MOBILIZATION OF MERCURY TO A FIRST-ORDER COASTAL PLAIN STREAM IN NEW JERSEY

Mercury (Hg) concentrations in the unfiltered waters of Squankum Branch in southern New Jersey have exceeded the State’s Surface Water Quality Standard, ranging from 50 to 250 ng/L at a long-term sampling site; wet conditions promoted higher concentrations. At a residential area 600 m upstream on the left bank, domestic wells tapped water where the drinking-water Hg maximum contaminant level (MCL) of 2 μg/L (2,000 ng/L) was exceeded (one of many such areas in the New Jersey Coastal Plain). Following rainfall, shallow groundwater discharging to the left bank of this first-order Coastal Plain stream contained > 5,000 ng/L. Upstream, on the right bank of Squankum Branch, shallow groundwater discharge contained nearly 2,000 ng/L of Hg from an unknown source, showing that concentrations of Hg exceeding the MCL occur in shallow as well as deep groundwater from this aquifer system. Elevated concentrations of chloride (10-21 mg/L) and nitrate (0.5-7 mg/L as N) in most samples of shallow groundwater discharging to the stream indicated anthropogenic effects on local groundwater quality. The Hg/dissolved organic carbon (DOC) ratio in shallow groundwater was high, and the relation between DOC and Hg in groundwater discharging to Squankum Branch was statistically weak. Binding of Hg to DOC in groundwater may be inhibited by low pH (< 5) and elevated chloride concentrations; the latter may be responsible, in some instances, for transporting dissolved Hg as Hg-Cl complexes. Unlike the shallow groundwater, there was a strong positive relation between Hg and organic carbon in soil and sediment samples. Where high-Hg groundwater discharged, bankside soils and sediments contained up to 12 mg/kg of HCl-leachable Hg. Upstream from these areas, bed-sediment Hg concentrations were <0.3 mg/kg. We had observed little downstream transport of Hg-rich sediments, apparently because flow is low in the studied stream segment, even after rainfall. Thus Hg contamination in the stream appears localized. High Hg concentrations in the area groundwater, which have yet to be traced to point sources, may result from mobilization of Hg in soils (from atmospheric deposition and pesticide applications) and aquifer sediments by components of septic-system effluent, which contains high concentrations of chloride and nitrogen species.