Guidelines for Selenium in Irrigation Waters

Because of the relatively high concentration of Se in the agricultural drainage waters of the west side of the San Joaquin Valley (SJV) in California and the potential for reuse of these waters on salt tolerant crops, a reassessment of quality criteria for Se in irrigation waters for this area was needed. The specific conditions for this area that justified a reassessment were (i) the Se in the drainage waters of the West Side is largely in the selenate (SeO₄) form which is not appreciably adsorbed by soils and is thus readily leached from the root zone, (ii) the waters containing high concentrations of Se have substantial concentrations of sulfate (SO₄) that dramatically reduce SeO₄ adsorption by plants, (iii) the salinity of the waters that contain high Se concentrations demand leaching fractions (LF) that insure that Se is also leached from the root zone, and (iv) kinetics of Se-species transformations. Recent research with alfalfa (Medicago sativa L.), which is one of the most sensitive as crop growth for Se accumulation, suggests that this crop can tolerate an average soil solution Se concentration (Seₛₛ) of 250 µg/L without exceeding the limit of 4 mg Se/kg in the dry weight of forage that has been set to protect bovine animals from Se toxicity. Based on a model the Se in the irrigation water (Seᵢw) can increase of 100 µg/L at LFs of 0.2 to 0.3 without exceeding the maximum (Seₛₛₘ) established for this crop when irrigated with sulfate waters. Thus, the recommended maximum Seᵢw for alfalfa grown on the West Side of the SJV is 100 µg/L. Guidelines for other crops can be established if the relationships among Seᵢw, Seₛₛₘ, LF, the Se in harvested products (Seₕₚ), and the maximum Seₕₚ for various uses of products for protection of wildlife, domestic animals, and humans are known. The senior author was on leave from the Volcani Center, A.R.O., Bet Dagan, 50-250, Israel. Contribution from the Dep. of Soil and Environmental Sciences, Univ. of California, Riverside. Financial support of the California Water Resources Control Board is gratefully acknowledged.