TRANSPORT OF TOTAL AND DISSOLVED MERCURY FROM THE CHEROKEE WATERSHED, SACRAMENTO RIVER BASIN, NORTHERN CALIFORNIA

Gold Rush-era hydraulic mining at the Cherokee Mine in Butte County, California has left a dominant imprint on the surrounding Cherokee Watershed. This watershed is a tributary of Butte Creek within the Sacramento River Watershed of northern California. The overall watershed of approximately 243 km² is comprised of several small, intermittent tributaries that feed into the Cherokee Canal. Hydraulic mining at the Cherokee Mine released large volumes of sediment during the latter half of the 19th Century. Dry Creek was the tributary most impacted by the hydraulic mining at the Cherokee Mine. Peak flows continue to transport reworked sediment from floodplains in the lower foothills to the floor of the Sacramento Valley. Ongoing gravel mining operations exploit these hydraulic mining sediments. Water quality is the primary watershed management concern among the Cherokee Watershed stakeholders. In addition to sediment releases, questions have been raised over the potential for mercury pollution stemming from historic mining. A water quality survey was conducted from November 2003 through December 2004. Chemical constituents in the survey included total (unfiltered) and dissolved mercury, and samples were collected on a monthly schedule and during three rainfall-runoff events. Dissolved mercury was determined after passing samples through a 0.45 mm filter. Fish tissue samples were collected for mercury analysis from the Cherokee Canal as axial muscle fillets from Sacramento suckers 200-280 mm in length. Ambient flows between storms contained relatively low total mercury concentrations, less than 8.0 ng/L. The highest concentrations of total mercury detected in the Cherokee Canal during two February 2003 rain-event-driven discharge events were 17.9 ng/L and 15.4 ng/L, respectively. A third rain event from the Cherokee Canal in December 2004 had runoff containing 26.5 ng/L of total mercury. However, for that same event, Dry Creek contained total mercury at 279 ng/L with dissolved mercury at 8.3 ng/L. Fish tissue samples showed a mean wet weight mercury concentration of 0.10 mg/g. Based on observed total and dissolved mercury concentrations, mercury transport from the Cherokee Watershed appears to be mainly in the solid phase as part of rain-event-driven runoff events.