Selenium removal from irrigation drainage water flowing through constructed wetland cells with special attention to accumulation in sediments

A flow-through experimental wetland system has been under investigation since 1996 to remove selenium (Se) from agricultural drainage water in the Tulare Lake Drainage District at Corcoran, California, U. S. A. The system consists of ten cells which have dimensions of 15 x 76 m continuously flooded and various substrates planted. The objectives of this article are to present the overall performance in Se removal after establishing the wetland for three years, and to examine factors affecting Se removal with special attention to accumulation in the sediments. In 1999, The wetland cells reduced Se from inflow water by 32 to 65% in concentration and 43 to 89% in mass. Vegetation plays an important role in Se removal as non-vegetated cell showed the least removal of Se. The inflow drainage water was dominated by selenate (Se(VI), 91%) with smaller percentages of selenite (Se(IV), 7%) and organic Se (org-Se(II-), 2%). The outflow water from the cells contained an average of 47% Se(VI), 32% Se(IV) and 21% org-Se indicating reduction processes occurring in the wetland cells. The surface sediment appears as a large sink of Se removal. The highest Se concentration was found in fallen litter, followed by the fine organic detrital layer on the sediment surface. The sediment Se concentration dramatically decreased with increasing sediment depth. The mass distribution of Se, however, was sediment (0-20 cm) > fine detrital matter > fallen litter. Fractionation of surface sediment (0-5 cm) reveals that elemental Se was the largest fraction (ave. 47%) followed by organic matter-associated Se (34%). Soluble, adsorbed, and carbonate-associated Se accounted for 1.2, 3.1 and 2.5% of the total sediment Se, respectively. The major Se sink mechanism in the cells is the reduction of selenate to elemental Se and immobilization into the organic phase of the sediments.