IODINE AS A SENSITIVE TRACER FOR DETECTING INFLUENCE OF ORGANIC-RICH SHALE IN SHALLOW GROUNDWATER

Public and regulatory agencies in New York have been concerned over the potential for drinking water contamination caused by high-volume hydraulic fracturing (hydrofracking) of the Marcellus shale in New York State (NYS), where exploitation of has not yet begun. Regulators need unique natural tracers to distinguish formation and flow-back water produced with Marcellus gas from natural variability in background chemistry as well as other sources of contamination. We present the results of an initial geochemical background study and show the utility of iodine with other halogens as a tracer for fluids associated with the Marcellus Formation of the Appalachian Basin of the eastern United States. The Br/I ratio for 19 stream waters sampled averaged about 28, similar to that found in waters contaminated or affected by dissolving halite. In contrast, Br/I ratios in 56 potable shallow ground waters sampled from shallow bedrock aquifers ranged from 0.2 to 27. These data separate into multiple groups. NYS formation waters found in the Marcellus and other deep gas bearing Devonian age formations have relatively high Br/I, ratios ranging from 17 to 49, depending upon the producing formation. In contrast, Marcellus produced waters from Bradford County, Pennsylvania have Br/I ratios of 23 to 27. The combination of iodine and bromine concentrations may be a new fingerprinting tool to differentiate the influence of deep formation waters from other sources of salinity.