sign in sign up
<<
Home , Methane

Article

pubs.acs.org/est

Methane and in Drinking- Overlying the Eagle Ford, Fayetteville, and Haynesville Shale Production Areas † ‡ § ∥ ⊥ Peter B. McMahon,*, Jeannie R.B. Barlow, Mark A. Engle, Kenneth Belitz, Patricia B. Ging, # ∇ ○ ◆ ¶ Andrew G. Hunt, Bryant C. Jurgens, Yousif K. Kharaka, Roland W. Tollett, and Timothy M. Kresse † U.S. Geological Survey, Colorado Water Science Center, Denver Federal Center, Bldg 53, MS 415, Denver, Colorado 80225, United States ‡ U.S. Geological Survey, Lower Mississippi-Gulf Water Science Center, 308 South Airport Road, Jackson, Mississippi 39208, United States § U.S. Geological Survey, Eastern Energy Resources Science Center, Department of Geological Sciences, University of Texas at El Paso, El Paso, Texas 79968, United States ∥ U.S. Geological Survey, New England Water Science Center, 10 Bearfoot Road, Northboro, Massachusetts 01532, United States ⊥ U.S. Geological Survey, Texas Water Science Center, 1505 Ferguson Lane, Austin, Texas 78754, United States # Crustal Geophysics and 17 Science Center, Denver Federal Center, Bldg 95, MS 963, Denver, Colorado 80225, United States ∇ U.S. Geological Survey, California Water Science Center, 6000 J Street, Placer Hall, Sacramento, California 95819, United States ○ U.S. Geological Survey, National Research Program, 345 Middlefield Road, Menlo Park, California 94025, United States ◆ U.S. Geological Survey, Lower Mississippi-Gulf Water Science Center, 3095 West California, Ruston, Louisiana 71270, United States ¶ U.S. Geological Survey, Lower Mississippi-Gulf Water Science Center, 401 Hardin Road, Little Rock, Arkansas 72211, United States

*S Supporting Information

ABSTRACT: Water wells (n = 116) overlying the Eagle Ford, Fayetteville, and Haynesville Shale hydrocarbon production areas were sampled for chemical, isotopic, and groundwater-age tracers to investigate the occurrence and sources of selected in groundwater. Methane isotopes and hydrocarbon compositions indicate most of the methane in the wells was biogenic and produced by the CO2 reduction pathway, not from thermogenic . Two samples contained methane from the fermentation pathway that could be associated with hydrocarbon degradation based on their co-occurrence with hydrocarbons such as and . Benzene was detected at low concentrations (<0.15 μg/L), but relatively high frequencies (2.4−13.3% of samples), in the study areas. Eight of nine samples containing benzene had groundwater ages >2500 years, indicating the benzene was from subsurface sources such as natural hydrocarbon migration or leaking hydrocarbon wells. One sample contained benzene that could be from a surface release associated with hydrocarbon production activities based on its age (10 ± 2.4 years) and proximity to hydrocarbon wells. Groundwater travel times inferred from the age-data indicate decades or longer may be needed to fully assess the effects of potential subsurface and surface releases of hydrocarbons on the wells.

■ INTRODUCTION on the effects of on water resources in the 1 Hydrocarbon production from and gas U.S. The Marcellus Shale production area in the northern (UOG) in the U.S. using horizontal drilling and Appalachian Basin is the most extensively studied UOG play hydraulic fracturing technologies has stimulated a substantial amount of research on the potential effects of these activities on Received: February 9, 2017 groundwater quality. Despite these efforts, the U.S. Environ- Revised: May 2, 2017 mental Protection Agency concluded that data gaps limited Accepted: May 13, 2017 their ability to fully assess the degree of impact in its final report

This article not subject to U.S. Copyright. Published XXXX by the American Chemical A DOI: 10.1021/acs.est.7b00746 Society Environ. Sci. Technol. XXXX, XXX, XXX−XXX Environmental Science & Technology Article

Figure 1. Br/Cl mass ratios as a function of chloride concentration in groundwater samples from the (A.) Eagle Ford, (B.) Fayetteville, and (C.) Haynesville study areas. Circles in each panel are color-coded according to the methane concentration in the sample. Data for shale water from this study (A. and C.−gray stars) and ref 9 (B.). In B., data for sewage/septic-tank effluent from ref 22, samples with the lowest Br/Cl ratios contained tritium and could be mixed with septic-tank effluent in those rural homeowner wells. In C., data for Wilcox from ref 23. Percentages indicate fractions of shale water in groundwater-shale water mixture. with respect to understanding the effects of UOG development County, Texas in 2010, and five samples of produced water − on groundwater quality.2 6 The Barnett (Texas) and Niobrara were collected from oil and condensate wells in the Eagle Ford (Colorado) Shale play areas have also been the focus of several Shale in Gonzales and Lavaca Counties, Texas in 2015. Water − studies.6 8 Fewer peer reviewed studies have examined wells were screened in aquifers composed of semiconsolidated groundwater quality in other major U.S. shale gas and tight sands and clays to interbedded shale, sandstone, and limestone oil play areas such as the Bakken (Montana and North Dakota), that were generally hundreds to thousands of meters above the Eagle Ford (Texas), Fayetteville (Arkansas), Haynesville UOG-bearing shales (SI Section 1, Figures S1−S4). Water (Louisiana and Texas), and Permian Basin (Texas and New wells were selected at random from populations of existing 9−12 Mexico). wells located ≤1 and >1 km from UOG wells to examine This paper examines the occurrence and sources of methane methane and benzene in groundwater in relation to hydro- and benzene in 116 drinking-water wells overlying the Eagle - proximity (SI Section 2). Distances to UOG wells Ford (EF), Fayetteville (FV), and Haynesville (HV) Shale ranged from 0.11 to 25 km, maximum of hydrocarbon production areas using chemical, isotopic, gas, and ground- wells of any kind within 1 km of the water wells ranged from 0 water-age tracers (Supporting Information (SI) Figures S1− to 62 wells, and drill years of the nearest hydrocarbon well of S3). Studies in other areas have suggested possible links any kind ranged from 1929 to 2014 (SI Figure S5, Table S2). between some methane and benzene in groundwater and UOG The 2015 IHS Global data set for oil and gas wells and State production activities.1,2,6,13 High methane concentrations in 14 databases were used to determine the number and distance of drinking water can present a potential explosion hazard, hydrocarbon wells from sampled water wells.17 UOG wells whereas high benzene concentrations can result in an increased 15 refer to hydrocarbon wells completed in the Eagle Ford, cancer risk, among other health concerns. And whereas Fayetteville, or Haynesville Shales, and hydrocarbon wells of methane in groundwater is almost entirely from subsurface any kind refer to UOG wells plus hydrocarbon wells completed sources, benzene in groundwater can be derived from 13,16 in other formations in the study areas. subsurface and surface sources. Water-well samples were analyzed for major , nutrients, This work expands the areal coverage of previous studies that and trace elements; methane, methane H and C isotopic examined methane in groundwater in parts of the FV and HV − 9,12 fi compositions, and C1 C5 ; H and O isotopic play areas, provides some of the rst data on co-occurrences composition of water; noble gas concentrations and isotopic of methane, benzene, and other VOCs in drinking-water wells fl − − compositions; tritium, hexa uoride (SF6), carbon-14 in in the EF FV HV play areas, examines possible links between dissolved inorganic carbon (DIC), and δ13C-DIC; and volatile biogenic methane and hydrocarbon degradation, and provides − fi organic compounds (VOCs) (SI Tables S3 S5). Water from the rst systematic analysis of groundwater age in the play hydrocarbon wells was analyzed for a subset of these areas, which is used to evaluate the relative importance of constituents (SI Table S6). Details on the methods of sample subsurface and surface sources of benzene. The use of collection and analysis are in SI Section 2. Details on assessing consistent sampling and analytical protocols and sets of VOC detections in groundwater are in SI Section 3. analytes provides an opportunity to compare the occurrence Tritium, SF , carbon-14, and tritiogenic -3 concen- and sources of methane and benzene among UOG play areas 6 trations were modeled using the software TracerLPM18 to representing a range of hydrogeologic conditions and hydro- determine fractions of post-1950s groundwater in the samples carbon development histories. and mean ages of the pre- and post-1950s fractions. Pre- and post-1950s groundwater are defined as water recharged before ■ MATERIALS AND METHODS or after the early 1950s start of above ground nuclear weapons In 2015 and 2016, samples of groundwater were collected from testing, respectively. Details on the analysis of groundwater ages 30 domestic wells in each play area (Supporting Information are in SI Section 4. (SI) Table S1). Twenty-six public-supply wells were also Mann−Whitney tests on ranked data were used to test for sampled in EF and HV. One sample of produced water was significant differences in methane concentrations between collected from a gas well in the Haynesville Shale in Rusk water wells ≤1 and >1 km from hydrocarbon wells (SI Table

B DOI: 10.1021/acs.est.7b00746 Environ. Sci. Technol. XXXX, XXX, XXX−XXX Environmental Science & Technology Article

δ13 δ2 − Figure 2. C composition of methane as a function of H CH4 and C1/C2+C3 in groundwater samples from the (A., D.) Eagle Ford, (B., E.) Fayetteville, and (C., F.) Haynesville study areas. Zone boundaries from refs 31 and 32. In A. and D., data for shale gas from this study (star symbols) and ref 30 (dashed gray box), data for spill sites from refs 33, 34, and data for the methanogenic consortium from ref 35. In B. and E., data for shale gas from ref 27. In C. and F., data for shale gas from refs 28 (Texas) and 29 (Louisiana) and data for Wilcox coalbed gas from refs 36 (sample PA-2) and 37 (all others).

S7).19 Spearman correlation analysis was used to examine Several EF samples with relatively high methane and chloride relations between concentrations of methane and location, concentrations had Br/Cl ratios that appeared to be more , and age of hydrocarbon wells (SI Table S8). A consistent with dilute groundwater mixing with small amounts significance level of 0.05 was used for each test. of a saline endmember having a Br/Cl ratio resembling that of than with Eagle Ford Shale water (Figure 1A). Saline ■ RESULTS AND DISCUSSION formation with Br/Cl ratios similar to seawater have been reported elsewhere in the Texas coastal plain,24,25 but Major- of Groundwater. Total dissolved seawater is used as an analog here because the source of the solids (TDS) concentrations in groundwater ranged from 99− saline water is unknown. EF3 was unusual among the mixed 7750 mg/L in EF (median = 493 mg/L), 34−509 mg/L in FV samples because of its high TDS concentration (SI Figure S6), (median = 159 mg/L), and 76−1401 mg/L in HV (median = 511 mg/L) (SI Table S3). The lowest TDS waters were large fraction of saline water (>10%) (Figure 1), and methane characterized as calcium−sodium-chloride (pH < 5) to calcium- isotopic composition (discussed below). FV17 had the highest chloride, sodium, and boron bicarbonate and mixed cation-bicarbonate (pH > 6) types (SI − Figure S6). The highest TDS waters were mostly characterized concentrations among FV samples. Although the Cl Br/Cl as sodium-bicarbonate to sodium-chloride and sodium-sulfate data indicate FV17 could have contained a small (<1%) fraction types. These TDS and major-ion characteristics are similar to of Fayetteville Shale water (Figure 1B), the sodium and boron previously published data for the areas and have been attributed seem less likely to have been derived from shale water. The to varying amounts of calcite, silicate, gypsum, and halite sodium and boron concentrations were highly correlated (rho = dissolution; ion exchange; iron and sulfate reduction; and 0.90, p < 0.001) (SI Figure S7L), and a previous study − possible mixing with small amounts of saline groundwater.9,12,20 concluded on the basis of boron isotopic data that the sodium Cross plots of major-ion concentrations from this study are boron relation in groundwater was related to boron generally consistent with those processes (Figure 1 and SI mobilization from clays in the aquifer by ion-exchange Figure S7). processes rather than substantial mixing with boron-rich shale 9 Mixing with even small amounts of brine from shales could water. This apparent lack of shale-water input is consistent contribute substantial amounts of salt and gas to ground- with methane isotopic data for FV17 (discussed below). water.6,9,21 Data for water isotopes, chloride concentrations, Several HV samples with higher chloride concentrations also and Br/Cl mass ratios in groundwater and shale water indicate appeared to contain chloride from a source other than shale the groundwater contained little (<1%), if any, water from the water based on their low Br/Cl ratios (although we only have Eagle Ford, Fayetteville, or Haynesville Shales (Figures 1 and one sample to characterize Br/Cl in Haynesville Shale water) − S8),.9,22 24 Groundwater samples had water isotopic compo- (Figure 1C). Halite dissolution might be a relatively important sitions that plotted on or near global or local meteoric water source of chloride in HV groundwater. Several of the highest lines (SI Figure S8), whereas shale water was isotopically TDS samples were sodium-chloride type waters that plot along enriched compared to groundwater from the same study area. sodium-chloride concentration trend lines with molar ratios

C DOI: 10.1021/acs.est.7b00746 Environ. Sci. Technol. XXXX, XXX, XXX−XXX Environmental Science & Technology Article near 1 (SI Figures S6 and S7J), suggesting chloride fraction of Fayetteville Shale water (Figure 1B). However, − contributions from halite dissolution. Halite is present in the FV17 did not contain detectable concentrations of C3 C5 subsurface throughout much of the HV region, particularly in hydrocarbons and its methane isotopic data indicate a the adjacent East Texas and North Louisiana Salt Basins (SI predominantly biogenic gas (CO2 reduction) component in Figure S3).26 HV16 and HV25 had more enriched Br/Cl ratios the sample (Figure 2B and E). δ13 − than most other high methane samples in HV and also are C CH4 and C1/(C2+C3) values for HV16, HV25, HV29, discussed below. and HVP98 plot between the biogenic and thermogenic fields Concentrations of Methane in Groundwater. Methane in Figure 2F, suggesting a mixed source for methane in those was detected at concentrations ≥0.001 mg/L in 81% (EF), 89% samples and/or alterations due to methane oxidation. Methane ff δ13 − δ2 − (FV), and 100% (HV) of the groundwater samples (SI Table oxidation processes clearly a ected the C CH4 and H S4; Figures S1−S3). Concentrations exceeded 10 mg/L, a CH4 values of HVP98 (Figure 2C). However, HV16, HV25, proposed action level for methane in groundwater,14 in 7% and HV29 had δ2H values for methane and water close to what (EF), 7% (FV), and 12% (HV) of the samples. The exceedance could be expected for biogenic methane produced by the CO2 rate for FV was about 3 times higher than the rate (2%) reduction pathway (Figure 3),31 consistent with the formation previously reported for a smaller part of the play area in Faulkner and Van Buren Counties.9 For HV, the exceedance rate was lower than the rate (17%) reported for the Texas part of the play area;12 however, that study focused its sampling on a methane hotspot near the Mount Enterprise fault zone where we also detected methane >10 mg/L (well HV16) in this study (SI Figure S3). The highest methane concentrations mostly occurred in higher TDS sodium-bicarbonate waters (SI Figure S6), consistent with findings from several other stud- ies.3,9,10,12,20 Some sodium-chloride waters in EF and HV also contained high methane concentrations. Methane concen- trations were not significantly correlated with well depth (SI Table S8), but in HV, there was a noticeable clustering of high concentrations in the 40−100 m depth interval, similar to previous findings in that area (SI Figure S9).12 Figure 3. δ2H isotopic compositions of water and methane in Methane concentrations were not spatially correlated with groundwater samples from the study areas. Isotopic relations for the hydrocarbon well locations in any of the study areas. There fermentation and CO2 reduction pathways for were no statistically significant differences in methane from refs31 and32, respectively. concentrations between water wells located ≤1 km and >1 km from UOG wells or from hydrocarbon wells of any kind (SI pathway in almost all the other water samples that contained Table S7); nor were there significant correlations between biogenic methane. The three HV wells were completed in the methane concentrations and distance to either UOG or Wilcox Group and had methane isotopic compositions similar hydrocarbon wells of any kind (SI Table S8). There were to in the Wilcox (Figure 2C). Wilcox coalbed also no significant correlations between methane concen- methane collected east of the study area in north-central trations and number of hydrocarbon wells within 1 km of the Louisiana was considered primarily biogenic in origin and 37 water wells, or between methane concentrations and drilling produced by the CO2 reduction pathway. year of the nearest hydrocarbon well. Lignite in relatively close vertical proximity to water wells Sources of Methane in Groundwater. Most samples that could increase the likelihood of coalbed methane mixing with contained enough methane to measure its isotopic composition groundwater. Lignite sample PA-2 and well HV16 were located δ13 − − ‰ δ2 − − had values of C CH4 < 60 , H CH4 between 250 in southern Panola County, Texas and had similar methane − ‰ and 150 , and C1/C2+C3 molar ratios >1000 (Figure 2); isotopic compositions and C1/C2+C3 ratios, although PA-2 − and 87% (FV) to 95% (HV) of the samples that had detectable contained detectable C3 C5 hydrocarbons and HV16 did not methane concentrations did not have detectable concentrations (Figures 2C, 2F).36 PA-2 was collected from a depth of 112 − 36 of C3 C5 hydrocarbons (SI Tables S4 and S5). These m, and HV16 from 64 m (SI Table S1). Previous studies geochemical characteristics are typical of biogenic gas produced suggested the Mount Enterprise fault in Panola County could 8−10 primarily by the CO2 reduction pathway. The predom- be a migration pathway for coalbed methane to enter overlying inance of biogenic gas in FV and HV is generally consistent aquifers.12 Thirty-four to 44% of the domestic wells in Caddo with previous studies in smaller parts of those areas.9,12 and Bossier Parishes, Louisiana, where HV25 and HV29 were Methane isotopic and hydrocarbon-gas compositional data located, had at least one lignite layer described in their boring 38 − from this and previous studies indicate shale gas in the study logs. HV25 and HV29 also did not contain C3 C5 areas was thermogenic and compositionally different from most hydrocarbons. Groundwater interaction with lignite could − of the groundwater methane (Figure 2).27 30 The data indicate account for the Br/Cl enrichment in HV16, given that lignite the UOG plays were not important sources of methane in the samples from the Wilcox Group (n = 44) had Br/Cl ratios sampled wells. This conclusion is supported by the Br/Cl data ranging from 0.006 to 0.076 (median 0.027),23 well above the and lack of correlation between groundwater−methane Br/Cl ratio in HV16. Boron enrichment in several higher concentrations and hydrocarbon well locations, densities, and methane HV samples, including HV16 and HV25, relative to drilling years. lower methane samples, could be further evidence for FV17 was the only FV sample with C1/C2+C3 < 1000, and groundwater interaction with lignite (SI Figure S7M). Co- Cl−Br/Cl data indicate it could have contained a small (<1%) occurrence of high boron and biogenic methane concentrations

D DOI: 10.1021/acs.est.7b00746 Environ. Sci. Technol. XXXX, XXX, XXX−XXX Environmental Science & Technology Article in groundwater were also reported in the lignite-rich Fort methane produced during crude oil degradation, with Union Formation in the northern Great Plains.10 subsequent isotopic alteration by oxidation processes. EF3 13C enrichment of Wilcox coalbed methane in north-central contained trace amounts of butane, high chloride and helium-4 Louisiana relative to typical biogenic gas (Figure 2C) has been concentrations relative to the shallow depth of the well (SI attributed to substrate depletion during methanogenesis.37 Figures S11C, S11D), and high sodium and boron concen- δ13 − δ13 − fl C CH4 and C DIC data for HV16, HV25, and HV29 trations, indicating mixing with a deeper saline uid (Figure 1A appear consistent with that process (SI Figures S10A, S10B). and SI Figure 7K). EF3 was essentially tritium (<0.1 TU) and HV25 was the only high-methane sample that appeared to be carbon-14 (<1% modern carbon, pmc) dead (SI Table S4), so substantially stripped of light noble (SI Figure S10C), any hydrocarbons potentially mixed in EF3 were likely from a suggesting direct contact with a local gas accumulation.6,39 subsurface source such as natural hydrocarbon migration or a Given that methane in HV25 was compositionally different leaking hydrocarbon well. Three UOG wells were located from Haynesville Shale gas (Figures 2C, 2F) and other lines of within 1 km of EF3 (the closest well at 500 m) (SI Table S2), evidence presented here, it seems unlikely that the inferred but Br/Cl data suggest groundwater in EF3 mixed with a saline local gas accumulation was from the Haynesville Shale. endmember other than Eagle Ford Shale water (Figure 1A). − EFP72 was the only sample with a C3 C5 hydrocarbon Importantly, FV7, the other sample besides EF3 containing detection (0.0004 mol % ) that also had a methane fermentation methane, clearly contained fuel hydrocarbons δ13 − − ‰ concentration >1 mg/L (6.7 mg/L), C CH4 > 50 ethylbenzene, xylenes, butane, and the manufactured fuel − ‰  ( 43.7 ), and C1/(C2+C3) < 1000 (551), indicating it additive methyl tert-butyl ether (MTBE) (SI Table S5) contained thermogenic gas. EFP72 was also unusual in that it possibly indicating a link between fermentation methane and − was the deepest (1299 m) and hottest (water temperature of hydrocarbon degradation in that sample.33 35 The presence of 60.8 °C) water well sampled. The depth and temperature of MTBE and tritium (0.96 TU) in FV7 suggest a surface source that well are consistent with geothermal gradients previously for the hydrocarbons in that sample. If fermentation methane in reported for that area (2.7−3.6 °C/100 m)40 (SI Figure S11A), EF3 and FV7 was associated with degradation of oil or fuel, it suggesting this was not an anomalous temperature associated would have implications for interpreting the presence of with leakage of deeper, hotter fluids. The water isotopic biogenic methane in shallow groundwater in hydrocarbon composition and Br/Cl ratio of EFP72 indicate it contained production areas. Biogenic methane in shallow groundwater in little (<1%), if any, Eagle Ford Shale water, and its TDS those settings is not typically thought to be related to − concentration was comparable to some of the low-methane EF hydrocarbon reservoirs or production,8 10 but data for EF3 samples (SI Figure S6). Cumulatively, the data indicate and FV7 may indicate methane produced by the fermentation thermogenic methane in EFP72 was not due to substantial pathway was associated with degradation of hydrocarbons. mixing between groundwater and gas-rich formation water Concentrations of Benzene in Groundwater. The from the Eagle Ford Shale such as might occur from large scale highest benzene concentration detected in any water sample natural brine migration21 or leakage from a nearby UOG well was 0.127 μg/L (HV29, 2015 sample; SI Table S5), nearly 40 (closest UOG well 1.8 km away). The methane in EFP72 also times lower than the 5 μg/L federal drinking-water standard.15 does not appear to be from substantial mixing with a migrated Even though benzene concentrations were low, benzene free-gas phase based on its 20Ne/36Ar ratio (0.16) relative to air- detection frequencies (DFs) were relatively high. Benzene saturated water (∼0.15) (SI Figure S11B). Elevated 20Ne/36Ar was detected at concentrations ≥0.013 μg/L (long-term ratios (0.4 to >1) were observed in some groundwater method detection level; SI Section 3) in 9.3% (EF), 13.3% containing thermogenic methane in the Marcellus play area (FV), and 2.4% (HV) of the water samples, which are about and attributed to partitioning of light noble gases into 1.5−8 times higher than benzene DFs reported in two national a migrated free-gas phase.6 Given the depth and temperature of groundwater data sets with similar detection levels.42,43 A EFP72 and that the top of the Eagle Ford Shale in that area was previous study in the EF play area did not detect benzene in ∼2000 m deeper than EFP72,41 the simplest explanation of the groundwater,11 which could be due in part to differences in data is that there was some connection between the deep benzene detection levels that were used (ref 11 did not report a aquifer in that location and a relatively shallow accumulation of benzene detection level). Benzene and methane were weakly thermogenic gas, although not through extensive direct contact correlated in EF but not correlated in FV or HV (SI Table S8). given the limited 20Ne and 36Ar gas stripping in the sample (SI Benzene detections occurred in more varied water types and Figure S10C). Similar conditions were observed in groundwater TDS ranges than methane (SI Figure S6), probably reflecting, in parts of the Barnett UOG play area in north Texas.39 in part, contributions of benzene to both shallow groundwater Evidence for Methane from Hydrocarbon Degrada- from surface sources and to deeper groundwater from tion. EF3 and FV7 were notably different from the other subsurface sources. samples that contained biogenic methane in that they appeared Surface versus Subsurface Sources of Benzene in to contain relatively large fractions of methane produced by the Groundwater. Groundwater recharged after the onset of acetate fermentation pathway based on their methane and UOG production activities in ∼2004 (FV) to ∼2008 (EF, HV) water δ2H values (Figure 3). Although the methane isotopic could potentially contain benzene and other hydrocarbons from composition of EF3 was somewhat unusual, a similar result was surface releases associated with those activities such as obtained when the well was resampled (Figure 2A). EF3 had an accidental spills or pipeline leaks. Tritium, tritiogenic helium- isotopic composition similar to some methane at fuel-spill sites 3, SF6, and carbon-14 were used to evaluate the timing of in Minnesota and California that was produced by the recharge. Age-dating has been used in previous studies to fermentation pathway and subjected to varying degrees of analyze sources of hydrocarbons in groundwater overlying oil oxidation (Figure 2A).33,34 Methane can also be produced by and gas fields,10,13,16 but this is the first systematic analysis of the fermentation pathway during crude oil degradation (Figure groundwater age in these play areas. Tritium was detected at 2A).35 The EF3 data could be consistent with fermentation concentrations >0.1 to 0.2 TU in 87% (FV), 20% (EF), and

E DOI: 10.1021/acs.est.7b00746 Environ. Sci. Technol. XXXX, XXX, XXX−XXX Environmental Science & Technology Article

Figure 4. Mean age of the pre-1950s (lower half of y-axes) and post-1950s (upper half of y-axes) fractions of water as a function of the fraction of post-1950s water in groundwater samples from the (A.) Eagle Ford, (B.) Fayetteville, and (C.) Haynesville study areas. Blue bands indicate the of UOG development relative to the sample year in each study area. Error bars represent 1-sigma errors. Orange symbols indicate samples with hydrocarbon detections. Hydrocarbon abbreviations: B, benzene; E, ethylbenzene; T, ; X, xylenes; MTBE, methyl tert-butyl ether; C3, propane; C4, ; C5, . Samples with benzene detections highlighted in red.

14% (HV) of the samples (SI Figure S12), indicating they mean ages ranging from 2,600 to >30,000 years, indicating the contained at least some groundwater recharged after the early benzene in those samples was from subsurface sources such as 1950s (post-1950s) (SI Section 4).18,44 The large percentage of natural hydrocarbon migration or leaking hydrocarbon wells. FV samples containing post-1950s recharge likely reflects the Those samples were sodium-bicarbonate type waters with shallow water-well depths there (SI Table S1), and higher relatively high TDS concentrations (Figure S6). The recharge rates driven by the relatively low mean annual air occurrence of benzene in old groundwater is consistent with temperature and high mean annual in FV.45,46 previous work near California oil fields.16 The results differ Conversely, 86% (HV), 80% (EF), and 13% (FV) of the from findings in the Marcellus play where a predominantly samples were tritium dead (<0.1 to 0.2 TU) and appeared to surface source for diesel-range organic compounds and other only contain pre-1950s recharge. For the pre-1950s samples in hydrocarbons in groundwater was observed,13 which could EF and HV, median carbon-14 and helium-4 concentrations reflect differences in recharge conditions and histories of were 10 and 5 pmc, respectively, and 4.4 × 10−7 and 1.6 × 10−6 hydrocarbon production between the areas. cm3STP/g (∼10−100 times air saturation levels), respec- Previous studies have shown that old groundwater ages can H2O − tively, suggesting many of those samples had ages older than be indicators of slow groundwater velocities.10,47 49 A study in several thousand years.6,10,39 the EF area reported groundwater velocities on the order of 1− The presence of post-1950s recharge in some samples does 3 m/year in parts of an aquifer containing groundwater >5000 not necessarily mean the water was recharged after the onset of years old,47 suggesting any benzene releases into old ground- UOG development, so the lumped-parameter modeling water from leaking UOG wells might have only traveled short software TracerLPM18 was used to estimate fractions of post- distances in the eight years since UOG production commenced. 1950s water in the samples and the mean ages of the pre- and The closest UOG well to an EF water well with a benzene post-1950s fractions (SI Table S9)(SI Section 4). Despite the detection was 1.26 km away (SI Table S2), suggesting the large percentage of FV samples that contained post-1950s benzene detections probably were not from UOG wells. recharge, only three of them had mean ages that overlapped the An UOG source can also be ruled out for the highest period of UOG development (Figure 4). The data indicate benzene concentration detected in pre-1950s groundwater groundwater in the depth zones of the drinking-water wells was (HV29). Benzene was detected in HV29 by the USGS in 2004 not particularly vulnerable to surface releases of contamination (SI Table S5), about 4 years before to the start of UOG potentially associated with UOG activities, at least not in the production in the HV play area but well after the drilling year ∼7−11 year time frame of UOG development relative to when (1955) of the nearest hydrocarbon well (0.13 km away) (SI the wells were sampled (blue bands in Figure 4). The wells Table S2). HV29 also had 62 hydrocarbon wells within 1 km, could become more vulnerable with time as potentially 92% of which predated UOG activity. This information contaminated groundwater overlying the wells moves deeper indicates benzene in HV29 was not from UOG production into the aquifers. This is particularly relevant in FV where mean activities, but an association with older hydrocarbon production ages of post-1950s recharge suggest groundwater travel times activities is possible given the proximity and age of those from recharge areas to many of the wells were on the order of hydrocarbon wells. HV29 also contained trace amounts of 15−40 years. and that could be asso- Of the nine samples with benzene detections, only FV12 ciated with well chlorination, a recommended well-maintenance contained benzene that could be from a surface release practice in parts of Louisiana.50 associated with UOG production activities based on its Implications. UOG operations did not contribute sub- groundwater age (Figure 4). FV12 contained about 80% stantial amounts of methane or benzene to the sampled post-1950s water, had a mean age for that fraction (10 ± 2.4 drinking-water wells. However, groundwater travel times years) that overlapped UOG development, and was 0.36 km inferred from the age-data indicate decades or longer may be from an UOG well. FV-12 was a low-TDS, mixed-cation- needed to fully assess the effects of potential subsurface and bicarbonate type water, consistent with the groundwater being surface releases of hydrocarbons on the wells.10 In EF and HV, relatively young and less chemically evolved (Figure S6). The where there was a long history of non-UOG hydrocarbon remaining eight samples only contained pre-1950s water with production (Figure S5), the few water wells with detections of

F DOI: 10.1021/acs.est.7b00746 Environ. Sci. Technol. XXXX, XXX, XXX−XXX Environmental Science & Technology Article − benzene or C3 C5 generally appeared to be closer to non- Stephen Opsahl and anonymous reviewers for the journal. This UOG hydrocarbon wells than to more recently drilled UOG work was funded by the USGS National Water-Quality wells (Figure 5), suggesting a connection between hydrocarbon Assessment (NAWQA) Project and Energy Resources Program. Any use of trade, firm, or product names is for description purposes only and does not imply endorsement by the U.S. Government.

■ REFERENCES (1) U.S. Environmental Protection Agency, Hydraulic Fracturing for Oil and Gas: Impacts from the Hydraulic Fracturing Water Cycle on Drinking Water Resources in the United States (Final Report). EPA-600- R-16-236Fa; Washington, D.C.: 2016. https://www.epa.gov/hfstudy. (accessed December 13, 2016). (2) Osborn, S. G.; Vengosh, A.; Warner, N. R.; Jackson, R. B. Methane contamination of drinking water accompanying gas-well drilling and hydraulic fracturing. Proc. Natl. Acad. Sci. U. S. A. 2011, 108 (20), 8172−8176. (3) Molofsky, L. J.; Connor, J. A.; Wylie, A. S.; Wagner, T.; Farhat, S. Figure 5. Distance of sampled water wells to nearest unconventional K. Evaluation of methane sources in groundwater in northeastern hydrocarbon (UOG) well (circles) or hydrocarbon well of any kind Pennsylvania. Groundwater 2013, 51 (3), 333−349. (triangles). Samples are color-coded according to whether they had a (4) Baldassare, F. J.; McCaffrey, M. A.; Harper, J. A. A geochemical detection of selected hydrocarbons. In general, only UOG wells were context for stray gas investigations in the northern Appalachian Basin: present in the Fayetteville study area. Location data for hydrocarbon Implications of analyses of natural gases from Neogene-through wells are listed in SI Table S2. Devonian-age strata. AAPG Bull. 2014, 98 (2), 341−372. (5) Siegel, D. I.; Smith, B.; Perry, E.; Bothun, R.; Hollingsworth, M. Pre-drilling water-quality data of groundwater prior to shale gas detections and older wells (e.g., HV29). We cannot, however, drilling in the Appalachian Basin: Analysis of the Chesapeake Energy rule out a possible natural subsurface source for some Corporation dataset. Appl. Geochem. 2015, 63 (1), 37−57. hydrocarbon detections (e.g., EFP72).16 (6) Darrah, T. H.; Vengosh, A.; Jackson, R. A.; Warner, N. R.; Our results may not be representative of the entire study Poreda, R. J. Noble gases identify the mechanisms of fugitive gas contamination in drinking-water wells overlying the Marcellus and areas. Only 130 hydrocarbon wells of any kind, out of a − combined population in the study areas of >100 000 wells (SI Barnett Shales. Proc. Natl. Acad. Sci. U. S. A. 2014, 111 (39), 14076 17 14081. Figure S5), were within 1 km of the sampled water wells. (7) Fontenot, B. E.; Hunt, L. R.; Hildenbrand, Z. L.; Carlton, D. D.; Sampling near more hydrocarbon wells could be warranted. Oka, H.; Walton, J. L.; Hopkins, D.; Osorio, A.; Bjorndal, B.; Hu, Q. Generally, samples were not collected at the water table where H.; Schug, K. A. An evaluation of water quality in private drinking the aquifers are most vulnerable to surface releases of water wells near extraction sites in the Barnett Shale contamination. We also did not collect samples from multiple formation. Environ. Sci. Technol. 2013, 47 (17), 10032−10040. depths at any one location. It could be helpful to understand (8) Li, H.; Carlson, K. H. Distribution and origin of groundwater vertical variability in groundwater chemistry in close proximity methane in the Wattenberg oil and gas field of northern Colorado. − to hydrocarbon wells. Environ. Sci. Technol. 2014, 48 (3), 1484 1491. (9) Warner, N. R.; Kresse, T. M.; Hays, P. D.; Down, A.; Karr, J. D.; ■ ASSOCIATED CONTENT Jackson, R. B.; Vengosh, A. Geochemical and isotopic variations in * shallow groundwater in areas of the Fayetteville Shale development, S Supporting Information north-central Arkansas. Appl. Geochem. 2013, 35 (1), 207−220. The Supporting Information is available free of charge on the (10) McMahon, P. B.; Caldwell, R. R.; Galloway, J. M.; Valder, J. F.; ACS Publications website at DOI: 10.1021/acs.est.7b00746. Hunt, A. G. Quality and age of shallow groundwater in the Bakken Further information on study design, sample collection Formation production area, Williston Basin, Montana and North − and analysis, assessing VOC detections, and groundwater Dakota. Groundwater 2015, 53 (S1), 81 94. age-dating approach; additional figures and tables. The (11) Hildenbrand, Z. L.; Carlton, D. D.; Meik, J. M.; Taylor, J. T.; Fontenot, B. E.; Walton, J. L.; Henderson, D.; Thacker, J. B.; Korlie, S.; data are also available for download from https://doi. Whyte, C. J.; Hudak, P. F.; Schug, K. A. A reconnaissance analysis of org/10.5066/F77D2SC4 (PDF) groundwater quality in the Eagle Ford shale region reveals two distinct bromide/chloride populations. Sci. Total Environ. 2016, 575 (1), 672− ■ AUTHOR INFORMATION 680. Corresponding Author (12) Nicot, J.-P.; Larson, T.; Darvari, R.; Mickler, P.; Slotten, M.; *E-mail: [email protected]. Aldridge, J.; Uhlman, K.; Costely, R. Controls on methane occurrence in shallow aquifers overlying the Haynesville Shale Gas Field, East ORCID Texas. Groundwater 2017, DOI: 10.1111/gwat.12500. Peter B. McMahon: 0000-0001-7452-2379 (13) Drollette, B. D.; Hoelzer, K.; Warner, N. R.; Darrah, T. H.; Notes Karatum, O.; O’Connor, M. P.; Nelson, R. K.; Fernandez, L. A.; The authors declare no competing financial interest. Reddy, C. M.; Vengosh, A.; Jackson, R. B.; Elsner, M.; Plata, D. L. Elevated levels of diesel range organic compounds in groundwater near ■ ACKNOWLEDGMENTS Marcellus gas operations are derived from surface activities. Proc. Natl. Acad. Sci. U. S. A. 2015, 112 (43), 13184−13189. We thank well owners who allowed us to sample their wells, (14) Eltschlager, K. K., Hawkins, J. W., Ehler, W. C., Baldassare, F. and Brian Varela for doing the GIS analysis of the IHS dataset. Technical measures for the investigation and mitigation of fugitive This article was improved by the constructive reviews of methane hazards in areas of mining. U.S. Department Interior Office

G DOI: 10.1021/acs.est.7b00746 Environ. Sci. Technol. XXXX, XXX, XXX−XXX Environmental Science & Technology Article of Surface Mining. 2001; Pittsburgh, PA; http://www.osmre.gov/ (32) Schoell, M. The and carbon isotopic composition of resources/blasting/docs/MineGasesDust/Methane.pdf (accessed No- methane from natural gases of various origins. Geochim. Cosmochim. vember 29, 2016. Acta 1980, 44 (5), 649−661. (15) U.S. Environmental Protection Agency, Table of regulated (33) Conrad, M. E.; Templeton, A. S.; Daley, P. F.; Alvarez-Cohen, drinking water contaminants. https://www.epa.gov/ L. Isotopic evidence for biological controls on migration of − dwstandardsregulations (accessed September 19, 2016). hydrocarbons. Org. Geochem. 1999, 30 (8), 843 859. (16) Landon, M. K.; Belitz, K. Geogenic sources of benzene in (34) Revesz, K.; Coplen, T. B.; Baedecker, M. J.; Glynn, P. D. aquifers used for public supply, California. Environ. Sci. Technol. 2012, Methane production and consumption monitored by stable H and C − isotope ratios at a crude oil spill site, Bemidji, Minnesota. Appl. 46 (16), 8689 8697. − (17) IHS Global, U.S. Well History and Production Database−2015: Geochem. 1995, 10 (5), 505 516. IHS Global, Englewood, Colo.. http://www.ihsenergy.com. (accessed (35) Richnow, H. H.; Zengler, K.; Gehre, M.; Michaelis, W.; Widdel, December 13, 2016). F. Methane formation during the degradation of n- and total (18) Jurgens, B. C., Böhlke, J. K., Eberts, S. M., 2012, TracerLPM crude oils. J. Conf. Abstr. Goldschmidt 2000, 5 (2), 844. (36) Warwick, P. D., SanFilipo, J. R., Karlsen, A. W., Barker, C. E. (Version 1): An Excel® workbook for interpreting groundwater age et al. Results of Coalbed Methane Drilling in Panola County, TX, Open- distributions from environmental tracer data. Techniques and Methods File Rep 2005−1046; U.S. Geological Survey: VA, 2005. Report 4-F3; U.S. Geological Survey: VA, 2012. (37) McIntosh, J. C.; Warwick, P. D.; Martini, A. M.; Osborn, S. G. (19) Helsel, D. R., Hirsch, R. M. Statistical Methods in Water Coupled hydrology and of - coal Resources; Elsevier: New York, 1992. beds, northern Gulf of Mexico. Geol. Soc. Am. Bull. 2010, 122 (7/8), (20) Grossman, E. L.; Hahn, R. W.; Fritz, S. J. Origin of gaseous 1248−1264. hydrocarbons in the Sparta Aquifer in Brazos and Burleson Counties, (38) Carlson, D. Lignite’s occurrence within aquifers of northern − Texas. Trans. Gulf Coast Assoc. Geol. Soc. 1986, 36 (1), 457 470. Louisiana. Proc Baton Rouge Geol Soc, First Annual Groundwater (21) Warner, N. R.; Jackson, R. B.; Darrah, T. H.; Osborn, S. G.; Symposium 2007, 1 (1), 43−53. Down, A.; Zhao, K.; White, A.; Vengosh, A. Geochemical evidence for (39) Wen, T.; Castro, M. C.; Nicot, J. P.; Hall, C. M.; Larson, T.; possible natural migration of brine to shallow Mickler, P.; Darvari, R. Methane sources and migration mechanisms in aquifers in Pennsylvania. Proc. Natl. Acad. Sci. U. S. A. 2012, 109 (30), shallow groundwaters in Parker and Hood Counties, Texas−A heavy 11961−11966. noble gas analysis. Environ. Sci. Technol. 2016, 50 (21), 12012−12021. (22) Katz, B. G.; Eberts, S. M.; Kauffman, L. J. Using Cl/Br ratios and (40) Pfeiffer, D. S.; Sharp, J. M. Subsurface temperature distributions other indicators to assess potential impacts on groundwater quality in south Texas. Gulf Coast Assoc. Geol. Soc. Trans. 1989, 39 (1), 231− from septic systems: A review and examples from principal aquifers in 245. the United States. J. Hydrol. 2011, 397 (3−4), 151−166. (41) U.S. Energy Information Administration, Update to the EIA (23) Palmer, C. A., Oman, C. L., Park, A. J., Luppens, J. A. The U.S. Eagle Ford Play Maps. http://www.eia.gov/maps/pdf/ Geological Survey coal quality (COALQUAL) database version 3.0. EIA%20Eagle%20Ford%20Play%20update%2012-29-14.pdf. (accessed Data Series 975; U.S. Geological Survey: VA, 2015. https://pubs.er. January 30, 2017). usgs.gov/publication/ds975 (accessed January 6, 2017). (42) Zogorski, J. S., Carter, J. M., Ivahnenko, T., Lapham, W. W., (24) Blondes, M. S., Gans, K. D., Rowan, E. L., Thordsen, J. J., Reidy, Moran, M. J., Rowe, B. L., Squillace, P. J., Toccalino, P. L. The Quality ’ − ’ M. E., Engle, M. A., Kharaka, Y. F., Thomas, B. U.S. Geological Survey of Our Nation S Waters volatile Organic Compounds in the Nation S National Produced Waters Geochemical Database v2.2 (PROVI- Groundwater and Drinking Water Supplies, Circular 1292; U.S. SIONAL). http://energy.usgs.gov/EnvironmentalAspects/ Geological Survey: VA, 2006. (43) Arnold, T. L., DeSimone, L. A., Bexfield, L. M., Lindsey, B. D., EnvironmentalAspectsofEnergyProductionandUse/ProducedWaters. Barlow, J. R., Kulongoski, J. T., Musgrove, M., Kingsbury, J. A., Belitz, aspx#3822349-data (accessed December 13, 2016). K. Groundwater Quality Data from the National Water-Quality (25) Land, L. S.; McPherson, G. L. Origin of saline formation waters, Assessment Project, May 2012 through December 2013. Data Series Cenozoic section, Gulf of Mexico sedimentary basin. AAPG Bull. 1992, − 997; U.S. Geological Survey:VA, 2016. 76 (9), 1344 1362. (44) Michel, R. M. Tritium Deposition in the Continental United States, (26) Adams, R. L. Basement tectonics and origin of the Sabine Uplift. 1953−1983, Water-Resources Investigations Report 89-4072; U.S. − Gulf Coast Assoc. Geol. Soc. Trans. 2009, 59 (1), 3 19. Geological Survey:VA, 1989. (27) Zumberge, J.; Ferworn, K.; Brown, S. Isotopic reversal (45) McMahon, P. B.; Plummer, L. N.; Böhlke, J. K.; Shapiro, S. D.; ‘ ’ ( rollover ) in shale gases produced from the Mississippian Barnett Hinkle, S. R. A comparison of recharge rates in aquifers of the United and Fayetteville formations. Mar. Pet. Geol. 2012, 31 (1), 43−52. States based on groundwater-age data. Hydrogeol. J. 2011, 19 (4), (28) Stolper, D. A.; Lawson, M.; Davis, C. L.; Ferreira, A. A.; Santos 779−800. Neto, E. V.; Ellis, G. S.; Lewan, M. D.; Martini, A. M.; Tang, Y.; (46) National Oceanic and Atmospheric Administration, 1981−2010 Schoell, M.; Sessions, A. L.; Eiler, J. M. Formation temperatures of climate normals. https://www.ncdc.noaa.gov/data-access/quick- thermogenic and biogenic methane. Science 2014, 344 (6191), 1500− links#normals (accessed September 25, 2016). 1503. (47) Pearson, F. J.; White, D. E. Carbon 14 ages and flow rates of (29) Darling, B. K. Stable isotope forensics as a means of identifying water in Carrizo Sand, Atascosa County, Texas. Water Resour. Res. the source of methane in a domestic water well, Caddo Parish 1967, 3 (1), 251−261. Louisiana. http://loga.la/wp-content/uploads/2014/09/LOGA- (48)Plummer,L.N.;Eggleston,J.R.;Andreasen,D.C.; Conference-Paper-BK-Darling-rev.pdf (accessed September 22, 2016). Raffensperger, J. P.; Hunt, A. G.; Casile, G. C. Old groundwater in (30) Harrington, J., Whyte, C., Muehlenbachs, K., Darrah, T. Using parts of the upper Patapsco aquifer, Atlantic Coastal Plain, Maryland, noble gas and hydrocarbon gas geochemistry to source the origin of USA: evidence from radiocarbon, -36, and helium-4. Hydro- − fluids in the Eagle Ford Shale of Texas, USA. AAPG Datapages Search geol. J. 2012, 20 (7), 1269 1294. and Discovery Article #41710. http://www.searchanddiscovery.com/ (49) McMahon, P. B.; Thomas, J. C.; Hunt, A. G. Groundwater ages and mixing in the Piceance Basin natural gas province, Colorado. pdfz/documents/2015/41710harrington/ndx_harrington.pdf.html − (accessed December 13, 2016). Environ. Sci. Technol. 2013, 47 (23), 13250 13257. (31) Whiticar, M. J.; Faber, E.; Schoell, M. Biogenic methane (50) Schellinger, D., Iqgal, J., Carney, B. Testing your water well for safety. Louisiana State University Research and Extension; http:// formation in marine and freshwater environments: CO reduction vs. 2 www.lsuagcenter.com/nr/rdonlyres/20ab91b7-7efd-4d0b-a2eb- acetate fermentation−Isotopic evidence. Geochim. Cosmochim. Acta 11e1bddaedf3/18042/watertest.pdf (accessed December 13, 2016). 1986, 50 (5), 693−709.

H DOI: 10.1021/acs.est.7b00746 Environ. Sci. Technol. XXXX, XXX, XXX−XXX