Lower American River Mercury

Science Center Objects

Dredging operations for gold along the American River began in the 1860s and continued through the early part of the 20th century, ending about 1962. More than one billion cubic yards of gravel were dredged, making the American River dredge field the second largest in California.

Preliminary results for 22 fish samples from Lake Natoma during Aug. 2000 indicate mercury concentrations greater than 0.3 ppm in edible fillets of bass and catfish, levels of potential concern to human health and ecosystems. Recent sewage spills and other land uses have added nutrient loads to some Lake Natoma tributaries. A comparative study of mercury contamination and bioaccumulation in two tributary watersheds to Lake Natoma is needed to evaluate the impacts of development and urbanization on mercury transport and bioaccumulation in dredged areas. 

The largest municipal wastewater treatment discharger in the Sacramento metropolitan area has begun a dialogue among stakeholders and regulators with regard to possible mercury offsets, a pollutant trading program that would involve cleanup of upstream problems in exchange for relief on its NPDES permit as a more cost-effective alternative to tertiary wastewater treatment to remove mercury. The lower American River watershed below Nimbus Dam is a prime candidate for potential mercury offsets projects, as it is contains gold-mining wastes downstream of the lowermost dams in the system.

A sampling program evaluates mercury and methylmercury concentrations and loads in two tributaries to Lake Natoma, the Willow Creek and Alder Creek watersheds. Organic wastewater contaminants are analyzed to trace sources of nutrients and organic carbon that may influence mercury methylation. Analyses of water, sediment, and biota are used to determine relative levels of methylmercury exposure and to assess the potentially differential effects of varying development and management practices. Mercury methylation potential and demethylation potential are measured in sediments to assess spatial variations. In the lower American River below Nimbus Dam, water, sediment, and biota are sampled in four tributaries that drain dredge tailings areas as well as four tributaries that drain urbanized areas unaffected by historical mining activities. Biological sampling will focus on invertebrate and amphibian taxa to develop a basis for comparing methylmercury bioaccumulation. The main stem American River is sampled in at least three locations on a monthly basis, plus more frequent sampling during storms, to evaluate concentrations and loads of mercury and methyl mercury in relation to inputs from mine wastes and urban tributaries. Samples of resident sport fish (largemouth bass, bluegill, and catfish) in Lake Natoma and in the main stem American River below Nimbus Dam are collected and analyzed for total mercury in edible fillet portions. Ponds along the flood plain are sampled for bass and bluegill. The data are supplied to public health agencies for evaluation of public health risks.