Selenium Fractionation and Speciation in a Wetland System

Selenium fractionation and speciation in Benton Lake National Wildlife Refuge (Montana), a wetland system containing moderate levels of selenium, were studied to determine the biogeochemical processes of selenium in wetlands. Results showed that selenate was a major selenium species of dissolved selenium in drainage water. It decreased substantially through the pond system as the relative percentages of organic selenium and selenite increased. Elemental selenium and selenium associated with organic materials were the major fractions in sediments, accounting for a mean of 46% and 33% of total selenium, respectively. Concentrations of soluble selenium and adsorbed selenium were relatively low, respectively accounting for a mean of 5% and 13%. Within the soluble and adsorbed fraction of selenium in sediment, selenate and organic selenium were the major selenium species. Selenium associated with oxides was very low (less than 4%). In this wetland environment, microbial reduction of selenate to elemental selenium, selenium uptake by wetland organisms, and incorporation of these organisms into wetland sediment are the major processes removing selenium from the water column; selenium adsorption is relatively less important.