Characterizing the association between oil and gas development and water quality at a regional scale

Over the past ∼15 y, unconventional oil and gas (UOG) production utilizing hydraulic fracturing and horizontal drilling has increased dramatically in the Marcellus Shale in the Appalachian region of the USA. The area also has a long history of conventional oil and gas (COG) production as well as coal mining, and the 3 extractive activities (UOG, COG, and coal mining) often occur in the same watersheds. Produced water and wastewater from oil and gas (OG) extraction can have very high concentrations of dissolved metals, radionuclides, and other solutes. Previous studies have documented streamwater pollution associated with spills and releases of these fluids, but it remains unclear whether OG development has broad regional effects on streamwater quality. This study assessed whether COG and UOG well densities in watersheds were associated with water-quality metrics, including specific conductance, dissolved metals, nutrients, radium isotopes, and anions, when controlling for coal mining and land cover. Water quality was measured at 120 stream sites with non-nested watersheds in the Marcellus Shale region. Linear modeling revealed that COG well density was associated with higher concentrations of Mg and Cl−, and UOG well density was associated with higher 228Ra:224Ra ratios, suggesting that this ratio might be a useful indicator of UOG development in watersheds. Adding OG well density to linear models explaining the concentrations of water-quality variables improved R2 by ≤0.04, indicating little increase in explanatory power. However, linear regression coefficients indicated that an increase in COG well density from 0 to 11 wells/km2 (the maximum well density observed in our study area) could be associated with 35× increases in Mg and Cl− concentrations, although the SE of these estimates was large. This research suggests that OG well density may have small but measurable effects on streamwater quality; however, the effects of other human activities and land uses, such as coal extraction and urban development, are likely larger and more environmentally relevant.