Keith E. Dennett

NET METHYL MERCURY PRODUCTION VERSUS WATER QUALITY IMPROVEMENT IN CONSTRUCTED WETLANDS: TRADE-OFFS IN POLLUTION CONTROL

In a system with mercury contamination, there are trade-offs between beneficial functions of a wetland and environmental risk of methyl mercury (MeHg) production. This project used five wetland mesocosms with three different experimental designs to assess the potential for nutrient, sediment, and total mercury (THg) removal and MeHg production associated with a proposed a large-scale wetland system. The latter was suggested for the mouth of Steamboat Creek (Nevada, USA) at the confluence with the Truckee River. Steamboat Creek has been documented to have high mercury concentrations and is a major source of nutrients to the river. Mesocosms that had creek sediments as the base and creek water as inflows resulted in decreasing THg concentration by 72–82%. Average percent nitrogen and phosphorus and suspended solids removal were 43%, 30%, and 70%, respectively. Net MeHg production was observed during spring, summer, and fall months; however, in the winter, these mesocosms acted as a sink. One wetland mesocosm with sediments low in mercury and creek water showed similar trends. Mesocosms with creek sediments and water low in mercury were a source of MeHg year round, with outflow concentrations 10 to 200 times that in the inflow. Based on the developed data, the environmental risk of the proposed large-scale wetland would be an increase of methyl mercury concentration in creek water that reaches the Truckee River by as much as 20 to 75%. However, the wetland would also be a significant sink for nutrients, suspended solids, and total mercury, decreasing the amount of mercury available for methylation downstream.

Watershed Restoration and Flood Control along Steamboat Creek Using Constructed Wetlands

Channel restoration along Steamboat Creek through revegetation and construction of wetlands has the potential to significantly improve water quality. Agricultural and stormwater runoff from the watershed feeding Steamboat Creek results in nonpoint source loadings of pollutants (e.g., nitrogen, phosphorus, and suspended solids) to the Truckee River. Steamboat Creek is also fed by geothermal hot springs as it passes through Washoe Valley. This results in high boron concentrations and increased total dissolved solids (TDS). Boron inhibits plant growth, which may have contributed to a loss of riparian vegetation along the stream channel. The loss of vegetation has resulted in bank erosion and increased total suspended solids (TSS). The combination of nutrients, TSS, boron, and TDS degrades water quality in Steamboat Creek, making it the most polluted tributary of the Truckee River. Channel revegetation will promote the uptake of nutrients, improved the stability of stream banks, and reduce erosion. A small-scale wetlands system was constructed and routinely monitored in order to quantify the effectiveness of wetlands to reduce nonpoint source pollution in Steamboat Creek. Results indicate that the system can effectively reduce nitrogen, phosphorus, and suspended solids. Typical removal efficiencies range from 65% to 99% for nitrate and nitrate, from 30% to 80% for ammonia, 20% to 60% for total phosphorus, 20% to 75% for orthophosphate, and 80% to 95% for suspended solids. Ultimately, data from this research project will be used to predict water quality improvements and develop preliminary design criteria for a large-scale wetlands system.