Marcellus Shale: Iodine as a sensitive tracer for detecting influence of organic-rich shale in shallow groundwater

Halogens are helpful for distinguishing the influence of formation water and/or flow-back water on drinking water contamination related to high-volume hydraulic fracturing of the Marcellus shale in Pennsylvania and in New York State (NYS). Halogen concentrations, particularly Br and I, to characterize natural variability of baseline water chemistry in the southern tier of NYS, were assessed. Majority of streams and drinking water wells have Br and I concentrations below 1 and 0.1 μM, respectively, a range typical for relatively pristine surface water and shallow groundwater. Wells that have higher Br and I concentrations are likely affected by formation waters. Br/I ratios indicate two different sources of formation waters in these wells, possibly controlled by geologic settings. These results suggest that I, combined with other halogens, may be a novel and sensitive tool for fingerprinting trace levels of formation water signal in drinking water sources.