Assessment of Drinking Water Risk from a Hypothetical Marcellus Shale Flowback Water Spill

One source of human health risk associated with shale gas development is possible exposure to chemicals present in a spill of flowback water from the horizontal hydraulic fracturing process. In the Marcellus Shale, water with sand and low concentrations of chemical additives are used to hydraulically fracture a horizontal well. These additives may include friction reducers, corrosion inhibitors, oxygen scavengers, scale inhibitors, and biocides that are blended into the water and sand mixture that is injected into a well at high pressures to fracture the shale rock. Following fracturing, a portion of the injected water flows back (i.e., flowback water) out of the well and is collected in tanks and/or impoundments. Flowback water contains salts, metals, and organic compounds from the formation and the chemicals that were introduced as additives to the influent stream. The potential exists for flowback water to be spilled during handling, transport, and storage.

In 2009, the Gas Technology Institute (GTI) published the findings of sampling and laboratory analysis of flowback water from 19 shale gas wells drilled into the Marcellus Shale in Pennsylvania and West Virginia. The chemical analysis results from the GTI study are used in this paper to characterize the chemical composition of an assumed spill of flowback water. A risk assessment is presented that quantitatively evaluates possible human health risk from a hypothetical scenario where 10,000 gallons of this flowback water is spilled on the ground, infiltrates into groundwater that is a source of drinking water, and a person located downgradient drinks the groundwater. Key uncertainties encountered when estimating risk are given explicit quantitative treatment using Monte Carlo Analysis. Chemicals significantly contributing to estimated health risks are identified, as are key uncertainties and variables to which risk estimates are sensitive.