Selenium in the Southern Coast Range of California: Well Waters, Mapped Geological Units, and Related Elements

A reconnaissance survey of 151 irrigation and stock wells was conducted in the southern coast range of California bounded by Alameda and San Joaquin counties in the north (37°45′N) and Ventura and Santa Barbara counties in the south (34°15′N). Selenium concentrations in 11 wells exceeded 20 µg/L, the recommended water quality guideline for irrigation water. Wells with Se concentrations >2 µg/L were associated with nearby surface Pliocene and Miocene (P < 0.001) marine rocks based on Pearson's chi-square analysis. Selenium was linearly related (P < 0.001) to electrical conductivity (EC), SO₄, and P concentrations. Improved regression fits resulted from stratification by Pliocene and Miocene marine surface rocks; stratification also improved the regression fit (P < 0.001) between Se and Cl. The multiple linear regression equation relating Se to EC and P within 2.5 km of Pliocene surface rocks produced an R² of 0.74 and a standard error of estimate of 11 µg Se/L. We conclude that either both Pliocene and Miocence rocks are Se sources, or that Pliocene rocks are positional markers locating recently uplifted, Sebearing Miocene rocks. Also, surface rock geology and well water composition (EC, P) would be useful information to locate wells that are likely to contain high levels of Se. Contribution from Coop. Ext., Univ. of California, Riverside.