Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 10, 1996
Publication Date: N/A
Interpretive Summary: The surface exposed shales of the Moreno and Kreyenhagen Formations in the Diablo coastal range are considered to be the sources of selenium (Se) that have enriched soils of the west-central San Joaquin Valley of California. The mobility, bioavailability and toxicity of Se is controlled by the concentration of individual Se species in a sample rather than a total Se content. Although the total Se content for the source shales and the resulting alluvial soils has been reported, the distribution of Se species has not been determined in these materials. This study investigates the distribution of Se species from the seleniferous source shales, to the adjacent alluvial soils, the San Luis Drain sediment and final disposal in the Kesterson evaporation ponds.
Technical Abstract: The exposed Moreno shale (5.21 mg total Se kg-1) had half of the Se content of the lesser exposed Kreyenhagen shale (10.94 mg total Se kg-1). The selenate, selenite and selenide concentrations were lower in the Moreno shale as compared with the Kreyenhagen shale. The Se in the San Luis Drain sediment (83.8 mg total Se kg-1) was enriched in insoluble Se fractions (91.4% of Se inventory). The soils of the Kesterson evaporation pond 4 (47.8 mg total Se kg-1), 7 (6.7 mg total Se kg-1), and 11 (5.4 mg total Se kg-1) were enriched in Se. The organic C content decreased in the sequence, San Luis Drain, Kesterson pond 4, 7 and 11, from 33.3, 26.7, and 19.3 g C, to 15.1 g C kg-1 soil material, respectively, suggesting a relationship between Se concentrations and organic C levels. For the nine soil materials, an exponential relationship (R2 = 0.96) was determined for the Se content and organic C contents suggesting that the Se in the materials tested was highly associated with the soil organic matter fraction. An additional 12 Se values and organic C contents from Se contaminated semi-arid soils or irrigation drainage water evaporation basins obtained from the literature closely fit the exponential function established for the nine soil materials. The results suggest that an initial rapid release of organic-associated Se, along with a long-term release of Se would be expected with C oxidation, due to the reversion of the former Kesterson wetland evaporation pond ecosystem to a native semi-arid grassland.