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Description
Management of agricultural drainage water contaminated with selenium is one of the most important environmental issues in California. A constructed wetland is an attractive water treatment option due to its low cost and minimal maintenance. The removal of volatile selenium from wetlands is called phytoremediation and is important for its potential in cleaning water polluted with selenium. Selenium ecotoxicity is generally encountered in arid or semiarid regions with alkaline and seleniferous soil derived from marine sediments. Selenium is mobilized and transported by irrigation drainage via tile drains and canals to wetlands and other water-bodies, where bioaccumulation of selenium to toxic levels occurs. The primary goal of constructed wetlands is to remove or decrease pollutants from waterbodies. Toxicity to wild life should also be taken in consideration when evaluating efficiency of removal of selenium in these wetlands, as they are crucial in supporting wildlife habitat. The objective of the present study was to quantitatively determine the rate of selenium (Se) removal by biological volatilization in the Imperial constructed wetlands in Imperial Valley, California. Volatile Se was collected continuously using an open flow chamber system throughout a 24 h sampling period at an airflow rate of 0.43m_ h__ from vegetated and non-vegetated areas of the Imperial constructed wetlands. Volatile Se in the air was trapped using an alkaline peroxide trap solution (30% H_O_ and ~0.05 M NaOH). Selenium volatilization was measured in the Imperial wetlands site during the months of May. June, July, September and October of 2010. The chemical form of volatile Se measured was dimethyl selenide. Rates for the most dominant plant species, saltmarsh bulrush, varied during the months that Se volatilization were measured. In May the rates for vegetated sites were 342.8 µg Se m__ day__; this rate corresponded to a percent removal of 10.4%. In June, July, September and October the rates for vegetated sites were 46.0, 3.2, 2.1 and 0.9 µg Se m__ day__ and for non-vegetated sites were 7.0, 1.1, 0.4 and 0.2 µg Se m__ day__; these rates correspond to percent removals of 1.4%, 0.1%, 0.06%, 0.03% and 1.6%, 0.3%, 0.1%, 0.04% respectively. Using mean rates that were measured for biological volatilization, we estimated that the Imperial constructed wetlands might have removed as much as 2.4% and 0.5% of the total Se removed from the inflow through Se volatilization for vegetated and non-vegetated sites respectively. This study concluded that biological volatilization as a means of Se removal in constructed wetlands is a significant pathway that should be further evaluated.
