by Shannon Julius
Shale gas development can affect surface water quality by means of runoff from well construction and discharge from wastewater treatment facilities. Olmstead et al. (2013) conducted a large-scale statistical study of the extent to which these two activities affect surface water quality downstream. This study is different than most current literature related to the regional water impacts of shale gas development in that it focuses on impacts to surface water bodies as opposed to groundwater bodies. Researchers consulted online databases to retrieve locations of shale gas wells and wastewater treatment facilities within Pennsylvania. These were spatially related to downstream water quality monitors using Geographic Information Systems (GIS). Concentrations of chloride (Cl–) and total suspended solids (TSS) were used as indicators of water quality because both are associated with shale gas development and are measured by water quality monitors. Shale gas wastewater typically has a high concentration of Cl–, which can directly damage aquatic ecosystems and is not easily removed once dissolved in water. TSS, which harm water quality by increasing temperature and reducing clarity, can potentially come from the construction of well pads, pipelines, and roads associated with well drilling, especially when precipitation creates sediment runoff. Results of the study suggest that wastewater treatment facilities are responsible for raised concentrations of Cl– downstream and that the presence of gas wells are correlated with raised concentrations of TSS downstream.
Olmstead, et al. collected publicly-available data from national and Pennsylvania state websites. TSS and Cl– concentration observations came from the Storage and Retrieval Data Warehouse (STORET) database of the US EPA. The locations and permitting and drilling dates of shale gas wells came from the websites of the Pennsylvania Department of Environmental Protection (PADEP) and the Pennsylvania Department of Conservation and Natural Resources (PADCNR). Shipments of wastewater and the locations of their destination treatment plants also came from PADEP. In order to test for the impacts of treated wastewater on surface water Cl– concentrations, the authors measured the density of wastewater treatment facilities and the volume of total wastewater shipments upstream of water quality monitors. Wastewater shipments to both publicly owned treatment works (POTWs) and centralized waste treatment (CWT) facilities were included, as long as they had NPDES permits making them eligible to treat shale gas waste. An additional cause of Cl– contamination is accidental release of wastewater during the hydraulic fracturing process, so density of upstream shale gas wells was also considered. TSS contamination models tested for impacts from waste disposal and from well construction, focusing on the time period when land clearing and well pad construction take place. Controlled factors included precipitation and seasonal changes in contaminant levels that are considered “normal” in different watersheds. For example, road salt is a source of Cl– in certain times of year in certain watersheds. The authors controlled for these “fixed effects,” thus eliminating other potential sources of raised contamination levels besides shale gas development.
The most significant statistical factor on raised downstream Cl– concentrations is the density of upstream wastewater treatment facilities. An increase of 1.5 facilities per watershed (one standard deviation) increases Cl– concentrations by 10–11%. In general, there was a weak positive correlation between raised levels of Cl– and the quantity of wastewater treated upstream. This result reflects the fact that many wastewater treatment plants do not have the capacity to treat very high concentrations of Cl–. The density of upstream shale gas wells have an insignificant effect on Cl–concentration, even when measurements are limited to wells that were developed within 0–3 and 3–6 months before a water sample was taken. This limitation should account for accidental releases of hydraulic fracturing fluid.
The presence of shale gas wells in a monitor’s watershed correlated with high concentrations of TSS, while the presence of waste treatment facilities had a statistically insignificant effect on downstream TSS. Increasing the average density of well pads by one standard deviation—or 18 well pads per watershed—leads to a 5% increase in TSS concentrations in surface water. The specific relationship between well pad density and downstream TSS concentration remains unclear. It seems likely that well pad construction would be a cause of raised TSS concentrations, however the data showed no change in downstream concentrations when water quality measurements are limited to the time window when well pads are in construction. TSS concentrations also do not increase when well pad density is considered along with precipitation, even though a change would be expected if TSS are transported from construction sites by stormwater. Further research could determine if road or pipeline construction, spills, or other emissions from well sites are responsible for the measured increase in TSS. Further research is also needed to investigate costs, benefits, and goals for controlling water contamination levels due to shale gas development impacts.
Olmstead, S., Muehlenbachs, L., Shih, J., Chu, Z., Krupnick, A., 2013. Shale gas development impacts on surface water quality in Pennsylvania. Proceedings of the National Academy of Sciences 110, 4962–4967.