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Trace
elements...essential for survival, yet potentially deadly. Scientists have long recognized that trace elements are naturally occurring in certain sedimentary rocks that formed in ancient oceans. Many of these trace elements, such as arsenic, cobalt, chromium, copper, iron, manganese, molybdenum, nickel, selenium, vanadium, and zinc are considered essential or beneficial to humans, and yet they can also become potentially toxic in overabundant amounts (Pais and Jones, 1997).
In the 1930's and 40's in the United States it became clear that soils with high selenium content were often associated with marine shales of the Cretaceous period. In the 1980's and 90's studies by the U.S. Geological Survey identified more specifically that certain organic-carbon-rich marine shales, such as the Miocene Monterey Formation of California, contain intervals that are very enriched in trace elements. These studies also indicated that trace-element enriched zones in the Monterey Formation could be located using a gamma ray tool -- both in the subsurface and in surface exposures. Even with these studies, relatively little is known about the distribution and amounts of selenium and other trace elements in the many sedimentary rock formations that are a major source of these elements. In California, marine shales of Cretaceous and Eocene age have been identified as the original source of the selenium and other trace elements that became concentrated to toxic levels in the water and soil of Kesterson National Wildlife Refuge. We summarize new trace element analyses of the Cretaceous Moreno Formation from two oil wells drilled in this region indicating that it is relatively enriched in selenium. From studies of Kesterson, where selenium is considered the major problem trace element causing birth defects and reproductive disorders in fish and waterfowl, Presser and others (1990) describe the pathways of selenium from its source in the rocks to organisms at the refuge (Figure 1). Follow-up studies have been conducted by the U.S. Geological Survey to determine and explain Se loads in the San Joaquin River and the San Francisco Bay/Delta ecosystems(e.g., Presser and Piper, 1998; Luoma and Presser, 2000).
About this Web Site This web site was created by Margaret A. Keller and Kevin R. Evans: 1) to highlight recent results from USGS Energy Program studies of petroleum source rocks, sedimentary rocks that are enriched in organic carbon as well as potentially hazardous trace elements such as selenium; 2) to describe results from related trace element studies in order to show the larger context of this work, and so that the selenium problem won't need to be "rediscovered" again as Seiler (1998) points out for Kesterson; and 3) to encourage research on trace elements in rocks. Studies of naturally occurring trace element distributions in rocks that are known to contain abundant trace elements, such as petroleum source rocks, ultimately will allow us to make predictive models of potential related health hazards. It will also give us important baseline chemical information to determine the pathways and budgets for selenium and other trace elements in problem areas that require remediation. To learn more about trace element concentrations in petroleum source rocks and USGS efforts to understand them better, take a look at the issues and research. To see a complete description of the contents and layout of this web site, click on "contents" below. http://geology.wr.usgs.gov/wreg/env/home.html USGS Home | Biology | Geology | Mapping | Water | Energy Program | Western Region Ask-a-Geologist | Search USGS | USGS Privacy Statement | Disclaimer
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