GEOLOGIC UNITS IN CALIFORNIA AS POTENTIAL SOURCES OF MINERAL HAZARDS

The California Geological Survey (CGS) recently completed a preliminary statewide assessment of mineral hazards in California. Prepared over several years for the California Department of Transportation (Caltrans), this project evaluated, among other natural and man-made features, geologic units throughout the state as potential sources of mineral hazards. We define “mineral hazards” here as minerals and elements that occur naturally in elevated, potentially harmful, concentrations in rocks, soils, and certain fluids. Such hazards may adversely affect public health and safety and the natural environment. The geologic complexity of California results in many sites of known or potential mineral hazards. The Caltrans project focused on geologic units that contain naturally-occurring asbestos (NOA); other fibrous minerals, including erionite; regulated metals (Ag, Ba, Be, Cd, Co, Cr, Cu, Hg, Mo, Ni, Pb, Tl, V, Zn) and metalloids (As, Sb, Se); and a few non-regulated metals (Mn, U). Although not part of the Caltrans project, the CGS has also evaluated various geologic units in parts of California as sources of radon gas, a known carcinogen. More than 90 geologic units in the state were identified as known or potential sources of mineral hazards. Together, these geologic units were highlighted on unpublished maps that Caltrans staff can use as screening tools to aid design of new projects, routine maintenance of highways, and emergency removal of geologic materials from road surfaces. Of these units, serpentinite/ultramafic rocks, the Franciscan Complex, the Monterey Formation and similar units, and contact-metamorphosed carbonate rocks warrant the most attention mainly because of their internal properties and widespread distribution. In addition, many areas of anomalous metallic mineralization (e.g., gold, mercury, metal sulfides) in geologic units were identified by general outlines on maps or through the plotted distribution of individual mines and prospects associated with this mineralization. The methods and products developed for the Caltrans project can be applied to many other uses where there are obligations to protect public health and safety and the environment.