Abstract
This study examined the relative contribution of total mercury (THg) and methylmercury (MeHg) from upstream historical mercury-mining districts to Lake Berryessa, a reservoir with water quality impaired by mercury. The third and fourth largest historical mercury producing mining districts in California are within Lake Berryessa’s three largest tributary watersheds. These mining districts, which lie within mercury-bearing deposits are located within the Pope Creek, Upper Putah Creek, and Knoxville-Eticuera Creeks watersheds. Downstream of the reservoir, Lower Putah Creek drains into the Yolo Bypass, a major source of THg and MeHg to the Sacramento–San Joaquin Delta. Study objectives include (a) determine if tributaries downstream of historical mercury mining districts and draining to Lake Berryessa are sources of THg and MeHg; (b) identify correlations in water chemistry among collected parameters; (c) characterizing variability of parameters in upstream and downstream reaches of each creek; and (d) estimate THg and MeHg loads entering the reservoir from each tributary. Water samples were collected from October 2012 to September 2014 during 55 non-storm and 9 storm events along Pope, Upper Putah, Lower Putah, and Knoxville-Eticuera Creeks and analyzed forfield parameters (pH, alkalinity, hardness, specific conductance, turbidity, dissolved oxygen, temperature, salinity, and redox potential), total suspended solids (TSS), and total particulate matter (TPM). Discharge measurements were made at the time of sample collection; flow and concentration data were combined to compute instantaneous loads of selected constituents. Additionally, water samples were collected during four non-storm and one storm events for analysis of unfiltered THg, MeHg, TSS, and TPM along the tributaries. One hundred thirty-five streambed sediment samples were collected to determine the spatial variation of THg, organic content, and grain size distribution. Based on results of water sampling, all three tributaries contribute THg and MeHg to Lake Berryessa. Some consistent spatial trends in THg (water) concentrations were observed over multiple sampling events; THg (water) decreased from upstream to downstream in all three tributaries. Tributary reaches with elevated THg in streambed sediment (“mercury hot spots”) are near or downstream of historical mercury mines and mercury enriched ore deposits. The importance of TSS and TPM relative to THg and MeHg transport in the most contaminated stream reaches indicates that erosion control may be a useful remediation tool for decreasing THg and MeHg loads to Lake Berryessa.