Determination of Mercury Loads for Cache Creek Settling Basin Inflow and Outflows and Related Investigations

Science Center Objects

Mercury (Hg), a legacy pollutant from mining of mercury deposits in the Coast Ranges as well as gold deposits in the Sierra Nevada, has contaminated surface waters throughout northern . Methylmercury (MeHg) is an organic form of Hg that bioaccumulates in food webs to elevated concentrations that threaten ecological health and human health through consumption of sport fish. 


Cache Creek Settling Basin area on aerial photo, showing sampling locations for baseline and storm-event water samlpling.

Cache Creek Settling Basin area on aerial photo, showing sampling locations for baseline and storm-event water samlpling.

(Public domain.)

The Cache Creek Settling Basin (CCSB) is an area managed by the California Department of Water Resources where Hg-contaminated sediment from the is trapped. There is considerable interest in the CCSB from the Regional Water Quality Control Board – Central Valley Region, as a place where potentially significant load reductions of Hg and MeHg may be achieved in the context of pending regulation under the Total Maximum Daily Loads (TMDL) process. There is also concern that wetlands and agricultural fields within the CCSB are areas where Hg is methylated. There is currently a lack of reliable data on loads of Hg and MeHg in and out of the CCSB, and no data on the distribution of Hg and MeHg in sediments within the CCSB.

The main purpose of this study is to quantify Hg and MeHg loads into and out of the CCSB during various flow regimes. A secondary goal is to assess the temporal and spatial variability of Hg-sediment relationships and to begin to develop a conceptual model of Hg methylation within the CCSB that can be used as a basis for the development of potential mitigation strategies that will result in reduced loads of Hg and MeHg out of the CCSB to the Yolo Bypass.

The study approach includes: a) collection of surface water flowing into and out of the CCSB and analysis for Hg, MeHg, suspended sediment, and other constituents during baseline conditions (Task 1) and particulate Hg and MeHg and suspended sediment during storm events (Task 2) over two water years (November 2009 through September 2011); b) collection and analysis of surface water (Task 3) and shallow sediment (Task 4) along transects within the CCSB to assess spatial and temporal gradients of Hg species and ancillary constituents that may affect formation of Hg(II)R and MeHg; c) collection and analysis of sediment in vertical profiles (drill cores) at selected locations within the CCSB to assess long-term temporal and spatial variations in trapped Hg (Task 5); and d) analysis of THg and MeHg loads for inflows and outflows (Task 6).