Formation and prevention of pipe scale from acid mine drainage at iron Mountain and Leviathan Mines, California, USA

Journal Pre-proof Formation and prevention of pipe scale from acid mine drainage at iron Mountain and Leviathan Mines, California, USA Kate M. Campbell, Charles N. Alpers, D. Kirk Nordstrom PII:

S0883-2927(20)30002-0

DOI:

https://doi.org/10.1016/j.apgeochem.2020.104521

Reference:

AG 104521

To appear in:

Applied Geochemistry

Received Date: 21 February 2019 Revised Date:

31 December 2019

Accepted Date: 2 January 2020

Please cite this article as: Campbell, K.M., Alpers, C.N., Nordstrom, D.K., Formation and prevention of pipe scale from acid mine drainage at iron Mountain and Leviathan Mines, California, USA, Applied Geochemistry (2020), doi: https://doi.org/10.1016/j.apgeochem.2020.104521. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

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Formation and prevention of pipe scale from acid mine drainage at Iron Mountain and Leviathan Mines, California, USA

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Fe(III)-rich precipitate that forms inside the pipelines and requires periodic and costly cleanout

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and maintenance. Pipelines at Iron Mountain Mine (IMM) and Leviathan Mine (LM) in

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California carry acidic water from mine sources to a treatment plant and have developed pipe

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scale. Samples of scale and AMD were collected from both mine sites for mineralogical,

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microbiological, and chemical analysis. The scale mineralogy was primarily schwertmannite

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with minor amounts of poorly crystalline goethite. Although the bulk composition of the scale

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was similar along the length of the pipeline at IMM, the number of iron-oxidizing bacteria and

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concentrations of associated trace elements decreased along the flow path inside the pipeline.

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Laboratory batch experiments with unfiltered AMD from IMM and LM showed that Fe(II)

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oxidation was driven by microbial activity when the pH was <5. A remediation strategy of

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decreasing the pH to <2.2 was tested through geochemical modeling and laboratory experiments.

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These experiments indicated that scale formation could be prevented by decreasing the pH,

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which could be achieved at IMM by mixing source waters. However, the presence of Fe(III)-

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rich scale in a pipeline buffers the pH to higher values that may affect the efficacy of this

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remedial approach.

Kate M Campbell , Charles N Alpers , and D Kirk Nordstrom 1,*

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U.S. Geological Survey, Geology, Geophysics, and Geochemistry Science Center, Denver, CO, USA U.S. Geological Survey, California Water Science Center, Sacramento, CA, USA U.S. Geological Survey, Boulder, CO, USA corresponding author. [email protected], (303) 236-2441, mailing address: 3215 Marine Street, Boulder, CO, 80303, USA.

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Abstract Pipelines carrying acid mine drainage (AMD) to treatment plants commonly form pipe scale, an

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Keywords: acid mine drainage; pipe scale; microbial iron oxidation; schwertmannite; remediation

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Highlights: • • • • •

Precipitate forms in pipelines carrying acid mine drainage, causing costly maintenance requirements. Scale samples collected from acid mine drainage pipelines at two mine sites were composed primarily of schwertmannite. Trace elements, including Al, As, Cu, V, and Zn, accumulate in pipe scale. Scale formation was driven by microbial Fe(II) oxidation at pH values <5. Modeling and laboratory experiments indicate that scale formation can be prevented by decreasing the pH to <2.2.

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1. Introduction Extraction of mineral resources from metal sulfide deposits, particularly at legacy mine

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sites, often creates conditions that favor the oxidation of sulfide phases and result in the

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formation of acid mine drainage (AMD). At massive sulfide deposits, the high concentrations of

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sulfide minerals and the low neutralization capacity of wallrocks and wastes can lead to severe

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AMD with pH values below 2 and elevated metal concentrations. The mechanisms of AMD

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generation are complex, but generally result from oxidation of pyrite in the presence of oxygen

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facilitated by microbially mediated oxidation of Fe(II) (Singer and Stumm, 1970; Nordstrom and

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Southam, 1997; Nordstrom and Alpers 1999a). The oxidative dissolution of pyrite and other

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metal sulfides generates high concentrations of Fe(II), sulfate, acidity, and other metal(loid)s

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associated with the deposit and potentially poses a hazard if the resulting AMD is released to the

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environment (Nordstrom and Alpers, 1999a).

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Because AMD has a low pH and high concentrations of Fe(II), acidophilic Fe(II)

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oxidation is an important process for microbial growth in AMD. Abiotic Fe(II) oxidation is

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kinetically limited at low pH (Singer and Stumm, 1970), but many chemolithoautotrophic

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organisms can grow by oxidizing Fe(II) in oxygenated waters (Baker and Banfield, 2003;

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Johnson and Hallberg. 2003). Acidophilic Fe(II) oxidizers are found worldwide and can thrive in

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waters with a wide range of metal concentrations, low pH, and even at moderately elevated

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temperatures (30-50 °C; summarized by Johnson, 2012). Their ubiquitous presence in

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moderately to extremely acidic waters is of great importance to the generation of AMD, the

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release of metals, and subsequent processes affecting mineral precipitation and metal

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sequestration.

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To control the release of AMD to the environment, site managers at the Iron Mountain

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Mine (IMM, Shasta County, CA) and Leviathan Mine (LM, Alpine County, CA) have installed

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onsite water treatment plants to treat AMD before it is released to the surrounding areas. A key

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component of the treatment process includes a series of pipelines to convey AMD from the main

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sources on site (e.g., adits, seeps) to the treatment plant, where the water is neutralized by slaked

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lime (Ca(OH)2) addition, precipitating an Fe-oxide-rich sludge. As acidic, Fe(II)-rich water is

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transported away from the pyrite source, continued Fe(II) oxidation in the pipelines causes

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accumulation of Fe(III)-rich precipitation, known as pipe scale, which can result in pipe clogging

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and AMD spillage if the pipeline is not properly maintained by removing the scale. At IMM, two

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major pipelines were installed to convey water from sources in two separate locations (Figure 1):

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(1) the PW3 underground workings pump station and Slickrock Creek Retention Reservoir

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(SCRR) and (2) the Richmond and Lawson adits. The Richmond/Lawson pipeline generally

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does not accumulate pipe scale, whereas the PW3/SCRR pipeline has scaling problems that

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require pipeline cleanout by physical scale removal every 2–4 years. At LM, two pipelines carry

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water from different sources (Delta Seep and channel underdrain (CUD)) to a collection pond

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(Pond 4; Figure 1C) where water is stored until pumped into the lime-neutralization treatment

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plant. Scaling occurs in the pipelines as well as pipes inside the treatment plant at LM, which are

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periodically replaced.

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At both the IMM and LM sites, as well as other AMD sites with pipelines, treatment of

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scale is costly; a better understanding of the fundamental biogeochemical processes controlling

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scale formation is needed to inform remediation or other site management options that would

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reduce the impact of scale formation. In addition, the biogeochemical processes creating pipe

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scale are also relevant to mechanisms, rates, and trace-element fate in natural acid rock drainage

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and other AMD systems where Fe(III)-rich precipitates form. The objectives of this study are to

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(1) characterize the mineralogy, morphology, microbial community, and trace element

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composition of Fe(III)-rich pipe scale; (2) determine effects of seasonal water chemistry

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variability on scale formation; (3) identify biogeochemical processes responsible for scale

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formation; and (4) identify and bench-test potential remediation strategies that would prevent or

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minimize scale formation.

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Figure 1. (A) Map of California showing the relative locations of Iron Mountain Mine (IMM) and Leviathan Mine (LM). Site maps of IMM (B) and LM (C), including pipelines (Richmond/Lawson and PW3/SCRR), Minnesota Flats treatment plant, and source waters (Richmond adit, Lawson adit, PW3, and Slickrock Creek Retention Reservoir (SCRR), and sampling locations.

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2. Methods

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2.1. Field collection of water and scale samples from IMM. Source water samples were

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collected near the influent of acidic water into the two pipelines (PW3/SCRR and

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Richmond/Lawson pipelines; Figure 1A). The PW3/SCRR pipeline regularly carries water

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pumped from underground workings (Old Mine#8 source at pump station PW3, referred

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hereafter as PW3) and periodically carries water from the Slickrock Creek Retention Reservoir

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(SCRR). SCRR stores water collected from the surface of the southwest side of the site that

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accumulates primarily in response to storm events. The Richmond/Lawson pipeline carries water

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draining from the Richmond adit (floor drain and 5-way drain) and the Lawson portal (Figure

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1a).

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The PW3/SCRR pipeline was sampled for water and scale along a 2.1 km reach via

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portals (“service saddles”) that allow direct access to the pipeline interior. Five service saddles

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were sampled: SS12, SS10, SS8, SS6, and SS24-1 (upstream to downstream, Figure 1a). The

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interior of the Richmond/Lawson pipeline was not sampled because it does not accumulate

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scale. Both pipelines are composed of high density polyethyene (HDPE). The PW3/SCRR

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pipeline had an interior diameter of 0.46 m (18 inches) from the start of the pipeline to SS24-1,

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where the diameter increased to 0.61 m (24 inches). Complete water chemistry samples from the

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sources (PW3, combined Richmond influent, and SCRR) as well as water and scale samples

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from the PW3/SCRR pipeline were collected in August 2012, December 2012, and April 2013,

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and analyzed for complete water chemistry. Flow in the pipelines was monitored on site at valve

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stations maintained by the site-management company. At the time of sampling, flow in the

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PW3/SCRR pipeline was substantially less than the maximum capacity of the pipe, resulting in

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turbulent flow through a partially filled pipe. Physical removal of the scale along the length of

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the pipeline had been performed in 2010, about 2 years prior to the first scale sample collection.

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In 2012, the PW3/SCRR pipeline was operated under two flow conditions to conduct a

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field test of how mixing SCRR water with PW3 water changes the water chemistry in the

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pipeline. The two flow conditions were (1) PW3 water only flowing into pipeline at ca. 284

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L/min, and (2) a mix of PW3 water and SCRR water flowing into the pipeline at a ratio of

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approximately 1:15 (PW3:SCRR) and a total flow rate of ca. 3,785 L/min. SS12 is upstream of

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the SCRR intake to the pipeline, and as a result, the pipeline contains only PW3 in that segment

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(see Figure 1).

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Water samples were pumped from the AMD sources or the pipeline with a peristaltic

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pump and field parameters (pH, Eh, conductivity, and temperature) were measured in a 500mL

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flow-through container and recorded after stabilization. The pH was measured with an Orion

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ROSS pH combination electrode calibrated with standard buffers (pH 1.68 and 4) if the pH was

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greater than 1.68; otherwise, the voltage of the sample was recorded for calibration with

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standardized sulfuric acid solutions in the laboratory (described below). The Eh was calculated

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from the electron motive force (emf) measured with a standard Pt combination electrode, with

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electrode performance verified by an ORP standard. Specific conductance (SC) was measured

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with a 1cm electrode (Thermo Orion) calibrated using a two-point curve with 100 µS/cm and

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1413 µS/cm standards. Water samples for laboratory analysis were filtered through a 0.45 µm

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capsule filter into pre-washed plastic sample bottles. Separate split samples were collected and

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preserved for cation analysis (100 mL sample acidified to 1% HNO by volume), Fe(II) and total

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Fe (Fe(T)) analysis (100 mL sample acidified to 1% HCl by volume in an opaque amber bottle),

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and pH/anion analysis (100 mL sample, unpreserved, in a deionized (DI) water-rinsed bottle).

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Samples were stored and shipped to the laboratory on ice and stored at 4 °C until analysis. Field

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blanks were collected by processing and preserving DI water (18.2 MΩ) identically to all three

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samples types. Field duplicates were collected at a rate of about 10%.

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Scale samples were collected after water samples were collected when the influent water

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to the pipeline was temporarily turned off. Samples were collected by removing 100-500 g of

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the precipitate from the pipeline with a clean, ethanol-sterilized chisel and placing the sample

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into an acid-washed glass container. The consistency of the scale was soft, particularly at SS12,

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but the scale was more condensed than typical freshly precipitated AMD sludge. The scale

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became increasingly consolidated downstream in the pipeline. The samples included subsamples

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collected across the pipe to the greatest extent possible. The thickness of the scale varied along

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the length of the pipeline but was typically 3–8 cm. Scale samples were shipped on ice and

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stored at 4 °C until analysis. A separate sample for DNA analysis was collected by transferring

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30–50 g of fresh scale directly to a sterile, 50 mL tube, flash frozen in the field on dry ice, and

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kept frozen until placed in an ultralow freezer (-70 °C) in the laboratory.

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A larger volume of water (2–8 L) was collected from PW3 and combined Richmond

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source water sites for laboratory oxidation experiments in April 2013. Several liters were

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transported unfiltered overnight to the laboratory with minimal headspace to preserve the active

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microbial community composition. An additional 2-3 L were collected simultaneously, filtered

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through a 0.45 µm cartridge filter on site, and filtered through a 0.1 µm sterile PVDF filter in the

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laboratory immediately upon receipt.

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2.2. Field collection of water and scale samples from Leviathan Mine. Water and scale samples

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were collected in September 2014 at the Leviathan Mine (see Figure 1C). All samples were

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collected and stored identically to methods used for the IMM sites. Water was collected from

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several locations along a pipeline carrying seep water (Delta Seep upper, lower, and combined)

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and at the pipeline outlet to the storage pond (Pond 4, Figure 1C). The storage pond held water

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from multiple sources for treatment at the plant. Additional samples were collected at two

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locations in a pipeline carrying seep water (channel under drain (CUD) at weir and at CUD inlet

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to the storage pond). Water and scale samples were also collected inside the treatment plant at

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the inlet to the reactor tank. Additionally, 2 L samples of unfiltered water and 1 L samples of

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filtered water were collected from the CUD at the weir and the Delta Seep (combined flow) for

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laboratory experiments. Scale samples were collected at each site where accessible by scraping

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as much mass as possible (typically 10–50g) into a centrifuge tube or acid-washed glass jar. The

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scale samples were shipped on ice and stored at 4°C until analysis.

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2.3. Water analysis. Major cations, trace elements, and total sulfur were determined by

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inductively coupled plasma optical emission spectrometry (ICP-OES; Perkin Elmer 7300DV).

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When the pH measured in the field was lower than the lowest standardized pH standard (pH

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1.68), the pH was calculated using the method of Nordstrom and Alpers (1999b). Briefly, a

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calibration curve was created with standardized sulfuric acid solutions and the voltage of the

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standards and samples was measured in the laboratory using an Orion ROSS pH electrode. The

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pH was calculated using geochemical speciation software with the Pitzer option for concentrated

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brines and the MacInnes assumption (PHREEQC; Parkhurst and Appelo, 2013). Sulfate was the

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dominant anion in all water samples, and total sulfur was determined to be an appropriate

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measurement for sulfate based on comparisons of results for sulfate by ion chromatography (IC,

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Dionex LC20 with an AS18 column) and total sulfur by ICP-OES with samples from IMM in

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August 2012. Sulfate measurements by IC were challenging, given the need to remove iron by

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ion exchange prior to IC analysis to prevent iron precipitation in the column from the addition of

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the bicarbonate eluent. Therefore, total sulfur by ICP-OES is reported as a proxy for total sulfate,

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and other anions are considered negligible. Charge balance calculations by PHREEQC indicate

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that this assumption was valid, with a calculated charge balance within 5% for all water samples

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reported. The iron oxidation state was measured using the ferrozine method (Stookey, 1970).

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Ferrous iron (Fe(II)) and Fe(T) concentrations were measured colorimetrically by absorbance at

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562 nm, and Fe(III) concentrations were calculated by difference (To et al., 1999).

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2.4. Scale characterization. For IMM, a 50–100 g of scale sample was homogenized in the

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laboratory and either air dried directly or gently washed with 5 mL deionized water followed by

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5 mL methanol before air-drying. The sample was then gently crushed with an agate mortar and

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pestle. Some samples showed evidence of layered color variations, presenting a banded structure

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with light and darker red-orange deposits (Figure S1). A physical separation of the darker and

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lighter bands was performed on a scale sample from SS8 collected in April 2013. Leviathan

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Mine scale samples, which did not show any visible laminations, were completely homogenized,

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washed, and dried in a similar manner.

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Mineralogy and composition of the scale were determined by powder X-ray diffraction

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(XRD), total carbon-nitrogen-phosphorus solid analysis, and microwave-HF digestions. For

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quantitative mineralogy, a 1-g aliquot of the washed, dry scale sample was mixed with 20%

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corundum by weight and ground in a micronizing mill with 4 mL of ethanol for 5 minutes. After

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drying at 60°C, the mixture was shaken with three acrylic balls and 200-800 µL Vertrel®

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solution (Dupont) to create randomly oriented aggregates. The powder was passed through a 250

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µm sieve and loaded into a sample holder. Samples were analyzed using a Siemens D500 X-ray

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diffractometer from 5 to 65 ° 2Θ using Cu Kα X-ray radiation, with a step size of 0.02 degrees

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and a dwell time of 2 seconds per step. Quantitative mineralogy was calculated using RockJock

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software (Eberl, 2003). Synthetic goethite, schwertmannite, and jarosite were prepared based on

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methods from Schwertmann and Cornell (2000) and added as new standards into the RockJock

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program after mineral purity and identity were confirmed by qualitative XRD. Qualitative XRD

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was performed by suspending the sample in <1 mL of ethanol and drying on a single crystal Si

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wafer mount. Qualitative XRD instrumental analysis conditions were identical to those for

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quantitative XRD analysis, and RockJock was run without corundum standard correction.

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Total carbon, nitrogen, and hydrogen content were measured on water-washed, air-dried

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scale samples from IMM using an Exeter CE-440 elemental analyzer. Selected samples from

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IMM were analyzed for total elemental composition was measured in selected samples using a

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microwave HF digestion followed by dilution in 1% HNO and analysis by inductively coupled

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plasma mass spectrometry (ICP-MS, Perkin Elmer NexIon 300) for samples from IMM.

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An array of chemical extractions were performed to understand mineralogical

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composition and trace element association. Four extractions were performed in parallel on

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rinsed, air-dried samples of SS6, SS8, SS10, and SS12 from IMM. In addition, splits of the

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synthetic goethite (FeOOH) and schwertmannite (ideal composition Fe O (OH) SO ) were used to

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verify the behavior of the chemical extractions. The extraction solutions were deionized water

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(DIW; pure water leach), 0.2M ammonium acetate (weak extraction for metals), 0.5M HCl

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(poorly crystalline iron phases), and 0.5M HCl with 0.5M hydroxylamine hydrochloride

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(HCl/HA; more aggressive extraction of iron phases with reducing agent), based on Lovley and

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Phillips (1987) and Tessier et al. (1979). For Leviathan scale samples, only DIW and HCl/HA

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extractions were performed if scale sample mass allowed; if insufficient mass was available, then

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only a HCl/HA extraction was performed. Each sample was extracted at a solid to solution ratio

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of 1:20 (50 g/L) using at least 2 g of material in each extraction. Each extraction condition was

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equilibrated for 24 h, the solution was filtered (0.22 µm), and a fresh extraction solution was

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introduced, using careful mass balance accounting at each step. The second extraction solution

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was allowed to equilibrate for one week, processed identically, and a third extraction solution

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was allowed to equilibrate for an additional week. This multi-step extraction on each scale

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sample was necessary because of the large amount of scale dissolution during extraction.

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Extraction blanks (no solid sample added) were conducted concurrently with sample extractions.

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The extraction solutions were diluted in 1% HNO for analysis by ICP-OES. The residual solids

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from IMM extractions of samples from SS12 were gently washed with DIW and analyzed by

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qualitative XRD.

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2.5. Microbial community analysis and iron-oxidizing bacteria determination by MPN

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technique. Frozen scale samples were defrosted in a sterile laminar flow hood and 1–2 g

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aliquots were extracted with a PowerSoil DNA isolation kit (MoBio Laboratories, Inc.). The

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DNA was successfully amplified for the 16S rRNA gene (28F-519R primers for bacteria and

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340F-958R primers for archaea). Amplification and sequencing were performed at Research and

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Testing Laboratories (Lubbock, TX) on a Roche 454-FLX/FLX+ platform with approximately

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10,000 sequences returned per sample. Rarefaction analysis showed that the sequencing depth

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was appropriate to represent the microbial community. Sequences were denoised, aligned, and

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analyzed with QIIME (Caporaso et al., 2010) using the SILVA database (Quast et al., 2012).

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Operational taxonomic units (OTUs) were identified at 97% sequence identity and grouped

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taxonomically at the genus level.

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The most probable number (MPN) technique (Johnson, 1995) was used to estimate the

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abundance of iron-oxidizing bacteria present in unfiltered water (PW3, SCRR) and scale samples

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(SS2, SS6, SS8, SS10, SS12, SS24-1). The MPN medium used for serial dilutions was fresh, 0.1

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µm-filtered PW3 water, and MPNs were initiated within 24 hours of sample collection. For

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water samples, 1 mL of unfiltered sample was added to 9 mL of medium, mixed, and serially

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diluted by a factor of 10 up to 10 times. For scale samples, 2 g of wet sample was aseptically

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weighed, added to a tube and vigorously mixed with 40 mL medium and allowed to equilibrate

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for 1 hour. One mL of this solution was then used for MPNs as for water samples. All tubes

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were allowed to incubate at 25 °C for 10 days. Three control tubes of sterile medium without

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inoculum were incubated identically to the MPN series. After incubation, tubes were determined

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to be either positive or negative for growth, and the concentration of cells per mL (water

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samples) or cells per g (scale samples) was determined. For each water and scale sample, the

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MPN dilution series was performed in duplicate and the results were averaged. Biomass was

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also estimated by presence of carbon, nitrogen, and phosphorous, consistent with the low pH of

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the water and scale, and the absence of specific C, N, and P mineral phases.

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2.6. Laboratory Fe(II)-oxidation experiments. To determine the role of microbial Fe(II)

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oxidation on scale formation, a series of laboratory batch incubation experiments were

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performed with filtered and unfiltered AMD water within 24 hours of collection. Filtered water

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was passed through 0.45-µm filter in the field and a 0.1-µm filter upon receipt in the laboratory.

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Batch experiments were conducted in 1-L Teflon® bottles that had been sterilized with ethanol

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that had been allowed to evaporate under a laminar flow hood. Each bottle was filled with 700

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mL of solution and placed on an orbital shaker table at room temperature with individual, sterile

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glass aerators supplying sterile air so oxygen was not limiting in the experiments. Three

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conditions were tested: (1) unfiltered water, (2) unfiltered water with composited scale, and (3)

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filtered water. For IMM samples, water from PW3 was used. The composited scale for condition

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(2) was made by homogenizing 100 g of wet, fresh scale from SS6 and SS10 in equal

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proportions by weight and mixing with 700 mL of unfiltered water from PW3. Because scale

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samples were limited for Leviathan Mine, only conditions (1) and (3) were tested with AMD

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from CUD at the weir and Delta Seep. Samples were collected periodically for Fe(II) and Fe(T)

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determination using the ferrozine method. The pH, temperature, and Eh were monitored using

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sterilized electrodes. At the end of the experiment, the solids were collected from the bottle and

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analyzed for mineralogy by qualitative XRD.

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2.7. Scale dissolution and precipitation laboratory experiments with geochemical modeling.

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Potential remediation treatments were explored using geochemical modeling and laboratory

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batch experiments. Dissolution and precipitation equilibrium reactions were modeled using the

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code PHREEQC with an augmented WATEQ4F thermodynamic database (Ball and Nordstrom,

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1991) with additional constants from Bigham et al. (1996) for schwertmannite and Baron and

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Palmer (1996) for K-jarosite. The saturation indices for schwertmannite, goethite, jarosite, and

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ferrihydrite were calculated for solution compositions using a wide range of Richmond/PW3

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mixtures (1%-75% Richmond water, balance PW3 water) and over a range of Fe(II) to Fe(III)

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ratios simulating microbial oxidation. Based on the modeling results, laboratory batch

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experiments were conducted with IMM samples by mixing filtered Richmond water with filtered

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PW3 water in three ratios: 1% Richmond, 99% PW3; 5% Richmond, 95% PW3; and 10%

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Richmond, 90% PW3. The water mixtures were inoculated with a fresh culture of

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microorganisms growing in PW3 water in the laboratory to test the extent of precipitation as

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Fe(II) was microbially oxidized to Fe(III) in the various mixes. The effect of preexisting scale

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was tested in a parallel set of experiments with the same water mixtures amended with a

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composited air-dried scale sample at a solid to solution ratio of 20 g/L. In addition, composite

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scale was mixed with 100% unfiltered Richmond water for comparison. All bottles were

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agitated at low speed on an orbital shaker and individual bottles were bubbled with sterile air.

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Each condition was performed in duplicate. Aqueous samples were collected periodically for

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Fe(II) and Fe(T) analysis, while pH, temperature, and Eh were monitored at a higher frequency.

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3. Results and Discussion

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3.1. Water composition and seasonality. All source waters at IMM were acidic and contained

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elevated concentrations of iron, sulfate, and other metal(loids), including Al, As, Cu, and Zn

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(Table 1). The Richmond and Lawson portal discharges had the lowest pH values (0.8–1.2 and

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1.9, respectively), whereas PW3 and SCRR waters were less acidic (2.6 and 3.0,

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respectively). Metal concentrations generally followed the pattern Richmond > Lawson > PW3

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> SCRR. Scale was observed only in the PW3/SCRR pipeline, but not in the Richmond/Lawson

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pipeline. Most of the Fe in Richmond, Lawson, and PW3 water (86%-99%) was present as

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Fe(II), whereas SCRR water contained variable but higher amounts of Fe(III) (27%-99% of

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Fe(T)) because of storage in a reservoir and higher exposure to oxygen.

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Flows from the IMM sources varied seasonally, with the highest flows occurring during

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the winter and spring, when storms brought rainfall and occasional snow to the area; summer and

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fall were largely dry (Figure 2). Because the flows from PW3 and SCRR were controlled, there

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were periods with no flow in the PW3/SCRR pipeline, and flows at other times were variously

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mixed between PW3 and SCRR depending on site management needs. The water composition

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also varied seasonally, with dilution from precipitation apparent during the winter and spring

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(high flow) compared to the summer (low or base flow) (Table 1a; additional data in Alpers et al.

342

(1992) and Campbell et al. (2019)). During high flow, PW3 and Richmond waters had a higher

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fraction of Fe(III) (14-18%), but concentrations of most other metals are similar or lower than

344

those under base flow conditions. The iron oxidation state and concentration varied substantially

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in SCRR water depending upon precipitation events because SCRR collected water from surface

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drainage at the site. Consequently, the influent to the PW3/SCRR pipeline varied depending

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upon the fraction of SCRR water and the composition of SCRR water routed to the pipeline; this

348

effect was seen in the field test of mixing PW3 and SCRR waters (Table 2).

349

Along the PW3/SCRR pipeline, water was sampled when only PW3 water was

350

introduced to the pipeline and when a mixture of PW3 and SCRR water (1:15 PW3:SCRR) was

351

flowing, similar to the range of pipeline operating conditions (Figure 2, Table 2). Travel time

352

between SS12 and SS24-1 was estimated to be approximately 45 minutes at an average flow rate

13

353

of 284 L/min with only PW3 flowing. Ferrous Fe was rapidly oxidized (11-35%) as water flowed

354

along the pipeline under both conditions. In addition, 7-15% of the total dissolved iron was lost

355

from solution, indicating that oxidation and precipitation occurred directly in the pipeline. The

356

pH increased from 2.62 in PW3 to 2.71 at SS24-1, consistent with the oxidation of Fe(II):

357 358

4Fe

+ O + 4H → 4Fe

+ 2H O

(1)

359 360

361 362 363 364 365 366 367 368 369

Figure 2. Flow rates from Richmond, Lawson, SCRR, and PW3 at the flow control valves, as reported by Iron Mountain Operations, the IMM site managers. Dashed line indicates the average flow for each site. Flow was reported in hourly increments and was converted to daily averages. Water from LM had generally higher pH values than IMM, ranging from 2.99 at Pond 4 to 5.99 in the Delta Seep, and overall metal concentrations were lower than IMM, but elevated

14

370

enough to require water treatment (Table 1b). Iron in the storage pond (Pond 4) and in the

371

treatment plant reactor tank water was present primarily as Fe(III) (97%) whereas the CUD water

372

was primarily Fe(II) (93%-95%) and Delta Seep water had a mixture of Fe(II) and Fe(III) (27%-

373

48% Fe(II)). The Fe(II) in Delta Seep water was oxidized as it flowed from seep intakes to the

374

pond. Further oxidation of the Fe(II) flowing into the pond from combined CUD and Delta Seep

375

flows occurred within the pond.

376 377

3.2. Scale composition. At both IMM and LM, the scale consisted of red-orange precipitates

378

that accumulated as a coating inside the pipes and varied in thickness from a few millimeters to

379

up to 8 cm thick (Figure S1). At IMM, the scale exhibited visible biomass on the surface,

380

particularly at SS12, which had a layer of biomass 2–6 mm thick. The physical appearance of the

381

scale became more consolidated and laminated downstream in the pipeline, with distinct light

382

and dark layers at SS8 and SS6. Mineralogical (XRD) analysis revealed that the scale at both

383

IMM and LM consisted of mainly schwertmannite with minor amounts of poorly crystalline

384

goethite (Table 3, Figure S2). At IMM, scale from SS12, SS10, SS8, and SS6 was consistently

385

1–2% goethite with 98–99% schwertmannite, indicating similar bulk mineralogical composition

386

for a large portion of the pipeline. Slightly higher amounts of goethite were present at SS24-1

387

(5.3%). In addition, the light layers at SS8 had more goethite than the dark layers (14.2%

388

compared to 2.7%), suggesting that the changing composition of the water (i.e., lower total Fe,

389

lower Fe(II), and higher pH) with the periodic introduction of SCRR water to the pipeline may

390

have caused an increase in the relative amount of goethite relative to schwertmannite

391

precipitated. This interpretation is supported by the lack of lamination at SS12, which is

392

upstream of the intake of SCRR to the pipeline. Seasonal changes in water chemistry and total

393

water volume therefore are likely to have affected the amount and composition of the scale.

394

Schwertmannite is a poorly crystalline iron oxyhydroxysulfate with a nanoparticulate,

395

molecular tunnel-like structure, high surface area, and variable composition that forms

396

aggregates with needle-like morphology (Bigham et al., 1990, 1994, 1996; Schwertmann et al.,

397

1995; Fernandez-Martinez et al., 2010; French et al., 2012; Caraballo et al., 2013). It has a

398

variable stoichiometry of Fe O (OH) (SO ) (x = 0.5–2), which indicates a sensitivity to sulfate

399

concentration; in waters with higher sulfate concentrations, the structure incorporates more

400

sulfate and less hydroxide (Bigham et al., 1996; Caraballo et al., 2013). The stability field for

8

8

8-2x

4 x

15

401

schwertmannite is approximately pH 2–5 (Bigham et al., 1996), and the maximum Fe:S molar

402

ratio is 8. The coexistence of schwertmannite and goethite is common in AMD systems, due to

403

the metastable nature of schwertmannite and paragenetic relationship of schwertmannite to

404

jarosite and goethite (Bigham et al. 1996; Bigham and Nordstrom, 2000). Schwertmannite with

405

trace amounts of goethite is favored over jarosite because of the pH of the system, the rate of

406

Fe(II) oxidation, and relatively low K concentrations in PW3 water (Bigham et al., 1996; Gramp

407

et al., 2008; Bai et al., 2012; Huang and Zhou, 2012).

408

The extraction results support the mineralogical XRD results for IMM scale (Table 4),

409

with the amount of mass dissolved and Fe:S ratios consistent with a sulfate-containing Fe-

410

oxyhydroxide within the variable composition of schwertmannite (molar Fe:S = 5–6 in the HCl

411

and HCl/HA extractions). The amount of S and Fe extracted was similar for all IMM scale

412

samples (Campbell et al., 2019), confirming that the bulk composition of scale was relatively

413

constant. In acidic extractions where most of the scale had dissolved, the remaining solid was

414

primarily goethite and jarosite; jarosite was possibly present in the original scale sample but at

415

concentrations too low to detect (i.e., <1% by weight) in the bulk sample by XRD. Scale samples

416

from the reactor inlet, CUD outlet, and CUD at the weir at LM also showed qualitative XRD

417

patterns and molar Fe:S (6–7) consistent with schwertmannite with possibly a slightly higher

418

proportion of an amorphous iron (oxyhydr)oxide such as ferrihydrite due to the slightly higher

419

pH at LM (Table 4, Figure S2). The precipitate from the Delta Seep combined flow, however,

420

had a molar Fe:S of 24, indicating that the precipitate was primarily amorphous ferrihydrite

421

(ideal composition of Fe5HO8·4H2O (Jambor and Dutrizac, 1998)), consistent with the

422

qualitative XRD pattern and a higher pH of 5.71 compared to the other sites.

423

Although the bulk composition was relatively constant at IMM, trace element, biomass

424

(represented by C, N, and P), and iron-oxidizing bacterial abundance generally decreased

425

downstream (Table 3, Table 5, Table 6). A notable exception is SS8, which tended to have more

426

lamination and slightly lower trace-metal concentrations than SS6. This section of pipeline may

427

have had slightly lower velocity flow, which would potentially affect trace-metal accumulation

428

in the scale. Textural changes in the scale along the flow path of the PW3/SCRR pipeline were

429

documented by Williams et al. (2017). In general, the scale is elevated in As, Cu, V, and Zn,

430

with a variety of other metals accumulating to a lesser extent (Table 6). Iron-oxidizing bacteria

431

MPNs indicate that active iron-oxidizing organisms are at least 3–4 orders of magnitude more

16

432

abundant at the most upstream location, SS12, than at other sites, with abundance generally

433

decreasing downstream (Table 5). Total C, N, and P concentrations also serve as indicators of

434

total biomass in this low pH system; C, N, and P show a similar spatial trend as MPN data (Table

435

3, Table 6).

436

Culture-independent microbial community composition (DNA sequencing of 16S rRNA

437

gene) at SS12 and SS6 revealed a diverse community with multiple putative metabolisms

438

coexisting in the scale (Figure 3). As expected, many of the sequences were identified as

439

organisms that have not been cultured in the laboratory and have unknown metabolisms, but

440

many of the sequences are related to known acidophiles (e.g., Acidobacteriales). Other studies of

441

microbial diversity at IMM and other AMD sites worldwide have also found diverse

442

communities in water compositions broadly similar to PW3 water (Edwards et al., 1999a, 2000a;

443

Johnson et al., 2001; Johnson and Hallberg, 2003; Druschel et al., 2004; Johnson, 2012; Kuang

444

et al., 2013; Mesa et al., 2017). For sequences that were closely related or identical to cultured

445

organisms, there were a high proportion of acidophillic, metal- and S-oxidizing organisms. Iron-

446

oxidizing organisms typical of acid rock drainage were found in the scale, including

447

Leptospirillum (Nitrospira genus), Ferrimicrobium (Acidobacteriales genus), Thiomonas

448

(Burkholdariales genus) and Acidithiobacillus (Acidithiobacillales genus). In addition, S-

449

oxidizing organisms were prevalent, including Thiobacillus (Burkholdariales genus),

450

Halothiobacillus (Chromatiales genus), and Tumebacillus (Bacillales genus). Additional

451

metabolisms are present, such as N-cycling organisms such as Nitrospira (Nitrospirales genus)

452

and members of the Rhizobiales genus. Relative abundance of bacteria to archaea was not

453

measured, and the only archaeal genus found was Thermoplasmatales, an acidophilic group with

454

heterotrophic S metabolism (Edwards et al., 2000b). Even though scale does not form in the

455

Richmond/Lawson pipeline, many similar organisms that have been extensively studied in

456

Richmond water are found in the PW3/SCRR pipeline scale (Bond et al., 2000; Druschel et al.,

457

2004). The community composition differed between SS12 and SS6, indicating variation in the

458

relative proportions of various organisms along the length of the pipeline. Although it is not

459

possible to infer relative microbial activity between sites because many sequences were not

460

directly linked with putative metabolisms, the DNA data combined with the biomass and MPN

461

data for Fe-oxidizing activity support a complex and variable microbial population in the

17

462

pipeline. The laboratory experimental community was substantially less diverse than the

463

pipeline, as expected for a laboratory culture (Hugenholtz et al., 1998).

464

Microbial communities inside the mine (Richmond) were found to vary seasonally and

465

were dependent upon geochemistry of the mine water (Schrenk et al., 1998; Edwards et al.,

466

1999a,b). Seasonal variations in scale community are possible as well; measureing the effect of

467

community variations on rate of scale formation may be important for the prediction and

468

planning of remediation strategies.

469 470

471 472 473 474 475

Figure 3. 16S DNA results from two extracted pipe scale samples and a precipitate formed during microbial iron-oxidation laboratory experiments. Classifications are presented at the genus level. Organisms present at <1% abundance were grouped into a single category. Archaea were also sequenced using a different primer set; only Thermoplasma was

18

476 477 478

detected. “Laboratory” refers to batch oxidation experiment with unfiltered PW3 water only; the solid precipitated during the experiments was collected and DNA extracted from the solid.

479 480

The presence of visible biofilms on the surface of the scale combined with microbial diversity

481

with multiple metabolisms shows a complex, interdependent community. The abundance of

482

Fe(II) flowing past the biofilm provides ample electron donor to support the growth of

483

acidophilic chemolithoautotrophic organisms as the base metabolism of the microbial

484

community. Similar biofilms that form in AMD and natural acid rock drainage sites show

485

coexisting, active S, organic carbon, N, H , and Fe metabolisms, and may contain geochemical

486

gradients within the biofilm (Bond et al., 2000; Baker and Banfield, 2003; Wilmes et al., 2009;

487

Méndez-García et al., 2014; Mesa et al., 2017).

2

488

Trace elements, including Al, As, Cu, V, and Zn, may be co-precipitating with

489

schwertmannite and goethite or accumulating scale-associated biomass. For elements forming

490

oxyanions at low pH, including As and V, sorption is electrostatically favorable, and strong

491

association of As with schwertmannite has been observed in multiple experiments (Acero et al.,

492

2006; Kumpulainen et al., 2007; Antelo et al., 2013; Sanchez-Espana et al., 2016; Zhang et al.,

493

2016). Uptake of di- and trivalent cations including Al, Zn, Cu, Ni, Co, Cd, and Mn has occurred

494

in schwertmannite-rich precipitates at other AMD sites, but to a lesser extent than for oxyanions

495

(Acero et al., 2006; Kumpulainen et al., 2007; Burgos et al., 2012; Antelo et al., 2013).

496

Geochemical speciation calculations using PW3 water chemistry indicate that in low-pH, high-

497

sulfate environments, many cationic metals form aqueous sulfate complexes that may make

498

adsorption more favorable to schwertmannite at low pH (Figure S3). In addition, sulfate

499

adsorption on the surface of schwertmannite may decrease the surface charge, increasing metal

500

adsorption through surface charge and ternary surface complexation (Bigham et al., 1990;

501

Jönsson et al., 2005, 2006; Peretzyzhko et al., 2009; Baleeiro et al., 2018). Adsorption of these

502

metal-sulfate species to schwertmannite will be explored in more detail in future experiments.

503

The nature of trace element binding to schwertmannite has implications for long-term

504

sequestration of these elements in pipe scale, as it has been shown that upon aging to goethite,

505

many of these elements (e.g., Cu, Zn, As) are released to solution (Antelo et al., 2013; Zhang et

506

al., 2016). The release of trace elements from schwertmannite upon aging is an important

507

consideration for evaluating remediation and waste storage options.

19

508

Schwertmannite in pipe scale was stable for at least 3-4 years, based on the interval

509

between pipeline cleanouts. Because schwertmannite is a metastable phase, it is expected to

510

transform completely to jarosite and/or goethite based on thermodynamic considerations. The

511

rate of schwertmannite conversion to more stable phases has been investigated in the laboratory

512

and in a wetland environment, and has a wide range of transformation rates that depend upon

513

aqueous chemistry (particularly pH and sulfate concentrations) and the presence of adsorbed

514

constituents, including As, Co, Mn, Zn, and organic matter. Complete transformation of

515

schwertmannite to goethite can occur in <100 days to 2 years under laboratory conditions

516

(Bigham et al., 1996; Jönsson et al., 2005; Schwertmann and Carlson 2005; Acero et al., 2006;

517

Kumpulainen et al., 2008; Antelo et al., 2013; Zhang et al., 2016), and within 6 years under field

518

conditions (Gagliano et al., 2004). The presence of elevated concentrations of trace elements,

519

high sulfate, constant temperature, and the presence of organic matter (biofilms) may stabilize

520

the pipe scale by conversion to more stable goethite in between pipeline cleanouts.

521 522

3.3. Biogeochemical mechanisms of scale formation. Laboratory batch studies were

523

conducted to determine the role of microbial activity on iron oxidation and scale formation with

524

water from IMM (PW3) and LM (Delta Seep and CUD). The filtered control bottles exhibit no

525

evidence of Fe(II) oxidation even after more than 150 hours, and were not affected by aeration.

526

These results are consistent with kinetically slow abiotic Fe(II) oxidation (Figure 4; Singer and

527

Stumm, 1970). In contrast, Fe(II) in unfiltered PW3 water (biologically active) was rapidly

528

oxidized within 100 hours and was oxidized slightly faster when scale was present from the start

529

of the experiment (Figure 4), likely due to increased cell concentrations associated with the scale.

530

Therefore, Fe(II) oxidation was primarily a biotic process in PW3 water. In the laboratory

531

experiment using PW3 water without scale, the microbial community was markedly simpler than

532

that observed in the scale samples in the field (Figure 3), and was dominated by a member of

533

Burkholdariales (85%) that does not have a cultured representative, and a member of

534

Acidithiobacillus (12%).

535 536

20

537 538 539 540 541 542

Figure 4. Results of laboratory iron-oxidation experiments with PW3 water from IMM. Controls consisted of PW3 water that had been filtered, whereas experimental conditions were unfiltered PW3 water with and without added scale from the PW3/SCRR pipeline. Error bars represent measurements in triplicate experimental bottles.

543

Total aqueous Fe concentrations also decreased rapidly over time (Figure 4), indicating

544

precipitation of schwertmannite, which was confirmed by XRD analysis of the residual

545

precipitate in the unfiltered PW3 water-without-scale condition. The pH in the unfiltered PW3-

546

without-scale condition initially increased due to Fe(II) oxidation (equation 1). After ~75 hours,

21

547

the pH decreased, corresponding to the removal of Fe from solution due to schwertmannite

548

precipitation:

549 550

8Fe

+ SO

+ 14H O → Fe O (OH) SO

( )

+ 22H

(2)

551 552

The delay between the onset of Fe(II) oxidation and the removal of Fe(III) via precipitation

553

shows that schwertmannite is oversaturated and precipitation is kinetically limited. Positive

554

saturation indices of schwertmannite were calculated during this period. When scale was present

555

from the start of the experiment, the initial increase in pH was not observed and the saturation

556

index was not oversaturated. A likely explanation for the difference in pH behavior is that the

557

scale seeded the precipitation of new schwertmannite, thus increasing the rate of precipitation.

558

For LM samples, CUD water precipitated schwertmannite and decreased pH only in the

559

unfiltered water experiment, similar to the PW3 results for IMM (Table 7), but Delta Seep water

560

showed no difference between filtered and unfiltered experiments. Additionally, the initial pH

561

was higher in Delta Seep water (5.5) and increased to 7.5 after iron oxidation, possibly caused by

562

CO degassing over the course of the experiment. Because the initial pH was substantially higher

563

in the Delta Seep compared with any of the other waters in this experiment, the Fe(II) oxidation

564

was dominated by abiotic processes, as opposed to the primarily biotically mediated Fe(II)

565

oxidation observed in PW3 and CUD waters. This behavior is consistent with a decrease in rate

566

of abiotic oxidation with decrease in pH as described by Singer and Stumm (1970). These results

567

also indicate that biotic oxidation at pH < ~5 is the cause of pipe scaling at IMM and some areas

568

of LM, including the treatment plant.

569

2

The pH values of waters in the collection pond (Pond 4) and inside the treatment plant at

570

LM (about 3.0) were substantially lower than those in influent waters (4.4 for CUD at Pond 4,

571

and 5.7 for Delta Seep at Pond 4) (Table 1). In addition, most of Fe(II) was oxidized while the

572

water was held in the pond prior to treatment. Although the relative amounts of influent water

573

contributing to the pond are unknown, the composition of the pond water suggests that the

574

precipitation of schwertmannite (eqn. 2) and/or ferrihydrite can explain the lower pH in the pond

575

compared to the influent waters.

576

22

577

3.4. Bench-scale demonstration of source water mixing as a potential remediation strategy at

578

IMM. Pipe scaling at IMM is driven by microbial Fe(II) oxidation, producing Fe(III) that then

579

precipitates as schwertmannite with minor amounts of goethite. Microbial Fe(II) oxidation may

580

be challenging to control at a large scale in the field, but precipitation of schwertmannite is

581

sensitive to pH. Geochemical calculations using PHREEQC indicate that decreasing the pH to

582

2.2 or less will prevent schwertmannite precipitation, even if all the dissolved iron is present as

583

Fe(III). The Fe(III) must hydrolyze before it can precipitate at pH ≃ pK1, which is pK1 = 2.2 for

584

Fe(III). At lower pH values, there is insufficient hydrolysis to allow precipitation. Mixing

585

calculations were performed to test the proportion of Richmond water needed to be added to

586

PW3 water to decrease the pH to below saturation for schwertmannite, using the measured water

587

composition from December 2012 for both the Richmond adit and PW3 (Table 1). A minimum

588

of 5% Richmond water by volume was calculated to be necessary to decrease the pH of the

589

mixed water to <2.2.

590

Laboratory batch experiments were conducted to verify the modeling results by mixing

591

unfiltered Richmond water to 1%, 5%, and 10%, balanced by PW3 water. In the absence of

592

scale, all of the Fe(II) was oxidized, but solid precipitate formed only in the 1% Richmond

593

condition (Figure 5); total dissolved Fe remained constant in the 5% and 10% Richmond

594

conditions, and the characteristic pH decrease caused by scale precipitation was only observed in

595

the 1% condition. These experimental results confirmed the modeling results, suggesting that

596

≥5% Richmond water amendment to the PW3/SCRR pipeline would prevent scale formation.

597

In the presence of scale, total Fe concentrations increased initially in the batch

598

experiments because of dissolution of the scale at lower pH values, but reprecipitation of scale

599

occurred in all three mixing scenarios. For comparison, when scale was placed in 100%

600

Richmond water, approximately 60% of the scale dissolved and the pH increased from 1.0 to 1.5.

601

In addition, the pH values at the end of the mixing experiment with scale converged to pH 2.2,

602

indicating that the presence of scale strongly buffered the pH. Therefore, if scale has already

603

formed in the pipeline, the addition of Richmond water to PW3 water will be less successful in

604

preventing scale formation than if it is added directly following a cleanout. Periodic soaking of

605

the scale-affected pipeline in 100% Richmond water is potentially another option, although

606

complete scale dissolution is not expected based on laboratory results.

607

23

608

609 610 611 612 613

Figure 5. Dissolved total iron, dissolved Fe(II), and pH results from PW3-Richmond laboratory mixing experiments, with and without added pipe scale.

614

3.5. Implications for pipe-scale remediation or mitigation at mine sites. Pipe scaling is a

615

common issue at mine sites where AMD is transferred via pipelines. When the pH is < ~5,

616

microbial Fe(II) oxidation is likely the underlying process that produces Fe(III), which then

617

precipitates initially as schwertmannite in the pipelines. A pipeline is an ideal environment for

618

microbial growth, as a steady supply of Fe(II) and dissolved oxygen continually flows across the

619

biofilms on the surface of the scale. From an engineering perspective, preventing microbial

620

growth is a challenging endeavor, and, as a result, other hydrogeochemical options are preferred.

621

In addition, the ability of the scale to accumulate trace elements may be an important factor in

622

managing waste from scale removal, particularly as trace elements may be released from the

623

scale upon long-term storage as the schwertmannite converts to more stable hydrous Fe(III) 24

624

oxide phases such goethite. In the case of IMM, the on-site availability of extremely low pH (<1)

625

water from the Richmond adit provides a unique opportunity to treat pipe scale with the mixing

626

or soaking approach described above. If very low pH water is not readily available, acid addition

627

may be considered as an alternative, depending on reagent costs and additional lime and sludge

628

management consequences at treatment facilities.

629

Another possible treatment option is to manage flows to increase the flow rate in the

630

pipeline, decreasing the residence time of water within the pipe. This could result in less frequent

631

operation of the pipeline with periods of no flow, theoretically decreasing the overall rate of in

632

situ Fe(II) oxidation. The efficacy of this type of option may depend on treatment plant

633

operating conditions and the ability to manage flows by mechanisms such as water

634

storage. Based on the measured loss of total Fe from PW3 to SS24 at IMM, a flow rate of 284

635

L/min (PW3 only, average flow) over 2.1 km of pipeline, and assuming a travel time of

636

approximately 45 minutes, approximately 1.4 g/m/day of schwertmannite precipitates in the

637

PW3/SCRR pipeline. If the pipeline operated continuously under these conditions, 9 kg/m/yr of

638

pipe scale would precipitate, which is clearly an overestimate of the rate of scale precipitation

639

given the operational conditions described (Figure 2). Because water in the PW3/SCRR pipeline

640

has variable flow and composition, depending on seasonal composition and with SCRR, the rates

641

of Fe(II) oxidation and precipitation rate need to constrained over a range of conditions to test

642

whether increasing flow rate would result in reduced scale precipitation. Development of a

643

reactive transport model would be beneficial to test remediation strategy options. Similarly,

644

testing variable compositions of water chemistry and treatment plant operations at LM would

645

help to determine the best approach for minimizing or eliminating pipe scale. Understanding the

646

fundamental biogeochemical and hydrologic processes that produce and control pipe scale, as

647

described here, is an important step in managing scaling of AMD pipelines at mine sites.

648 649

4. Acknowledgements

650

The authors thank Lily Tavassoli (U.S. Environmental Protection Agency, EPA), James Sickles

651

(EPA), Gary Riley (EPA), Rudy Carver (Iron Mountain Operations, IMO), Theron Elbe (IMO),

652

Don Odean (IMO), Tyler Kane (U.S. Geological Survey, USGS), Deb Repert (USGS), Dale

653

Brokaw (USGS), JoAnna Barrel (Colorado School of Mines/USGS), Gary Campbell (USGS),

654

and Blaine McCleskey (USGS). Review comments provided by Chris Mills (USGS) and two

25

655

anonymous reviewers were greatly appreciated. Funding for this work was provided by the U.S.

656

Environmental Protection Agency, the USGS Water Mission Area, and the USGS Minerals

657

Resources Program in the Energy and Minerals Mission Area. Associated data can be found in a

658

USGS data release (Campbell et al., 2019). Any use of trade, firm, or product names is for

659

descriptive purposes only and does not imply endorsement by the U.S. Government.

660 661 662 663 664 665

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666 667 668 669 670 671 672

Alpers, C.N., D.K. Nordstrom,, and J.M. Burchard (1992). “Compilation and interpretation of waterquality and discharge data for acidic mine waters at Iron Mountain, Shasta County, California, 194091.” U.S. Geological Survey Water-Resources Investigations Report 91-4160, 173 p.

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Bai, S., Z. Xu, M. Wang, Y. Liao, J. Liang, C. Zheng and L. Zhou (2012). "Both initial concentrations of Fe(II) and monovalent cations jointly determine the formation of biogenic iron hydroxysulfate precipitates in acidic sulfate-rich environments." Materials Science and Engineering C 32(8): 2323-2329.

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Baker, B. J. and J. F. Banfield (2003). "Microbial communities in acid mine drainage." FEMS Microbiol Ecol 44(2): 139-152.

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Baleeiro, A., S. Fiol, A. Otero-Farina, and J. Antelo (2018). “Surface chemistry of iron oxides formed by neutralization of acidic mine waters: removal of trace metals.” Applied Geochemistry 89: 129-137.

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Ball, J. W. and D. K. Nordstrom (1991). User's manual for WATEQ4F, with revised thermodynamic data base and test cases for calculating speciation of major, trace, and redox elements in natural waters, U.S. Geological Survey Open-File Report 91–183: 193.

683 684

Baron, D. and C. D. Palmer (1996). "Solubility of Jarosite at 4-35 C." Geochimica et Cosmochimica Acta 60(2): 185-195.

685 686

Bigham, J. M., L. Carlson and E. Murad (1994). "Schwertmannite, a new iron oxyhydroxysulphate from Pyhasalmi, Finland, and other localities." Mineralogical Magazine 58(4): 641-648.

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Bigham, J. M. and D. K. Nordstrom (2000). "Iron and aluminum hydroxysulfates from acid sulfate waters; Sulfate minerals; crystallography, geochemistry, and environmental significance." Reviews in Mineralogy and Geochemistry 40: 351-403.

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Bigham, J. M., U. Schwertmann and G. Pfab (1996). "Influence of pH on mineral speciation in a bioreactor simulating acid mine drainage." Applied Geochemistry 11(6): 845-849.

Acero, P., C. Ayora, C. Torrentó and J. M. Nieto (2006). "The behavior of trace elements during schwertmannite precipitation and subsequent transformation into goethite and jarosite." Geochimica et Cosmochimica Acta 70(16): 4130-4139.

Antelo, J., S. Fiol, D. Gondar, C. Pérez, R. López and F. Arce (2013). "Cu(II) incorporation to schwertmannite: Effect on stability and reactivity under AMD conditions." Geochimica et Cosmochimica Acta 119: 149-163.

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30

830 831

Tables

31

Table 1. Selected results from water analysis of (A) IMM and (B) LM samples. Elements consistently below detection on ICP-OES are not reported here; complete water chemistry results can be found in Campbell et al. (2019). Total sulfur is reported and is present as sulfate. A. Lawson 24-Apr2013 1.93 19

Date pH Temperatur e Eh SC Fe(II) Fe(T) Fe(II) Sulfur Al As Ca Cd Co

°C V µS/cm mg/L mg/L % mg/L mg/L mg/L mg/L mg/L mg/L

0.65 16110 3150 3560 88% 13600 458 4.9 216 2.19 0.64

Cr Cu K Mg Mn Na Pb Si

mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L

Zn

mg/L

31Aug2012 2.62

PW3 12Dec2012 2.50 17.3

Iron Mountain Mine (IMM) Richmond 24Apr2013 2.59

30-Aug2012 0.84

12-Dec2012 1.01 39.5

24-Apr2013 1.15

SCRR 31-Aug2012 3.09 20.2

12-Dec2012 3.00 10

24-Apr2013 3.02 17.3

0.64 5829 679 1220 56% 6090 368 <0.4 89.5 0.28 0.39 0.04 1 77 0.74 245 8.3 6.9 <0.2 56.3

18.1 0.66 6490 976 1090 90% 6030 409 <0.2 108 0.42 0.42

32.90 0.63 76200 12500 12600 99% 49400 772 27.7 292 7.9 0.81

0.62 44610 7190 8380 86% 31400 445 19.8 308 4.9 0.57

31.9 0.64 79300 9380 11400 82% 48200 700 30.45 320 9.1 0.76

0.65 1815 59 80 73% 1540 118 <0.04 30. 5 0.093 0.11

0.79 959 <0.1 30 <1% 511 36.4 <0.08 23.6 0.030 0.065

0.67 1598 50 49 101% 972 84.8 <0.04 25.6 0.064 0.087

0.15 55 40.7 332 7.1 45.4 0.316 54.9

20.9 0.60 7250 1440 1460 99% 6980 465 <0.4 91 0.32 0.44 0.04 9 86 0.64 294 10.2 7.5 <0.2 59.4

0.051 78 0.95 281 9.5 6.2 <0.2 58.4

0.14 130 86.4 430 9.2 109 3.68 75.3

<0.1 158 54.4 256 5.8 67.1 3.4 50.7

0.19 145 95.5 392 8.2 104.5 3.435 74.3

0.011 13 0.83 88 2.7 6.8 0.0565 49.1

0.0035 4.3 0.61 31.65 1.6 3.2 0.0505 18.8

0.010 12 0.70 64.1 2.0 5.9 <0.04 40.5

284

36.5

30.1

34.6

1030

607

1020

10.1

3.9

7.2

32

B. Leviathan Mine (LM)

Sample Date pH

Pond 4

Reactor Tank

CUD @ Pond 4

CUD @ Weir

Delta Seep @ Pond 4

Delta Seep Lower

Delta Seep Upper

Delta Seep Combined Flow

3-Sep-2014

3-Sep-2014

3-Sep-2014

3-Sep-2014

3-Sep-2014

3-Sep-2014

3-Sep-2014

3-Sep-2014

2.99

3.01

4.43

4.74

5.66

5.59

5.99

5.71

Temperature

°C

14.9

15.1

12.8

9.4

17.2

10.9

11.5

11.8

Eh

V

0.72

0.72

0.45

0.38

0.42

0.30

0.29

0.29

SC

µS/cm

2792

2796

2669

2700

1732

1780

1703

1748

Fe(II)

mg/L

2.2

1.8

315

326

3.5

9.2

7.3

9.4

Fe(T)

mg/L

64.3

64.0

333

352

13.0

21.5

15.5

19.4

Fe(II)

%

3%

3%

95%

93%

27%

43%

47%

48%

Sulfur

mg/L

1660

1600

1710

1750

980

971

957

992

Al

mg/L

20.9

20.3

24.1

24.9

0.75

0.66

0.40

0.83

As

mg/L

<0.2

<0.2

0.25

0.39

<0.2

<0.2

<0.2

<0.2

Ca

mg/L

371

360

298

301

255

253

247

257

Cd

mg/L

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

Co

mg/L

0.58

0.57

0.58

0.57

0.17

0.18

0.14

0.18

Cr

mg/L

0.016

<0.01

0.011

0.021

<0.01

<0.01

<0.01

<0.01

Cu

mg/L

0.012

0.013

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

K

mg/L

14.2

13.9

16

16.3

5.12

4.69

4.91

5.21

Mg

mg/L

87.2

84.6

69.3

70.2

70.1

69.4

69.3

70.7

Mn

mg/L

18.7

18.2

15.8

16

11.5

11.8

9.85

11.7

Na

mg/L

32

31.2

26

26.2

28

27.9

27.7

28.1

Pb

mg/L

<0.2

<0.2

<0.2

<0.2

<0.2

<0.2

<0.2

<0.2

Si

mg/L

38.7

37.9

33.8

33.9

24.9

25.7

23.5

25.3

Zn

mg/L

0.30

0.30

0.34

0.33

0.08

0.06

0.06

0.09

33

Table 2. Selected water chemistry of source water and water collected during steady-state flow conditions along the PW3/SCRR pipeline at IMM under two source-water conditions: PW3 only, and a mix of PW3 and SCRR. Total sulfur is reported and is present as sulfate. Pipeline Influent Source Site Name

Source Water

Date pH

PW3

PW3

PW3

PW3

PW3

SCRR+PW3

SCRR+PW3

SCRR+PW3

PW3

SCRR

SS12

SS10

SS8

SS6

SS24-1

SS10

SS6

SS24-1

8/31/12

8/31/12

8/31/12

8/31/12

8/31/12

8/31/12

8/31/12

8/31/12

8/31/12

8/31/12

2.62

3.09

2.63

2.71

2.73

2.74

2.71

2.89

2.89

2.90

Temperature

°C

20.9

20.2

23.3

22.4

24

25.1

24.2

21.7

21.6

20

Eh

V

0.60

0.65

0.60

0.62

0.62

0.62

0.62

0.64

0.65

0.66

SC

µS/cm

7250

1810

6900

6600

6440

6340

6260

3290

3260

3080

Fe(II)

mg/L

1440

59

1400

1320

1040

1060

886

354

312

260

Fe(T)

mg/L

1460

80

1410

1390

1360

1360

1360

439

369

373

Fe(II)

%

99%

73%

100%

95%

76%

78%

65%

81%

85%

70%

Sulfur

mg/L

6980

1540

7050

6750

6870

6690

8370

3350

3060

3140

Al

mg/L

465

117.5

452

420

453

453

456

209

194

198

Ca

mg/L

91

30

94

94

94

94

95

52

49

49

Cd

mg/L

0.32

0.093

0.30

0.29

0.30

0.30

0.31

0.19

0.17

0.18

Co

mg/L

0.44

0.11

0.43

0.44

0.42

0.44

0.44

0.23

0.21

0.21

Cr

mg/L

0.049

0.011

0.048

0.052

0.048

0.052

0.044

0.023

0.02

0.021

Cu

mg/L

85.8

13.0

84.6

78.3

83.7

84.7

84.9

33.3

30.1

30.9

Mg

mg/L

294

88

289

286

290

282

289

148

139

142

Mn

mg/L

10.2

2.72

10.3

10.2

10.5

10.3

10.5

5.0

4.68

4.78

Na

mg/L

7.53

6.83

7.37

7.28

7.34

7.17

7.47

7.14

7.01

7.06

Ni

mg/L

0.092

0.0435

0.089

0.089

0.063

0.082

0.084

0.0625

0.0565

0.0555

Si

mg/L

59.4

49.1

59.4

59.4

60.8

59.4

59.8

53.5

52.1

53.1

Zn

mg/L

36.5

10.1

34.3

32.6

33.8

33.3

34.8

17.9

16.7

17.1

34

Table 3. Quantitative XRD mineralogy results, water content, total C, and total N of scale collected from PW3/SCRR pipeline at IMM. Results in percent by weight. “--” indicates no data collected for given parameter. Site SS12 SS10 SS8 SS6 SS24-1 SS8 dark SS8 light

Goethite Schwertmannite Water Total C Total N % % % % % 60 1.13 0.16 1.2 98.8 46 0.58 0.10 2.3 97.7 39 0.52 0.09 2.5 97.5 37 0.50 0.07 1.9 98.1 ---5.3 94.7 ---2.7 97.3 ---14.2 85.8

35

Table 4. Chemical extraction and digestion results for pipe scale sample, including synthetic schwertmannite and goethite minerals. Mineralogy of the remaining solid was determined using qualitative XRD for IMM samples. DIW indicates a deionized water extraction; n/a indicates solids were not analyzed due to insufficient mass.

Treatment

DIW - IMM

DIW - LM

Ammonium Oxalate

HCl

HCl/HA - IMM

HCl/HA - LM

Microwave HF

Sample SS6 SS8 SS10 SS12 CUD Outflow Delta Combined SS6 SS8 SS10 SS12 Schwertmannite Goethite SS6 SS8 SS10 SS12 Schwertmannite Goethite SS6 SS8 SS10 SS12 Schwertmannite Goethite Reactor Tank CUD Outflow CUD @ Weir Delta Combined SS6 SS8 SS10 SS12

% Mass Dissolved 8% 9% 9% 8%

Fe:S (mol/mol) 0.2 0.3 0.3 0.1 0.03

---

0.0 0.5 0.5 0.5 0.5 --5.6 5.2 5.3 5.6 --

20% 22% 22% 19% 43% 0% 88% 94% 90% 88% 100% 1% 94% 96% 94% 93% 100% 3%

-5.8 5.3 5.5 5.4 --7.3 6.1 6.2

-----

24.3 5.5 4.7 4.9 4.9

100% 100% 100% 100%

36

Remaining Minerals Schwertmannite, goethite

n/a

Schwertmannite (partial dissolution), Goethite

Goethite, Jarosite

Jarosite, Goethite (partial dissolution)

n/a

none

Table 5. Most probable number (MPN) results for iron-oxidizing bacteria at IMM. Two lab replicates of PW3 water are presented; each value is an average of experimental duplicates. Water cells/mL PW3 rep 1 5E+06 PW3 rep 2 3E+05 SCRR 1E+05 Scale cells/g SS12 1E+10 SS10 5E+06 SS8 1E+05 SS6 1E+04 SS2 2E+05 SS24-1 2E+03

37

Table 6. Trace-element concentrations from microwave HF digestion of pipe scale for elements present at >1 mg/kg at IMM. HCl-Hydroxyamine results for LM are presented in Table S1. Li, Mo, Rb, and Sb are not reported here, because they were below analytical detection limits.

SS12

SS10

SS8

SS6

45.6

45.1

45.3

46.7

5.3

5.3

5.5

4.9

Al

Units weight % weight % weight %

0.1

0.3

0.2

0.2

Mg

mg/kg (ppm)

231

320

188

158

As

mg/kg (ppm)

219

79.3

66.9

86.8

Ba

mg/kg (ppm)

2.8

8.8

8.4

6.0

Ba

mg/kg (ppm)

4.0

9.3

11.0

6.9

Bi

mg/kg (ppm)

3.8

1.1

0.8

1.4

Ca

mg/kg (ppm)

195

141

141

101

Cr

mg/kg (ppm)

9.7

7.1

9.7

5.3

Cu

mg/kg (ppm)

586

213

171

375

Ga

mg/kg (ppm)

20.8

12.8

11.6

11.4

K

mg/kg (ppm)

360

270

209

187

Mn

mg/kg (ppm)

29.0

24.7

25.4

21.8

Mo

mg/kg (ppm)

11.1

4.0

3.0

4.1

P

mg/kg (ppm)

1043

424

336

349

Pb

mg/kg (ppm)

8.9

4.7

3.4

5.5

Sb

mg/kg (ppm)

6.2

1.8

1.5

1.8

Sc

mg/kg (ppm)

6.6

2.0

1.3

0.9

Se

mg/kg (ppm)

3.7

1.8

2.0

2.5

Te

mg/kg (ppm)

17.6

4.6

3.4

4.8

V

mg/kg (ppm)

76.0

80.4

74.0

77.2

Y

mg/kg (ppm)

1.2

1.5

1.1

1.1

Zn

mg/kg (ppm)

52.6

25.5

21.5

23.6

Zr

mg/kg (ppm)

2.7

5.9

4.4

4.8

Element Fe S

38

Table 7: Batch laboratory experimental results with water from LM.

pH

Fe(T) (mg/L)

initial final

% Fe(II)

initial

final

initial

final

Precipitate Formed

Delta Seep Unfiltered

5.52

7.56

6.9

0.0

50%

--

Yes

Delta Seep Filtered

5.52

7.46

6.7

0.0

50%

--

Yes

CUD Unfiltered

4.67

3.33

345

303

100%

99%

Yes

CUD Filtered

4.67

4.27

345

342

100% 100% No

39

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: