Establishing baseline water quality for household wells within the Marcellus Shale gas region, Susquehanna County, Pennsylvania, U.S.A.

Applied Geochemistry xxx (2015) xxx–xxx

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Establishing baseline water quality for household wells within the Marcellus Shale gas region, Susquehanna County, Pennsylvania, U.S.A. Amy L. Rhodes a,⇑, Nicholas J. Horton b a b

Department of Geosciences, Smith College, Northampton, MA 01063, United States Department of Mathematics and Statistics, Amherst College, Amherst, MA 01002, United States

a r t i c l e

i n f o

Article history: Available online xxxx Editorial handling by Brian W. Stewart

a b s t r a c t Flowback fluids associated with hydraulic fracturing shale gas extraction are a potential source of contamination for shallow aquifers. In the Marcellus Shale region of northeastern Pennsylvania, certified water tests have been used to establish baseline water chemistry of private drinking water wells. This study investigates whether a single, certified multiparameter water test is sufficient for establishing baseline water chemistry from which possible future contamination by flowback waters could be reliably recognized. We analyzed the water chemistry (major and minor inorganic elements and stable isotopic composition) of multiple samples collected from lake, spring, and well water from 35 houses around Fiddle Lake, Susquehanna County, PA that were collected over approximately a two-year period. Statistical models estimated variance of results within and between households and tested for significant differences between means of our repeated measurements and prior certified water tests. Overall, groundwater chemistry varies more spatially due to heterogeneity of minerals within the bedrock aquifer and due to varying inputs of road salt runoff from paved roads than it does temporally at a single location. For wells located within road salt-runoff zones, Na+ and Cl concentrations, although elevated, are generally consistent through repeated measurements. High acid neutralizing capacity (ANC) and base cation concentrations in well water sourced from mineral weathering reactions, and a uniform stable isotopic composition for well water, suggests long flowpaths for groundwater that dampen seasonal variability of most elements. Exceptions occur for two wells within road salt runoff zones that show the greatest range of concentrations for Na+ and Cl, suggesting that these wells have a faster pathway to surficial recharge. Additionally, sampling protocols can induce variability for Fe, Mn, and Pb, making other elements identified in flowback fluids (Ba, Br, Ca, Cl, Mg, Na, Sr) more dependable indicators of contamination. Although there is general concordance between our repeated measurements and the certified test results, characterizing baseline chemistry is strengthened when results from multiple households are combined to establish regional upper baseline limits that will have a low probability of being exceeded by future samples unless conditions have changed. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The Marcellus Shale is projected to be the largest contributor of growth to the United States natural gas supply (EIA, 2012). Its development has become viable due to horizontal drilling technology combined with hydraulic fracturing techniques, which increase the permeability of tight shales such as the Marcellus to liberate natural gas from the shale formation (Vidic et al., 2013). These techniques use vast quantities of water, between 8000 and 38,000 m3 per well (Kargbo et al., 2010; Howarth et al., 2011; ⇑ Corresponding author. E-mail address: [email protected] (A.L. Rhodes).

Chapman et al., 2012; Rahm et al., 2013), and they generate large quantities of wastewater that contain high concentrations of total dissolved solids (TDS), heavy metals, radionuclides, oils, greases, and soluble organic compounds (Kargbo et al., 2010; Gregory et al., 2011; Howarth et al., 2011; Chapman et al., 2012; Barbot et al., 2013; Haluszczak et al., 2013). The biggest wastewater quantity—flowback waters—can comprise 10% of the injected fluid volume, and these are recovered from the well following hydraulic fracturing but prior to natural gas production (Rahm et al., 2013). Produced waters make up the majority of the remaining wastewater, and they consist of extremely salty brine fluids that are recovered from the well during natural gas production in small volumes, 1–2 m3 per day over the lifespan of the well (Rahm et al., 2013).

http://dx.doi.org/10.1016/j.apgeochem.2015.03.004 0883-2927/Ó 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Rhodes, A.L., Horton, N.J. Establishing baseline water quality for household wells within the Marcellus Shale gas region, Susquehanna County, Pennsylvania, U.S.A. Appl. Geochem. (2015), http://dx.doi.org/10.1016/j.apgeochem.2015.03.004

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Management of hydraulic fracturing wastewater is a large environmental concern (Rahm et al., 2013) because wastewater treatment plants are not equipped to treat the high concentrations of dissolved salts and heavy metals or handle the large volumes of wastewater (Ferrar et al., 2013). Therefore, wastewater is commonly transported offsite for disposal by deep injection into bedrock or recycled and reused in future hydraulic fracturing operations (Rahm et al., 2013). In Pennsylvania, development of natural gas from the Marcellus Shale has been particularly rapid. Within the last five years, over 14,250 permits for unconventional wells were granted and more than 6700 wells drilled (PA DEP, 2014a,b). This rapid pace of development has raised concern in the scientific community and amongst the public regarding accidental contamination of rivers, lakes, and groundwater aquifers (Entrekin et al., 2011; Howarth et al., 2011; Vidic et al., 2013). In anticipation of natural gas development within their watershed, 35 homeowners living in Ararat Township, Susquehanna County, Pennsylvania have participated in a study to establish the baseline chemistry of their well water and nearby Fiddle Lake. Many of these households had already conducted their own certified water tests in 2010 in response to Pennsylvania legal recommendations to establish pre-drill conditions of water quality. Pennsylvania subsequently strengthened its 1984 Oil and Gas Act by passing Act 13 (58 Pa.C.S. sec. 3218, 2012), which presumes an unconventional gas well operator responsible for pollution of a water supply located within 2500 feet (762 m) of a gas well if pre-drill conditions were not determined within 12 months of drilling, and if water quality was diminished within 12 months post drilling. Landowners within 3000 feet (914 m) of a proposed unconventional well also must be notified by the gas company in advance of drilling, which will enable homeowners to voluntarily conduct a certified water test. These laws will encourage testing of water supplies by both homeowners and oil and gas companies. However, recommendations on how to document pre-drill conditions are not uniformly explained, nor are specific parameters required, leaving the homeowner or gas company to decide what, when, and how often to measure water quality. For example, Pennsylvania’s regulatory agency, the PA Department of Environmental Protection (PA DEP), recommends a certified analysis for 20 different parameters but ranks these parameters with different levels of importance (PA DEP, 2014a,b). Penn State Extension recommends analysis for 29 parameters but provides priority rankings that differ from the PA DEP (Penn State Extension, 2012). Penn State Extension (2012) does provide more context for basing a decision, and it further encourages a certified water test within a few months prior to drilling. It, however, is not a regulatory body, and neither organization specifically recommends more than one pre-drill certified test. Therefore, the potentially short timeframe between notification and drilling and the high cost ($500) of a multi-parameter test may preclude use of more than a single test to serve as a baseline. We question whether one water test is sufficient for establishing baseline water quality conditions given potential variability in hydrologic conditions over a multiple year timeframe. Therefore, the primary objective of this study is to establish estimates of variability of different constituents in groundwater through repeated measurements to evaluate the robustness of using a single, certified water test for characterizing baseline water inorganic chemistry. This study also aims to characterize the geochemistry and spatial variability of well water chemistry for different households surrounding Fiddle Lake due to possible heterogeneity of the aquifer and variable inputs of nonpoint source pollution, such as road salt, that already exists within the watershed. Additionally, we establish upper baseline concentrations for several inorganic

components recognized in flowback water (e.g. Cl, Br, Na, K, Ca, Mg, Sr, Ba) and characterize Cl/Br mass ratios for the Fiddle Lake region so that possible accidental additions of pollution from hydraulic fracturing activities can be reliably recognized. Contested cases (Molofsky et al., 2011; Osborn et al., 2011) of possible contamination of groundwater resources in Pennsylvania by methane due to shale gas drilling demonstrate the need for baseline water quality studies (Davis, 2011; Schon, 2011). Several researchers (Hasbargen et al., 2013; Vidic et al., 2013; Llewellyn, 2014; Johnson et al., 2014; Lautz et al., 2014) have pursued baseline water quality studies of inorganic constituents in the Marcellus Shale region of Pennsylvania and New York, and the rapid development of unconventional natural gas requires more of this work. Much of this baseline research has focused on distinguishing between effects of varied land use activities and presence of Appalachian basin brine fluids on groundwater. No baseline studies have addressed the issue of variability with muliple measurements, which is fundamental to establishing the validity of baseline water quality measurements prior to gas drilling.

2. Description of study area 2.1. Hydrologic setting, surficial geology, and land use The study area is located in Ararat Township of eastern Susquehanna County, northeastern Pennsylvania (Fig. 1). Fiddle Lake (41.7880, 75.5314) is a 25.5 ha water body within a 112 ha rural watershed, as defined by its outlet on the south shore. The lake is nestled within gently rolling hills with 25 m of relief on a high elevation, regional plateau. It is one of the highest elevation lakes (610 m) in Pennsylvania and creates a headwater tributary to the Lackawanna River, which flows to the Susquehanna River and ultimately into Chesapeake Bay. Created by glacial activity, Fiddle Lake is set in moderately thick glacial till that was deposited 16–17 ka (Braun, 2010). Its long axis (1.1 km) is oriented northeast–southwest, consistent with the direction of the most recent ice advance during the late Wisconsinan (Braun, 2010). A shallow lake, its average depth is 3 m. Fiddle Lake has no perennial inlet streams, but it does receive surface runoff during hydrologic events, and two perennial springs occur on its northern shoreline. Primarily, however, it is fed by groundwater. Average annual precipitation recorded between 2007 and 2013 at a nearby weather station in Thompson, PA (41.883, 75.5833, 15720 elevation) is 1360 mm, and precipitation is fairly evenly distributed throughout the year, averaging 114 (±24) mm per month (National Climate Data Center, NOAA, 2014). Approximately 80 houses, hayfields and pasture, and one summer camp border the lake, and a small dairy farm is within its watershed. The community consists of both year-round residents and vacation homes. Paved roads pass through the northern and southwestern parts of the watershed, and private dirt roads provide access to houses on the western and eastern shorelines. Houses in the community connected to a regional sanitary sewer system in the year 2000. The study area is identified as a potential location for natural gas development from the Marcellus Shale (Middle Devonian), which is much deeper than the aquifer (depth = 1800–2100 m; 6000–7000 ft) and 70 m thick (Piotrowski and Harper, 1979; DCNR, 2013). Several gas lease sites have been identified near by but outside of the Fiddle Lake watershed. As of October 30, 2012, thirty-seven lease sites have been permitted within a 20 km radius of Fiddle Lake (PA DEP, 2012), and three gas pads have recently been approved or are under production within 3–4 km of Fiddle

Please cite this article in press as: Rhodes, A.L., Horton, N.J. Establishing baseline water quality for household wells within the Marcellus Shale gas region, Susquehanna County, Pennsylvania, U.S.A. Appl. Geochem. (2015), http://dx.doi.org/10.1016/j.apgeochem.2015.03.004

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Fig. 1. The Fiddle Lake watershed showing sample locations for houses, springs, and the lake. Wells also having certified tests are shown with an ‘‘X’’. Topography from Thompson Quadrangle, PA, 7.5 Minute Series. Lower map frame shows regional topography; contour interval = 20 m.

Lake (SRBC, 2014). Some landowners in the Fiddle Lake community have signed natural gas lease agreements, but no gas well development has occurred before or during water sampling for this study.

2.2. Description of bedrock aquifer The Catskill Formation (Late Devonian) underlies the glacial material, and it is recognized as an important, regional bedrock

Please cite this article in press as: Rhodes, A.L., Horton, N.J. Establishing baseline water quality for household wells within the Marcellus Shale gas region, Susquehanna County, Pennsylvania, U.S.A. Appl. Geochem. (2015), http://dx.doi.org/10.1016/j.apgeochem.2015.03.004

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aquifer (Warner et al., 2012; Wilson, 2014; Taylor, 1984). The Catskill Formation consists of fine- to coarse-grained sandstone, siltstone and shale units that formed as part of a massive, prograding deltaic complex. Called the Catskill Delta, these sediments originated from mountains constructed during the Acadian Orogeny, and they were transported westward by fluvial activity and deposited in the Appalachian Basin, which subsided and deepened during the orogeny (Faill, 1985). Stratigraphic members of the Catskill Formation in Susquehanna County have not been mapped in detail, but the units have been correlated (Berg et al., 1983) with the delta plain and alluvial plain environments of the Catskill Delta described further south (Berg, 1975; Smith and Rose, 1985; Sevon and Berg, 1978; Glaeser, 1974). White (1881) provides the only published detailed descriptions of bedrock outcrops in Ararat Township with two stratigraphic sections. The stratigraphy includes massive, grayish-white sandstone (9 m thick), overlain by red shale (30.5–33.5 m thick), which is overlain by calcareous breccias (1.5–3 m thick) that contain shale pieces, pebbles, and fish bone fragments. The calcareous units, which can be locally discontinuous, occur at the bases of gray sandstone units (4.5–6 ft thick) that contain current beds with alternate orientations. Sandstone units sometimes contain strings of coal that are 1–2.5 cm thick (White, 1881). These historical descriptions of the bedrock stratigraphy generally agree with the thicknesses of rock units documented in groundwater well logs for households at Fiddle Lake. Depths to bedrock at well locations range approx. 2.5–33 m (PAGWIS, 2013), and well casings typically extend 10–36 m below the surface. Depths of household drinking water wells range 40–90 m. The wells cut through interlayered gray and brown sandstone and brown and red shale bedrock. Gray sandstone at the base of the wells is the primary water-bearing unit; red shale is noted as an additional water-bearing layer for some wells (PAGWIS, 2013). Groundwater yields range from 18–151 liters per minute (5–40 gallons per minute) (PAGWIS, 2013).

3. Methods 3.1. Sampling A total of 150 well water samples were collected from 35 houses between 15 March 2011 and 26 May 2013, and between 1 and 8 samples were collected from each house during this time frame (Fig. 1). Nineteen households had 5–8 repeated measurements, and timing of collection tended to occur between early spring through mid fall when most houses were available for sampling. In addition, 16 lake surface water samples were collected from 2 locations in Fiddle Lake: at the outlet and at the northern shore. Two perennial groundwater springs located near the Fiddle Lake shoreline (n = 11) were collected. Four snow samples were collected for stable isotope analysis in March 2011. Well water samples were collected from homeowners’ houses at locations similar to where water would be sampled for a certified laboratory test. Typically, they were collected from the kitchen sink; in some cases, samples were collected from a spigot outside of the house or directly from a homeowner’s water tank. Some houses had sediment filters between the well intake and their water tank; two of these houses were sampled upstream and downstream of the sediment filter to assess changes in chemistry due to filtration. No houses with water softeners were included in the analysis. All well water samples were collected after running the water at high flow for 3–5 min. All samples were collected in acid-washed polyethylene bottles after rinsing the bottle and cap three times with the sample. A 125 ml split of each sample was immediately acidified with 2 ml of concentrated nitric acid, and then filtered

through a 0.45 lm CorningÒ SFCA syringe filter approximately 24–72 h following acidification. The intent behind filtering post acidification was to mimic the sampling methods used by the certified, independent laboratory (see Section 3.3 below) and to keep metals in solution, particularly iron, that readily precipitate if oxidized within household water systems or within the well borehole. Waters for stable isotope analysis were collected in 4 ml Qorpak glass bottles with polyethylene caps that prevented evaporation. All samples were transported in a cooler and kept refrigerated until analysis. 3.2. Laboratory measurements Collected samples were analyzed in the Center for Aqueous Biogeochemical Research (CABR) at Smith College. Major cations (Ca2+, Mg2+, Na+, K+), trace metals (Ba2+, Cd2+, Cr2+, Cu2+, FeTOTAL, Li+, Mn2+, Pb2+, Sr2+, Zn2+), and silica were measured on acidified, filtered splits using a Teledyne Leeman Prodigy High Dispersion Inductively Coupled Plasma Spectrometer (ICP-OES). Arsenic was measured on acidified, filtered splits using an atomic absorption graphite furnace (Perkin–Elmer AAnalyst 600). Major anions 2 (F, Cl, Br, NO 3 , SO4 ) were measured on filtered (0.45 lm syringe filter), unacidifed samples using ion chromatography (Dionex ICS-3000). Lower method of quantification limit (MQL) values for AsTOTAL, Ba2+, Br, Cd2+, Cr2+, Cu2+, Li+, Mn2+, Pb2+, Sr2+ and Zn2+ were determined by calculating 10 ⁄ standard deviation of 7 standards of low concentration (Harris, 2010). Lower MQL values for all other ions and silica were based on the lowest standard of the instrumental calibration curve. Acid neutralizing capacity (ANC) and pH were measured with a ManTech autotitrator with Titra-Flo pH electrode, and ANC was calculated by the inflection point method. Specific conductance was measured using a Fisher-Scientific benchtop meter. Total dissolved solids (TDS) were calculated based on the concentrations of all constituents measured. The stable isotopic composition of water (d18O and d2H) was measured using a Picarro L1102i cavity ringdown spectrometer, and results were calibrated against internal laboratory and USGS standards (W-43146 and W-32615). Maximum relative errors for standards are believed to be no more than 2% for cations and silica; 1% for trace metals; and 5% for anions. The average cation–anion balance is 21 (±51) leq/L (0.5 ± 1.7%), with the percent ion imbalance calculated as follows: (Rcations  Ranions)/(Rcations + Ranions) ⁄ 100. Precision for d18O and d2H values is ±0.05 and ±1.0 per mil, respectively. Acid neutralizing capacity (ANC), which is defined as the equivalent sum of bases minus the concentration of H+ (Drever, 1997), 2 2 is assumed to equal [HCO 3 ] + 2[CO3 ] in solution, and [CO3 ] is calculated to be no more than 1% of the total ANC due to near-neutral pH measurements. 3.3. Certified laboratory water tests Certified water tests were performed by a single, independent analytical laboratory for fourteen of the houses in this study and for Fiddle Lake surface water (n = 15). The laboratory was certified by the PA DEP via accreditation by the National Environmental Laboratory Accreditation Program (NELAP). Samples were collected between 6/27/2010 and 8/16/2010 by the laboratory’s field technician from household sinks or water tanks after running the water for 3 min. Parameters measured by the certified laboratory that overlap with the CABR included: pH, TDS, Ca2+, Mg2+, Na+, Cl, 2+ 2+ 2+ 2+ 2 NO 3 , SO4 , AsTOTAL, Ba , FeTOTAL, Mn , Pb , and Sr . Some certified test results, however, did not report Cl (n = 12), SO2 4 (n = 13), and As (n = 14). Concentrations of anions were measured by ion chromatography according to EPA Method 300.0. Concentrations of

Please cite this article in press as: Rhodes, A.L., Horton, N.J. Establishing baseline water quality for household wells within the Marcellus Shale gas region, Susquehanna County, Pennsylvania, U.S.A. Appl. Geochem. (2015), http://dx.doi.org/10.1016/j.apgeochem.2015.03.004

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cations and trace metals were measured by ICP-AES according to EPA Method 200.7 (total recoverable analytes) on samples that were acidified at the time of collection and then were subsequently passed through a 0.45 lm filter 24 h following acidification. Samples having turbidity values P1 NTU were analyzed by the ICP-AES following acid digestion of the particulate material according to EPA Method 200.7, and five of the certified samples had elevated turbidity values ranging 1.0–4.0 NTU.

proportion of SBC that is contributed by salts (road salt or saline solutions derived from bedrock) can be distinguished by subtracting the concentration of Cl from SBC (Rhodes et al., 2001).

3.4. Statistical analysis

Overall, well water geochemistry is classified as calcium bicarbonate water, which is representative of groundwater affected predominately by chemical weathering of silicate and carbonate minerals within a bedrock aquifer. Values for acid neutralizing capacity (ANC) and silica are fairly high (405–2299 leq/L and 4.5–11.0 mg/L, respectively), and base cation concentrations (Ca2+, Mg2+, Na+, K+) are dominated by calcium (Table 1). Sulfate concentrations in lake and groundwater (3.4–21.2 mg/L) are elevated relative to average regional precipitation (0.79 ± 0.39 mg/L); elevated sulfate in groundwater may be derived from weathering of pyrite and copper sulfide minerals, which have been reported in shale rocks of the Catskill Formation and in association with fossil plant material (Smith and Rose, 1985). Wells located within drainage areas of paved roads (houses 12–23; 30–34) have elevated average concentrations of Na+ (7.9 ± 3.3 mg/L) and Cl (19.6 ± 9.7 mg/L) compared to houses outside of these drainage areas (Na+ = 3.8 ± 3.5 mg/L;  Cl = 1.5 ± 1.2 mg/L; Table 1, Fig. 2A and B). Elevated concentrations of sodium and chloride are also present in Fiddle Lake

Water chemistry distribution is described by a number of summary statistics, including the minimum, mean, median, 95th percentile, and maximum values. To account for repeated measurements on wells within the same household, random intercept models (Fitzmaurice et al., 2011) were fit using the nlme package (Pinheiro et al., 2014) in version 3.1.1 of R (R Core Team, 2014). These models allow estimation of variance components associated with household effects (between household, or standard deviation between household means) and residual error (within household standard deviation). Models were first fit with no predictors using just the CABR measurements from all homes to assess between and within household variability. A second model was fit including the certified tests (wells and lake), and a test of difference in means was carried out comparing certified vs. CABR measures (after accounting for household effects). This second model was repeated using only wells with certified results (no lake), and then excluding House #35 (due to aberrant results evident upon visual analysis of the certified results; see results section). For all models, an alpha level of 0.05 was used to establish statistical significance, and no adjustment for multiple comparisons was undertaken. For parameters having concentrations below MQL values, we substituted a value that was ½ of the MQL value (Helsel and Hirsch, 2002) so that the effect of substitution would yield estimates midway of other approaches that might substitute a value of zero or the MQL value. Although this approach does not have any theoretical statistical basis (Helsel and Hirsch, 2002), a sensitivity analysis determined that means and standard deviations showed little difference (up to ±0.5 lg/L) for our data whether we substituted a value of zero, ½ of the MQL, or the MQL value when <60% of the results fell below the MQL. Therefore, we expect our calculations of means and standard deviations to be similar to alternative calculation methods that address results that fall below reporting limits (Helsel and Hirsch, 2002), and we did not calculate means and standard deviations for Cr2+, Cd2+, Li2+, and Pb2+, which had >95% of results below the MQL. We also did not quantify differences between the certified test results and CABR data for pH because of the inherent variability in pH due to different degrees of air equilibration. 3.5. Characterizing effects of mineral weathering on major element geochemistry Mineral weathering of carbonate and silicate minerals in bedrock will produce an equivalent concentration of base cations (Ca2+, Mg2+, Na+, K+) relative to the concentration of acid neutralizing capacity (ANC), when measured in units of eq/L (Drever, 1997). Therefore, waters whose chemistry is controlled primarily by mineral weathering of carbonate and silicate minerals should have a 1:1 relationship between the sum of base cations (SBC) and ANC (Rhodes et al., 2001). If SBC > ANC, additional cations may have been added due to dissolution of road salt, other salt minerals, brine fluids in bedrock, or dissolved salts added from farming or septic effluent. SBC > ANC can also result from a loss of ANC due to addition of strong acids, such as from the weathering of sulfide minerals like pyrite or from acidic precipitation. The

4. Results 4.1. Summary of well, lake and spring geochemistry

Table 1 Median concentrations of dissolved elements measured by the Center for Aqueous Biogeochemical Research (CABR), this study. C notes if parameter was also measured by certified laboratory. Well results are also distinguished by whether they are located within (salt) or outside (no salt) regions that receive road salt runoff from paved roads. Numbers of observations (n) vary by parameter because some samples were not analyzed for AsTOTAL, Li2+ and Sr2+. Parameter Sample sites Samples (n) SC pH TDS ANC Ca Mg Na K F Cl Br NO3–N SO4 SiO2 As Ba Cd Cr Cu Fe Li Mn Pb Sr Zn d18O d2H

Units

lS/cm lab mg/L leq/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L lg/L lg/L lg/L lg/L lg/L lg/L lg/L lg/L lg/L lg/L lg/L per mil per mil

All wells

Springs

Lake

Wells salt

Wells no salt

C

35

2

2

16

19

14

144–150 173 8.0 115 1465 29.7 2.8 5.6 0.6 0.05 5.9 0.013 0.23 10.0 7.08 <0.4 22.3 <1 <2 20.4 11.1 <10 <2.0 <10 62.2 10.2 9.4

12 70.2 7.58 41.4 503.9 10.6 0.82 1.06 0.53 0.03 0.52 0.008 0.75 6.60 5.75 <0.4 29.0 <1 <2 <2 36.3 <10 65.8 <10 21.6 1.5 9.34

14 65.2 7.33 33.1 274 6.53 0.69 6.16 0.63 0.03 10.49 0.006 0.11 4.49 0.80 0.60 12.3 <1 <2 <2 36.6 <10 65.8 <10 21.6 1.5 6.53

70–74 179 8.0 116.4 1392.3 30.0 2.96 7.47 0.66 0.05 20.2 0.01 0.23 8.49 6.62 <0.4 16.5 <0.5 <1.0 25.6 17.6 <10 <2.0 <10 48.8 9.9 8.94

74–76 147.8 8.07 110.2 1600.7 29.5 2.5 2.4 0.58 0.05 1.22 0.01 0.23 10.44 7.6 0.42 44.4 <1 <2 18.5 <10 <10 <2.0 <10 67.3 11.6 9.53

59.2

59.9

45.2

57.6

60.6

X X X X X

X X X X X

X X X X

Please cite this article in press as: Rhodes, A.L., Horton, N.J. Establishing baseline water quality for household wells within the Marcellus Shale gas region, Susquehanna County, Pennsylvania, U.S.A. Appl. Geochem. (2015), http://dx.doi.org/10.1016/j.apgeochem.2015.03.004

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Fig. 2. Box and whisker diagrams of chloride (A) and sodium (B) concentrations for wells, springs, and lake surface within (salt) and outside (no salt) of road salt runoff zones. NE Wells (wells #6-11) on the northeastern shore of Fiddle Lake have elevated sodium and chloride concentrations compared to other wells outside of road salt runoff zones; distribution of wells (no salt) also includes the NE Wells. (C) Sulfate concentrations in wells, springs, and lake are elevated compared to concentration in regional precipitation, suggesting a mineral source for sulfate in regional groundwater. (D) After accounting for the input of base cations from salt, the sum of base cations (SBCChloride) is greater for wells than it is for the springs and lake, suggesting a longer groundwater flowpath with more water–rock interaction for well groundwater. Precipitation data are monthly averages (n = 23) measured between 2012 and 2013, recorded at NADP Site PA-98, Luzerne, PA (NADP, 2014).

(Na+ = 6.4 ± 0.7; Cl = 11.0 ± 1.2 mg/L) and a groundwater spring (Na+ = 12.6 ± 0.9 mg/L; Cl = 19.2 ± 16.2 mg/L) located downstream of a paved road, whereas a groundwater spring upstream of a paved road has much lower average sodium and chloride concentrations (Na+ = 0.9 ± 0.2 mg/L and Cl = 0.5 ± 0.1 mg/L; Fig. 2A and B). Plotting SBC vs. ANC (Fig. 3A) shows that the chemistry of samples outside of road salt runoff zones is primarily controlled by mineral weathering of carbonate and silicate minerals within the watershed. SBC for these wells and spring plot slightly above a predicted 1:1 line, and the slope of the regression is close to one. Although all these samples contain minor chloride, the deviation between SBC and ANC is balanced primarily by sulfate (Fig. 2C), possibly sourced from mineral weathering of pyrite, and to a lesser extent by nitrate. In contrast, the lake, spring, and groundwater samples located within road salt runoff zones all have SBC values that greatly exceed ANC (Fig. 3A). Subtracting chloride concentrations from SBC brings the SBC:ANC imbalance of these samples within range of the other well samples and spring (Fig. 3B).

Flowback and production waters from hydraulic fracturing of the Marcellus Shale have been shown to have elevated concentrations of minor elements (Ba, Sr, Br, Fe, and Mn) (Chapman et al., 2012; Barbot et al., 2013; Haluszczak et al., 2013). In contrast, concentrations of minor elements in lake and groundwater at Fiddle Lake are generally low throughout the study area, and in particular for barium, bromide, and strontium. Descriptive statistics for all the groundwater samples (Table 2) show relatively uniform groundwater concentrations for several elements (ANC, Br, Ca, F, K, Mg, SiO2, Sr) within the study area, as illustrated by similarities between the lower and upper quartile values (e.g. Q25 and Q75). In contrast, As, Cl, Cu, Fe, Mn, Na, and Zn vary by at least one order of magnitude over the study area. For some households (n = 8), national drinking water regulations (EPA, 2012) for secondary maximum contaminant levels (SMCL) were exceeded for FeTOTAL (>0.3 mg/L), and the SMCL for Mn2+ (>0.05 mg/L) was exceeded for 2 households. For most of these households, the SMCL was exceeded for just one test; yet

Please cite this article in press as: Rhodes, A.L., Horton, N.J. Establishing baseline water quality for household wells within the Marcellus Shale gas region, Susquehanna County, Pennsylvania, U.S.A. Appl. Geochem. (2015), http://dx.doi.org/10.1016/j.apgeochem.2015.03.004

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geochemistry results for wells, lake, and springs for each parameter are presented in Supplementary Tables 1–4. 4.2. Stable isotope geochemistry The stable isotopic composition (d2H and d18O; Fig. 4; Supplementary Tables 2 and 4) of most well water falls on a trend line between the composition of snow and springs that parallels the global meteoric waterline (GMWL), defined as d2H = 8.13 ⁄ d18O + 10.8‰ (Rozanski et al., 1993). Monthly local meteoric water (LMW) values, modeled using the lake’s latitude, longitude, and altitude values (Bowen et al., 2005; Bowen, 2013), plot around both these lines. The d2H and d18O of lake surface samples are enriched relative to the groundwater samples, and they plot to the right of the GMWL. Well waters on the northwestern shore of Fiddle Lake (wells #12–20) also plot on an evaporation trend, suggesting that water in these wells may be coming in part from Fiddle Lake. Isotopic compositions of formation waters from the Marcellus Shale and Upper Devonian sandstone units (Sharma et al., 2014), shown for reference, are more enriched in heavy isotopes than all the Fiddle Lake samples (Fig. 4). 4.3. Variability of geochemical results 4.3.1. Statistical model of variability Statistical models were used to quantify the variability of the geochemical results for each well and among all wells within the study area. For the majority of elements, variation in water chemistry at a single house is smaller than the variation in water chemistry between houses (Table 3). Exceptions occur for some minor elements (As, Cu, Fe, and Zn), which all had equal or larger within-house standard deviations than the variability observed geographically. Additionally, the standard deviations of these elements (Fe and Zn) within households are greater than their mean concentrations. However with the exception of Fe, the withinhouse standard deviations of elements expected in hydraulic fracturing wastewater (Na, Cl, Br, Ba, Sr) are very low and below mean values.

Fig. 3. (A) Sum of base cations (SBC) vs. acid neutralizing capacity (ANC) for wells, springs, and lake within (salt) and outside (no salt) of road salt runoff zones. Data are plotted in relation to 1:1 line predicted for weathering of silicate and carbonate minerals. Deviation to left of predicted 1:1 line results from addition of SBC from dissolved salts or loss of ANC due to addition of strong acids. Dashed line shows regression for wells and springs outside of road salt runoff zones. B. Subtracting Cl from SBC (SBC-Chloride) removes proportion of SBC due to chloride-bearing salts. Remaining deviation from 1:1 line results from regional inputs of SO4 and NO3bearing salts or acids to groundwater. The lower ANC values for Fiddle Lake and the springs as compared to the wells suggests that the lake and springs source groundwater from surficial sediments rather than the bedrock aquifer.

one house (well 17) consistently exceeded the SMCL for FeTOTAL, and a different house (well 15) consistently exceeded the SMCL for Mn2+. The national drinking water standard for primary maximum contaminant levels (MCL) for Pb2+ (>0.015 mg/L) was exceeded for 2 tests, each from a different household. For the two houses where samples were collected before and after sediment filtration systems, no significant difference in water chemistry was observed for one house, and a reduction in iron after filtration (from 61 to <10 lg/L) was observed in the other. All

4.3.2. Examination of geographic and seasonal variability For most parameters, variation in well water concentrations show a geographic pattern around Fiddle Lake, which we illustrate with Ca2+, Na+, Cl, Ba2+, Mg2+, and Sr2+ (Fig. 5). As expected, wells within drainage areas of paved roads have high concentrations of Na+ and Cl due to runoff of deicing agents. However, several wells outside of these ‘‘road salt runoff zones’’ on the northeastern shoreline (houses 6–11) also have elevated concentrations of Na+ and slightly elevated concentrations of Cl compared with other wells outside of road salt runoff zones (e.g. houses 1–5, 24–29, 35). Additionally, higher concentrations of Ca2+ and ANC (not shown) occur in wells located on the eastern shore of Fiddle Lake, whereas Ba2+ is higher for a cluster of houses (#21–29) on the western shore. FeTOTAL is generally elevated in the northwestern region of Fiddle Lake, and one house (Well 17) has consistently high iron. Sr2+ is uniformly low (<0.15 mg/L) in all wells, except for house #22, which has relatively high concentrations of both Sr2+ and Ba2+ (740 lg/L and 210 lg/L, respectively). Overall, concentrations of lake and groundwater geochemistry do not show a robust seasonal pattern or consistent response with monthly rainfall depths (Fig. 6). In Fig. 6, Well 7 shows variations in concentration over time that are most representative of households having 5–8 samples over the study period. Conversely, Well 18 illustrates one of three wells with high variation in geochemistry; Wells 17 and 31 (not shown) are the others. Both Wells 18 and 31 are within the salt runoff zones and show the biggest range in chloride concentrations (15 mg and 25 mg/L,

Please cite this article in press as: Rhodes, A.L., Horton, N.J. Establishing baseline water quality for household wells within the Marcellus Shale gas region, Susquehanna County, Pennsylvania, U.S.A. Appl. Geochem. (2015), http://dx.doi.org/10.1016/j.apgeochem.2015.03.004

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Table 2 Summary statistics for dissolved elements measured in groundwater wells by the CABR. SD = standard deviation (±1r). Mean and SD values not reported if more than 60% of results were below the method of quantification limit (MQL). Results below MQL were substituted with a value half of the MQL. Parameter

Units

n

Mean

SD

Min

Q25

Median

Q75

Q95

Max

SC pH TDS ANC Ca Mg Na K F Cl Br NO3–N SO4 SiO2 As Ba Cd Cr Cu Fe Li Mn Pb Sr Zn d18O d2H

lS/cm

150 150 150 150 150 150 150 150 149 150 150 150 150 150 144 150 150 150 150 150 144 150 150 144 150 148 148

166 7.8 111.2 1458 29.4 2.7 6.0 0.68 0.05 11.2 0.013 0.36 96 7.37 0.45 35.46 – – 34.6 112.6 – 15.7 – 72.4 33.3 9.3 59.3

43.6 0.29 26.4 476.8 7.7 0.9 4.0 0.24 0.02 11.6 0.006 0.51 3.4 1.48 0.49 32.35 – – 39.6 318.0 – 61.8 – 81.7 82.4 0.47 2.28

60 6.9 55.9 404.8 13.3 1.0 1.0 0.41 <0.01 0.4 0.004 <0.02 4.0 4.48 <0.4 7.32 <0.5 <1.0 <1.0 <5.0 <5.0 <1.0 <5.0 31.4 <0.5 10.2 66.1

136 7.9 89.5 1181 25.1 1.8 2.4 0.54 0.04 1.2 0.009 0.06 6.1 6.20 <0.4 15.14 <0.5 <1.0 7.3 <5.0 <5.0 <1.0 <5.0 53.5 3.43 9.6 60.9

173 8.0 115.0 1465 29.7 2.8 5.6 0.6 0.05 5.9 0.013 0.23 10.0 7.08 <0.4 22.28 <0.5 <1.0 20.4 11.1 <5.0 <1.0 <5.0 62.2 10.2 9.4 59.2

192 8.2 133.2 1805 33.2 3.2 10.2 0.7 0.06 19.7 0.015 0.38 11.5 8.48 0.59 44.88 <0.5 <1.0 41.4 56.6 <5.0 3.8 <5.0 72.3 25.68 8.9 57.6

225 8.3 146.5 2191 42.3 4.2 12.5 1.26 0.09 30.4 0.022 1.88 15.4 10.09 1.11 91.57 <0.5 <1.0 112.2 639.0a <5.0 31.8 <5.0 99.3 137.0 8.4 55.6

328 8.5 161.8 2299 47.8 4.4 15.7 1.91 0.13 46.4 0.03 2.30 21.2 11.02 4.15 219.60 2.23 <1.0 217.8 2142a 8.1 377.3a 25.9a 746.5 611.0 8.1 54.7

lab mg/L leq/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L lg/L lg/L lg/L lg/L lg/L lg/L lg/L lg/L lg/L lg/L lg/L per mil per mil

a Maximum measured concentration exceeds drinking water standard. Present-day SMCL for Fe = 0.3 mg/L (300 lg/L). SMCL for Mn = 0.05 mg/L (50 lg/L). MCL for Pb = 0.015 mg/L (15 lg/L) (EPA, 2012). SMCL = secondary maximum contamination limit set for potable water for cosmetic reasons. MCL = maximum contamination limit established for human health.

Fig. 4. Stable isotope composition of groundwater wells, lake, spring, and snow samples collected at Fiddle Lake, shown with Devonian Sandstone and Marcellus Shale production waters by Sharma et al. (2014). Modeled precipitation data (Bowen, 2013) plot around both the GMWL and a best-fit line defined by the measured snow and spring data, which may represent a local meteoric water line (LMWL) trend. Lake water follows an evaporitic trend from the GMWL and LMWL. A subset of wells (#12–20) also follow an evaporitic trend between the other wells and the lake. These evaporitic trends also point toward isotopic values for production waters of the Devonian Sands and Marcellus Shale, illustrating potential difficulty of using stable isotope tracers to recognize pollution from natural gas production if used independently of other tracers.

respectively). The changes and direction of Cl concentrations are synchronous with Na concentrations at these locations, and both wells show relatively higher Na and Cl values during July 2012 that coincide with a decrease in Ca, ANC, and d18O values. Minor elements show little variation with time; the greatest variation occurs for FeTOTAL in Well 17, where it varies by approximately 2 mg/L (Fig. 5). Samples from this well consistently contained red sediment, and as with all analyses, samples were acidified prior to being passed through a 0.45 lm filter. After the homeowners installed a sediment filter to the household plumbing system in 2012, the FeTOTAL concentrations for this well dropped below 0.5 mg/L. Concentrations for manganese vary the most for Fiddle Lake (by 100 lg/L); however, concentrations for other elements in Fiddle Lake remain relatively consistent over time (Fig. 6). The stable isotopic composition of well water (shown in Fig. 6 as d18O) shows little variation in groundwater over time (within 0.5 per mil), and within a much lower range than the d18O values modeled for precipitation in Fiddle Lake region. (By comparison, d18O of the modeled monthly data vary by 12 per mil.) Other than a synchronous lower value on July 2012 for several wells, the small changes in d18O values for individual wells tend not to correlate between well locations. The strongest seasonal signal in d18O values occurs for Fiddle Lake, where d18O values increase from winter to summer due to timing of recharge by winter precipitation (which has heavier values) and effects of summertime evaporation (which causes isotopic enrichment of surface lake water). 4.4. Comparison to certified tests For the 15 sample sites (wells and lake) with a certified test by a commercial analytical laboratory, we statistically compared the values reported for the certified tests against results from the multiple analyses performed by us (CABR). Results from the statistical model (Table 4) initially showed strong statistical differences

Please cite this article in press as: Rhodes, A.L., Horton, N.J. Establishing baseline water quality for household wells within the Marcellus Shale gas region, Susquehanna County, Pennsylvania, U.S.A. Appl. Geochem. (2015), http://dx.doi.org/10.1016/j.apgeochem.2015.03.004

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Table 3 Variation in concentrations between household wells (geographic variability) and within individual household wells, represented as standard deviations (SD) of means. Standard deviation ratios (SD ratio) < 1 show that variability within households is greater than geographic variability. Households with only one sample were excluded; the number of households = 30. Parameter

Units

Observations (n)

SD between households

SD within households

SD ratio (Within:Between)

TDS ANC Ca Mg Na K F Cl Br NO3–N SO4 SiO2 As Ba Cd Cr Cu Fe Li Mn Pb Sr Zn d18O d2H

mg/L leq/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L lg/L lg/L lg/L lg/L lg/L lg/L lg/L lg/L lg/L lg/L lg/L per mil per mil

150 150 150 150 150 150 150 150 149 150 150 150 144 150 150 150 150 150 144 150 150 144 150 148 148

28.0 480.0 8.18 0.92 4.0 0.19 0.024 12.1 0.006 0.48 3.2 1.48 0.31 38.9 n.a. n.a. 24.4 204 n.a. 49.0 n.a. 116 55.4 0.4 1.93

5.6 77.4 1.43 0.17 0.90 0.11 0.011 2.84 0.003 0.16 0.97 0.33 0.37 3.7 n.a. n.a. 29.4 223 n.a. 24.6 n.a. 6.0 69.8 0.2 1.13

0.20 0.16 0.17 0.18 0.23 0.58 0.46 0.24 0.50 0.33 0.30 0.22 1.19 0.10 n.a. n.a. 1.20 1.09 n.a. 0.50 n.a. 0.05 1.26 0.50 0.59

n.a.: Not analyzed because >60% of results were below analytical reporting limit.

(p < 0.01) for 3 out of 12 parameters (Ca2+, Mg2+, and Cl) and a weak statistical difference (p = 0.02) for TDS. To focus on groundwater, we repeated the statistical model for just the well samples, and the same parameters (except for TDS) still showed significant differences between the certified tests and CABR analyses; barium also showed a weak statistical difference (p = 0.047). The greatest difference between the certified results and CABR measurements occurs for House #35, where concentrations of Ca2+, Mg2+, Na+, Sr2+, and other cations are 1–2 orders of magnitude lower than repeated (n = 7) CABR measurements, and the ion balance for the certified results deviates strongly toward excess anions. Conversations with the homeowner and with the chemist at the commercial laboratory did not identify a reason for these large differences. The homeowner did not make any changes to the well or plumbing, and the house does not use filters or a water softener. Likewise, the chemist reviewed the analysis, quality controls, and verified the chain of custody; however, the laboratory did not save the sample to enable a repeat analysis. Due to the large cation–anion imbalance and the strong deviation from our repeated measurements, we argue that the certified test results for metals for House #35 are in error. Therefore, we reran the statistical model comparing the certified results against our measurements for wells, excluding House #35. Without House #35 and lake, the model results showed significant differences between certified and CABR groundwater data for Mg2+ and Cl (p = 0.002, p = 0.007, respectively; Table 4). 5. Discussion Overall, our study shows that water chemistry for an individual well varies little over approximately a 2.5 year period, between the time of certified tests collected during summer 2010 and CABR samples collected through May 2013. The results establish a baseline water chemistry for the bedrock aquifer, from which potential contamination by hydraulic fracturing could be reliably recognized. For houses outside of salt-runoff zones, concentrations of the parameters most indicative of pollution by wastewater from

hydraulic fracturing (Ba, Br, Ca, Cl, Mg, Na, Sr) are consistently low over the study period (Fig. 3 and Table 3). No strong geochemical evidence for naturally occurring brine fluids is apparent in the groundwater chemistry. The elevated Na+ in wells on the northeastern shore (houses #6–11) is not balanced by Cl (Fig. 3A and B) and Br concentrations are low. Chemical weathering of hornblende could be producing elevated Na+ for these households, in addition to slightly higher concentrations of Cl and Mg2+, compared to the other wells outside of the salt runoff zones. For wells located within salt-runoff zones, Na+ and Cl concentrations, although elevated, are still generally consistent over this 2-year timeframe. Our baseline characterization of Fiddle Lake is based on samples of the lake surface and outlet, which also may consist predominately of the lake’s surface water. Our characterization of the lake’s baseline chemistry would be improved had we sampled the lake at multiple depths over time to assess for potential chemical stratification and effects of seasonal turnover because dense elements, such as those associated with hydraulic fracturing wastewater, could accumulate toward the lake bottom. The shallow average depth (3 m) of Fiddle Lake relative to its surface area (25.5 ha), and the similar geochemistry between the lake and spring water, suggest that the groundwater inflow to the lake is shallow and that the lake is not strongly stratified for most of the year. However, further work is needed to more thoroughly characterize the lake’s baseline geochemistry. Despite the overall consistency of groundwater chemistry of the bedrock aquifer, we observe some geochemical variability. A statistical difference between the certified results and CABR analyses did occur for three parameters (Mg2+ and Cl, p = 0.002 and p = 0.007) once Fiddle Lake and House #35 were eliminated from the statistical model. While the statistical differences may be due to chance, given that we carried out the test using 12 ions, we note that the CABR measurements for magnesium have very low variability (mean wells = 2.7 ± 0.9 mg/L), and road salt is the primary contributor of chloride within the watershed (Table 3). These and other outlying observations may be better understood when interpreted within the geologic context of the aquifer.

Please cite this article in press as: Rhodes, A.L., Horton, N.J. Establishing baseline water quality for household wells within the Marcellus Shale gas region, Susquehanna County, Pennsylvania, U.S.A. Appl. Geochem. (2015), http://dx.doi.org/10.1016/j.apgeochem.2015.03.004

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Fig. 5. Variation in water geochemistry between sample sites in reference to certified test results for select parameters. Houses #1–34 are organized in geographical order around the lake, starting at the southeastern-most location and continuing counter clockwise to the southwestern part of Fiddle Lake. House #35 is located outside of the Fiddle Lake watershed, and ‘‘salt zone’’ identifies wells receiving road salt runoff from paved roads. See Fig. 1 for sample site locations.

5.1. Groundwater recharge and geochemical variability Fiddle Lake is surrounded by a small watershed, and it is nestled within gently sloped hills on a high elevation plateau. The bedrock aquifer that underlies glacial till consists of nearlyhorizontal sedimentary units that dip to the SE by <10°. This regional topography and underlying bedrock structure suggest that the hydraulic gradient of the aquifer is low and that groundwater recharge is localized primarily within the watershed, although groundwater elevation data and groundwater age are needed to corroborate this hypothesis. Nevertheless, elevated concentrations of chloride are limited to samples collected within the runoff zone of paved roads, which shows that groundwater recharge occurs locally and nonpoint source pollution within the watershed reaches homeowners wells. Additionally, the stable isotope data indicate that some wells on the northwestern shore (houses 12–20) are recharged in part by the lake, illustrating a local hydrologic connection to the surface. In fact, the house with the highest d18O and d2H values also

has the highest Mn2+ concentrations, suggesting that lake water may be a source of this well’s elevated Mn values. In contrast, Fiddle Lake and the springs are likely recharged by groundwater within glacial sediments that follow shorter and shallower flowpaths than the deeper flowpaths that recharge the bedrock aquifer and supply groundwater to wells. Not only is the lake shallow, but the lake and spring waters have the lowest ANC and SBC concentrations, after accounting for inputs of sodium from road salt (Figs. 2D and 3). This suggests less influence by water– rock interactions. Variation in the mineral composition of the glacial till and bedrock also will account for some differences in concentration between the springs and different wells. For example, calcite cement in sandstone layers occurs in varying proportions within bedrock, and limestone boulders are unevenly distributed in the overlying till (White, 1881). Even a small percentage of calcite cement in siliciclastic rocks or sediments will have a dominating effect on water chemistry, producing high concentrations of calcium and ANC due to the high solubility of carbonate minerals (Mast et al., 1990).

Please cite this article in press as: Rhodes, A.L., Horton, N.J. Establishing baseline water quality for household wells within the Marcellus Shale gas region, Susquehanna County, Pennsylvania, U.S.A. Appl. Geochem. (2015), http://dx.doi.org/10.1016/j.apgeochem.2015.03.004

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11

Fig. 6. Illustrative examples of variability of water chemistry for two household wells and for surface lake water. Well 7 shows little variation and is representative of the majority of wells sampled in which concentrations do not show robust seasonal changes. Well 18 illustrates one of two wells with the greatest variability (note Na, Cl, and d18O) that may have a more direct connection to surface runoff or access groundwater from different bedrock layers of the aquifer over time. Lake surface water shows high variability for d18O (due to surface evaporation) and Mn (due to lake redox processes) but otherwise has consistent element concentrations. Monthly precipitation totals are represented by bars. Lines connecting data points are intended as visual aids and are not interpolations of values between points.

The longer flowpath of groundwater to household wells not only increases concentrations of major elements, but it dampens the variability in concentration of most elements and the Table 4 Model-based estimate of differences between CABR measurements and certified test results. Results were evaluated for all samples having certified results (including the lake), all wells with certified results (wells only), and all wells with certified results, excluding House #35. Negative differences show that concentrations reported for certified results were greater than CABR measurements. Certified results are significantly different if noted as follows: ⁄ p < 0.05, ⁄⁄ p < 0.01, ⁄⁄⁄ p < 0.001. Numbers of observations (n) vary by parameter because some CABR samples were not analyzed for AsTOTAL and Sr2+, and some certified analyses did not report results 2+ for Cl, SO2 was excluded because of high number of results below 4 , and AsTOTAL. Pb analytical reporting limits. Parameter

Units

Model 1: wells and lake

Model 2: wells only

Model 3: wells without house #35

TDS Ca2+ Mg2+ Na+ Cl NO 3 –N SO2 4 AsTOTAL Ba2+ FeTOTAL Mn2+ Sr2+

mg/L mg/L mg/L mg/L mg/L mg/L mg/L lg/L lg/L lg/L lg/L lg/L

7.1⁄ 1.81⁄⁄ 0.28⁄⁄⁄ 0.03 2.59⁄⁄ 0.01 0.11 0.21 1.38 34.2 9.38 3.3

2.8 1.96⁄⁄ 0.30⁄⁄⁄ 0.07 2.55⁄⁄ 0.01 0.04 0.26 1.31⁄ 37.1 12.1 3.4

5.0 0.43 0.17⁄⁄ 0.11 2.77⁄⁄ 0.02 0.02 0.28 1.23 39.1 11.8 0.5

102–107 12–15

87–90 11–14

80–82 10–13

(n) CABR (n) Certified

variability of the stable isotopic composition of the water (Fig. 6). Concentrations do not show a robust change with season. Exceptions occur for Na+ and Cl concentrations at Wells 18 and 31, suggesting that these wells have a faster connection to surficial recharge or receive different proportions of groundwater from different stratigraphic units within the aquifer over the year. Well 31 in particular has some of the highest concentrations of Cl (>35 mg/L) despite being located more than 300 m from the road, indicating a more direct connection with surface runoff. However overall, our study shows that the water chemistry at an individual well varies little because the long groundwater flowpaths to wells have greater geochemical equilibration with the aquifer. Groundwater chemistry varies more spatially due to lithologic heterogeneity than temporally due to possible seasonality. 5.2. Comparison between certified tests and CABR results The overall concordance between the repeated CABR measurements and the certified tests suggests that a single certified test generally represents baseline water chemistry of groundwater prior to gas drilling (Table 4), particularly for those elements most characteristic of flowback fluid chemistry (Ba, Br, Ca, Cl, Mg, Na, Sr). However, this is a cautionary conclusion, and multiple baseline measurements always will characterize a household’s water chemistry better than a single test. The CABR measurements, which at most consist of 8 repeated measurements, may have missed—or captured—large precipitation events that could cause turbidity and anomalously high concentrations for some elements, such as

Please cite this article in press as: Rhodes, A.L., Horton, N.J. Establishing baseline water quality for household wells within the Marcellus Shale gas region, Susquehanna County, Pennsylvania, U.S.A. Appl. Geochem. (2015), http://dx.doi.org/10.1016/j.apgeochem.2015.03.004

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Fe and Mn. Coupled with turbidity events, sampling protocols will strongly influence Fe and Mn concentrations. Wells may not have been fully purged prior to sample collection for either the CABR and certified water tests because field indicator parameters (pH, temperature, conductivity, dissolved oxygen, and turbidity) were not monitored. Both the CABR and certified water samples were acidified prior to filtration through a 0.45 lm membrane, which can elevate concentrations of metals due to dissolution of colloids and sediment (Eby, 2004). For example, the homeowners of the well having high Fe (Well #17) report incidents of red sediment in their water, particularly following several weeks of well inactivity, and this observation is consistent with the presence of red shale in the aquifer. High concentrations of Fe and Mn are reported regionally for bedrock wells within the Catskill Formation (Taylor, 1984), and Fe and Mn concentrations in groundwater can be affected by changing redox conditions (McGuire et al., 2000). Therefore given the inherent variability of Fe and Mn concentrations that can be attributed to sampling methods and changing redox conditions, Fe and Mn are not robust indicators of wastewater contamination from hydraulic fracturing fluids, particularly within bedrock aquifers containing Fe- and Mn-bearing minerals such as the Catskill Formation. Additionally, sampling at the wellhead or upstream of a homeowner’s water tank will be a better indicator of aquifer conditions because plumbing is a possible source of trace metals to household water. The two samples that had elevated Pb concentrations (>0.015 mg/L) were collected at the homeowner’s kitchen sink, and household plumbing may have influenced the results. Therefore, results of all tests must take into account sampling protocols, the geologic context of the aquifer, potential effects of water systems, potential seasonal variability, drought, and changing redox conditions. If only one certified test is possible to establish baseline conditions prior to natural gas drilling, monitoring potential changes post drilling will be better informed if samples are collected under the same hydrologic conditions as the baseline sample. In all cases, results from an individual homeowner’s well will be better interpreted if placed in context of baseline chemistry data of a broader region. We attribute the large and significant discrepancy between the certified results measured for House #35 and CABR data to analytical error by the certified laboratory. If our speculation is correct, this highlights the point that laboratories can make mistakes, and the majority of homeowners do not have the expertise to recognize an unreasonable result. The certified laboratory reports do not present an anion–cation balance for the data, which is one method to identify an analytical error. However, complete anion–cation balances are not always possible if a client does not request measurement of all major ions, and errors in trace metal concentrations may be masked for waters having high TDS. A repeat analysis is another way to identify a large laboratory error, and choosing a different laboratory could be helpful if a systemic procedural error is the cause for inaccuracy. 5.3. Estimating regional baseline upper limits In their studies of baseline chemistry of large-scale, regional aquifers, Edmunds et al. (2003) and Lee and Helsel (2005) estimate the upper concentration limits for major and minor elements as the statistical 90th and 95th percentile of their data as a method for recognizing future changes in chemistry due to land use change. For the Fiddle Lake region, we select the maximum value of each parameter for our samples (n = 144–150) because of the limited number of well water sample sites (n = 35) within a small watershed (Table 2). Presumably, sampling a broader area or more wells could result in occurrences that exceed those already measured. Therefore, we select maximum value as a conservative upper

estimate for our data to identify baseline concentrations that have a low probability of being exceeded by subsequent samples unless conditions have been altered (Lee and Helsel, 2005). An exception occurs for Fe and Mn, which still show a wide range of concentration values that exceed the 95th percentile (likely due to sampling protocols), making these elements less reliable for detecting changes from nonpoint source pollution. Similar caution should be applied to elements such as Pb that can be influenced by household plumbing systems. In the case of pollution from hydraulic fracturing, the other elements identified in flowback waters from the Marcellus Shale (Cl, Br, Na, Ca, Mg, Sr, Ba) may be more dependable indicators of contamination for this region. Determining reasonable upper probability exceedance values requires a large data set. Data sharing between homeowners or through a public database such as the Shale Network (Brantley et al., 2012; CUAHSI, 2014) and other published baseline investigations for northeastern Pennsylvania and southern central New York (Johnson et al., 2014; Lautz et al., 2014; Llewellyn, 2014) will enable establishment of upper exceedance concentrations for a region of interest. 5.4. Distinguishing road salt contamination from brine fluids Elevated sodium and chloride concentrations in ground and surface waters due to deicing agents are an ubiquitous problem for watersheds and potable water supplies in the northern United States (Mullaney et al., 2009; Kelly et al., 2008; Godwin et al., 2003; Rhodes et al., 2001) and could mask possible contamination from flowback fluids that commonly contain high concentrations of in situ brines (Chapman et al., 2012; Haluszczak et al., 2013; Barbot et al., 2013). Baseline measurements prior to shale gas drilling have identified nonpoint source pollution in groundwater from various land use activities, including road salt, in northeastern Pennsylvania and southern New York (Johnson et al., 2014; Lautz et al., 2014; Llewellyn, 2014). Appalachian basin brine (ABB) fluids that naturally occur in Devonian-aged bedrock aquifers in northeastern Pennsylvania and New York (Poth, 1962; Dresel and Rose, 2010; Warner et al., 2012; Johnson et al., 2014; Lautz et al., 2014; Llewellyn, 2014) also could be sources of elevated sodium and chloride that characterize the baseline geochemistry of regional groundwater. For example, Lautz et al. (2014) identified ABB as the primary source of salinity in 35% of the groundwater wells that they sampled in southern New York state that had >20 mg/L Cl. As has been established by other researchers (Johnson et al., 2014; Llewellyn, 2014; Mullaney et al., 2009; Panno et al., 2006; Whittemore, 1995), the mass ratios of Br and Cl are useful to graphically distinguish brine fluids from other Cl sources, such as halite from road salt. Fig. 7 shows Cl/Br mass ratios vs. Cl for baseline water samples at Fiddle Lake relative to illustrative mixing curves for halite and for ABB characterized by Johnson et al. (2014) at Salt Spring, Salt Spring State Park in northern Susquehanna County, PA. As expected, well, spring, and lake samples impacted by road salt follow the halite mixing curve between the most dilute well water sample we measured and a halite compositional end member defined by Mullaney et al. (2009). The curve estimates that the elevated Cl concentrations we measured in groundwater wells (up to 38 mg/L) constitute no more than 0.3% halite to groundwater. ABB is not a recognizable component of groundwater for our samples. Therefore, accidental additions of Marcellus Shale flowback fluids within the Fiddle Lake watershed should be easily recognized if Br and Cl are analyzed. The effect would cause groundwater chemistry to trend toward chloride and Cl/Br ratios representative of brine fluids, as shown on Fig. 7 by the composition of ABB from Devonian-aged sedimentary rocks (Warner et al., 2012; Dresel and Rose, 2010; Osborn and McIntosh, 2010) and from Marcellus Shale flowback fluids (Haluszczak et al., 2013; Warner et al., 2012; Hayes, 2009).

Please cite this article in press as: Rhodes, A.L., Horton, N.J. Establishing baseline water quality for household wells within the Marcellus Shale gas region, Susquehanna County, Pennsylvania, U.S.A. Appl. Geochem. (2015), http://dx.doi.org/10.1016/j.apgeochem.2015.03.004

A.L. Rhodes, N.J. Horton / Applied Geochemistry xxx (2015) xxx–xxx

Fig. 7. Cl/Br mass ratio vs. Cl for baseline water samples at Fiddle Lake relative to illustrative mixing curves for halite and for ABB from northeastern Pennsylvania. Well, spring, and lake samples impacted by road salt follow the halite mixing curve, and no baseline samples trend toward ABB. Potential future mixing between samples at Fiddle Lake and Marcellus Shale flowback waters would cause samples to trend toward chloride and Cl/Br ratios representative of brine fluids. Flowback waters from the Marcellus Shale (Hayes, 2009; Haluszczak et al., 2013; Warner et al., 2012) and ABB samples from Upper Devonian sandstone and shale units (Dresel and Rose, 2010; Osborn and McIntosh, 2010; Warner et al., 2012) shown for reference. Mixing curve end members were calculated based on the minimum concentrations for non-road salt impacted well samples at Fiddle Lake (Cl = 0.39 mg/L; Cl/Br = 29) and concentrations for road salt and brine reported in the literature. Halite end member: Cl = 20,000 mg/L; Cl/Br = 10,000 (Mullaney et al., 2009). Brine end member from Salt Springs State Park, Susquehanna County, PA: Cl = 4678 mg/L, Cl/Br = 105 (Johnson et al., 2014).

Although we were able to graphically characterize the Cl/Br mass ratios of the CABR analyses, Br was not measured by the certified laboratory for the Fiddle Lake residents. Br also is not listed by the PA DEP as a primary parameter to be measured in advance of shale-gas drilling for baseline measurements. Penn State Extension does highlight Br as an important baseline parameter (Penn State Extension, 2012). In these cases, and when low concentrations of Br are below method detection limits for different laboratories, other mass ratios, such as (Ba + Sr)/Mg, could be useful alternative indicators of ABB and/or flowback fluids from shale gas drilling in regional groundwater (Johnson et al., 2014). Additionally at Fiddle Lake, the stable isotopic composition of multiple water samples collected for the lake and a subset of households (wells #12–20) follow an evaporitic trend that could be misinterpreted as a mixing line between regional groundwater and wastewater from shale gas production (Fig. 4). Potentially low volumes of brine water in target shale formations could have minimal effects on stable isotopic composition of wastewater, which likely is more reflective of the isotopic composition of the injection fluid. Therefore, stable isotope tracers may be less robust indicators of contamination by shale gas drilling activities, and their use as a tracer may be strengthened if baseline isotopic signatures for a region’s groundwater were previously determined.

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groundwater from the Catskill Formation, Susquehana County, PA. Concentrations of major and minor elements in groundwater show similar or greater geographic variability than the variation in chemistry at a single well. Long hydrologic flowpaths for groundwater dampen seasonal variability of major and minor elemental and stable isotopic composition of groundwater at a particular location. Two wells showing larger variation in Na and Cl concentrations (fluctuating up to 25 mg/L for Cl) likely have greater connectivity to the surface that delivers road salt runoff, and they may be recharged through multiple lithologies within the aquifer. Fe and Mn can be highly variable and less reliable indicators of pollution by flowback waters. Varying redox conditions and water–rock interactions affect mobilization of Fe and Mn. Differing sampling protocols can affect concentrations of these elements greatly, including the sequence of acidification relative to filtering, because of variation in turbidity and field conditions at the time of sample collection if wells are not fully purged. Additionally, household plumbing systems can introduce metals such as Pb that are not reflective of aquifer conditions. Other elements that show much less variability overall and that are more characteristic of flowback fluid chemistry (Ba, Br, Ca, Cl, Mg, Na, Sr) will be more dependable indicators of contamination from gas drilling. Mass ratios of Cl/Br show that road salt is the source of elevated Na and Cl concentrations and that ABB is not an important component of groundwater in the study area. Certified water tests should include Br so that the sources of elevated Na and Cl may be better interpreted. The general consistency of inorganic chemistry of an individual homeowner’s well and the overall concordance between the CABR measurements and the certified test results supports the use of a single certified test for characterizing baseline geochemistry of a private well. Repeated measurements will improve the characterization of baseline chemistry. Defining baseline water quality conditions may be further strengthened after combining data from multiple households and using maximum values to establish the upper baseline concentrations for a region that would have a low probability of being exceeded unless environmental conditions are altered.

Acknowledgements We greatly appreciate and thank the Fiddle Lake residents who shared their water and certified test results. This work was supported by Smith College research funds for faculty. The Center for Aqueous Biogeochemical Research (CABR) was funded by NSF-MRI Grant CHE-0722678. Writing of this manuscript was supported by NSF ADVANCE Grants 0620101 and 0620087. Marc Anderson of the CABR (Smith College) helped develop analytical methods; E. Gillespie, E. Olmsted, and K. Thornton assisted with geochemical analyses; and E. Olmsted helped with literature review. M. Lavine and N. Thai (University of Massachusetts) provided several useful suggestions regarding the statistical analysis. We thank E. Olmsted, N. Moreira, R. Newton, and A. Guswa for helpful comments on an earlier version of the manuscript. We appreciate detailed and thoughtful comments by G. Llewellyn and an anonymous reviewer, which greatly improved the manuscript, and editorial handling by B. Stewart.

Appendix A. Supplementary material 6. Conclusions This study establishes estimates of variability over 2-year period for baseline inorganic chemistry measurements of

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apgeochem.2015. 03.004.

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