Development of a comprehensive watershed model to assess the impact of atmospheric deposition of mercury on surface water

Hundreds of streams, lakes, and estuaries are considered impaired due to increased mercury concentrations. Exposure to high levels of mercury can cause serious health hazards including nervous system damage, kidney damage, and neurological developmental abnormalities. Mercury in the form of methylmercury is of greatest concern because it bioaccumulates in predatory species of the aquatic food chain, and the common route of exposure is ingestion. This research focuses on developing a comprehensive watershed model that fills this gap by utilizing the outcome of recent studies and advancement in modeling tools.; The most important source of mercury in watershed environment is the atmospheric deposition, which varies significantly in space and time. Mercury forms various complexes in water and exists in particulate and dissolved phases. Methylmercury, the pollutant of most concern, forms through biodegradation of dissolved mercury complexes. Biodegradation and adsorption/desorption characteristics of mercury complexes are expected to vary depending on ambient conditions and concentrations of other water quality constituents. Therefore, the capability of modeling mercury complexes under varying environmental conditions is essential for accurately simulating the fate and transport of mercury in surface water. A speciation algorithm was developed and integrated with an existing watershed model—the Hydrologic Simulation Program Fortran (HSPF), to develop a comprehensive mercury model. HSPF is a continuous and public domain model capable of simulating many processes involving fate and transport of pollutants. Further enhancement of HSPF by adding a speciation submodel makes the model more suitable for modeling mercury and methylmercury in surface water.; The model was applied to the Sheboygan River watershed in Wisconsin to demonstrate the applicability of the model and prove the importance of speciation modeling. The average modeled partition coefficients for mercury and methylmercury were 160,000 L/kg and 215,000 L/kg, respectively. The mercury biodegradation rate was 1.0 day-1. For the observed ranges of pH and chloride concentrations in the Sheboygan River HgCl₄−2, HgCl ₃− and HgCl₂ were the three most important complexes of mercury. On the average 81%, 17% and 2% of total mercury were HgCl₄−2, HgCl₃− and HgCl₂, respectively, for an average chloride concentration of 10−3 M and an average pH of 8.2.