In the early 1980's, selenium problems from agricultural waters that drained into Kesterson National Wildlife Refuge in California's San Joaquin Valley (Figure 2) led to investigations in other parts of the western United States (Figure 3) and to increased involvement of Federal, State, and local governments in the need for environmental remediation or regulation of selenium (Presser, 1994a; 1994b).

	Figure 2. Map showing locations of Kesterson National Wildlife Refuge, sedimentary basins, oil and gas fields, and petroleum source rock areas in California. (click for larger image) 23K
	Figure 3. Map showing marine sedimentary rocks of Cretaceous and Tertiary ages, study areas, and results of evaluations of selenium contamination from the Department of Interior (DOI) National Irrigation Water Quality Program. (click for larger image) 27K

Rocks are one of the primary sources of selenium that may eventually accumulate in soil, water, and plants. The enrichment of selenium in soils derives mainly from the weathering of marine organic-carbon-rich mudstones (Figure 3; Tourtelot et al., 1960; Lakin, 1961; Vine and Tourtelot, 1970; Herring, 1991). Organic-carbon-rich marine mudstones (or shales) are one of the principal rock types that have a high content of trace elements. This type of rock forms from the sedimentation of mud and organic remains on the ocean floor.

Organic-carbon-rich marine mudstones, are today found in all kinds of geographic settings at the earth's surface and in the subsurface of sedimentary basins, and are studied extensively as the source rocks for petroleum. Where these rocks are subject to weathering and erosion or the action of groundwater, their trace elements may be mobilized to groundwater and surface water including wetlands. Thus, the naturally occurring trace elements in these rocks can have deleterious effects on water quality and health if concentrated to toxic levels by secondary processes.

In the United States as a whole, 70% of seleniferous soils are attributed to Cretaceous mudstones (Trelease, 1942) - the major petroleum source rocks of the mid-continent and Great Plains. In California, selenium-enriched soils have been associated primarily with the Cretaceous Moreno Formation and Eocene Kreyenhagen Formation (Tidball et al., 1986; Presser and Swain, 1990; Presser et al., 1990; Presser, 1994a), petroleum source rocks locally distributed in the San Joaquin Valley (Beyer and Bartow, 1988). Presser (1994a) also suggested that Miocene rocks of the Coast Ranges could be seleniferous based on selenium in the Tulare Lake Bed Area evaporation ponds, for which the Miocene rocks were a suspected source. New studies by the USGS show that the Miocene Monterey Formation, the major petroleum source rock in California, is also seleniferous (e.g. Piper and Isaacs, 1995a).

The USGS Energy Resources Program has studied the distribution of various trace elements, including uranium and selenium, in certain petroleum source rocks in the United States. Learn more about these USGS studies, including the Moreno Formation and the Monterey Formation.

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