MAPS OF POTENTIAL MINERAL AND MINING-CHEMICAL HAZARDS ALONG PARTS OF STATE HIGHWAYS 128 AND 299 IN NORTHERN CALIFORNIA

Certain minerals, when sufficiently concentrated in geologic materials, can be potentially harmful to human health and the environment. Chemicals used to mine and process ore can present similar problems. Such minerals and mining chemicals may be encountered locally during the construction and maintenance of highways.

At the request of the California Department of Transportation (Caltrans), the California Geological Survey prepared maps of potential mineral and mining-chemical hazards along a 16-mile-long corridor of State Highway 128 in Napa County and a 175-mile-long corridor of State Highway 299 in the Klamath-Cascade region. The corridors consist of the highways and one-mile-wide buffers on each side. Because streams can transport geologic materials and mining chemicals to the highways from sites outside the corridors, the maps also include the watersheds that drain into the corridors. Information on these maps is used by Caltrans to help identify potential environmental and safety issues in planning for road construction, routine maintenance, and emergency removal of debris deposited on roads by natural processes.

Physical features analyzed for their potential as sources of mineral hazards included bedrock, faults, alluvial deposits, landslide deposits, small-stream sediment, and mines and prospects. Each feature was rated as high, medium, or low for its potential to 1) contain naturally occurring asbestos (NOA) and 2) locally equal or exceed threshold concentrations of each of 17 metals (CAM17) that Caltrans routinely evaluates as possible contaminants in earth materials. Based on this analysis, an “overall” rating was assigned to each physical feature and shown on the maps by color-coding. Mines and prospects were also evaluated as potential sources of mining chemicals. They were not rated, however, because of insufficient historic information about mining and processing activities. Instead, estimates of types or degrees of ore-processing are presented for most mines and prospects.

Potential mineral hazards of most concern along SH128 are associated with NOA, chromium, and nickel. Those along SH299 are associated with NOA, antimony, arsenic, cadmium, chromium, copper, lead, nickel, silver, and zinc; the mining chemical of most concern is mercury.