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Occurence and Distribution of Mercury in the Surficial Aquifer, Long Neck Peninsula, Sussex County, Delaware,2003-04

By Michael T. Koterba, A. Scott Andres, Joseph Vrabel, Dianna M. Crilley, Zoltan Szabo, John F. DeWild, George R. Aiken, and Betzaida Reyes-Padro

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In January 2001, mercury (Hg) was detected (500 nanograms per liter, ng/L, or greater) in the distribution system of the Long Neck Water Company (LNWC), Pot Nets, Delaware. By April 2001, two LNWC production wells had been taken off-line because discharge concentrations of total mercury (HgT) either had exceeded or approached the Federal limit of 2,000 ng/L. From October 2003 through January 2005, the U.S. Geological Survey, Delaware Geological Survey, and Delaware Department of Natural Resources and Environmental Control conducted a cooperative study to (a) determine if the Hg contamination was widespread, (b) identify possible forms of Hg in Groundwater, and (c) examine Hg occurrence in relation to (geo)chemical conditions and characteristics of Groundwater and sediment in the surficial aquifer on the Long Neck Peninsula, Sussex County, Delaware.

An initial water-quality survey conducted with samples from 22 production wells revealed that concentrations of HgT in Groundwater in the surficial aquifer ranged from 0.11 to 1,820 ng/L. Shallow Groundwater (less than 120 feet below land surface) throughout most of the peninsula, including that which contained elevated concentrations of HgT (exceeding 100 ng/L), appeared to be affected by human activities. All samples contained volatile organic compounds (VOCs) and elevated nitrate-nitrogen (NO3-N, exceeding 0.4 milligrams per liter, mg/L). Most (16 of 22) samples had elevated specific conductance (SC, in excess of 100 microsiemens per centimeter at 25 degrees Celsius). Elevated concentrations of HgT, however, only occurred in five production wells in the Pot Nets Bayside and Lakeside communities.

The vertical distribution of HgT in shallow Groundwater (less than 80 feet below land surface) was determined with samples collected at 5 to 6 vertical-nest short-screened (2- 5-foot length) monitoring wells installed near Bayside and Lakeside production wells with the highest HgT concentrations (exceeding 1,000 ng/L). Elevated concentrations ofHgT (100-6,380 ng/L) occurred in the shallow aquifer near each well at different depths. Chemical analyses of selected soil, fill, and aquifer sediment samples, obtained during the installation of nested wells, indicated that little HgT occurred in soil or fill at either site (40 micrograms per kilogram, µg/kg, or less). No HgT was detected (less than 20 µg/kg) in aquifer sediment samples. These low HgT concentrations imply that neither the soil, fill, nor aquifer sediment was a likely source of the elevated Hg in Groundwater. Given Hg occurrence appeared to be a Groundwater transport phenomenon, the forms of Hg in transport were investigated. Differences in HgT concentrations between raw and filtered (0.1- and (or) 0.4-absolute micrometer pore size) samples from nested wells were random in sign and similar in magnitude to the variability in measuring HgT attributed to field and laboratory methods (± 5-10 percent, for HgT concentrations exceeding 100 ng/L). Thus, Hg transport likely occurred in a dissolved or fine-colloidal nonparticulate phase.

Methyl mercury (HgMe) only was detected at low concentrations (0.06 ng/L or less) in nested-well samples with low to moderate concentrations of HgT (less than 366 ng/L). Whether HgMe occurred at similar concentrations in samples with high HgT concentrations was unresolved due to a sample-matrix interference problem. Potential complex forms of Hg were investigated in relation to the occurrence of selected ligands (organic carbon, sulfide, and chloride concentrations) and geochemical conditions (for example, pH and dissolved oxygen concentrations). Only dissolved organic carbon (DOC) appeared directly related to Hg occurrence. Elevated concentrations of HgT and DOC co-occurred in Groundwater at both Pot Nets sites. The average concentration of DOC was about four times greater in samples from the Pot Nets wells with the highest HgT concentrations (exceeding 1,000 ng/L) than in most Pot Nets or peninsula samples with low HgT concentrations (less than or equal to 20 ng/L).

The relation between Hg and DOC does not appear to be coincidental. Nested-well samples with elevated concentrations of HgT and DOC generally had a higher specific ultraviolet absorption at 280 nanometers than samples with low concentrations of DOC and HgT. The highest acid-neutralization capacities (ANCs) occurred in Groundwater samples with the highest concentrations of DOC and HgT at both sites, and DOC and ANC were positively correlated (r2=0.90). In addition, the fraction of hydrophobic carbon, measured for samples with the highest concentrations of HgT at each site, equaled or exceeded by several fold the total DOC found in samples with low concentrations of HgT at each site. These findings indicated the possible presence of humic aromatic and aliphatic carbon compounds in the hydrophobic carbon fraction. Other studies in different locations, but with similar hydro-chemical conditions, have shown such carbon fractions can strongly interact with simple chemical forms of Hg -ionic (HgII), sulfide (HgS), or methyl (HgMe).

Shallow Groundwater at each Pot Nets site appeared affected by human activities. Detectable VOCs, and elevated SC, NO3 --N, and (or) magnesium occurred at all sampled depths, including those depths that corresponded to elevated concentrations of HgT. In addition, the apparent dates of recharge and ages of Groundwater, estimated from sample concentrations of chlorofluorocarbon compounds (CFCs), indicated that Groundwater with elevated concentrations of HgT at both Pot Nets sites was similar in relation to apparent recharge dates and ages (circa 1970 to 1985, or 20-35 years). Groundwater samples with low concentrations of HgT, which were obtained from nested wells screened at deeper and shallower depths than the wells with high concentrations, of HgT, had apparent recharge dates that corresponded to times before and after this period, respectively.

The elevated concentration of Hg in Groundwater near the Pot Nets wells appears to be of local, anthropogenic, and relatively recent origin, and possibly from a single source. High concentrations of HgT (exceeding 1,000 ng/L) first occurred at the Lakeside production well just 2 months after high concentrations led to the shutdown of the Bayside well. Elevated concentrations of HgT only have been found in shallow Groundwater in the vicinity of these two Pot Nets production wells. No measurable Hg occurred in the aquifer sediment at either production-well site. Groundwater with elevated concentrations of HgT appeared relatively young and similar in apparent age at both sites. In addition, its quality at all sampled depths at both sites reflected chemical indicators of recent human activities typically associated with residential, or agricultural followed by residential, development.

The occurrence of elevated Hg at both Pot Nets community sites in Groundwater that also appeared affected by residential and (or) agricultural development also has been widely observed in similar settings and coastal aquifers in New Jersey. It is not possible, however, to determine from the limited historical data on Hg in the shallow aquifer, and this single study, whether or not there could be a potentially more widespread but scattered pattern of Hg occurrence in the shallow surficial aquifer in Delaware.

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