Quantifying Groundwater to Sediment and Surface Water Loads for Mercury and Lead in Tidal Wetlands: Case Study

Portions of the marsh sediments at the LCP Chemicals Georgia Site (Site) were dredged and removed in 1999. However, sediment samples obtained four years later showed 0.54 $\mathrm{mg}/\mathrm{kg}$ average mercury concentrations within the remediated portion of the marsh. Groundwater was investigated as a potential contributor to recontamination. A mass discharge study on a transect running along the marsh/upland boundary was used to calculate mass discharges from upland groundwater to sediment and surface water for mercury and lead. Using conservative assumptions, the groundwater loading analysis showed that the maximum spatially averaged groundwater contribution of mercury to the sediments was $0.02\mathrm{mg}/\mathrm{kg}$, less than the measured sediment concentration ($0.54\mathrm{mg}/\mathrm{kg}$) by an order of magnitude. The discharge analysis also showed that low marsh sediment porewater concentrations result in an infinitesimal change in the water quality in the stream when diluted by mixing with surface water. The magnitude of the mass discharge, in accordance with an established classification system, indicated that both mercury and lead plumes were classified as Mag 5 plumes at the Site. That is, a stream flowing at 0.028 ${\mathrm{m}}^3/\mathrm{s}$ [1 cubic foot per second (cfs)] would not be threatened by the discharge from this Site, and flow at this Site is significantly higher than $0.028{\mathrm{m}}^3/\mathrm{s}$ (1 cfs). This work demonstrated that the groundwater pathway for lead and mercury does not contribute significantly to the degradation of sediments or water quality in this marsh. This paper demonstrates the use of relatively straightforward mass balance calculations using data collected from strategically positioned monitoring wells to evaluate the potential offshore flux of groundwater contaminants and their potential impact on surface sediment and surface water.