MERCURY TRANSPORT IN CLEAR CREEK, A DRAINAGE IN NORTHERN CALIFORNIA IMPACTED BY HISTORICAL GOLD MINING

Clear Creek, a tributary of the Sacramento River, drains most of the mineralized area of the French Gulch district, the most productive lode gold district in the Klamath Mountains region of California. Downstream from the French Gulch district, late Tertiary and Quaternary alluvial deposits have been mined for gold by various placer methods including hydraulic mining and dredging, resulting in mercury contamination of sediments. In addition, mercury-contaminated lode-mine tailings released from the French Gulch district are entrained in active stream sediments.

Mercury concentrations in waters of lower Clear Creek have been investigated under various flow conditions, and in other environments including flood plain ponds, impoundments in dredge tailings, wetlands ponds constructed in tailings, and drainage from gravel (dredge tailings) wash ponds. Under all seasonal conditions in these environments a significant proportion (usually >50%) of the mercury present is associated with particles larger than 0.45 µm, as is the case for Al and Fe, suggesting that Hg may be associated with Al- and Fe-bearing phases. The proportion of particulate Mn is highly variable, but some Hg may be associated with Mn-bearing particles as well. During high flow conditions both Al and Hg increase markedly, indicating that clay mineral particles may be particularly important in stream transport of Hg. In pond environments some Hg may be associated with Fe oxyhydroxide, Mn oxide, and clay particles, and some may be associated with particulate carbon.