The role of groundwater transport in aquatic mercury cycling

Mercury, which is transported globally by atmospheric pathways to remote aquatic environments, is a ubiquitous contaminant at very low (nanograms Hg per liter) aqueous concentrations. Until recently, however, analytical and sampling techniques were not available for freshwater systems to quantify the actual levels of mercury concentrations without introducing significant contamination artifacts. Four different sampling strategies were used to evaluate ground water flow as a mercury source and transport mechanism within aquatic systems. The sampling strategies employ ultraclean techniques to determine mercury concentrations in groundwater and pore water near Pallette Lake, Wisconsin. Ambient groundwater concentrations are about 2–4 ng Hg L⁻¹, whereas pore waters near the sediment/water interface average about 12 ng Hg L⁻¹, emphasizing the importance of biogeochemical processes near the interface. Overall, the groundwater system removes about twice as much mercury (1.5 g yr⁻¹) as it contributes (0.7 g yr⁻¹) to Pallette Lake. About three fourths of the groundwater mercury load is recycled, thought to be derived from the water column.