Correspondence/Rebuttal pubs.acs.org/est
Response to Comment on “An Evaluation of Water Quality in Private Drinking Water Wells Near Natural Gas Extraction Sites in the Barnett Shale Formation”
M
cHugh et al.1 question our study of groundwater quality near natural gas extraction sites in the Barnett Shale.2 Here, we respond to their comments and provide additional clarification.
■
COMPARISON OF ACTIVE AND NONACTIVE/REFERENCE DATA SETS Comparison of Maximum Concentration. The authors’ argument that unequal sample sizes resulted in higher concentrations is misleading without context about the nature of these data. The statistical artifact they invoke assumes that sample variance will increase proportionately with sample size, but constituent concentrations are determined by their surrounding geologic and hydrologic conditions, which exhibit only moderate underlying variation.3−5 In other words, the range of variation for a given constituent is constrained by its aquifer of origin, and deviations from this range are unlikely without aquifer disturbance. Concentration versus Distance to Nearest Gas Well. If our patterns are due to unequal sample sizes, then graphing historical concentrations of constituents versus distance to the nearest gas well should show an exaggerated difference among active (n = 268), nonactive (n = 76), and reference samples (n = 13). However, the range for arsenic and selenium are similar among data sets (Figure 1). Levene’s test also shows homogeneous variance among the three populations (arsenic: F = 0.071, p = 0.931; selenium; F = 0.179, p = 0.836). These data demonstrate that unequal sample sizes cannot inflate constituent concentrations beyond the natural range of variation. Statistical Comparison. We chose the relatively conservative Mann−Whitney U test for its relaxed assumptions that allow unequal sample sizes, heteroscedasticity, and nonnormality, all of which characterize these data. The smaller nonactive/reference data set likely resulted in a failure to detect a statistical difference, but the high concentrations we observed are no less meaningful. McHugh et al. also used regression to suggest there is no relationship between gas well proximity and constituent concentration. These data violate regression assumptions since the residuals are heteroscedastic and not normally distributed. Therefore, using regression on these data to either prove or disprove any inference is inappropriate.
Figure 1. Historical (1989−99) concentration of (A) arsenic and (B) selenium versus distance to the nearest natural gas well in private water well samples in the Barnett Shale. Dashed lines represent the Environmental Protection Agency’s Maximum Contaminant Limit (MCL).
because they differ in location, geologic history, industrial practices, waste management, and aquifer hydrology.8 We disagree that our selenium detection limit (10 μg/L) created a false pattern. The drinking water maximum contaminant limit (MCL) for selenium is 50 μg/L, and our detection limit is well below that value. Even with a lower detection limit, we would have observed at least two MCL exceedances in ten samples collected over one summer. In contrast, the historical data show only one of 329 selenium samples collected over ten years equaled the MCL.2
■
EVALUATION OF THE HISTORICAL DATA SET McHugh et al. suggest that increases in arsenic and strontium and decreases in TDS and barium are inconsistent with natural gas impacts, yet they provide no explanation to support this assertion. They cite two studies6,7 of natural gas impacts in the Marcellus Shale, not the Barnett Shale. While these formations are similar in some aspects,8 drawing conclusions about the Barnett Shale based on Marcellus Shale research is ill-advised © 2014 American Chemical Society
Published: March 3, 2014 3597
dx.doi.org/10.1021/es500425j | Environ. Sci. Technol. 2014, 48, 3597−3599
Environmental Science & Technology
Correspondence/Rebuttal
Groundwater analyses commonly present dissolved metal concentrations, but we analyzed unfiltered total concentrations because it is more relevant to actual consumption of groundwater.9 The MCL value for arsenic in drinking water (10 μg/L) is expressed as total arsenic to represent the actual risk from drinking unfiltered water,10 and Focazio et al.11 show that filtered and unfiltered arsenic measurements are nearly equal beyond 10 μg/L. Furthermore, many of the concentrations we observed would have exceeded the MCL value even in a filtered sample.
discussion of alternative explanations for the patterns we observed.
Brian E. Fontenot† Zacariah L. Hildenbrand†,‡ Doug D. Carlton, Jr.† Jayme L. Walton§ Kevin A. Schug*,† †
■
PATTERNS NOT CONSISTENT WITH IMPACTS FROM NATURAL GAS EXTRACTION The authors’ assertion that we provided “little or no discussion” of alternative mechanisms is incorrect. We discussed several possible mechanisms other than natural gas extraction (pesticide application, historical crop defoliation, regional drought, municipal and rural groundwater withdrawals, and aquifer layer turnover). None of the alternative explanations matched our observations more accurately than natural gas extraction. Absence of Hydraulic Fracturing Indicators. Failing to detect suspected components of a generic fracturing fluid “recipe” is not a failure to detect natural gas impacts. Flowback and produced waters often include elevated metals concentrations6 similar to what we observed. McHugh et al. state that detection of methanol and ethanol in reference areas is evidence against fracturing fluid contamination, but they fail to acknowledge our statement that alcohols could be due to mechanisms other than natural gas extraction. We also state that alcohol concentrations were not correlated with distance to nearest gas well, and we never attributed alcohols to casing leaks or surface releases. Absence of Positive Correlation between Parameters. We did observe a positive correlation between TDS and arsenic, which supports potential impacts from natural gas extraction. Our nonparametric analyses were unable to detect a correlation between TDS and other metals, but this could be due to low statistical power, especially in the case of selenium. Additionally, a positive correlation between arsenic and turbidity is not necessary to support mechanical disturbance impacts. If pH values in the water are near 8.5 (as in 37% of our samples), the adsorption of arsenic greatly decreases,12 and arsenic released from mechanical disturbance could remain in a dissolved state after particulate matter settles and turbidity decreases. Correlation with Depth. We agree that a negative correlation of constituents with depth does not rule out a geologic origin as we stated in our paper, but we also could not rule out casing leaks,6 surface spills,13 or some combination of the two as scenarios that could potentially cause a negative depth correlation. Contrary to the suggestion by McHugh et al, we never claimed that mechanical disturbance resulted in a negative depth correlation with metals. Additionally, we do not feel that a lack of negative depth correlation between TDS and selenium is necessarily suggestive that there are no natural gas impacts because our selenium sample size is too low for statistical inference and TDS is naturally high (i.e., >500 mg/L) at all depths in these aquifers, which would obscure any correlation of TDS and well depth. We maintain that we cannot rule out natural gas extraction as a potential contributor to elevated constituent concentrations, and we welcome further
■
Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, Texas 76019, United States ‡ Inform Environmental, LLC, Dallas, Texas 75227, United States § SWCA Environmental Consultants, 2201 Brookhollow Plaza Drive, Suite 400, Arlington, Texas 76006, United States
AUTHOR INFORMATION
Corresponding Author
*Phone: 817-272-3541. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) McHugh, T.; Molofsky, L.; Daus, T.; Connor, J. Comment on “An Evaluation of Water Quality in Private Drinking Water Wells Near Natural Gas Extraction Sites in the Barnett Shale Formation. Environ. Sci. Technol. 2014, DOI: 10.1021/es405772d. (2) Fontenot, B. E.; Hunt, L. R.; Hildenbrand, Z. L.; Carlton, D. D., Jr.; Oka, H.; Walton, J. L.; Hopkins, D.; Osorio, A.; Bjorndal, B.; Hu, Q. H.; Schug, K. A. An evaluation of water quality in private drinking water wells near natural gas extraction sites in the Barnett Shale formation. Environ. Sci. Technol. 2013, 47 (17), 10032−10040. (3) Reedy, R. C.; Scanlon, B. R.; Walden, S.; Strassberg, G. Naturally Occurring Groundwater Contamination in Texas; Texas Water Development Board: Austin, TX, 2011. (4) Scanlon, B. R.; Nicot, J. P.; Reedy, R. C.; Kurtzman, D.; Mukherjee, A.; Nordstrom, D. K. Elevated naturally occurring arsenic in a semiarid oxidizing system, Southern High Plains aquifer, Texas, USA. Appl. Geochem. 2009, 24, 2061−2071. (5) George, P. G.; Mace, R. E.; Petrossian, R. Aquifers of Texas; Texas Water Development Board: Austin, TX, 2011. (6) Vidic, R. D.; Brantley, S. L.; Vandenbossche, J. M.; Yoxtheimer, D.; Abad, J. D. Impact of Shale Gas Development on Regional Water Quality. Science 2013, 340, 6134 DOI: 10.1126/science.1235009. (7) Chapman, E. C.; Capo, R. C.; Stewart, B. W.; Kirby, C. S.; Hammack, R. W.; Schroeder, K. T.; Edenborn, H. W. Geochemical and strontium isotope characterization of produced waters from Marcellus Shale natural gas extraction. Environ. Sci. Technol. 2012, 46, 3545−3553. (8) Bruner, K. R.; Smosna, R. A. Comparative study of the Mississippian Barnett Shale, Fort Worth Basin, and Devonian Marcellus Shale, Appalachian Basin, DOE/NETL-2011/1478; Department of Energy, National Energy Technology Laboratory: Morgantown, WV, 2011. (9) Nielson, D. M.; Nielson, G. L. Groundwater Sampling. In Practical Handbook of Environmental Site Characterization and GroundWater Monitoring, 2nd ed.; Nielson, D. M., Ed.; CRC Press: Boca Raton, FL, 2006. (10) U.S. EPA. Arsenic and Clarifications to Compliance and New Source Contaminants Monitoring, Final Rule (66 FR 6976). In Federal Register; United States Environmental Protection Agency: Washington, DC, 2002; pp 6976−7066. (11) Focazio, M. J.; Welch, A. H.; Watkins, S. A.; Helsel, D. R.; Horn, M. A. A Retrospective Analysis on the Occurrence of Arsenic in GroundWater Resources of the United States and Limitations in Drinking-Water3598
dx.doi.org/10.1021/es500425j | Environ. Sci. Technol. 2014, 48, 3597−3599
Environmental Science & Technology
Correspondence/Rebuttal
Supply Characterizations; United States Geological Survey: Golden, CO, 2000 (12) Goldberg, S.; Johnson, C. T. Mechanisms of arsenic adsorption on amorphous oxides evaluated using macroscopic measurements, vibrational spectroscopy, and surface complexation modeling. J. Colloid Interface Sci. 2001, 234 (1), 204−216. (13) Healy, R. W.; Bartos, T. T.; Rice, C. A.; Mckinley, M. P.; Smith, B. D. Groundwater chemistry near an impoundment for produced water, Powder River Basin, Wyoming, U.S.A. J. Hydrol. 2011, 403 (1− 2), 37−48.
3599
dx.doi.org/10.1021/es500425j | Environ. Sci. Technol. 2014, 48, 3597−3599
