Mercury and methylmercury related to historical mercury mining in three tributaries to Lake Berryessa, Putah Creek Watershed, California

Abstract:

This study examined the relative contribution of total mercury (THg) and mono-methylmercury (MMHg) from upstream historical mercury-mining districts to Lake Berryessa, a reservoir with impaired water quality because of mercury. The third and fourth largest historical mercury-producing mining districts in California are within Lake Berryessa’s three largest tributary watersheds: Pope, (Upper) Putah, and Knoxville-Eticuera Creeks. Downstream of the reservoir, Putah Creek drains into the Yolo Bypass, a major source of THg and MMHg to the Sacramento–San Joaquin Delta. Water samples were collected from October 2012 to May 2014 during 37 non-storm and 8 storm events along Pope, (Upper) Putah, and Knoxville-Eticuera Creeks and analyzed for field parameters (temperature, pH, specific conductance, dissolved oxygen, and turbidity). Additionally, water samples collected during five of the non-storm and storm events were analyzed for unfiltered THg and MMHg and total suspended solids (TSS). Discharge was measured during sampling to calculate instantaneous loads. More than 120 streambed sediment samples were collected to determine the spatial variation of THg and organic carbon content (loss on ignition). Across the watersheds, unfiltered THg (in water) samples ranged from 2.3 to 125 ng/L and unfiltered MMHg (in water) samples from 0.12 to 1.0 ng/L. Concentrations of THg ranged from less than 0.0001 to 122 mg/kg in streambed sediment. Tributary reaches with elevated mercury concentrations (“hot spots”) are near or downstream of historical mercury mines and have: (1) strong positive correlations between THg (in water) or MMHg (in water) and TSS (R²> 0.88, n=5); (2) higher instantaneous loads of suspended sediment, THg and MMHg than reaches with low THg and MMHg concentrations; and (3) elevated sediment organic carbon content. Tributary reaches with weaker correlations among THg, MMHg, and TSS in unfiltered water may reflect non-mining sources of dissolved THg and MMHg, such as geothermal springs and groundwater influx from shallow aquifers. The importance of suspended particulate matter relative to THg and MMHg transport in the most contaminated stream reaches indicates that erosion control is likely to be a critical factor in successful remediation efforts in the Upper Putah Creek watershed.