Water selenium speciation and sediment fractionation in a California flow-through wetland system

A now-through wetland system was established in the Tulare Lake Drainage Di strict (TLDD) in California to determine if selenium (Se) from saline irrig ation drainage can be removed prior to impoundment in evaporation basins to reduce potential toxicity to waterbirds. The objective of this research wa s to evaluate Se speciation, accumulation, and fractionation in the waters and sediments of the newly developed wetland system. The inlet,vater was do minated by selenate [Se(VI), 92%], with smaller percentages of selenite [Se (IV), 5%] and organic Se [org-Se(-II), 3%]. For the outflow water, the aver age percentage of Se(VI) was 72% in November 1997 and 59% in February 1999. This change may be due to an increase in either residence time and/or accu mulation of organic detrital matter, which may enhance Se(VI) reduction pro cesses. Selenium accumulation, transformation, and incorporation with the s olid phase were all intensified in the surface sediment (<20 cm). The highe st total Se concentrations in the sediments were found in the top 5 cm and concentrations dramatically decreased with depth. Elemental Se [Se(0)], as extracted by Na2SO3 was the largest fraction (average of 46%) of the total sediment Se, followed by organic matter-associated Se (OR I-Se) extracted b y NaOH (average of 34%). Soluble, adsorbed, and carbonate-associated Se, as extracted by KCI, K2HPO4 (pH 8.0), and NaOAc (pH 5.0), were about 3, 10, a nd 3% of the total sediment Se, respectively. After establishing the wetlan d for 2 yr, significant Se removal from the flowing water was observed. The major sink mechanisms in the sediment are reduction to Se(0) and immobiliz ation into the organic phase.