MERCURY CYCLING, TRAPPING, AND EXPORT ASSOCIATED WITH A NORTHERN CALIFORNIA, USA, MULTI-HABITAT SETTLING BASIN UNDERGOING LAND-USE CHANGE (Invited Presentation)

The Cache Creek Settling Basin (CCSB), constructed in 1937 and expanded in 1993, is a 1600-hectare multi-use floodplain area in Yolo County, California. The CCSB was designed to trap sediment from the upper watershed and to reduce sediment transport into the Yolo Bypass, a floodwater conveyance protecting the city of Sacramento. With several natural and anthropogenic mercury (Hg) sources in the watershed (including inactive Hg mines, gold mines where Hg was used for amalgamation processing, geothermal springs, and erosion of Hg-enriched geologic material), the CCSB traps significant amounts of particulate-bound Hg. During relatively wet years, the CCSB may remain flooded for days or weeks, providing an environment where inorganic Hg likely transforms to toxic methylmercury (MeHg) by microbial processes. As a result, the CCSB has been identified as a significant source of MeHg to the Yolo Bypass and San-Francisco Bay Delta, where MeHg bioaccumulation in fish and other wildlife contributes to ecological and human-health risks. This presentation will present findings from more than a decade of Hg research associated with the CCSB, including: a) estimates of within-basin cycling, trapping, and (or) export of particulate and dissolved Hg and MeHg; b) effects of grain size on Hg concentration and trap efficiency; ; c) Hg speciation and MeHg production potential rates within CCSB soils as a function of land use (including agricultural, naturally vegetated floodplain, riparian zone, and stream channel); d) the effect of land-use change (conversion of floodplain to agricultural use) on Hg speciation in CCSB soils; and e) recent efforts to use surface-water optical properties and other parameters that are continuously monitored (such as turbidity, pH, and dissolved oxygen) in the development surrogates for particulate and dissolved Hg and MeHg species. The surrogates will allow high-temporal-resolution estimates of concentrations at stations where water flows into and out of the basin, and should reduce long-term monitoring costs. These results are currently being used to inform California state agencies as to the status of the CCSB as both a source and sink for various Hg species. Quantifying Hg and MeHg loads from the CCSB is of importance to those responsible for managing and regulating the downstream Hg-impacted Yolo Bypass and San Francisco Bay and Delta.