ASSESSMENT OF MERCURY TRANSFORMATION IN CONSTRUCTED WETLANDS

Constructed wetlands have been proven to be a reliable and cost-effective method of improving water quality by reducing biochemical oxygen demand (BOD), total suspended solids (TSS), nutrients (i.e., nitrogen and phosphorus), as well as metals, trace organics, and pathogens from overlying water. Wetlands usually function as sinks for total mercury (THg), but may also release previously deposited Hg when bottom sediments are disturbed or when the environmental conditions change. In addition, wetlands are known to be potential sites of methyl mercury (MeHg) production. Steamboat Creek (SBC), NV, is the largest source of nitrogen and phosphorus to the Truckee River, which ends in a terminal water body, Pyramid Lake. In addition the stream's sediments and waters are highly contaminated with Hg containing mine waste. A proposal for construction of a wetland at the mouth of the creek prompted the study of the potential for nutrient removal and MeHg production using mesocosms with sediment and water from the creek. Four replicated experimental designs were applied. Two designs included as base sediments, sand and gravel with very low Hg concentrations, and two contained sediments from SBC with elevated Hg concentrations. The mesocosms receive water from SBC or effluent from the Truckee Meadows Water Reclamation Facility (TMWRF). THg, MeHg, and nutrients, in the wetland influent and effluent flows, were monitored routinely over several years, along with other water quality parameters. The impact of drying and wetting cycles on the effluent water quality and net methyl Hg production was also assessed. Wetland trains with SBC water and SBC base sediments acted as sink for MeHg during the winter months and source of MeHg during summer months. Wetlands with SBC sediments and TMWRF effluent were a source of MeHg all year round. After drying and re-wetting, an increase in the MeHg released from the wetlands was observed. However, after 25 days, effluent concentrations returned to that measured before the drying. Other conditions that were systematically manipulated included hydraulic retention time and the addition of chemicals (sulfate and nitrate). The impact of these manipulations on the MeHg releases from the wetland will be discussed.