Selenium Speciation of Marine Shales, Alluvial Soils, and Evaporation Basin Soils of California

The surface exposed shales of the Moreno and Kreyenhagen Formations are considered to be the sources of selenium (Se) that have enriched soils of the west-central San Joaquin Valley of California. Although the total Se content for the source shales and the resulting alluvial, seleniferous soils has been reported, the distribution of Se oxidation states has not been determined in these materials. This study investigates the Se speciation trends in the seleniferous source shales, the adjacent alluvial soils, the San Luis Drain sediment to the Kesterson evaporation pond soils. The surface-exposed Moreno shales (5.21 mg total Se kg⁻¹) had half of the Se content of the less exposed Kreyenhagen shale (10.94 mg total Se kg⁻¹). The elemental Se (Se⁰) concentration was similar in the two shales, but the selenate (Se + VI), selenite (Se + IV), and selenide (Se-II) 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 the Se⁰ and the Se-II 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 also enriched in Se with 86, 76, and 48% of the Se inventory present in the Se⁰ and Se-II fractions, respectively. The organic C content also 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. A linear relationship between the sum of (Se-II and Se⁰)-Se and organic C levels (R² = 0.96*; significant at the 0.05 level) was noted for the analyzed San Luis Drain sediment and the Kesterson soils. For the nine soil materials, an exponential relationship (R² = 0.96) was determined for the sum of (Se-II and Se⁰)-Se 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 analyses and organic C contents from Se contaminated semiarid and irrigation drainage water evaporation basin soils obtained from the literature closely fit the exponential function established for the nine soil materials. The results suggest that an initial release of organic-associated Se would be expected with increased C oxidation, along with a slower, long-term release of the refractory Se, due to the reversion of the former Kesterson wetland evaporation pond ecosystem back to a native semiarid grassland.