Mercury contamination poses a significant threat to aquatic ecosystems, with reservoirs in arid regions providing ideal conditions for microbial transformation of inorganic mercury into neurotoxic methylmercury. This transformation, driven by redox-sensitive biogeochemical processes, leads to bioaccumulation and biomagnification of methylmercury in aquatic food webs. Historic mercury mines act as sources of inorganic mercury to local watersheds, particularly in areas impacted by mining such as the California Coastal Range. Presently, 131 reservoirs across the state of California have been listed as mercury-impaired and in need of remediation. Guadalupe Reservoir, in the New Almaden Mining District of Santa Clara County, is among these impaired reservoirs and exhibits record-high fish mercury concentrations statewide. Remediation strategies, including use of sorbent amendments, are under exploration to mitigate methylmercury formation in the system. However, critical knowledge gaps regarding the biogeochemical controls and locations of methylmercury production hinder the effective remediation of mercury concentrations in fish. This study aims to integrate biogeochemical measurements and microbial metagenomics to identify the primary locations of methylmercury production within Guadalupe Reservoir and determine the key biogeochemical and microbial processes responsible for its production. In this assessment, we conducted in-situ incubations in the water column and sediments throughout the stratification cycle along with monthly biogeochemical monitoring to understand mercury methylation processes in varying redox conditions. These experiments will be complemented by metagenomic analyses to identify the microbial clades responsible for mercury methylation under specific conditions in the water column. Ultimately, this research aims to identify the locations and rates of mercury methylation in Guadalupe Reservoir, which in turn will assist water managers in targeted remediation strategies.