MERCURY IN A NORTH ATLANTIC COASTAL PLAIN AQUIFER SYSTEM: A FLOWPATH STUDY

Transport of mercury (Hg)—a widespread contaminant in water from the unconfined Kirkwood-Cohansey (CKKD) aquifer system—was examined during a National Water Quality Assessment (NAWQA) Program study. The CKKD, a major drinking-water source in New Jersey’s Coastal Plain, is a thin quartzose sand aquifer with intermittent kaolinitic clay layers. Mercury concentrations have exceeded the Maximum Contaminant Level (2 µg/L) in water from more than 700 domestic wells and also were near this level from a few public supply wells. Point sources of Hg to most affected wells have not been identified. Clusters of observation wells were installed along a flowpath to a representative public supply well, where water is treated because of past Hg exceedences. Water from about half the observation wells contained Hg at background levels (<0.010 µg/L), but exceeded background levels in the well clusters nearest the supply well, with the highest concentration (0.63 µg/L) at a depth of 40 m—the same depth as the top of the screen in the supply well. Deep (up to 40 m) wells upgradient and near the supply well yielded waters with traces of pesticides and chlorinated solvents and above-background levels of calcium, magnesium (Mg), sodium, chloride, and bromide (Br) — that entered the aquifer from the land surface from industrial, road-salt, and soil-amendment leachates. Concentrations of Hg correlated weakly and directly to those of Mg and Br. The supply well pumps at a rate of about 290,000 gallons/day; if the raw water contains Hg at 2 µg/L, this would result in a mass of Hg > 2 grams/day being withdrawn. The flowpaths with the highest Hg presumably were not captured by the observation wells. Concentrations of Hg in the sandy aquifer sediments were low (< 0.1 mg/kg), but ranged up to 0.13 mg/kg in organic-rich kaolinitic clays at depth. Whether the large mass of Hg in water at depth has been transported effectively from surficial soils where Hg is deposited atmospherically, is dispersed with urban waste, or has been applied as pesticides, or whether additional Hg is mobilized from clayey aquifer sediments (both facilitated by yet unidentified chemical agent(s)), remains the focus of investigation.