A GEOCHEMICAL FRAMEWORK FOR UNDERSTANDING THE SOURCE AND MIGRATION OF NATURAL HYDROCARBON BRINES INTO SHALLOW GROUNDWATER AQUIFERS: A CASE STUDY FROM SOUTH-CENTRAL NEW YORK

The shale gas boom has focused significant attention on the environmental and hydrogeological implications for the presence of methane in shallow aquifers, specifically in the northern Appalachian Basin. Some workers suggest that shale gas drilling increases methane concentrations in a subset of wells (Jackson et al, 2013), while others suggest that methane is controlled by natural hydrogeological factors (Molofsky et al, 2011; 2013). Overshadowed by the debate regarding the potential impacts of hydraulic fracturing on groundwater quality, the possible human health implications of poor ambient groundwater quality has received scant attention. In the northern Appalachian Basin, the Alleghanian Orogeny, which occurred ~300 Ma ago, mobilized hydrocarbon-rich formation brines from the organic-rich Marcellus shale into the shallow Upper Devonian formations that serve as modern aquifers in our study area. These metal-rich brackish (diluted brine) fluids have poorly characterized chemistry and toxicity, and therefore represent unknown human and agricultural health risks. To begin the process of characterizing these potential risks, we conducted a case-study of 75 pre-drill groundwater wells from south-central NY. To assess the extent (i.e., geographic distribution) and magnitude of potentially toxic geological fluids, our analysis includes water (salts, metals, and water stable isotopic composition) and gas (hydrocarbon and noble gas) chemistry. This multi-parameter approach is essential to distinguish among the sources of contamination (e.g., natural brines vs. chloride from road salt or methane from agricultural sources). We have identified several spatially distinct clusters with elevated methane, barium, iron, etc. that have geochemical evidence for geological brine migration. These clusters align along ~N-S trending valleys with identifiable strike slip offsets. The faults may have served as the conduits for the migration of potentially toxic brine components to shallow Upper Devonian strata in our study area.