Pilot Study: Selenium in the Moreno Formation of the western San Joaquin Valley, California

For the western San Joaquin Valley between Panoche and Cantua Creeks, studies by Presser and others (1990) and Presser (1994a) indicate that selenium enrichment in soils and agricultural drainage waters derives from the marine sedimentary rocks of the Kreyenhagen Shale and Moreno Formation. Presser and others (1990) report selenium contents ranging from 0.8-23 ppm for 17 rock samples from the Moreno Formation. In order to further characterize the lateral and vertical variation in the contents of selenium and other potentially hazardous trace elements within the Moreno Formation of this region (Figure 10), 24 core samples of mudstone from 2 widely spaced wells were analyzed for major and minor element chemistry. These wells were selected based on prior studies (McGuire, 1988 and Fonseca, 1997) describing the stratigraphy, depositional setting, and hydrocarbon potential of the Moreno Formation in this region (Figure 11).

Figure 10. Map showing outcrop of the Moreno Formation and location of wells sampled in this study. (click for larger image) 61k

Figure 11. Stratigraphy of the western San Joaquin Valley. The Moreno Formation is shown in cyan and the Kreyenhagen Shale is light green. (click for larger image) 25k

Results of the minor and trace element analyses from this reconnaissance study of the Moreno Formation are summarized in Figure 12 (Keller, Evans, and Presser, unpublished data, 1999). These results indicate a significant positive correlation beween total selenium and total organic carbon in both wells (Figure 13). These results also show that the selenium (4-34 ppm) and other trace element contents of the Moreno Formation are variable both laterally or geographically within this region as well as vertically within the formation. Note, however, that these values may be higher or lower in other well or outcrop sections of the Moreno, and indeed in parts of the Moreno Formation not available in the core samples from the two wells analyzed. These results corroborate the prior findings of Presser and others (1990) regarding the high selenium content of the Moreno Formation, and additionally give results for other potentially hazardous elements. Further study is needed to determine the extent of the zones of high content of potentially hazardous trace elements.

	Figure 12. Elemental analyses of the Moreno Formation. (click for larger image) 12k
	Figure 13. Correlation of organic carbon and selenium in samples from the Cheney Ranch #1 and Magnet Fearon #2 oil wells. (click for larger image) 12 k

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http://geology.wr.usgs.gov/wreg/env/moreno.html
Contact: Margaret A. Keller (mkeller@usgs.gov)
Western Region Energy Group–Environmental Studies
Modified: July 31, 2002

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