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Contents[this page; see also the Site Map]. Issues and ResearchHow did selenium first come to public attention in the United States? How are rocks related to selenium enrichment in surface water, ground water, plant, and soil? This web page provides background and research results on certain sedimentary rocks that are enriched in trace elements, including selenium. Figure 2 Map of Petroleum source rocks and sedimentary basins in Californiashows sedimentary basins, onshore oil and gas fields, and principal areas of California where trace element enrichment may occur. Figure 3 Selenium Study Areas in the Arid and Semi-Arid Western United Statesshows the relationship of Cretaceous and Tertiary marine mudstone outcrops to selenium contamination at study sites investigated by the Department of the Interior (DOI) National Irrigation Water Quality Program. Trace Elements in the Monterey FormationThe Monterey Formation is the principal petroleum source rock of California. It also contains abundant trace elements, including selenium and uranium. Figure 4 Stratigraphy of the Monterey Formation in the Santa Maria Basin, central coast Californiashows age and stratigraphic relationships of rocks, including the Monterey Formation, in the Santa Maria Basin. Figure 5 Chart of Trace Element Enrichment in the Monterey Formationshows that trace elements in the Monterey Formation are enriched compared to the world shale average (WSA). Figure 6 Correlation Diagram of Selenium and Organic Carbon in the Monterey Formationshows the strongly positive correlation between organic carbon and selenium. Figure 7 Uranium and Selenium Content of Arco Bixby #1shows the relationship between uranium, selenium, and the gamma-ray well-log signature of the Monterey Formation. Depositional Model for Trace Element Enrichment of the Monterey FormationMarine mudstones may become enriched in trace elements during deposition. Using Gamma-Ray Logs to Estimate Selenium Content of RocksIn the Monterey Formation, where uranium and selenium are highly correlated, gamma-ray logs provide a proxy indication of selenium content. Figure 8 Block Diagramshows well log and outcrop gamma-ray methods for detecting uranium-rich marine mudstones. Figure 9 Average Annual Precipitation in the Western United Statesshows the high aridity of the western U.S., a region where selenium becomes concentrated in soils due to evapo-transpiration processes. Recent Studies: Trace Elements in the Moreno ShaleSamples of the Cretaceous Moreno Formation are enriched in selenium. Analyses show correlation with organic-carbon-rich intervals in the formation, however, the correlation is not as strongly positive as in the Monterey Formation. Figure 10 Exxon Cheney Ranch #1 and Magnet Fearon #2shows the location of two wells where the Moreno Formation was sampled in the Cheney Ranch and Coalinga oil fields. Figure 11 Stratigraphy of the Moreno Formation in the western San Joaquin Basin, central Californiashows the age and stratigraphic relationships of the Moreno Formation in the San Joaquin Basin. Figure 12 Trace Element Content in Moreno Formation-shows that trace elements are somewhat enriched in this formation. Figure 13 Correlation Diagram of Selenium and Organic Carbon in the Moreno Formation-shows the positive correlation between organic carbon and selenium, much like the relationship between these components in the Monterey Formation. Related Studiescurrent USGS studies in the Minerals Program are addressing trace element contamination issues associated with phosphate mining in Idaho. Further studies are needed to refine our understanding of other means by which selenium and other trace elements can pose a hazard to health. Frequently Asked Questions (FAQ) about Selenium Answers to questions about the uses of selenium and the toxicity of selenium to humans and wildlife. LinksTrace elements on the world-wide web. Further ReadingsA short list of USGS and other literature about selenium and a more comprehensive list of references mentioned in this web site. Site MapThe graphics-rich equivalent of this site and an additional navigational tool. http://geology.wr.usgs.gov/wreg/env/content.html |
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