Effects of Oxygen and Nitrate on Mercury Cycling in the profundal Zone of Lakes and Reservoirs

Thesis (Ph.D.), Department of Civil and Environmental Engineering, Washington State UniversityMethylmercury, a neurotoxin, has been identified as a serious public health concern. To date, effective remediation techniques for preventing and remediating methylmercury contamination have remained elusive, mainly due to the lack of knowledge in regard to how methylmercury is generated and degraded in the aquatic environment. This dissertation evaluated methylmercury accumulation in both replicate experimental chambers containing undisturbed sediment-water interface samples from mercury-contaminated lakes in northern California, and a two-year field study evaluating spatial and temporal patterns of nitrate, oxygen and methylmercury during summer stratification in Occoquan Reservoir, Virginia. The primary objective of this research was to access how oxidants, including nitrate, affect the cycling of mercury in the profundal zone of lakes and reservoirs. Results demonstrated that methylmercury accumulation in both experimental chamber water and field sampling was lower under aerobic and anoxic (no oxygen but nitrate present) conditions versus anaerobic conditions. In Guadalupe Reservoir, California, MeHg efflux measured in replicate bench-scale chambers averaged 5.5 ng/m2/d under aerobic conditions and 22 ng/m2/d under anaerobic conditions. In Almaden Lake, California, MeHg efflux was -2.3 ng/m2/d (uptake) under aerobic conditions and 11 ng/m2·d under anaerobic conditions. Like oxygen, nitrate repressed MeHg efflux from sediments from Almaden Lake to < 0.2 ng/m2/d. In Occoquan Reservoir, Virginia, MeHg was < 0.1 ng/L in a tributary enriched with nitrate from treated wastewater effluent compared to 0.04-1.1 ng/L in a low nitrate reference tributary. MeHg accumulation correlated with elevated iron and manganese concentrations. Nitrate addition to the reservoir likely kept downstream bottom waters from going highly reduced, thereby reducing MeHg accumulation in bottom waters. This dissertation also describes a fellowship performed with the World Bank, as part of the WSU NPSIRE IGERT program, evaluating environmental and economic policy in the water/wastewater sector in Cartagena, Colombia. The investigation found that Cartagena's water supply coverage has increased from 73% to 99.9% and sewer coverage from 61% to 90%. However, a lack of transparent environmental policy regarding public participation has limited the effectiveness of achieving meaningful benefits without negatively impacting the public.Department of Civil and Environmental Engineering, Washington State Universit