Big Ledge Mine, Nevada, USA Connor P. Newman, Simon R. Poulson, Karl W. McCrea

PII:

S0009-2819(19)30061-3

DOI:

https://doi.org/10.1016/j.chemer.2019.125552

Reference:

CHEMER 125552

To appear in:

Geochemistry

Received Date:

6 August 2019

Revised Date:

23 October 2019

Accepted Date:

2 November 2019

Please cite this article as: Newman CP, Poulson SR, McCrea KW, Contaminant generation and transport from mine pit lake to perennial stream system: Multidisciplinary investigations at the Big Ledge Mine, Nevada, USA, Geochemistry (2019), doi: https://doi.org/10.1016/j.chemer.2019.125552

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Pit-lake hydrology and contaminant transport at the Big Ledge Mine

Contaminant generation and transport from mine pit lake to perennial stream system: Multidisciplinary investigations at the Big Ledge Mine, Nevada, USA

Nevada Division of Environmental Protection, 901 S. Stewart St., Suite 4001, Carson City,

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Connor P. Newman (ORCID 0000-0002-6978-3440)1*, Simon R. Poulson2, Karl W. McCrea1

University of Nevada, Reno, Department of Geological Sciences and Engineering

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Nevada, 89701

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*Corresponding author, [email protected] , Current address: US Geological

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Survey, Denver Federal Center, Bldg. 53, MS-415, Denver, CO, 80225

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Graphical abstract

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Newman et al. - REVISED

Highlights: 

Inverse geochemical modeling and stable isotopes provide insight on water mixing and mineral dissolution reactions

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Discrete and diffuse acid mine drainage impact a stream downgradient from a mine pit

Sulfate salts temporarily store trace metals and reflect long-term dissolution of sulfide

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

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lake

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INSERT GRAPHICAL ABSTRACT HERE

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minerals

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Submitted to: Geochemistry (special issue on groundwater)

Abstract

The Big Ledge mine located in northeastern Nevada, USA is a former barite mine site that now hosts an acidic pit lake and highly concentrated waste rock seepage, which flows into an

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intermittent stream. In an effort to identify the source(s) of the seepage, stable isotopic analyses were conducted (δ2H, δ18OH2O, δ18OSO4, and δ34S) in the pit lake, seepage, and groundwater. Additionally, inverse geochemical modeling was utilized to evaluate mixing relationships and mineral equilibria. Mass-balance calculations were also performed on the seepage/stream system to quantify discrete and diffuse solute inputs to surface waters. The stable isotopic composition

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Pit-lake hydrology and contaminant transport at the Big Ledge Mine

of the pit lake and seepage display evaporative enrichment with the seeps being intermediate between the pit lake and groundwater, indicating that water in the seeps is partially sourced in the upgradient pit lake. Inverse modeling corroborates the isotopic mixing relationships, suggests that sulfide minerals could be dissolving along the flow path in the waste rock, and predicts the precipitation of sulfate salts from the concentrated seepage waters. Mineralogical analysis of efflorescent salts at the site, including epsomite (MgSO4·7H2O), pickeringite

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(MgAl2[SO4]4·22H2O), and melanterite (FeSO4·7H2O), corroborates the inverse modeling.

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Mass-balance calculations indicate the presence of diffuse acid mine drainage inflows along the creek, which, along with discrete inflows from waste rock seepage, contribute to sustained

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dissolved-metal transport in the perennial Tabor Creek downgradient from the mine.

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Keywords: stable isotopes; inverse modeling; sulfate salts; sulfide oxidation; mass balance;

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groundwater-surface water interactions; acid mine drainage; geochemical modeling

1. Introduction

Acid mine drainage (AMD) is a serious issue resulting from the exploitation of mineral

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deposits worldwide. The generation and transport of AMD may result in the contamination of surface waters and groundwaters in the vicinity of mine sites. The geochemistry of waters impacted by AMD varies widely (Nordstrom, 2011b), but regardless of specific contaminants, impacted streams and springs generally have decreased ecological productivity. The processes of contaminant mobilization and transport are therefore important to understand for land managers,

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Newman et al. - REVISED

regulatory agencies, and mining companies, so that negative impacts of mining may foreseen and remediated. One common legacy of modern mining operations is mine pit lakes, which form following the cessation of dewatering activities in open pit mines (Castendyk et al., 2015). These features are common in the state of Nevada, USA, where numerous open-pit mines are currently

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in operation or have closed to form pit lakes (Newman, 2016; Shevenell et al., 1999). Many existing pit lakes in Nevada display overall good water quality and in some instances are

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comparable to terminal natural lakes (Shevenell, 2000). However, some existing Nevada pit

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lakes are acidic with elevated contaminant concentrations, necessitating the application of water treatment (Croall et al., 2017). Pit-lake management strategies in Nevada are also highly

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dependent upon the hydrology of the pit lake (i.e., if it is a terminal groundwater sink or has a groundwater discharge component). Any pit lake that is part of a flow-through groundwater

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system is regulated by the applicable state agency (Nevada Division of Environmental Protection; NDEP) according to groundwater standards (NDEP, 2019a), which are more strict

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than water-quality standards for pit lakes (NDEP, 2019b). In addition to pit lakes, streams are commonly impacted by AMD. The loading of metals and other contaminants to streams can occur due to several processes, by either discrete input

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(e.g., from a flowing adit or seepage from mine waste rock) or by diffuse input through groundwater contributions (Byrne et al. 2017; Lachmar et al., 2006), and many sites have both diffuse and discrete sources of AMD (Johnston et al., 2017). It is possible in both situations to develop remediation plans for impacted streams, which commonly employ quantification of metal sources and geochemical modeling of in-stream processes (Caruso et al., 2008; Runkel et al., 2012). In addition to contributions of metals and acidity from diffuse groundwater inflows 4

Pit-lake hydrology and contaminant transport at the Big Ledge Mine

and discrete surface-water inflows, contaminants may be added to streams through the dissolution of efflorescent sulfate salts. These metal-sulfate salts are common in mined areas and have been associated with substantial contamination of surface waters (Sánchez-España et al., 2005). Salts tend to precipitate due to evaporation during dry periods and then dissolve upon rain events, releasing substantial acidity and contaminants to the environment (Hammarstrom et al.,

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2005). Stable isotopic studies have been applied to mine-impacted environments in a number of

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different hydrogeologic and geographic settings (e.g., Dogramaci et al., 2017; Edraki et al., 2005;

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Mayo et al., 1992; Nordstrom et al., 2007), and specifically in pit lakes to examine geochemical and hydrologic processes controlling contaminant fate and transport (Gammons et al., 2006;

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2009; 2013; Sánchez-España et al., 2014; Pellicori et al., 2005). Although these methods clearly provide useful information on hydrologic and geochemical processes operating in and

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downgradient of pit lakes, no such studies have been published in Nevada. In addition to stable isotopic methods, inverse geochemical modeling may be a useful method to examine mass

2015).

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balance and mixing relationships on mine sites (Glynn and Brown, 2012; Walton-Day and Mills,

This paper focuses on applying stable isotopic analyses, inverse geochemical modeling,

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mineralogical analysis, and evaluation of solute loading to AMD at a former barite mine, the Big Ledge mine. It is important from an environmental and human health perspective to determine the hydrologic character of this pit lake, the mass balance of secondary minerals present on the site, and the potential fate of contaminants in the downgradient stream system. The ultimate goal of this study is to provide for increased knowledge of the hydrologic and geochemical system and to support mine remediation efforts. 5

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2. Study site and background The Big Ledge mine is located in northeastern Nevada in the Snake Range (Fig. 1). The mine site is at relatively high elevation, ranging from approximately 2,300 to 2,500 meters above mean sea level (m amsl). The mine has been operated intermittently since the 1970s, with the most recent mining period ending in 2013. The ore deposit is a Paleozoic bedded-barite (BaSO4)

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type, similar to others located in northern Nevada (Jewell and Stallard, 1991). During periods in which mining has been inactive, a pit lake has formed in the open pit. This pit lake was

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dewatered prior to the most recent mining activity, and has since re-filled to a depth of

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approximately 18 m. The pit lake is assumed to be holomictic (undergoing complete mixing once per year), but this is not known with certainty as the only instance in which the pit lake has been

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water (increasing in stage elevation).

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profiled at depth is this study. As of the completion of these studies the pit lake is still gaining

Although the majority of Nevada is arid, the site is located in an area of relatively high

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precipitation rates (approximately 750 mm annually). Snow commonly accumulates at the site between the months of October and May (and makes up the majority of annual precipitation), making the site generally inaccessible through the winter because of the rugged terrain and lack of road maintenance. The mine is located immediately upgradient from an unnamed intermittent

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stream channel (Fig. 1), which flows to the perennial Tabor Creek, which is in turn tributary to the Humboldt River, a major river system in northern Nevada. The intermittent unnamed stream channel generally flows from May through August or September of each year. A variety of monitoring locations are associated with the site, for both surface water and groundwater (Fig. 1; Table 1). These locations are distributed geographically around the mine

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Pit-lake hydrology and contaminant transport at the Big Ledge Mine

site and both hydraulically upgradient and downgradient in order to quantify any potential waterquality impacts, and the general groundwater flow direction is shown in Fig. 1. Pit-lake water quality is monitored from the shore of the lake, at monitoring location PS-1 (Fig. 1). 1

Distances from the confluence refer to the stream distance measured between the confluence of

overland WRF seepage and the unnamed tributary. This confluence occurs essentially at BLW-

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03, although BLW-03 is a discrete seepage point. Negative distances indicate locations

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upgradient of the confluence.

Hydrologic location is relative to the pit lake, and is described in terms of surface waters based

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on topographic gradients (e.g., the land surface) and for groundwaters in terms of hydraulic

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gradient (e.g., the potentiometric surface).

Impacted refers to direct and clear geochemical (e.g., concentration) or hydrologic (e.g., water-

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level gradient) evidence of impacts from the pit lake, as determined in this investigation. Locations H2O-9 and H2O-10 are not shown in Fig. 1 due to the scale (both locations plot

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outside the map to the north). These locations are summarized in this table because they were used to calculate the average composition of unimpacted surface water (Table 2). In 2016, an area of overland seepage flow with low pH was discovered emanating from

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the toe of the western waste rock facility (WRF), in the areas designated on Fig. 1b as BLW-01, BLW-02, and BLW-03. The monitoring location BLW-04 is upgradient of the seepage discharge location on the unnamed tributary, and locations BLW-05 and BLW-06 are downgradient of the confluence of the WRF seepage and the unnamed tributary. The WRF seepage area displays filamentous algae and a number of secondary efflorescent sulfate salts, which form directly adjacent to the seepage channels (Fig. 2). 7

Newman et al. - REVISED

Beginning in June 2017, remediation efforts have been underway at the site. These efforts include constructing a trench and pond system to capture discrete surface seepage from BLW-02 and BLW-03, and potential subsurface seepage from the WRF. Water treatment using reverse osmosis and microfiltration has also been initialized to allow clean water to be discharged to the unnamed tributary. Prior to initiation of reverse osmosis/microfiltration, the captured seepage water was pumped to the top of the west WRF, where it was evaporated using active evaporation

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sprayers. The reverse osmosis/microfiltration treatment results in the production of a

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concentrated brine. Although the brine production is quantitatively minor compared to the

volume of clean water produced, the brine must be managed to preclude downstream release.

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Because of the high elevation and remote location of the site, the most efficient method of brine

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management has been pumping the brine to the pit lake, resulting in increasing SO4 and metal concentrations in the lake (data not shown in this study). This management strategy has therefore

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complicated the geochemistry of the lake and cannot be used in the long-term because this recycles water and solutes in the interconnected pit lake/seepage system. Active brine

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management was initiated after the conclusion of isotopic sampling described in this study. A portion of the stream chemistry data described in this study were collected after active remediation was initiated, allowing for successes of remediation to be evaluated.

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3. Data and methods

3.1 Sample collection and analysis Water-quality and potentiometric surface elevation data were collected on approximately

a quarterly basis from the mine site for regulatory purposes (see Fig. 1 for locations). The regulation of pit-lake water quality in Nevada is complex, and surface (epilimnion) chemistry is regulated on the basis of total concentration (not dissolved; NDEP, 2019b) while deeper 8

Pit-lake hydrology and contaminant transport at the Big Ledge Mine

(hypolimnion, monimolimnion) chemistry is regulated based on dissolved concentrations that could theoretically seep into the aquifer (NDEP, 2019a). Therefore, the epilimnion is subject to standards for the surface of pit lakes (NDEP, 2019b), while groundwater standards are applied to hypolimnion and monimolimnion waters (NDEP, 2019a). Because of these regulations pit-lake surface samples were not filtered upon collection, but samples of the pit lake at depth were

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filtered to 0.45 μm. Comparison of surface (unfiltered) and depth (filtered) samples indicated no systematic differences.

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Groundwater samples were collected at monitoring wells MW-01A, MW-02, MW-03A,

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MW-05, MW-06, and MW-07. All groundwater samples were filtered to 0.45 μm. Samples of surface water were collected at sites BLW-01, BLW-02, BLW-03, BLW-04,

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BLW-05, BLW-06, TA-1A, and TC-5. Of the surface-water sampling sites, BLW-01, BLW-02,

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and BLW-03 collect strictly WRF seepage discharge. BLW-04 collects unimpacted surface water. The other surface-water sites are variable mixtures of WRF seepage water and unimpacted

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streamflow.

Water-quality standards for streams in Nevada are dictated by state regulation, and the water-quality standards in Tabor Creek and its tributaries are specifically defined by the Nevada Administrative Code (NAC), chapter 445A.1486 (NDEP, 2018). Water-quality sampling

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procedures are dictated by the regulated concentration fraction of each given constituent (i.e., some constituents are regulated based on dissolved concentrations and some on total concentrations). Metal concentrations in streams are generally regulated on the dissolved fraction, so stream samples for metals are filtered to 0.45 μm. The fraction used in the analysis of stream-chemistry is unlikely to have major impacts on the conclusions, as previous research has

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indicated that dissolved constituent loads of Cd and Zn (the constituents of interest) in AMD impacted streams are similar to total loads (Runkel et al., 2013). Parameters analyzed included total alkalinity, Al, As, B, Be, Ca, Cd, Cl, Cr, Cu, F, Fe, K, Mg, Mn, Na, Ni, NO3-NO2, Pb, pH, Sb, SO4, and Zn. Cation samples were acidified to pH 2 with nitric acid and refrigerated. Metal concentrations were quantified by inductively-coupled plasma-

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atomic emission spectrometry. Anion samples were refrigerated and concentrations were quantified using ion chromatography. Alkalinity was determined by titration and pH was

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measured in the field. All samples were analyzed by commercial laboratories certified by State

to quality assurance by calculation of charge balance.

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of Nevada under the Clean Water Act. All water-quality data utilized in this study were subjected

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Stable isotopic samples were collected primarily in June through August 2017, and were

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subsequently analyzed for δ2H, δ18OH2O, δ18OSO4, and δ34SSO4 (hereafter referred to simply as δ34S). A portion of the stable isotope results were presented in an unpublished consulting report

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(Arcadis, 2017), but the interpretation in that report was not supported by data from a site-wide perspective, specifically impacts at downstream monitoring sites TA-1A and TC-5, and internal inconsistencies existed in the interpretation of stable-isotope data. Additional samples not available for Arcadis (2017) are also included in this analysis, expanding the available dataset.

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Samples of δ18OSO4 and δ34S were filtered through a 0.45 μm filter, and samples of δ2H and δ18OH2O were unfiltered. Groundwater and WRF seepage were not analyzed for δ18OSO4. Samples of the pit lake for isotopic analysis were collected both from the surface and at depth. Depth samples were collected using an unmanned aerial vehicle (UAV) equipped with a Niskin sample bottle and modified equipment to facilitate precise sampling of specific layers of the pit lake (Castendyk et al., 2018; Newman et al., 2018). Two depth samples were collected, at 10

Pit-lake hydrology and contaminant transport at the Big Ledge Mine

approximately 10 and 13 meters below the lake surface. These sample depths were selected based on the conductivity-temperature-depth profile of the pit lake at the time of sampling, which indicated thermal and chemical stratification. δ2H and δ18OH2O analyses were completed at two laboratories. Depth samples of the pit lake collected by the UAV were analyzed at the University of Utah Stable Isotope Ratio Facility

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for Environmental Research (SIRFER). Analytical procedures utilized at the SIRFER lab are detailed in Good et al. (2014). Reported uncertainties for δ2H and δ18OH2O analyzed at the

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SIRFER lab are respectively 0.33 and 0.02 per mil (‰). Analysis of all isotopic water samples

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other than the pit-lake depth samples was performed at the University of Nevada, Reno (UNR) Stable Isotope Laboratory by using a Picarro L2130-i cavity ringdown spectrometer, with

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analytical uncertainties for δ2H and δ18OH2O of 1‰ and 0.1‰, respectively. Results of δ2H and

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δ18OH2O are both reported in comparison to Vienna Standard Mean Ocean Water (VSMOW). Samples for analysis of δ18OSO4 and δ34S were quantitatively precipitated to produce

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BaSO4 according to method 4500 of Rice et al. (2012). BaSO4 precipitates were analyzed at the UNR Stable Isotope Laboratory. δ34S analyses were performed using V2O5 as a combustion aid, and followed the methods of Giesemann et al. (1994) and Grassineau et al. (2001). BaSO4 precipitates were analyzed for δ18OSO4 following the method of Kornexl et al. (1999). The

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analytical error, estimated by replicate analysis, was ±0.2‰ and ±0.4‰ for δ34S and δ18OSO4, respectively. Results for δ18OSO4 are reported in comparison to VSMOW, while results for δ34S are reported in comparison to Vienna Cañon Diablo Troilite (VCDT). Although 18OSO4 may be useful for process evaluation on mine sites, only two samples were analyzed for 18OSO4 as part of this study. As such, no further 18OSO4 results are discussed. Results of 18OSO4 will be discussed on a regional basis for Nevada pit lakes (including data from the Big Ledge Mine) in a separate 11

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analysis. Stream discharge was measured by two different methods (depending on the approximate discharge at the time of measurement). At high discharge rates (greater than approximately 10 L/s), the velocity-area method was used (Herschy, 1993). At lower discharge rates the timed volumetric streamflow method was used (Dobriyal et al., 2017).

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Grab samples of efflorescent sulfate salts found near the WRF seepage were collected for

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mineralogical characterization. Mineral specimens were collected and stored in air-tight bags to minimize the extent of dehydration reactions, which are known to occur in this family of

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hydrated mineral salts (Jerz and Rimstidt, 2003). One mineral specimen was subjected to quantitative x-ray diffraction (XRD) using a Panalytical X'Pert Pro X-ray Diffractometer with

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Co radiation, and quantification was performed using Panalytical High Score+ software with

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ICDD PDF-4 Minerals Database and integrated Reitveld Quantification Method. The sample was also analyzed using scanning electron microscopy (SEM) performed on an ASPEX Explorer

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Scanning Electron Microscope with Omegamax EDS detector. Geochemical composition was analyzed by inductively-coupled optical-emission spectroscopy (ICP-OES).

3.2 Mixing calculations and inverse geochemical modeling Under certain conditions, stable isotopes can be used as conservative tracers for

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calculating regional and site-specific water balances (Blasch and Bryson, 2007; Walton-Day and Poeter, 2009), and have been successfully utilized in pit lakes to identify discharge to groundwater (Gammons et al., 2013). In this study, δ2H is utilized as a tracer to complete mixing calculations according to simple two-component linear mixing: 𝛿 2 𝐻𝑠𝑒𝑒𝑝 = (𝛿 2 𝐻𝑝𝑙 × 𝑓𝑝𝑙 ) + (𝛿 2 𝐻𝑔𝑤 × 𝑓𝑔𝑤 )

(1) 12

Pit-lake hydrology and contaminant transport at the Big Ledge Mine

𝑓𝑝𝑙 + 𝑓𝑔𝑤 = 1

(2)

where δ2Hseep, δ2Hpl, and δ2Hgw respectively denote the compositions of the WRF seep, pit lake, and groundwater (all in ‰), and fpl and fgw respectively represent the fractions in the pit lake and groundwater present in the mixed seepage water (unitless). Values for δ2Hseep, δ2Hpl, and δ2Hgw are all known, so the values for fpl and fgw can then be calculated. Assumptions of the mixing

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calculations include that only two end-member components make up the seepage and that the seepage has a composition intermediate between the two end members. Results of stable-isotopic

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analysis (detailed below) support the seepage having an intermediate composition, but it is not

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known a priori if meteoric precipitation (i.e., rain, snow) plays a role in the water balance of seepage. The importance of meteoric precipitation was addressed through inverse geochemical

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modeling.

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The δ34S composition can also be used for mixing calculations (Trettin et al., 2007). However this would introduce substantial uncertainty because, although regional δ34S

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compositions of sulfate and sulfide minerals are available (Arehart et al., 1993; Jewell and Stallard, 1991; Johnson et al., 2008), no analyses of solid-phase δ34S have been conducted on the Big Ledge mine site. Moreover, although δ34S compositions are available for the aqueous phase, dissolution or precipitation of sulfide or sulfate minerals could impact the δ34S composition of

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the waters (Taylor and Wheeler, 1994). Therefore no mixing calculations using δ34S were performed.

In addition to mixing calculations using isotopes (or other conservative constituents),

inverse geochemical modeling has also been shown to be a useful method for illuminating mixing relationships and mass balance on mine sites (Glynn and Brown, 2012; Walton-Day and

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Mills, 2015), and similar inverse mass-balance applications have been used with success at Nevada mines in the past (Earman and Hershey, 2004). In this application, inverse geochemical modeling can be used to test if the composition of the WRF seepage can be explained through mixing and mass-balance modifications of the pit-lake and groundwater compositions. Inverse geochemical modeling has a number of assumptions (see Glynn and Brown [2012]), and these

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are generally met on the Big Ledge mine site (see Supplementary Material for details). Various combinations of initial and final water chemistries and mineral assemblages were

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utilized in the inverse modeling. The input water compositions that were used for modeling are

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summarized in Table 2. Theoretically, the timing of sample used as inputs to inverse modeling (Table 2) could impact the results (Glynn and Brown, 2012). The temporal separation between

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samples was based on the statistical technique of cross-correlations analysis (e.g., Lee et al., 2006), as described by Newman and Mann (2018) and calculations of pore-water velocity

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(Supplementary Material). Although these techniques did allow for an approximate temporal connection to be evaluated, the impact on calculations is likely minor because the compositions

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of all waters involved (groundwater, pit-lake water, and the WRF seepage) were relatively static during 2017 (Fig. 3), so no substantial uncertainty would be introduced by using analyses from different dates as inputs.

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Minerals that were included in modeling were barite, birnessite, calcite, epsomite, fluorite, galena, gibbsite, ferrihydrite, gypsum, melanterite, manganite, pyrite, and sphalerite. Inverse geochemical modeling was completed using the PHREEQC code (Parkhurst and Appelo, 2013) and the minteq.v4 thermodynamic database.

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Pit-lake hydrology and contaminant transport at the Big Ledge Mine

Each of the minerals included in modeling were utilized to provide mass-balance constraints on aqueous constituents shown in Table 2. Many have either been observed in the Independence Mountains (MacFarlane, 2001), at the Big Ledge mine (melanterite and epsomite, results below), at other barite mines in northern Nevada (Johnson et al., 2008), or are commonly assumed to be present at mine sites and/or in pit lakes (Eary, 1999; Nordstrom, 2009). In model number #2, several minerals (calcite, galena, pyrite, and sphalerite) were only allowed to

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dissolve, as opposed to model #1 where more freedom was allowed for all minerals to either

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dissolve or precipitate. This additional control was instituted to reduce the number of potential models, and because the low pH, high sulfate concentrations, and intermediate δ34S in the WRF

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seepage are conceptually consistent with sulfide dissolution.

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3.3 Groundwater-surface water interactions and contaminant mass balance In mine-impacted streams, the quantification of sources and sinks of contaminants is an

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important consideration for defining and guiding remediation efforts. Various methods for massload quantification are summarized in Kimball et al. (2009), and a brief summary of the method

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used in this study follows. The dissolved-mass transport of each constituent of interest was calculated at all surface-water monitoring locations for each sampling event according to equation 3:

(3)

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𝑀𝑐 = 𝐶𝑐 × 𝑄

where Mc denotes the dissolved-mass flux of constituent c (mg/s), Cc denotes the dissolved concentration of constituent c (mg/L), and Q denotes the discharge of the stream at the given location (L/s). As described above, dissolved-mass flux is assumed to correspond closely to total-mass flux for the constituents of concern (Runkel et al., 2013).

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Geochemical modeling may be useful for evaluating potential reactions occurring in the stream channel (e.g., Caruso et al., 2008; Lachmar et al., 2006). In this study PHREEQC (Parkhurst and Appelo, 2013) was applied to water-quality data to calculate the saturation indices of various minerals in surface waters. All simulations used the default minteq.v4 thermodynamic database.

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4. Results

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4.1 Site geochemistry and hydrology

The pit lake has shown variable geochemical characteristics over time, ranging from

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acidic with elevated concentrations of metals to circumneutral with relatively low contaminant

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concentrations (Fig. 3). When the pit lake is acidic it displays a typical chemistry for AMD, with elevated concentrations of Al, Cd, Zn, and SO4 (likely from sulfide oxidation). In particular, Cd

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is of interest at the site due to elevated concentrations of this contaminant in impacted waters (up to approximately 10 mg/L; Fig. 3d). These Cd concentrations are orders of magnitude higher

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than Cd concentrations associated with other contaminated sites around the world (Kubier et al., 2019), illustrating the extreme geochemical nature of the WRF seepage. The minimum and maximum concentration of all parameters for groundwater, WRF seepage, the pit lake, and unimpacted surface waters are summarized in Table 3. All analytical data related to the site are

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supplied in the Supplementary Material. The groundwater flow direction on the site is generally from the northwest to southeast

(Fig. 1; Supplementary Material), and water-level observations allow a conceptual hydrologic model of the site to be established (Fig. 4). The location of the cross section shown in Fig. 4 is

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Pit-lake hydrology and contaminant transport at the Big Ledge Mine

illustrated in Fig. 1, running from northwest to southeast. Water table elevations in monitoring wells near the cross section were projected to the section. The orientation of the water table through time in the vicinity of the pit lake has varied. In the time period 2007 through 2012 when MW-03 (not shown in Fig. 1) was actively monitored the pit-lake surface elevation was greater than that in MW-03, indicating a potential discharge

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from the pit lake to groundwater. Well MW-03 was abandoned in 2012, and a replacement well (MW-03A) was completed adjacent to the previous well, but was not drilled until 2017. The

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water table elevation in this well is slightly higher than the pit lake (by approximately a meter),

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although fluctuations closely match the pit-lake surface (see all hydrographs in Supplementary Material). The later water table orientation would suggest that the pit lake does not experience

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outflow. However, MW-03A was not an exact replacement for MW-03, as it was completed approximately 20 meters shallower (well depths on Fig. 4 are to scale). Given the indicated

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fracture-flow characteristics of the Big Ledge site (Supplementary Material), and the known complexity of groundwater flow in fractured bedrock mine environments (Lachmar, 1994), there

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is reason to believe that the new MW-03A may not be monitoring the same fracture system as the previous MW-03, which would complicate simple hydrologic determination of flowpaths downgradient from the pit lake. In addition to complication of groundwater-flow interpretation,

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contaminant transport in the vicinity of pits excavated in fractured bedrock has been shown to be heterogeneous and anisotropic, with enhanced solute transport in fractures (Farouk et al., 2015). Because well MW-03A likely does not intersect the same fracture system as was previously intersected by MW-03, it would not be expected to observe water-quality impacts of the pit-lake outflow in MW-03A.

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Similar to MW-03A, MW-05 also does not display water-quality impacts from the pit lake. The likely reason for the lack of apparent pit-lake outflow chemistry in MW-05 despite its intermediate location between the pit lake and main WRF seepage discharges at BLW-02 and BLW-03 is that MW-05 is located to the east of the most direct groundwater flow path from the pit lake to the seepage location (see Fig. 1). The hydrologic cross section in Fig. 4 also displays

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the intersection of the estimated potentiometric surface with the land surface in the vicinity of the original seepage monitoring location at BLW-01 (see Fig. 1 for surface locations).

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Overall the geochemical and hydrologic data suggest that the pit lake may be the source

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for the seepage, given the intersection of the potentiometric surface with the land surface in the simplified cross section, and the general geochemical similarities between pit-lake and WRF

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seepage waters (Fig. 3). However, the WRF seepage waters are substantially more concentrated than the pit lake, requiring some input of solute mass along the flow path. The likely source of

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this solute influx is minerals in the WRF, which the conceptual flow path runs subparallel to and which is known to contain acid-generating materials. This conceptual model of pit-lake outflow

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and interaction with gangue minerals in the WRF is further tested and refined below using stable isotope data, mixing calculations, and inverse modeling.

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4.2 Stable isotopic composition and mixing calculations

The stable isotopic composition of δ2H and δ18OH2O is illustrated in Fig. 5, which

includes an estimate of isotopes in precipitation from the online isotopes in precipitation calculator (OIPC; Bowen, 2017). Also plotted on Fig. 5 is the global meteoric water line (GMWL) of Craig (1961). 18

Pit-lake hydrology and contaminant transport at the Big Ledge Mine

The composition of groundwater extracted from wells on the Big Ledge site is most similar to winter and spring precipitation (November and April) as estimated by OIPC, indicating that groundwater recharge may be composed primarily of precipitation from these months. The regression line for groundwater (slope =5.58) dips less steeply than the GMWL (slope = 8) and the OIPC-LWML (slope = 7.66). Because the of the moderate differences between the OIPCLWML and the groundwater composition, the regression line for groundwater is herein termed

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the local groundwater recharge line (LGRL), which indicates precipitation water that has

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infiltrated to the groundwater table and has experienced some evaporation during recharge.

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The linear regression lines for WRF seepage and the pit lake display shallower slopes than the LGRL or any MWLs and are consistent with evaporative enrichment of groundwater.

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The pit-lake and WRF seepage regression lines are nearly parallel, with the WRF seepage regression line shifted to slightly lighter compositions. The regression lines appear to define

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local evaporation lines (LEL) representative of open-water evaporation, similar to the LEL observed at Butte, Montana (Gammons et al., 2006). While open-water evaporation is certainly

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ongoing in the Big Ledge pit lake, the WRF seepage areas do not host pools or other areas of open water. The similarity of the slopes therefore suggests that some of the water in the WRF seepage is derived from the upgradient pit lake. While shallow soil-water evaporation can result

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in formation of an LEL, an LEL formed in this manner would be expected to dip substantially more shallowly than an LEL formed by open-water evaporation (Gibson et al., 2008). The minor difference in slope of the pit-lake and WRF LELs on Fig. 5 do not support in-situ soil-water evaporation as the process responsible for fractionation of the WRF seepage waters. Sulfur isotope composition is plotted vs. SO4 concentrations in Fig. 6, illustrating that groundwater has a wide range in δ34S values (+5.8 to +9.0‰) and SO4 concentrations (15 to 200 19

Newman et al. - REVISED

mg/L), while the pit lake and WRF seepage have much less variability, with slightly heavier δ34S values (approx. +11 and +10‰, respectively) and substantially higher SO4 concentrations (approx. 2,000 and 20,000 mg/L, respectively). To facilitate conceptual evaluation of possible processes on the site, Fig. 6 also illustrates a comparison of δ34S values from this study with δ34S values from mineral deposits in northern Nevada (Arehart et al., 1993; Bawden et al., 2003; Mitchell, 1977). All δ34S values are from deposits hosted in Paleozoic rocks, although not all are

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directly applicable to stratiform barite deposits. Additional discussion of these ancillary data is

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included in the Supplementary Material.

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Two geochemical pathways are illustrated on Fig. 6a. The first pathway (labeled mixing only) illustrates the likely composition of the WRF seepage waters if they were composed only

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of conservative mixing between pit-lake waters and groundwater. It is clear that the WRF seepage waters do not lie on the mixing only line, and therefore cannot be explained by pure

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conservative mixing (i.e., mixing without further geochemical reaction). The second pathway (sulfide oxidation) illustrates a possible geochemical evolution if the pit-lake waters were to be

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modified by sulfide oxidation, which would cause both an increase in SO4 and a likely decrease in δ34S (because sulfide minerals in northern Nevada commonly have mean δ34S of near zero ‰; Fig. 6b). The δ34S values of WRF seepage and the pit lake are likely influenced by the presence

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of sulfate minerals (higher δ34S), and the slightly isotopically lighter δ34S of the WRF seepage may indicate the modification of pit-lake via sulfide oxidation, following discharge from the pit lake into the fractured aquifer. The results of stable isotopic mixing calculations are summarized in Table 4. Groundwater compositions could be represented by MW-03A (which is in the flow line from the

20

Pit-lake hydrology and contaminant transport at the Big Ledge Mine

pit lake to the WRF seepage) or MW-05 (which is adjacent to the seepage). The pit-lake waters themselves represent the pit-lake end member. Mixing calculations generally indicate that the pit lake could make up 70% or more of the WRF seepage, and the median of the calculations is that 76% of WRF seepage water could be derived from the pit lake (although one of the four permutations indicates 35% of the seepage

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being made up by the pit lake). Also, the calculations using MW-05 as an input (which is the closest groundwater monitoring well to the WRF seepage, see Fig. 1) are fairly consistent,

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indicating between 74% and 91% of the seepage is derived from the pit lake.

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4.3 Inverse geochemical modeling

Prior to conducting inverse geochemical modeling, the composition of each input water

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(Table 2) was evaluated for saturation with potential mineral phases (to evaluate if any minerals

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are likely dissolving or precipitating). This analysis indicated that ferrihydrite and gibbsite were supersaturated in the pit-lake and MW-05, but undersaturated in BLW-02. Gypsum was

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contrastingly undersaturated in the pit lake and MW-05, but at saturation (SI=0) in BLW-02. Melanterite was undersaturated in all waters, even though it has been observed in the vicinity of BLW-02 (see results below in Section 4.4 Mineralogy of Efflorescent Salts). Finally, two phases that are important for the inverse modeling (pyrite and manganite) were consistently

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undersaturated. Pyrite is important because pyrite oxidation in the WRF is conceptually the source of acidity in the WRF seepage waters. Manganite is important for its part in the redox chemistry of the geochemical evolution of waters, as will be discussed further below. It is important to note that although pyrite is undersaturated in all waters, it is closer to saturation in BLW-02 (SI=-38.1) than in the pit lake (SI=-85.1). Therefore, SI values are consistent with some pyrite dissolution along the flow path from the pit lake to the seepage, given that the “final” 21

Newman et al. - REVISED

water has become more saturated with respect to pyrite. A full table of SI values in all inverse modeling inputs is found in the Supplementary Material. Two different inverse geochemical models were utilized to evaluate potential mass balance and mixing relationships on the site. Model #1 uses the compositions of the pit lake and MW-05 as the initial waters and the composition of seepage at BLW-02 as the final water.

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Although MW-03A is between the pit lake and the seepage discharge point, this well was not used because appropriate records were not available for the temporal frequency identified by

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cross-correlations analysis (Newman and Mann, 2018) or pore-water flow velocities

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(Supplementary Material), as monitoring well MW-03 was replaced by MW-03A (see the gap in monitoring records in Fig. 3). Groundwater sampled in well MW-05 likely represents

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groundwater in close proximity to the seepage. Also the geochemistry of wells MW-03/03A and

different wells is likely minor.

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MW-05 is not substantially different (Fig. 3), and therefore the difference between using these

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Model #2 uses the same compositions for groundwater and the pit lake, but also includes the chemistry of precipitation to test the impact of including a third water-budget component on the modeling. Although meteoric precipitation is partially included in the groundwater and pitlake compositions by nature of the groundwater recharge process, explicitly including this

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component allows for the proportion of direct precipitation on the WRF to be evaluated. An average precipitation chemistry (for 1984 to 1997) was derived from station number NV01 (Saval Ranch) of the National Atmospheric Deposition Program (NADP) database. Site NV01 is approximately 40 miles from the Big Ledge mine. Although isotopes can be used in inverse modeling, no isotopic compositions were used due to uncertainties in the isotopic composition of solid phases. As the stable isotopic compositions of waters are not used in the inverse modeling, 22

Pit-lake hydrology and contaminant transport at the Big Ledge Mine

the isotopic mixing calculations and inverse modeling are independent of one another and may be used to evaluate results from the other method. Numerous solutions were realized for both models, as is common for inverse geochemical modeling (e.g., Glynn and Brown, 2012). Both model #1 and model #2 had nine possible solutions. Not all of these solutions are considered further, because while they are

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geochemically-feasible subset of realized solutions is presented here.

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mathematically feasible they may not be realistic for a variety of reasons. Only the most

Model #1 indicates pit-lake mixing fraction of 0.76 (Table 5), quite similar to the range

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calculated by isotopic mixing calculations (although these methods are independent of one another). The mineral masses transferred in model #1 generally indicate dissolution of minerals,

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aside from melanterite, manganite, and calcite, which are predicted to precipitate. It is important

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to note that while not all of these minerals have not directly been observed on the site, several have. Specifically, the precipitation of melanterite (FeSO4·7H2O), epsomite (MgSO4·7H2O) and

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pickeringite (MgAl2[SO4]4·22H2O) has been observed in the vicinity of the WRF seepage (Section 4.4). Gypsum, although generally common at mine sites, was not predicted to be involved in mass-transfers. This may be somewhat surprising given that the analysis of gypsum SI values in the seeps indicates that gypsum is at saturation.

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The results of the first realization for model #2 indicate that a portion of the water-budget

of the seepage could be composed of meteoric precipitation (0.17), and that no groundwater would be required. The second realization indicates that the entirety of the WRF seepage could be made up by the pit lake. Similar to the results of model #1, a variety of minerals are predicted to be dissolving in model #2. In this realization however melanterite is predicted to dissolve and

23

Newman et al. - REVISED

not precipitate, which is contrary to mineralogical observations at the site. Also, neither of the realizations for model #2 include dissolution of pyrite. Aside from the presence and mass-balance of melanterite in each realization, several other mineralogical aspects are noteworthy. Each model includes manganite (MnOOH) and birnessite (MnO2) in mass transfers, and all mass transfers of these Mn minerals are essentially

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equal in magnitude but opposite in direction (i.e. birnessite is dissolving and manganite is precipitating). These results suggest redox changes in the solution, as Mn4+ in birnessite is

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reduced to Mn3+ in manganite. The birnessite dissolution results in model #1 are also quite high

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(i.e., large molar quantities), possibly suggesting that one or more redox controls in the realworld waters are not implemented in the model. Although redox was not explicitly included in

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the modeling, redox measurements were collected throughout the sampling period. The results indicate that the Eh of pit-lake water ranges from 222 mV to 454 mV, while Eh in WRF seepage

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waters (both BLW-02 and BLW-03) typically has a smaller range from 360 mV to 375 mV. The redox chemistry of the seepage appears to be complex, and cannot be easily explained with the

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inverse modeling. However, the inverse modeling is still a useful tool to evaluate potential geochemical reaction pathways on the site (e.g., Walton-Day and Mills, 2015), although the modeling results should not be stated without a proper understanding of their uncertainty and

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ambiguity.

An additional noteworthy aspect of the inverse modeling is that the second-most

important sulfide mineral by mole transfers (after pyrite) dissolving in the reactions is sphalerite. Despite no direct observations of sphalerite at the site, the elevated Zn concentrations support its presence, as do general mineralogic assemblages observed at bedded barite mines (Johnson et al., 2017). Although it was not included in inverse modeling because it lacks a controlling solid 24

Pit-lake hydrology and contaminant transport at the Big Ledge Mine

phase, Cd is very important at the site due to the elevated concentrations in WRF seepage (Fig. 3d). Cadmium has been noted at other mine sites to be substituted into the sphalerite structure (Diehl et al., 2008; Schaider et al., 2014). A strong positive correlation in waters at the Big Ledge mine supports that sphalerite may be the source of Cd at the site (Fig. 7).

4.4 Mineralogy of efflorescent salts

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The active seepage area downgradient of the west WRF displays abundant efflorescent sulfate salts (Fig. 2). Results of quantitative XRD indicate that the sampled salts are 79%

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epsomite (MgSO4·7H2O), 12% pickeringite (MgAl2[SO4]4·22H2O), and 9% melanterite

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(FeSO4·7H2O). The predominance of Mg-minerals at the site is somewhat anomalous from the majority of sites in the literature, which tend to be dominated by Fe-minerals (e.g., Chou et al.,

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2013).

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Characterization completed by SEM (Fig. 8) illustrates that the salts are fine grained and vary compositionally over small scales. The spectra in Fig. 8a indicate that in the characterized

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portion of the sample the dominant cation is Mg, while Al is the dominant cation in the portion characterized in Fig. 8b. These two areas of the sample are likely epsomite and pickeringite respectively. Fig. 8a also shows minor Mn and Zn peaks, likely reflective of minor substitution of these species in the mineral structure, which has been shown to be important for contaminant

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mobility in surface waters impacted by AMD and secondary salt formation (Hammarstrom et al., 2005).

Substitution of other metals into the efflorescent salt mineral structure is also indicated by

the composition determined by ICP-OES, which showed elevated Cd (119 mg/kg), Co (199 mg/kg), U (75 mg/kg), and Zn (9,750 mg/kg). Full analytical results of ICP-OES are presented in

25

Newman et al. - REVISED

the Supplementary Material. The molar ratio of Cd:Zn in the aqueous phase (average [Cd]:[Zn]=0.026) is nearly four times greater than the Cd:Zn molar ratio in the solid phase ([Cd]:[Zn]=0.007), where square brackets denote molar concentrations. The correlation of Cd and Zn in the aqueous phase (Fig. 7) likely reflects Cd substitution into sphalerite, which may be dissolving in the WRF. The difference between the molar Cd:Zn ratios in the aqueous phase and efflorescent salts may indicate that precipitation of metal salts is not scavenging Cd as efficiently

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as Zn. While the trace-metal content in the solid phase shows that these salts are a potential sink

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of contaminants, dissolution of salts during storm events or in spring during greater stream flows

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would release these constituents into surface waters.

4.5 Mass-balance of contaminants in surface waters

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Typical for streams in mountainous regions, discharge rates for Tabor Creek and the unnamed tributary immediately downgradient from the mine site vary seasonally, with a clear

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effect of spring runoff (Supplementary Material). Since approximately June 2017, the WRF seepage at BLW-02 and BLW-03 has been captured in a trench and diverted into a lined pond to

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minimize contaminants from migrating into the unnamed tributary. The captured seepage has been either pumped to the top of the west WRF and evaporated using active evaporation units (approximately June 2017 through August 2018), or pumped back to the pit lake (approximately

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August 2018 onward), the latter of which complicates the chemistry and water balance of the pit lake. Excavation of the trench showed that low pH seepage was travelling along the interface between fractured bedrock and alluvium at a depth of approximately 4 meters below ground surface. Because of the excavation of this cutoff trench, discharge at BLW-02 and BLW-03 ceased in September 2017, although seepage was reinitiated near BLW-03 (at location BLW03A, not labeled on Fig. 1) in May 2018. 26

Pit-lake hydrology and contaminant transport at the Big Ledge Mine

Although the discharge rates from the seepage locations can no longer be directly monitored due to the excavation, the water balance of the seeps can be evaluated using the pumping rates from the capture pond (Fig. 9). Pumping rates from the pond are proportional to seepage discharge rates into the pond because the pond is kept at an operational level, meaning that pumping is increased when seepage discharge increases, and vice versa. Inspection of Fig. 9 indicates that diffuse seepage discharge rates into the interceptor trench since excavation have

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varied seasonally, similar to discrete seepage discharge rates prior to excavation of the trench

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and to natural streamflow (see Supplementary Material). Together these results indicate that, although some portion of the water budget of seepage is derived from the pit lake (as evidenced

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from stable isotopes), seasonal water-budget input from snowmelt and rainfall is likely. The

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pumping rate (and by association the seepage discharge rate) never reaches zero however, and apparently asymptotically approaches a value that may represent the long-term portion of the

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seepage derived from the pit lake, similar to groundwater baseflow in streams. It is also important to note that the pumping rates from the pond are generally greater than the sum of

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observed seepage discharge rates at BLW-02 and BLW-03. The greater pumping rate indicates that the entirety of seepage from the WRF likely was not discharging to the surface, but some was flowing in the shallow subsurface in the region now intercepted by the capture trench.

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Quantification of temporal variation in dissolved-mass transport in surface water for Al, Cd, and Zn (Fig. 10) illustrates that sampling location BLW-06, approximately 50 meters downstream from the confluence of the unnamed tributary and discrete WRF seepage discharge at BLW-02 and BLW-03 consistently has the greatest mass-transport rate for Al (until early 2018), while mass-transport rates for Cd and Zn are generally greater at site TA-1A (approximately 1,070 meters downstream of the mine at the confluence between the unnamed 27

Newman et al. - REVISED

tributary and Tabor Creek). Site BLW-04 (approximately 30 meters upgradient of the confluence of WRF seepage the unnamed tributary) has consistently low dissolved-mass transport, implying that the WRF seepage (both discrete and diffuse) is the main contributor of dissolved metal mass to the stream. Dissolved-load quantification also indicates that the unnamed tributary is receiving

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diffuse AMD in addition to the discrete WRF seepage locations (BLW-02 and BLW-03). Evaluation of the time period October 2016 through August 2017 on Fig. 10 shows that the

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combined dissolved-mass transport at sites BLW-02 and BLW-03 is substantially less than the

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dissolved-mass transport at BLW-06 (note the logarithmic scales). No other discrete seepage areas have been found in the reach between BLW-03 and BLW-06. The lack of other discrete

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sources, yet the greater mass transport of mining-related constituents at BLW-06, is likely due to diffuse seepage from the mine site that is being discharged from the subsurface into the unnamed

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tributary in the reach immediately downstream of the discrete WRF discharge. Such a situation would not be uncommon, in that former mine sites often have both discrete and diffuse sources

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of AMD into streams (Byrne et al., 2017; Johnston et al., 2017). Although diffuse AMD impacts are evident, the excavation of the seepage interceptor trench does appear to have reduced mass loading to the stream. The most apparent positive

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water-quality impacts are the substantially reduced dissolved loads in site BLW-06 in spring 2018 (February 2018 through May 2018) when compared to the same period in 2017 (Fig. 10). It is not known however how much of this inter-year variability is due to climatic drivers. In northern Nevada, water year 2017 (1 October 2016 through 30 September 2017) was extraordinarily wet, leading to increased discharge rates in spring of 2017. Therefore, some of

28

Pit-lake hydrology and contaminant transport at the Big Ledge Mine

the increased mass loading during spring 2017 may be linked to climatic drivers, and the decreased loading in 2018 may be due to less snowpack in winter 2018 than during winter 2017. Mineral equilibrium calculations using PHREEQC indicate that gibbsite is predicted to be supersaturated in the majority of surface-water sites over time, showing the potential for this phase to precipitate (Fig. 11). Amorphous Al(OH)3 however was generally undersaturated (Fig.

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11), which is consistent with observations by Nordstrom (2011a) that gibbsite commonly displays greater supersaturation than amorphous Al(OH)3 in circumneutral waters, despite field

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observations indicating that amorphous phases such as basaluminite are dominant (Nordstrom,

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2011a). The possible precipitation of an Al-phase is also evident in the dissolved-mass loading calculations, where dissolved Al-load is generally less at site TA-1A than at site BLW-06 (Fig.

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10a), whereas Cd (Fig. 10b) and Zn (Fig. 10c) mass loading displays the opposite pattern. Together with the mineral equilibrium calculations, the mass-transport calculations indicate that

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Cd and Zn are not.

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Al is being removed from the system in the stream reach between BLW-06 and TA-1A, whereas

These simple equilibrium calculations however do not incorporate the effects of mineral growth kinetics. In field studies, amorphous minerals are commonly observed to precipitate from solution first, and in specific reference to Al phases, amorphous Al-hydroxysulfates are

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commonly found near sources of AMD (Nordstrom, 2011b). Although no mineralogic determination was made, a white-colored mineral precipitate was observed to be forming immediately below the confluence of the unnamed tributary and Tabor Creek at site TA-1A (Fig. 11c). Based on the dissolved-mass transport results discussed above, it is considered plausible that this is an amorphous Al-phase.

29

Newman et al. - REVISED

5. Discussion The stable isotopic results collected from the Big Ledge pit lake, WRF seepage, and groundwater monitoring wells suggest that the pit lake is a flow-through system that discharges to downgradient groundwater. Following mixing and geochemical reactions in the subsurface, it is then likely that the water partially derived from the pit lake becomes surface water again when

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it discharges as the WRF seepage and as diffuse inflows to the unnamed tributary.

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Discharge from the pit lake could make up between 35% and 91% of the seepage waters, based on δ2H mixing calculations (Table 4). The seepage is highly concentrated compared to the

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pit lake water however (Fig. 3), so dissolution of minerals in the WRF is required to satisfy mass-balance constraints, and is indicated by the δ34S composition of the pit lake and WRF

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seepage (Fig. 6). The use of stable isotopes in this study are quite similar to the application in

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Gammons et al. (2013), where a pit lake in Montana, USA was also shown to be part of a groundwater flow-through system. There is an alternative explanation for the enrichment of δ2H-

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δ18O in the WRF seepage, namely that similar evaporative processes could be occurring as in the pit lake. However, no storage of water in pools occurred during the isotopic sampling, and therefore there was no hydraulic retention time for seepage water to evaporate and become isotopically enriched. Another possible explanation is that evaporation in the vadose zone

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occurred. However, shallow subsurface evaporation would be expected to result in an LEL with a much shallower dip than the open-water LEL observed from the pit lake (Gibson et al., 2008). No substantial difference in LEL slopes is observed in the data however. Additionally, the δ34S compositions of WRF seepage indicate sourcing in the pit lake. Therefore, an alternative conceptual model of separate pit-lake/WRF seepage systems is not considered to be reasonable.

30

Pit-lake hydrology and contaminant transport at the Big Ledge Mine

Inverse geochemical modeling (although more ambiguous than mixing calculations), produces similar results in that the pit lake is calculated to makeup between 76% and 100% of the seepage discharge waters (Table 5). While this model cannot “prove” that the simulated reactions are occurring (e.g., Nordstrom, 2012), it is nonetheless a useful tool for evaluating potential reactions. It is important to note that inverse modeling and isotopic mixing calculations are independent of one another because isotopic compositions were not included in inverse

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modeling. The relative agreement of the two methodologies therefore may support their

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individual results, and both methods indicate that the pit lake is part of a flow-through system.

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Although diffuse subsurface seepage is apparently adding mass of dissolved metals to Tabor Creek along much of the sampled reach, remediation efforts do appear to have decreased

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contaminant transport from the site. The total seepage pumped from the capture trench (Fig. 9), for the timeframe of July 2017 through November 2018, can be summed to estimate the total

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volume of seepage that the trench has impounded and kept out of the unnamed tributary. The calculation results in approximately 45,300 L of seepage effluent that has been captured.

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Multiplication of this volume by the average dissolved concentrations of Cd and Zn in BLW-02 and BLW-03 yields a rough estimate of the contaminant mass that has been intercepted. These calculations yield estimates of 0.49 kg of Cd and 22.5 kg of Zn that have been captured.

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Although these calculations are only approximate, they do indicate that excavation of the capture trench has reduced mass loading to the tributary. Dissolved-mass transport compared between years 2017 and 2018 also indicates that remediation has lessened the downstream transport (Fig. 10). The percent reduction from May 2017 to April 2018 in cumulative Cd dissolved flux was 94% at the confluence between Tabor Creek and the unnamed tributary (site TA-1A), indicating

31

Newman et al. - REVISED

the effectiveness of the remediation. It is important to note however that the impact of climatic fluctuations remains unclear given the differences in snow accumulation between the two years. It is worthwhile to discuss the different spatial and temporal scales over which AMD investigations are commonly performed in surface waters, and the notable aspects of this study. Many investigations utilize spatially-detailed and temporally-focused sample collection (i.e., tens

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of sample collection sites with samples collected at each site each hour for a period of 24-hours; Gammons et al., 2015; Kimball and Runkel, 2009). While this allows for intimate knowledge of

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the system on the day of the sample collection, such applications are generally lacking in terms

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of long-term trends. The study described herein contrasts with these methods in that there are fewer sample sites (6 sites), and samples are not collected in a synoptic manner. While this

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approach lacks fine temporal resolution, the dataset described herein is rare in the literature in the context of the long-term assessment it allows. By quantifying metal fluxes over a period of

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approximately 2 years it is possible to see the seasonal variability in diffuse discharge, and the impact of remediation becomes more evident. Therefore, while the approach may not describe

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diel effects it allows for an assessment of metal loading and attenuation on the watershed scale and an extended timeframe that is useful for land management agencies.

6. Conclusions

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The Big Ledge mine in northeastern Nevada is a dynamic system of interconnected

groundwater and surface-water resources. Mining-related contaminants including Al, Cd, and Zn are partially sourced in the pit lake on the mine site, and likely are influenced by mineral dissolution in the WRF. In this study, a multidisciplinary approach was utilized to quantify water balances and potential geochemical reactions on the Big Ledge site. Inverse geochemical

32

Pit-lake hydrology and contaminant transport at the Big Ledge Mine

modeling results are consistent with the mixing ratios determined using stable isotopes and support the presence of discharge from the pit lake. Isotopic mixing calculations and inverse modeling are completely independent of one another quantitatively, and therefore their relative agreement lends support to the overall calculated mixing ratios. Inverse geochemical modeling has indicated that a number of mineral phases may be dissolving in the WRF, and these results

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are corroborated by correlations of Cd and Zn, as well as the δ34S of pit-lake and seepage waters. Upon discrete surface discharge, WRF seepage waters are highly concentrated in

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contaminants. The elevated concentrations result in the precipitation of efflorescent metal-sulfate

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salts that are mineralogically dominated by Al-Fe-Mg species (pickeringite, melanterite, and epsomite respectively), but that also contain substantial Cd and Zn mass.

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Downstream dissolved-mass loading analysis indicates that the entirety of the AMD

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seepage is not present above the ground surface, and therefore may not be captured in the cutoff trench that has been constructed downgradient of the WRF. Although not all AMD related to the

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site is captured, the remediation efforts have somewhat decreased downstream loading of contaminants. Downstream transport also appears to be impacted by mineral precipitation for Al, but not for Cd or Zn, which are transported farther from the site in Tabor Creek. Although remediation has reduced downstream Cd transport, dissolved Cd concentrations following

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remediation are still greater than applicable regulatory limits. Further remediation is currently ongoing to address these regulatory concerns. The study of the Big Ledge mine discussed herein combined a number of techniques

including inverse geochemical modeling, stable isotopic analyses and mixing calculations, analysis of groundwater-flow dynamics, mineralogy of secondary salts, and dissolved-load

33

Newman et al. - REVISED

quantification in an attempt to outline a more complete understanding of this complex site. Despite the successes of these studies, work at the mine is ongoing. The multidisciplinary approaches used at the Big Ledge mine can serve as an example of the uses, and uncertainties, of these methods. The methods applied at the Big Ledge mine have shown themselves to be very useful for regulatory purposes, specifically to identify flow-through pit lakes and attempt to

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quantify groundwater-surface water interactions.

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Acknowledgements

The authors appreciate the help of Devin Castendyk, Brian Straight, Pierre Filiatreault, and

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Americo Pino in collecting depth samples of the pit lake with the UAV. Patrick Goldstrand, Tom Gray, Rob Kuczynski, Shawn Gooch, Joe Sawyer, Aili Gordon, Rob Hegemann, Dan Erbes, Bill

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Doherty, Jeff Mann, Greg Kipp, Patsy Moran, and John Barta all have contributed to

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understanding and managing the site. Joshua Zimmerman, Salt Anderson, and George Burke performed the XRD, SEM, and ICP-OES laboratory work. Financial support was received from

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the NDEP for isotopic sample preparation and analysis. Two anonymous reviewers provided

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helpful comments that improved the manuscript.

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Pit-lake hydrology and contaminant transport at the Big Ledge Mine

References Arcadis, 2017, Big Ledge mine – Isotope investigation, Unpublished consulting report concerned with isotope geochemistry of the Big Ledge mine, 96 pp.

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Arehart, G.B., Eldridge, C.S., Chryssoulis, S.L., and Kesler, S.E., 1993, Ion microprobe determination of sulfur isotope variations in iron sulfides from the Post/Betze sediment-

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hosted disseminated gold deposit, Nevada, USA, Geochemica et Cosmochemica Acta,

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vol. 57, pp. 1505-1519.

Bawden, T.M., Einaudi, M.T., Bostick, B.C., Meibom, A., Wooden, J., Norby, J.W., Orobana,

re

M.J.T., and Chamberlain, C.P., 2003, Extreme 34S depletions in ZnS at the Mike Gold

lP

deposit, Carlin Trend, Nevada: Evidence for bacteriogenic supergene sphalerite, Geology, vol. 31, no. 10, pp. 913-916.

ur na

Blasch, K. W. and Bryson, J. R., 2007, Distinguishing sources of ground water recharge by using δ2H and δ18O, Groundwater, vol. 45, no. 3, pp. 294–308, DOI:10.1111/j.17456584.2006.00289.x.

Bowen, G. J., 2017, The Online Isotopes in Precipitation Calculator, version 3.1.

Jo

http://wateriso.utah.edu/waterisotopes/index.html.

Bowen G. J., Wassenaar L. I. and Hobson K. A., 2005, Global application of stable hydrogen and oxygen isotopes to wildlife forensics, Oecologia, vol. 143,pp. 337-348, DOI:10.1007/s00442-004-1813-y.

35

Newman et al. - REVISED

Byrne, P., Runkel, R.L., and Walton-Day, K., 2017, Synoptic sampling and principal components analysis to identify sources of water and metals to an acid mine drainage stream, Environmental Science and Pollution Research, vol. 24, no. 20, pp. 17220-17240, DOI: 10.1007/s11356-017-9038-x. Caruso, B.S., Cox, T.J., Runkel, R.L., Velleaux, M.L., Bencala, K.E., Nordstrom, D.K., Julien,

of

P.Y., Butler, B.A., Alpers, C.N., Marion, A., and Smith, K.S., 2008, Metals fate and transport modelling in streams and watersheds: state of the science and USEPA workshop

ro

review, Hydrological Processes, vol. 22, pp. 4011-1042, DOI: 10.1002/hyp.7114.

-p

Castendyk, D.N., Eary, L.E., and Balistrieri, L.S., 2015, Modeling and management of pit lake

10.1016/j.apgeochem.2014.09.004.

re

water chemistry 1: Theory, Applied Geochemistry, v. 57, pp. 267-288, DOI:

lP

Castendyk, D., Hill, B., Filiatreault, P., Straight, B., Alangari, A., Cote, P., and Leishman, W., 2018, Experiences with Autonomous Sampling of Pit Lakes in North America using

ur na

Drone Aircraft and Drone Boats, pp. 1036-1041, in: Wolkersdorfer, C., Sartz, L., Weber, A., Burgess, J., and Tremblay, G. (eds.), Proceedings of the 11th International Conference on Acid Rock Drainage, South Africa.

Jo

Chou, I.M, Seal, R.R. II, and Wang, A., 2013, The stability of sulfate and hydrated sulfate minerals near ambient conditions and their significance in environmental and planetary sciences, Journal of Asian Earth Sciences, vol. 62, pp. 734-758.

Craig, H., 1961, Isotopic variations in meteoric waters, Science, vol. 133, pp. 1702–1703.

36

Pit-lake hydrology and contaminant transport at the Big Ledge Mine

Croall, J., White, P., Coleman, B., Schmidt, J. and Skidmore, S., 2017, Novel approach to acid pit lake treatment and management. Proceedings of the Conference of the International Mine Water Association, Lappeenranta, Finland, pp. 495-501. Diehl, S.F., Hageman, P.L., and Smith, K.S., 2008, Chapter A: What is weathering in mine waste? Mineralogic evidence for sources of metals in leachates, pp. 4-7, in: Verplanck,

of

P.L., ed., Understanding contaminants associated with mineral deposits: U.S. Geological

ro

Survey Circular 1328, pp. 96.

Dobriyal, P., Badola, R., Tuboi, C., and Hussain, S.A., 2017, A review of methods for

-p

monitoring streamflow for sustainable water resource management, Applied Water

re

Science, vol. 7, pp. 2617-2628, DOI: 10.1007/s13201-016-0488-y. Dogramaci, S., McLean, L., and Skrzypek, G., 2017, Hydrogeochemical and stable isotope

lP

indicates of pyrite oxidation in carbonate-rich environment; the Hamersley Basin, Western Australia, Journal of Hydrology, vol 454, pp. 288-298, DOI:

ur na

10.1016/j.jhydrol.2016.12.009.

Earman, S. and Hershey, R.L., 2004, Water quality impacts from waste rock at a Carlin-type gold mine, Elko County, Nevada, Environmental Geology, vol. 45, no. 8, pp. 1043-1053,

Jo

DOI: 10.1007/s00254-004-0982-7. Eary, L.E., 1999, Geochemical and equilibrium trends in mine pit lakes, Applied Geochemistry, v. 14, no. 8, pp. 963-987, DOI: 10.1016/S0883-2927(99)00049-9.

Edraki, M., Golding, S.D., Baublys, K.A., and Lawrence, M.G., 2005, Hydrogeochemistry, mineralogy, and sulfur isotope geochemistry of acid mine drainage at the Mt. Morgan

37

Newman et al. - REVISED

mine environment, Queensland, Australia, Applied Geochemistry, vol. 20, pp. 789-805, DOI: 10.1016/j.apgeochem.2004.11.004. Farouk, B.A., Michel, A., Richard, S., and René, T., 2015, Numerical simulations of water flow and contaminants transport near mining wastes disposed in a fractured rock mass, International Journal of Mining Science and Technology, vol. 25, pp. 37-45, DOI:

of

10.1016/j.ijmst.2014.11.003

ro

Gammons, C.H., Poulson, S.R., Pellicori, D.A., Reed, P.J., Roesler, A.J., and Petrescu, E.M., 2006, The hydrogen and oxygen isotopic composition of precipitation, evaporated mine

re

330, DOI: 10.1016/j.jhydrol.2005.12.005.

-p

water, and river water in Montana, USA, Journal of Hydrology, vol. 328, no. 1, pp. 319-

Gammons, C.H., Snyder, D.M., Poulson. S.R., and Petritz, K., 2009, Geochemistry and stable

lP

isotopes of the flooded underground mine workings of Butte, Montana, Economic Geology, vol. 104, pp. 1213-1234.

ur na

Gammons, C.H., Pape, B.L., Parker, S.R., Poulson, S.R., and Blank, C.E., 2013, Geochemistry, water balance, and stable isotopes of a “clean” pit lake, at an abandoned tungsten mine, Montana, USA, Applied Geochemistry, vol. 36, pp. 57-69, DOI:

Jo

10.1016/j.apgeochem.2013.06.011. Gammons, C.H., Nimick, D.A., and Parker, S.R., 2015, Diel cycling of trace elements in streams draining mineralized areas – A review, Applied Geochemistry, vol. 57, pp. 35-44, DOI: 10.1016/j.apgeochem.2014.05.008.

38

Pit-lake hydrology and contaminant transport at the Big Ledge Mine

Gibson, J. J., Birks, S. J., and Edwards, T. W. D., 2008, Global prediction of δA and δ2H- δ180 evaporation slopes for lakes and soil water accounting for seasonality, Global Biogeochemical Cycles, 22, GB2031, doi:10.1029/2007GB002997. Giesemann A., Jager H.J., Norman A.L., Krouse H.P. and Brand W.A.,1994, On-line sulfurisotope determination using an elemental analyzer coupled to a mass spectrometer.

of

Analytical Chemistry, vol. 66, pp. 2816-2819.

ro

Glynn, P. and Brown, J., 2012, Integrating field observations and inverse and forward modeling: Application at a site with acidic, heavy-metal-contaminated groundwater, pp. 181-233,

re

Vadose, and Geothermal Systems, CRC Press.

-p

in: Bundschuh, J. and Zilberbrand, M., eds., Geochemical Modeling of Groundwater,

Good S.P., Mallia D.V., Lin J.C., and Bowen G., 2014, Stable Isotope Analysis of Precipitation

lP

Samples Obtained via Crowdsourcing Reveals the Spatiotemporal Evolution of Superstorm Sandy. PLoS ONE, vol. 9, no. 3: e91117. doi:10.1371/journal.pone.0091117.

ur na

Grassineau N.V., Mattey D.P. and Lowry D., 2001, Sulfur Isotope Analysis of Sulfide and Sulfate minerals by Continuous Flow-Isotope Ratio Mass Spectrometry, Analytical Chemistry, vol. 73, pp. 220-225.

Jo

Hammarstrom, J.M., Seal, R.R. II, Meier, A.L., and Kornfield, J.M., 2005, Secondary sulfate minerals associated with acid drainage in the eastern US: Recycling of metals and acidity in surficial environments, Chemical Geology, vol. 215, no. 1-4, pp. 407-431, DOI: 10.1016/j.chemgeo.2004.06.053.

39

Newman et al. - REVISED

Herschy, R., 1993, The velocity-area method, Flow Measurement and Instrumentation, vol. 4, no.1, pp. 7–10. doi:10.1016/0955-5986(93)90004-3 Jerz, J.K. and Rimstidt, D.L., 2003, Efflorescent iron sulfate minerals: Paragenesis, relative stability, and environmental impact, American Mineralogist, vol. 88, no. 11-12, pp. 19191392, DOI: 10.2138/am-2003-11-1235.

of

Jewell, P.W. and Stallard, R.F., 1991, Geochemistry and paleoceanographic setting of central

ro

Nevada bedded barites, Journal of Geology, vol. 99, pp. 151-170.

Johnson, C.A., Emsbo, P., Poole, F.G., and O’Rye, R., 2008, Sulfur- and oxygen-isotopes in

re

pp. 133-147, DOI: 10.1016/j.gca.2008.10.011.

-p

sediment hosted stratiform barite deposits, Geochemica et Cosmochemica Acta, vol. 73,

Johnson, C.A., Piatak, N.M., and Miller, M.M., 2017, Barite (Barium), chap. D of Schulz, K.J.,

lP

DeYoung, J.H., Jr., Seal, R.R., II, and Bradley, D.C., eds., Critical mineral resources of the United States—Economic and environmental geology and prospects for future

ur na

supply: U.S. Geological Survey Professional Paper 1802, p. D1– D18, https://doi.org/10.3133/pp1802D. Johnston, A., Runkel, R.L., Navarre-Sitchler, A., and Singha, K., 2017, Exploration of diffuse

Jo

and discrete sources of acid mine drainage to a headwater mountain stream in Colorado, USA, Mine Water and the Environment, DOI: 10.1007/s10230-017-0452-6.

Kimball, B.A., Runkel, R.L., and Gerner, L.J., 2009, Methods and basic data from mass-loading studies in American Fork, October 1999, and Mary Ellen Gulch, Utah, September 2000: U.S. Geological Survey Data Series 443, 33p.; Available at http://pubs.usgs.gov/ds/443.

40

Pit-lake hydrology and contaminant transport at the Big Ledge Mine

Kimball, B.A. and Runkel, R.L., 2009, Spatially Detailed Quantification of Metal Loading for Decision Making: Metal Mass Loading to American Fork and Mary Ellen Gulch, Utah, Mine Water and the Environment, vol. 28, pp. 274-290, DOI: 10.1007/s10230-009-00855. Lachmar, T.E., 1994, Application of fracture-flow hydrogeology to acid-mine drainage at the

of

Bunker Hill Mine, Kellogg, Idaho, Journal of Hydrology, vol. 155, pp. 125-149.

ro

Lachmar, T.E., Burk, N.I., and Kolesar, P.T., 2006, Groundwater contribution of metals from an abandoned mine to the North Fork of the American Fork River, Utah, Water, Air, and

-p

Soil Pollution, vol. 173, pp. 103-120, DOI: 10.1007/s11270-005-9031-8.

re

Lee, L.J.E., Lawrence, D.S.L., and Price, M., 2006, Analysis of water-level response to rainfall and implications for recharge pathways in the Chalk aquifer, SE England, Journal of

lP

Hydrology, vol. 330, pp. 604-620, DOI: 10.1016/j.jhydrol.2006.04.025. Kornexl B.E., Gehre M., Höffling R. and Werner R.A.,1999, On-

18O measurement of

ur na

organic and inorganic substances, Rapid Communications in Mass Spectrometry, vol. 13, pp. 1685-1693.

Kupier, A., Wilkin, R.T., and Pichler, T., 2019, Cadmium in soils and groundwater: A review,

Jo

Applied Geochemistry, vol. 108, DOI: 10.1016/j.apgeochem.2019.104388. MacFarlane, M.J., 2001, Late Paleozoic Tectonism in the Northern Snake Mountains, Elko County, Nevada, Ph.D. Dissertation, University of Nevada, Reno.

41

Newman et al. - REVISED

Mayo, A.L., Nielsen, P.J., Loucks, M., and Brimhall, W.H., 1992, The use of solute and isotopic chemistry to identify flow patterns and factors which limit acid mine drainage in the Wasatch Range, Utah, Groundwater, vol. 30, no. 2, pp. 243-249. Mitchell, A.W., 1977, Geology of some bedded barite deposits north central Nevada, Masters Thesis, University of Nevada, Reno, pp. 65.

of

Newman, C.P., 2016, Integrating predictive hydrogeologic and geochemical assessments in pit-

ro

lake regulation and reclamation in Nevada, Geological Society of America Abstracts with Programs, vol. 48, no. 7, DOI: 10.1130/abs/2016AM-283051.

-p

Newman, C.P., Castendyk, D.N., Straight, B., Filiatreault, P., and Pino, A., 2018, Remote water-

re

quality sampling of Nevada pit lakes using unmanned aircraft systems, Geological Society of America Abstracts with Programs, vol. 50, no. 5, ISSN 0016-7592, DOI:

lP

10.1130/abs/2018RM-313820.

Newman, C.P. and Mann, J., 2018, Evaluating mine-waste seepage water and solute sources

ur na

using stable isotopes and inverse geochemical modeling, Geological Society of America Abstracts with Programs, vol. 50, no. 5, ISSN 0016-7592, DOI: 10.1130/abs/2018RM313818.

Jo

NDEP, 2018, Humboldt Region: Tabor Creek, water-quality standards applicable to Tabor Creek, Nevada Administrative Code Section 445A.1486, https://www.leg.state.nv.us/nac/nac-445a.html#NAC445ASec1486, accessed 20 November 2018.

42

Pit-lake hydrology and contaminant transport at the Big Ledge Mine

NDEP, 2019a, NDEP Profile I Parameter List, https://ndep.nv.gov/uploads/documents/20141027_Profile_I_List.pdf, accessed 8 January 2019. NDEP, 2019b, NDEP Profile III Parameter List, https://ndep.nv.gov/uploads/documents/20140701_Profile_III_List.pdf, accessed 8

of

January 2019.

ro

Nordstrom, D.K., 2009, Pitfalls and limitations of mineral equilibrium assumptions for

geochemical modeling of water-rock interactions at mine sites, pp. 11, Proceedings of the

-p

8th International Conference on Acid Rock Drainage, Skellefteå, Sweden.

re

Nordstrom, D.K., 2011a, Hydrogeochemical processes governing the origin, transport and fate of major and trace elements from mine wastes and mineralized rock to surface waters,

lP

Applied Geochemistry, v. 26, no. 11, pp. 1777-1791, DOI: 10.1016/j.apgeochem.2011.06.002.

ur na

Nordstrom, D.K., 2011b, Mine waters: Acidic to circumneutral, Elements, vol. 7, pp. 393-398. Nordstrom, D.K., 2012, Models, validation, and applied geochemistry: Issues in science, communication, and philosophy, Applied Geochemistry, v. 27, no. 10, pp. 1899-1919,

Jo

DOI: 10.1016/j.apgeochem.2012.07.007. Nordstrom, D.K, Wright, W.G., Mast, M.A., Bove, D.J., and Rye, R.O., 2007, Aqueous-sulfate stable isotopes – A study of mining-affected and undisturbed acidic drainage, pp. 386416, in: Church, S.E., von Guerard, P., and Finger, S.E., eds., Integrated investigation of environmental effects of historical mining in the Animas River Watershed, San Juan

43

Newman et al. - REVISED

County, Colorado: U.S. Geological Survey Professional Paper 1651, pp. 1096 plus CDROM. Parkhurst, D.L. and Appelo, C.A.J., 2013, Description of input and examples for PHREEQC version 3—A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations: U.S. Geological Survey Techniques and

of

Methods, book 6, chap. A43, pp. 497.

ro

Pellicori, D. A., Gammons, C. H., and Poulson, S. R., 2005, Geochemistry and stable isotope composition of the Berkeley pit lake and surrounding mine waters, Butte, Montana.

-p

Applied Geochemistry, vol. 20, no. 11, pp. 2116–2137. DOI:

re

http://doi.org/10.1016/j.apgeochem.2005.07.010

Rice, E.W., Baird, R.B, Eaton, A.D, and Clesceri, L. (Eds.), 2012, Standard Method for the

lP

Examination Water and Wastewater, American Water Works Association, 22nd edition. Runkel, R.L., Kimball, B.A., Walton-Day, K., Verplanck, P.L., and Broshears R., 2012,

ur na

Evaluating remedial alternatives for an acid mine drainage stream: a model post audit, Environmental Science and Technology, vol. 46, pp. 340–347, DOI: 10.1021/es2038504. Runkel, R.L., Walton-Day, K., Kimball, B.A., Verplanck, P.L., and Nimick, D.A., 2013,

Jo

Estimating instream constituent loads using replicate synoptic sampling, Peru Creek, Colorado, Journal of Hydrology, vol. 489, pp. 26–41, DOI:10.1016/j.jhydrol.2013.02.031.

Sánchez-España, J., Pamo, E.L., Santofimia, E., Aduvire, O., Reyes, J., and Barettino, D., 2005, Acid mine drainage in the Iberian Pyrite Belt (Odiel river watershed, Huelva, SW Spain):

44

Pit-lake hydrology and contaminant transport at the Big Ledge Mine

Geochemistry, mineralogy and environmental implications, Applied Geochemistry, vol. 20, pp. 1320-1356, DOI: 10.1016/j.apgeochem.2005.01.011. Sánchez-España, F.J., Diez-Ercilla, M., Pérez-Cerdán, F., Yusta, I., and Boyce, A., 2014, Hydrological investigation of a multi-stratified pit lake using radioactive and stable isotopes combined with hydrometric monitoring, Journal of Hydrology, vol. 511, pp.

of

494–508.

ro

Schaider, L.A., Senn, D.B., Estes, E.R., Brabander, D.J., and Shine, J.P., 2014, Sources and fates of heavy metals in a mining-impacted stream: temporal variability and role of iron

-p

oxides, Science of the Total Environment, vol. 490, pp. 456-466, DOI:

re

10.1016/j.scitotenv.2014.04.126.

Shevenell, L.A., 2000, Water quality in pit lakes in disseminated gold deposits compared to two

lP

natural, terminal lakes in Nevada, Environmental Geology, vol. 39, no. 7, pp. 807-815. Shevenell, L., Connors, K. A, Henry, C.D., 1999, Controls on pit lake water quality at sixteen

ur na

open-pit mines in Nevada, Applied Geochemistry, vol. 14, pp. 669–687, DOI:10.1016/S0883-2927(98)00091-2. Taylor, B.E. and Wheeler, M.C., 1994, Sulfur- and oxygen-isotope geochemistry of acid mine

Jo

drainage in the western United States, pp. 481-514, in: Alpers, C.N. and Blowes, D.W., eds., Environmental Geochemistry of Sulfide Oxidation, Washington D.C.: American Chemical Society, DOI: 10.1021/bk-1994-0550.ch030.

45

Newman et al. - REVISED

Trettin, R., Gläser, H.R., Schultz, M., and Strauch, G., 2007, Sulfur isotope studies to quantify sulfate components in water of flooded lignite open pits – Lake Goitsche, Germany, Applied Geochemistry, vol. 22, pp. 69-89, DOI: 10.1016/j.apgeochem.2006.07.020. Walton-Day, K. and Poeter, E., 2009, Investigating hydraulic connections and the origin of water in a mine tunnel using stable isotopes and hydrographs, Applied Geochemistry, vol. 24,

of

pp. 2266-2282, DOI: 10.1016/j.apgeochem.2009.09.015.

ro

Walton-Day, K. and Mills, T.J., 2015, Hydrogeochemical effects of a bulkhead in the Dinero mine tunnel, Sugar Loaf mining district, near Leadville, Colorado, Applied Geochemistry,

Jo

ur na

lP

re

-p

vol. 62, pp. 61-74, DOI: 10.1016/j.apgeochem.2015.03.002.

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Figure Captions

Fig. 1 Map of the Big Ledge mine illustrating the approximate potentiometric surface, general groundwater flow direction, monitoring locations, the pit lake, and west WRF. The inset shows the vicinity of the WRF seepage, and surface-water sampling locations are labeled with the distance in m from the confluence of the WRF seepage with the unnamed tributary (which occurs at BLW-03). Negative distances indicate upgradient locations of the WRF seepage and 47

Newman et al. - REVISED

tributary. Also shown is the location of hydrologic cross section on line A to A’ (which also

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coincides with the conceptual groundwater flow path from the pit lake to the WRF seepage).

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Fig. 2 Photos of the seepage area downgradient of the west WRF, a) efflorescent sulfate salts forming adjacent to seepage channel, b) efflorescent sulfate salts forming in an area of filamentous algae, c) likely formation of metal oxyhydroxides along the seepage channel, and d) schematic of confluence between unimpacted streamflow upgradient of the mine (entering from

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left of photo) and AMD seepage (entering from top of photo), white area in upper right is prominent efflorescent salts.

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Fig. 3 Plots of the geochemistry of the Big Ledge pit lake, groundwater, and surface-water monitoring locations over time; a) pH, b) SO4, c) Ca, and d) Cd. Note the different concentration

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scales.

Fig. 4 Hydrologic cross section through the Big Ledge mine site (vertical exaggeration=4.34). The ground surface was extracted from a 3-meter digital elevation model (DEM). Groundwater well depths are shown to scale. The approximate extent of the WRF is shown in dark brown. 49

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Potentiometric surface elevations in wells are shown by triangular symbols and are connected

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with solid blue lines where known and dashed blue lines where uncertain.

Fig. 5. Isotopic composition of estimated precipitation (OIPC), groundwater, WRF seepage, and the pit lake; a) compared to the GMWL of Craig (1961) and the OIPC-LMWL estimated for the Big Ledge site; b) linear regressions of Big Ledge site data for groundwater, the pit lake, and

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WRF seepage (OIPC-LMWL removed for clarity). The range of δ18OSO4 in the pit lake is also plotted for comparison (shaded area).

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Fig. 6. a) Sulfur isotopic composition versus SO4 concentrations in the aqueous phase at the Big

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Ledge mine with conceptual geochemical evolution pathways, and b) box and whisker plot of solid-mineral δ34S in other deposits hosted in Paleozoic rocks in northern Nevada (Arehart et al.,

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1993; Bawden et al., 2003; Mitchell, 1977). Note the different y-axis scales between panels to

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allow comparison while maintaining focus on data from the Big Ledge mine.

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Fig. 7. Correlation of Zn and Cd, which may indicate that Cd is substituting into sphalerite on the

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site.

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Fig. 8 SEM characterization of efflorescent salts; a) SEM image of likely epsomite salts at various scales and spectra indicating Mg dominated composition with minor Al, Mn, and Zn peaks, and b) SEM image of likely pickeringite salts at various scales and spectra indicating Al

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dominated composition with minor Mg and Mn peaks.

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Fig. 9 Pumping rates from the WRF seepage capture pond, which by association are proportional

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box.

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to diffuse discharge from the WRF. The time period of the isotopic study is shown in the gray

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Fig. 10 Dissolved-mass transport rates (milligrams per second [mg/s]) for a) Al, b) Cd, and c) Zn in surface waters downgradient of the Big Ledge mine site. Note the different dissolved load scales.

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Fig. 11 Values for the saturation index (SI) for a) amorphous Al(OH)3 and b) gibbsite over time (SI=0 is denoted with a dashed horizontal line), and c) white amorphous material at TA-1A potentially indicating precipitation of an Al-phase (photo by C. Newman, 7 June 2017).

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Pit-lake hydrology and contaminant transport at the Big Ledge Mine

Table 1. General characteristics of monitoring locations used in this analysis.

Type

Location

Distance from confluence (m)1

Hydrologic Location2

Impacted3

PS-1

Pit lake

Shore of pit lake

--

--

--

BLW-01

WRF seep

Original discovered seep; immediately at base of WRF

-110 (location now reclaimed)

Downgradient

Yes

BLW-02

WRF seep

Primary active seepage area

-12

Downgradient

Yes

BLW-03

WRF seep

Primary active seepage area

0

Downgradient

Yes

BLW-04

WRF seep

Upstream of confluence of WRF seepage and unnamed tributary

-30

Downgradient

No

BLW-05

WRF seep

Downstream of confluence of WRF seepage and unnamed tributary

23

BLW-06

WRF seep

Downstream of confluence of WRF seepage and unnamed tributary

50

TA-1A

Stream

At confluence of unnamed tributary and Tabor Creek

1070

TC-5

Stream

Downstream of confluence of unnamed tributary and Tabor Creek

H2O-9 4

Stream

Upstream of confluence of unnamed tributary and Tabor Creek

H2O-10 4

Stream

MW-01A

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Location ID

Yes

Downgradient

Yes

Downgradient

Yes

3290

Downgradient

Yes

--

Upgradient

No

In adjacent unnamed drainage north of mine site, feeds Tabor Creek

--

In adjacent drainage

No

Groundwater

West of pit

--

Upgradient

No

MW-02

Groundwater

North of pit

--

Cross gradient

No

MW-03

Groundwater

South of pit (now abandoned)

--

Downgradient

Yes

MW-03A

Groundwater

South of pit

--

Cross gradient

No

Groundwater

South of pit, adjacent to WRF seepage

--

Downgradient

No

Groundwater

East of pit

--

Downgradient

No

Groundwater

Northeast of pit

--

Downgradient

No

MW-06 MW-07

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MW-05

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Downgradient

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-- indicates not applicable

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Table 2. Water chemistries used in inverse modeling. Model #1 used the composition of the pit lake, MW-05, and BLW-02 as inputs. Model #2 used the pit lake, MW-05, BLW-02, and mean precipitation concentrations from National Atmospheric Deposition Program (NADP) site NV01. All units mg/L except pH, which is standard units. Mean Pit Lake

MW-05

BLW-02

6/2/2017

5/19/2017

1984-1997

Alkalinity

0

210

0

0

Al

0.4

0.054

370

0

Ca

350

62

370

Cl

1.1

1.4

20

F

2.1

0.47

110

0

Fe

0.1

Pb

0.001

Mg

280

re 22

0

0.00125

0.005

0

32

2500

0.028

19

0.032

290

0

6.51

7.67

3.75

5.46

6.8

8.6

24

0.14

Jo

Na

0.132

lP

pH

0.178

0.01

ur na

Mn

ro

5/16/2016

-p

of

Precipitation

Parameter

S

2200

82

15000

0.438

Zn

24

0.01

230

0

58

Pit-lake hydrology and contaminant transport at the Big Ledge Mine

Table 3. Summary of geochemical composition of groundwater, WRF seepage, the pit lake, and unimpacted surface waters. When samples were below the detection limit one-half the detection limit was used in calculations. All concentrations are in mg/L except pH which is standard units. Unimpacted Surface Groundwater

WRF Seepage

Pit Lake Water

Parameter Maximu

Minimu

Maximu

Minimu

Maximu

Minimu

Maximu

m

m

m

m

m

m

m

m

0

340

0

27

1

200

Alkalinity (Total as

1

ro

CaCO3)

of

Minimu

210

0.023

4.20

120

560

0.025

6.40

0.025

1.00

Antimony

0.001

0.04

0.001

0.001

0.001

0.018

0.001

0.011

Arsenic

0.001

0.25

0.009

0.022

0.001

0.004

0.001

0.003

Barium

0.02

0.84

0.05

0.25

0.005

1.3

0.014

0.360

Beryllium

0.0005

0.002

0.016

0.083

0.0005

0.003

0.001

0.002

Boron

0.025

0.1

0.5

1

0.025

0.05

0.000

0.100

Cadmium

0.0005

0.007

2.4

9.3

0.0005

1.4

0.001

0.025

Calcium

11

170

320

480

19

410

0.025

66

Chloride

0.7

5.2

10

20

0.5

10

0.500

8.3

re

lP

ur na

Chromium

-p

Aluminum

0.001

0.008

0.025

0.42

0.001

0.033

0.001

0.020

0.001

0.027

0.49

7.5

0.001

0.37

0.001

0.025

0.3

3.1

18

150

0.24

4.6

0.050

1.6

0.005

49

0.1

160

0.005

36

0.010

1.2

0.001

0.005

0.0025

0.005

0.001

0.005

0.001

0.005

Magnesium

4.7

65

850

3400

5.6

300

2.400

27

Manganese

0.002

4.9

75

400

0.15

28

0.001

2.2

Mercury

0.00005

0.0005

0.00005

0.0014

0.00005

0.001

0.00005

0.00005

Nickel

0.001

48

30

30

0.045

3.7

0.001

0.20

Nitrate + Nitrite (as N)

0.05

4.4

7.5

11

0.025

4.2

0.025

0.40

Potassium

1.1

3.9

10

23

0.95

6.1

0.250

2.0

Copper Fluoride Iron

Jo

Lead

59

Newman et al. - REVISED Unimpacted Surface Groundwater

WRF Seepage

Pit Lake Water

Parameter Maximu

Minimu

Maximu

Minimu

Maximu

Minimu

Maximu

m

m

m

m

m

m

m

m

Selenium

0.0025

0.01

0.35

0.67

0.005

0.1

0.003

0.010

Silver

0.001

0.0025

0.025

0.125

0.001

0.0025

0.001

0.003

Sodium

0.7

15

12.5

24

2

32

2.6

14

Sulfate

12

520

6400

20000

36

2200

2.5

170

TDS

84

970

6300

26000

120

3200

50

380

Thallium

0.0005

0.001

0.00175

0.005

0.0005

0.002

0.001

0.001

Zinc

0.005

3.1

86

350

0.08

30

0.005

0.750

pH

6.79

8.57

3.2

4.68

3.55

8.23

6.97

8.45

re

-p

ro

of

Minimu

Table 4. Fractions of the pit lake (fpl) calculated to be present in WRF seepage depending on

fpl

lP

using different isotopic compositions as end members (BLW-02 or BLW-03 for WRF seepage

MW-03A

MW-05

BLW-02

0.78

0.91

BLW-03

0.35

0.74

Jo

ur na

and MW-03A or MW-05 for groundwater).

60

Pit-lake hydrology and contaminant transport at the Big Ledge Mine

Table 5. Mixing fractions of end-member waters (pit lake, groundwater, and precipitation) and mole transfers for the inverse models considered potentially reasonable. Mixing fractions are unitless and transfers are in units of mol/L. Negative mole transfers indicate precipitation, positive transfers indicate dissolution. Phases marked with “-“ were not predicted to participate in the given realization. Model #2

of

Model #1

Realization One

Realization Two

Pit Lake

0.76

0.83

1.00

Groundwater

0.24

0

0

Precipitation

N/A

0.17

Birnessite

1.300

0.048

Calcite

-0.001

-

Epsomite

0.095

Fluorite

0.003

-p re

0.047

lP

-

0.093

0.003

0.003

1.94 × 10−8

2.06 × 10−8

1.98 × 10−8

0.014

0.014

0.014

Ferrihydrite

0.070

-0.026

-0.026

Melanterite

-0.166

0.027

0.027

Jo

Gibbsite

0

0.095

ur na

Galena

ro

Realization One

Manganite

-1.295

-0.043

-0.042

Pyrite

0.096

-

-

Sphalerite

0.003

0.003

0.003

61