Correspondence/Rebuttal pubs.acs.org/est
Comment on “An Evaluation of Water Quality in Private Drinking Water Wells Near Natural Gas Extraction Sites in the Barnett Shale Formation”
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n their Article,1 the authors conclude that, within areas of active natural gas extraction, concentrations of various metals in private wells are elevated relative to wells located in nonactive/reference areas and relative to historical conditions. We appreciate the authors’ attention to this important issue regarding the possible effect of shale gas development activities. However, we find that the authors’ data do not support their conclusions because (i) the comparison between the active area and nonactive/reference data sets is flawed, (ii) the comparison against the historical data set does not suggest current impacts, and (iii) the water quality patterns observed in these data sets are not likely related to natural gas extraction activities.
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COMPARISON OF ACTIVE AND NONACTIVE/REFERENCE DATA SETS Comparison of Maximum Concentration. The authors have inferred a water quality impact from the fact that the maximum value of the larger data set exceeds that of the smaller data set. However, comparing maximum concentrations between two disproportionately sized data sets (e.g., 91 measurements from the active extraction area versus only 9 measurements in the nonactive/reference areas) is not statistically valid or meaningful with regard to causal factors. Rather, the maximum concentration of a large data set is reasonably expected to be greater than that of a much smaller data set, solely as a result of the difference in the sample population size. For example, comparing the maximum well depths in the authors’ data sets shows the maximum water well depth in the active area (1400 ft) to be 2.9× the maximum well depth in the nonactive/reference (485 ft). Clearly, the difference in these well depths is not related to natural gas extraction activities but is rather a function of the disproportionate sample populations. Similarly, the difference in the maximum concentrations of metals between these data sets should not be interpreted as a causal outcome of gas extraction. Concentration versus Distance from Nearest Gas Well. The graphical analysis of concentration versus distance to the nearest gas well presented as Figure 2 of the Article1 is also inappropriate because of the difference in the number of measurements in the active area versus the nonactive/reference areas. Because there are a large number of measurements at locations close to gas wells and relatively few measurements at locations far away, the small number of higher concentration values will, by statistical probability, occur relatively close to gas wells, which creates a false visual impression of an association between concentration and distance. The fallacy of this graphical analysis can again be illustrated using well depth. For the data set presented in the article, a plot of well depth shows strikingly similar visual correlation to distance from nearest gas well, even though no such causal relationship exists (Figure 1). © 2014 American Chemical Society
Figure 1. Depth of private water well versus distance to the nearest natural gas well for Barnett Shale private water wells analyzed by Fontenot et al.1
Statistical Comparison. Using the Mann−Whitney U test, there is no statistically significant difference in concentration between the active area data set and the nonactive/reference data set for any of the parameters analyzed by the authors and consistently detected in both data sets: TDS (p = 0.76), arsenic (p = 0.57), strontium (p = 0.58), and barium (p = 0.68). In addition, regression analysis shows no statistically significant relationship between distance from the nearest gas well and the concentration of any of the parameters analyzed. In short, the data set provides no evidence that the concentrations of these parameters are higher in the active area, contrary to the suggestion by the authors.
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EVALUATION OF THE HISTORICAL DATA SET The authors suggest that, within the active area, differences in arsenic, selenium, strontium, and TDS concentrations between the current data set and an historical data set may indicate recent impacts by gas development. However, the authors observe statistically significant increases in arsenic and strontium concentrations and statistically significant decreases in TDS and barium concentrations compared to the historical data set, a pattern that is inconsistent with impacts associated with natural gas development.2,3 For selenium, the authors’ statistical analysis is inappropriate because the comparison is based only on detected results and the detection limit for selenium was much higher for the current data set (i.e., 10 μg/ L), thereby creating a false indication of higher concentrations in the current data set. In addition, the current data set presents total (unfiltered) metal concentrations, while the historical data set presents dissolved metal concentrations, a difference that limits the significance of any comparisons. Published: March 3, 2014 3595
dx.doi.org/10.1021/es405772d | Environ. Sci. Technol. 2014, 48, 3595−3596
Environmental Science & Technology
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Correspondence/Rebuttal
(4) Glenn, S.; Lester, J. L. An analysis of the relationship between land use and arsenic, vanadium, nitrate, and boron contamination the Gulf Coast aquifer of Texas. J. Hydrol. 2010, 389 (1−2), 214−226. (5) Fritch, T. G.; McKnight, C. L.; Yelderman, J. C., Jr.; Arnold, J. G. Environmental Auditing: An aquifer vulnerability assessment of the Paluxy Aquifer, central Texas, U.S.A., Using GIS and a modified DRASTIC approach. Environ. Manage. 2000, 25 (3), 337−345. (6) 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.
PATTERNS NOT CONSISTENT WITH IMPACTS FROM NATURAL GAS EXTRACTION The authors discuss several speculative mechanisms by which natural gas extraction activities could have impacted water quality. However, they provide little or no discussion of lines of evidence that show these water quality data are unlikely to be attributable to natural gas extraction: Absence of Hydraulic Fracturing Indicators. The authors did not detect 27 of 29 hydraulic fracturing indicator compounds in any of their water well samples. The two detected compounds (methanol and ethanol) were evenly distributed in active and nonactive/reference samples. These results suggest an absence of impacts associated with casing leaks or surface releases.2 Absence of Positive Correlations between Parameters. The authors did not observe consistently positive correlations between TDS and metals concentrations in private wells. This suggests an absence of impacts associated with casing leaks or surface releases of produced water.2,3 The author’s data set also shows no correlation between arsenic and turbidity, which is contrary to their suggestion that mechanical disturbance of the wells by gas extraction activities has resulted in elevated arsenic concentrations. Correlation with Depth . The authors observe a statistically significant negative correlation between metal concentration and well depth for all three of the most widely detected metals (i.e., arsenic, strontium, and barium concentrations were higher in shallower wells). However, as noted by the authors, this pattern has been observed for arsenic and other constituents in wells not located in natural gas extraction areas,4,5 and there is no scientific basis to conclude that this condition is related to either the mechanical disturbance or the casing leaks, as proposed by the authors. Furthermore, this negative correlation with depth was not observed for TDS and selenium, the two constituents more likely to be associated with surface releases.6 Thomas McHugh Lisa Molofsky Anthony Daus John Connor* GSI Environmental, Inc., 2211 Norfolk, Suite 1000, Houston, Texas 77027, United States
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AUTHOR INFORMATION
Corresponding Author
*Phone: 713-522-6033. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Fontenot, B. E.; Hunt, L. R.; Hildenbrand, Z. L.; Carlton, D. D.; 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. (2) 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. (3) 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. 3596
dx.doi.org/10.1021/es405772d | Environ. Sci. Technol. 2014, 48, 3595−3596
