Correspondence/Rebuttal pubs.acs.org/est
Response to Comment on “Impact to Underground Sources of Drinking Water and Domestic Wells from Production Well Stimulation and Completion Practices in the Pavillion, Wyoming Field” onnor et al.1 question our findings2 that activities related to acid stimulation and hydraulic fracturing directly impacted Underground Sources of Drinking Water (USDWs) and indirectly (as a result of leaching from at least 64 unlined pits where flowback and produced water were disposed prior to the mid-1990s) impacted domestic water wells in the Pavillion, WY Field. Briefly, they state that no impact to USDWs occurred because the formations below depths “actually utilized by the local community” for groundwater supply were “oil and gas reservoirs”, and because our finding2 of impact to USDWs relied on test results from EPA’s two monitoring wells, MW01 and MW02. They stated that concentrations of major ions in the two wells reflected “natural salinity” and that organic compounds in them reflected interactions with monitoring well construction materials rather than oil and gas operations. The basis of their last statement is that EPA monitoring wells had (1) “very different ionic signatures but an identical organic signature”, (2) organic compounds not detected in produced water and domestic water well samples, and (3) a “unique pH level” observed during sampling. They state that no impact occurred to domestic water wells because “there is no significant difference in the water quality of water wells located near to and far from the gas production wells.” As we discussed,2 impact to USDWs differs from impact to domestic water wells. The former can occur without the latter. EPA3 unequivocally concluded that the Wind River and Fort Union Formations, where hydraulic fracturing occurred in the Pavillion Field, are USDWs. The Wind River Formation is used extensively for water supply in the Pavillion area and throughout the Wind River Basin. A review of EPA’s aquifer exemption data set recently obtained through a Freedom of Information Act Request4 indicates that 51 aquifer exemptions in the Fort Union Formation were approved in the State of Wyoming which would not be required if this formation was not a USDW. Our finding of impact to USDWs was based on a preponderance of evidence indicating solute transport to water-bearing sandstone units within these USDWs which included:2 (1) leakoff of stimulation fluids, (2) injection of stimulation fluids, including undiluted diesel fuel, into waterbearing sandstone units, (3) advective-dispersive solute transport resulting from pressure buildup, (4) loss of zonal isolation during production well stimulation, and (5) observation of major ion anomalies in produced water from production wells. Data from EPA monitoring wells provided additional, but not the only, evidence of impact to USDWs. Overall, the evidence for solute transport to water-bearing sandstone units2 is not surprising given that over 41.5 million liters of stimulation fluids were injected into USDWs at the Pavillion Field.
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© XXXX American Chemical Society
Data from EPA monitoring wells indicated upward solute migration to depths of current groundwater use (∼300 m). As discussed,2 reasons for this migration included (1) shallow acid stimulation (e.g., 213 m) and hydraulic fracturing (e.g., 322 m), (2) localized upward hydraulic gradients, (3) induced upward hydraulic gradients during stimulation, (4) lack of a laterally extensive confining unit, and (5) issues related to production well integrity. These issues included potential loss of zonal isolation during stimulation in at least 11 production wells and lack of cement directly below shallow surface casing (median depth 185 m) at a majority (52%) of production wells. Shallow well stimulation increases the possibility of impact to domestic water well users requiring greater caution during oil and gas production.5 Connor et al.1 provided no evidence that major ion concentrations in produced water and MW02 reflect natural salinity. As we highlighted,2 concentrations of sodium, potassium, and chloride were higher, and sulfate concentrations lower, in produced water than expected concentrations in the Wind River Formation (P < 0.0001 in all cases). Elevated chloride and low sulfate concentrations (P < 0.0001 for each) at MW02 were also anomalous compared to expected concentrations in this formation. As we stated,2 the absence of major ion anomalies at MW01, the shallower of the two monitoring wells, may reflect a transition from primarily petroleum-based stimulation fluids prior to the mid-1990s to potassium chloridebased stimulation methods afterward. This may also explain the absence of major ion anomalies in most domestic water wells located near unlined pits. The statement that EPA monitoring wells had “identical” organic signatures is inconsistent with data presented in our paper.2 At MW01, benzene, ethylbenzene, toluene, and xylenes (BTEX) were either not detected or flagged as blank contaminants whereas at MW02 these compounds were detected at concentrations of 260, 710, 101, and 1260 μg/L, respectively. Concentrations of trimethylbenzenes, alkylbenzenes, naphthalenes, methylphenols, and alkylphenols showed a similar detection pattern as outlined in Tables SI E3a and E4a of our paper.2 Another compound, tert-butyl alcohol (TBA), was not detected in MW01 but was detected at 6,300 μg/L at MW02. Detection of TBA in groundwater has been attributed to degradation of tert-butyl hydroperoxide used for hydraulic fracturing.6 Also as we discussed,2 differences in organic compounds detected in EPA monitoring wells may reflect variation in well stimulation techniques over time. As outlined in SI Section C of our paper,2 methanol (up to 10% composition of some stimulation fluids), isopropanol, 2butoxyethanol, and petroleum hydrocarbons were used
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DOI: 10.1021/acs.est.6b04300 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Correspondence/Rebuttal
Notes
extensively for production well stimulation. It is highly unlikely that these compounds originated from monitoring well construction materials (see SI Section E of our paper2). Also, as discussed,2 detection of alkylphenols and high concentrations of low molecular weight organic acids likely reflect degradation products of organic compounds of anthropogenic origin. The statement that organic compounds detected in EPA monitoring wells were not detected in produced water samples analyzed by EPA is also inconsistent with data presented in our paper.2 BTEX compounds, trimethylbenzenes, and alkylbenzenes, and naphthalenes were detected in produced water samples and in particular at one “water” sample at extraordinary concentrations of 77500 mg/L (summation of BTEX components), 14980 mg/L, 2756 mg/L, and 35 mg/L, respectively as outlined in Table SI D3 of our paper.2 Other compounds were detected in both produced water and EPA monitoring wells including phenols. Phenol, 2-methylphenol, 2,4-dimethylphenol, and 3- and 4-methylphenol were detected in one of four produced water samples at concentrations of 6.96 mg/L, 7.76 mg/L, 5.00 mg/L, 6.76 mg/L, respectively. Heterogeneity in detection of organic compounds from the same formation suggests an anthropogenic source.7 Also, some compounds detected in EPA monitoring wells such as methanol, isopropanol, glycols, and alkylphenols were not analyzed in produced water samples. Potential causes of elevated pH during sampling of monitoring wells, including interaction with cement and degassing of carbon dioxide, were discussed in SI Section E.5 of our paper.2 Even if interactions with cement did contribute to the elevated pH observed during sampling, a hypothesis we dispute, this possibility does not change the general findings of our paper2 given the extensive data for chloride, sulfate, BTEX, hydrocarbons, 2-butoxyethanol, akylphenols, phenols, low molecular weight organic acids and more, which are unrelated to cement interaction. Finally, there was a clear difference in domestic water well quality with distance from unlined pits near production wells. As discussed,2 diesel range organics (DRO) were elevated (P = 0.003) in domestic water wells located
