Concentrations of total mercury (Hg) from 2 μg/L (the USEPA maximum contaminant level) to 72 μg/L in water from about 600 domestic wells in residential parts of eight counties in southern New Jersey have been reported by State and county agencies. The wells draw water from the areally extensive (7770 km²) unconfined Kirkwood–Cohansey aquifer system, in which background concentrations of Hg are about 0.01 μg/L or less. Hg is present in most aquifer materials at concentrations <50 μg/kg, but is at 100–150 μg/kg in undisturbed surficial soils. No point sources of contamination to the affected areas have been conclusively identified.

To determine whether high levels of Hg in ground water are related to a particular land use and (or) water chemistry, water samples from 105 wells that tap the aquifer system were collected by the United States Geological Survey. These included randomly selected domestic wells, domestic and observation wells in selected land uses, and sets of clustered observation wells—including two sets that are downgradient from residential areas with Hg-contaminated ground water. Hg concentrations in filtered samples (Hg_f) were at or near background levels in water from most wells, but ranged from 0.1 to 3.8 μg/L in water from nearly 20% of wells. Hg_f concentrations from 0.0001 to 0.1 μg/L correlated significantly and positively with concentrations of other constituents associated with anthropogenic inputs (Ca, Cl, Na, and NO₃) and with dissolved organic carbon. Hg_f concentrations >0.1 μg/L did not correlate significantly with concentrations of the inorganic constituents. Hg_f concentrations near or exceeding 2 μg/L were found only in water from wells in areas with residential land use, but concentrations were at background levels in most water samples from undeveloped land. The spatial distribution of Hg-contaminated ground water appears to be locally and regionally heterogeneous; no extensive plumes of Hg contamination have yet been identified.