Correspondence/Rebuttal pubs.acs.org/est
Response to Comment on “A Comprehensive Analysis of Groundwater Quality in The Barnett Shale Region”
I
Comparisons with historical data sets might be confounded by myriad different activities taking place at different times or through succession of natural processes following contamination. The authors mention differences in concentration of certain metals between our previous2 and current1 studies as evidence refuting UOG impacts, but failed to mention the context that the current study’s decreased concentrations also corresponded to a less reducing hydrogeological environment, which is less favorable for metal ion desorption (especially arsenic, sequestered in scale and rust). In the current study, we measured dissolved metals and therefore filtered samples prior to analysis. McHugh et al. suggest that this procedure alone resulted in reduced metal concentrations relative to the first study, which analyzed total metals.2 Although this procedure may have resulted in variation in mean sample concentrations between studies, the magnitude of differences (mean arsenic levels of 12.0 and 1.4 μg/L in the 2011 and 2014 studies, respectively) is likely too large to be explained by differences in sampling alone and we argue that other factors played a role as well. Therefore, McHugh et al.’s statement that the lower metals detected in the current study “indicates an absence of widespread UOG impacts.” is not supported and does not consider the other findings of our study. Although a decrease in constituent concentration with increased distance from UOG wells might provide evidence of UOG impacts, an absence of such relationships says nothing about the effects of UOG on groundwater. McHugh et al. point out that we did detect weak, but significant relationships opposite to those expected, but we also demonstrated that these particular relationships were likely spurious and influenced by a confounding variable, depth of water well. Although partial regression coefficients are theoretically held constant with respect to other predictor variables, they are also sensitive to variance inflation from multicollinearity, which could influence parameter estimates and p-values, and is especially problematic for determining significance of weak relationships. We therefore elected to minimize our analyses of these relationships in the current study.1 Finally, the authors noted that we did not make explicit comparisons with reference samples. Preferably, such samples should be a true time series at the same locations from which the “test” samples are taken before UOG activities began. Because this is rarely possible, a common practice is to collect samples from areas that have not experienced UOG activity. However, these “references” are of questionable value because they originate from different locations and naturally have different constituent concentrations (or can be influenced by other contaminants not affecting the study area). Thus, it would be difficult to interpret the meaning of these references, and they cannot be considered as adequate controls, a fact pointed
n a recent analysis of groundwater quality in the Barnett Shale region, we detected elevated levels of volatile organic compounds, chlorinated compounds, alcohols, and metal ions throughout the Trinity and Woodbine aquifers across 13 contiguous counties, and suggested that some impacts on water quality might have resulted indirectly or directly from unconventional oil and gas (UOG) activities. McHugh et al. present three main arguments to dispute this assertion: (i) Several elevated compounds did not covary strongly, (ii) some elevated constituents may not be associated with UOG activities, and (iii) certain lines of evidence used in previous studies to correlate contamination with UOG activity were lacking in the current study. Each of these arguments is flawed and collectively do not advance their argument that UOG activities have not impacted groundwater. McHugh et al. use rhetoric that incorrectly implies that UOG activities are simple and homogeneous. For example, the authors state that a lack of correlation between methanol and ethanol concentrations indicated “that they originate from two distinct sources unrelated to UOG.” In fact, this is an argument only that contamination did not result from the same source at the same time. Given the temporal and spatial scales, potential contamination sources, chemical degradation rates and interactions with different environmental factors, proprietary chemical formulas used by UOG operations, and different times of contamination events, it would be remarkable to observe strong correlations between concentrations of any constituents. Therefore, an absence of strong constituent covariation is not inconsistent with UOG activity, especially at the scale of our study. McHugh et al. also state that many of these compounds are not on the 2011 Congressional report on hydraulic fracturing or listed as frequently used chemicals on the national hydraulic fracturing chemical registry. However, these general statements do not provide evidence of compounds that might be, or actually are, associated with UOG activity. As UOG drilling techniques progress, the proprietary “recipes” used have also changed, and the 2011 Congressional report should not be considered inclusive of all chemicals used in UOG activities. The authors also point to a lack of relationships consistent with UOG impacts, specifically concentrations of inorganics with respect to (i) historical data sets, (ii) proximity to nearest gas well, and (iii) reference area comparisons. Although our reliance on these forms of inference was limited in the current study, we did use these methods in a previous assessment of water quality in the Barnett Shale,2 which found that groundwater in areas of active UOG extraction had elevated levels of metals and alcohols compared to historical data. Therefore, we feel these issues pertain more to differences in methodological choices between two studies rather than to overall inferences, but we will nonetheless address them. The relationships between historic data sets, distance to nearest gas well, and reference areas with UOG activity are expected to be tenuous and noisy when detectable. © 2015 American Chemical Society
Published: December 16, 2015 498
DOI: 10.1021/acs.est.5b05582 Environ. Sci. Technol. 2016, 50, 498−499
Environmental Science & Technology
Correspondence/Rebuttal
out by these very same authors in a previous commentary3 on our earlier study.2 McHugh et al. note many of the problems associated with correlative studies, but do not provide convincing alternative explanations for elevated concentrations of a suite of harmful compounds in a region of high UOG activity. Contrary to their assertions, we have acknowledged the limitations of our study as well as the “negative” findings. In fact, the two specific concerns of our analyses (metal extraction methods and distance to gas well relationships) were discussed in our study; unfortunately, McHugh et al. provide nothing new or compelling to the argument.
Henderson, D.; Kadjo, A. F.; Roelke, C. E.; Hudak, P. F.; Burton, T.; Rifai, H. S.; Schug, K. A. A Comprehensive Analysis of Groundwater Quality in The Barnett Shale Region. Environ. Sci. Technol. 2015, 49 (13), 8254−62. (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) McHugh, T.; Molofsky, L.; Daus, A.; 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, 48 (6), 3595−6.
Jesse M. Meik†,‡ Brian E. Fontenot†,○ Zacariah L. Hildenbrand*,†,§ Doug D. Carlton, Jr.†,∥ Jayme L. Walton† Josh T. Taylor§ Jonathan B. Thacker∥ Stephanie Korlie∥ C. Phillip Shelor∥ Drew J. Henderson∥ Akinde F. Kadjo∥ Corey E. Roelke†,⊥ Paul Hudak# Taylour Burton∇ Hanadi Rifai∇ Kevin A. Schug*,†,∥ †
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Affiliate of the Collaborative Laboratories for Environmental Analysis and Remediation, The University of Texas at Arlington, Arlington, Texas 76019, United States ‡ Department of Biological Sciences, Tarleton State University, Stephenville, Texas 76401, United States § Inform Environmental, LLC, Dallas, Texas 75206, United States ∥ Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, Texas 76019, United States ⊥ Department of Biology, The University of Texas at Arlington, Arlington, Texas 76019, United States # Department of Geography, University of North Texas, Denton, Texas 76203, United States ∇ Department of Civil and Environmental Engineering, University of Houston, Houston, Texas 77204, United States
AUTHOR INFORMATION
Corresponding Authors
*(Z.L.H.) Phone: 915-694-7132; e-mail:
[email protected]. *(K.A.S.) Phone: 1 817-272-3541; e-mail:
[email protected]. Present Address ○
Water Quality Protection Division, United States Environmental Protection Agency, Dallas, Texas 75202, United States.
Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Hildenbrand, Z. L.; Carlton, D. D., Jr.; Fontenot, B. E.; Meik, J. M.; Walton, J. L.; Taylor, J. T.; Thacker, J. B.; Korlie, S.; Shelor, C. P.; 499
DOI: 10.1021/acs.est.5b05582 Environ. Sci. Technol. 2016, 50, 498−499
