NATURALLY OCCURRING ASBESTOS ASSESSMENT IN THE COPPER HILL VOLCANICS OF CALIFORNIA

In the interests of public health and safety, geologists have been evaluating areas under development in the Copper Hill Volcanics Formation of the Sierra Nevada Foothills of California for naturally occurring asbestos (NOA) since early 2003. Although clearly present higher concentration geologic occurrences of NOA can be identified and managed (typically through engineering controls), broad areas of lower concentrations of NOA that may still pose health risks are present and can actually present a greater assessment challenge for the geologist.

Over the last several years, the California Geological Survey and the California Environmental Protection Agency have been developing and providing a suite of tools useful for managing NOA in earthwork for homes, businesses, and schools. Engineering geologists, whom normally work in the fields of rock mechanics and seismology, have also had to review their training in understanding metamorphic processes and apply the tools within the framework of the geochemistry of asbestos petrogenesis and paramorphism.

Amphibole NOA in the Copper Hill Volcanics appears to be associated with both low-grade metamorphism of mafic volcanic rock and with hydrothermal deposition in veins. Amphibolite schists in the region have been found to contain NOA at levels that potentially trigger regulatory mitigation requirements due to possible health risks. Altered clino-pyroxenes are likely one source of tremolite. Veins and fracture coatings of amphibole asbestos, along with what are likely to be partially paramorphosed amphiboles with coatings of both actinolite and tremolite asbestos have all been identified in the Copper Hill Volcanics.

This carries profound implications for similar types of rock in similar types of metamorphic environments in California. As development continues, it is important that we understand the extent of NOA associated with metamorphosed mafic rock.