ENVIRONMENTAL LEGACY OF THE CALIFORNIA GOLD RUSH: 1. MERCURY CONTAMINATION AND BIOACCUMULATION ASSOCIATED WITH ABANDONED PLACER-GOLD MINES

Widespread mercury contamination of watersheds has resulted from the extensive use of elemental mercury in the mining and recovery of gold during the late 19th and early 20th centuries along the western slope of California’s Sierra Nevada. The Bear and Yuba watersheds were selected for detailed study because of concerns over numerous abandoned mines on both public and private lands, previous bioaccumulation studies, and observations of visible elemental mercury at several placer-gold mines and in river sediments. Samples of water, sediment, and various aquatic fauna, including insects, frogs, and fish, from mine sites and receiving waters were analyzed for total mercury (Hg-T) and methylmercury (MeHg) to determine the extent of contamination and to identify potential remediation targets. Sampling focused on sites of historical hydraulic mining that exploited thick, auriferous gravel deposits of the ancestral (early Tertiary) Yuba River; a large part of the elemental mercury used in hydraulic mining was lost to the environment.

The most elevated Hg-T concentrations in water and sediment were found in ground and tunnel sluices—the sites of historical placer gold ore processing. Elevated MeHg concentrations were detected in water from some tunnel sluices and pit lakes formed by partially blocked drain tunnels. Isotopic rate measurements of mercury methylation potential [using ²⁰³Hg(II)] and mercury demethylation potential (using ¹⁴C) are consistent with relatively high levels of mercury bioaccumulation in organic-rich, near-neutral pit lakes. Predatory insects, such as water striders (family Gerridae) and dragonflies (order Odonata, suborder Anisoptera), were shown to be reliable indicators of bioavailable mercury, making them useful for comparing mercury contamination among sites. Elevated Hg-T concentrations in sport fish, especially black bass (Micropterus spp.) in reservoirs and brown trout (Salmo trutta) in streams prompted local officials to issue consumption advisories. A long-term monitoring program is needed to evaluate the local and regional benefits of recent and future mine-site remediation projects.