Elevated concentrations of nutrient in Steamboat Creek make it the most polluted tributary of the Truckee River. Along with high nutrient loads, high mercury (Hg) concentrations have also been documented in both creek water and sediments. The reduction of nutrients in the creek is an important factor in reducing the potential for eutrophication in the lower Truckee River. Construction of wetlands along the creek has been proposed as one method to improve the water quality by reducing nutrients and sediments from non-point pollution sources. Although wetlands are considered to be a significant sink for total Hg and nutrients, they may also be potential sites for methyl mercury (MeHg) production. MeHg is a known neurotoxin, and is readily bioaccumulated by aquatic organisms. Thus, the presence of elevated concentrations of Hg in the creek water and sediments raises concerns regarding the use of constructed wetlands because they could increase inputs of MeHg to the lower Truckee River.
In order to quantify the nutrient removal and the potential risk of MeHg production in a wetland system along the creek, a pilot-scale wetland system was constructed and monitored for several years along the creek at the Truckee Meadows Water Reclamation Facility (TMWRF) in Sparks, Nevada. Ten parallel wetland mesocosms with four different experimental designs were utilized to assess seasonal variations in the removal of nutrients, total suspended solids (TSS), and total mercury (THg), as well as the net production of MeHg. Experimental designs included: (1) both water and sediments with elevated concentrations of Hg; (2) water with elevated concentrations of Hg and clean sediments; (3) clean water and sediments with elevated concentrations of Hg; and (4) clean water and clean sediments. The effects of environmental conditions (i.e., drying and wetting, changing hydraulic retention times (HRTs), and high sulfate and nitrate loadings) on the production of MeHg and nutrient removal were also evaluated. Furthermore, a simulation model, WWQM (Wetlands Water Quality Model), was developed to evaluate nitrogen, phosphorus, and sediment retention in the constructed wetland system.
Experimental results suggested that no seasonal variation in THg removal was observed with any of the experimental designs throughout this study. Seasonal variations in MeHg production were observed for designs containing Hg contaminated sediments, with values peaking in the summer; other designs did not show any consistent seasonal variations. Sulfate and temperature were determined to be the most critical parameters driving the production of MeHg in all designs.
The manipulation of environmental conditions by drying and wetting along with the variation in flow rates that would be expected to occur seasonally in wetland systems located in arid regions were shown to influence the net production of MeHg and removal of nutrients and total suspended solids. Drying followed by rewetting temporarily increased the concentrations in the effluent due to flushing of the MeHg that accumulated in sediments. Sulfate concentrations were present above optimum range required for Hg methylation year round; therefore, Hg methylation was not influenced by further increase in sulfate concentration. Also, the nitrate inputs were not high enough to show any inhibitory effects on Hg methylation. Longer HRTs (reduced flow during a dry water year) improved water quality by reducing nutrients and TSS concentrations, whereas, shorter HRTs (high flow during storm events) impaired water quality within the wetlands.
The WWQM model which was developed predicted nutrient removal and TSS retention reasonably well and results were consistent with the experimental data collected from the pilot-scale wetland system. Model results revealed that construction of wetlands along the creek could remove nitrogen, phosphorus, and TSS by approximately 62%, 38%, and 84%, respectively, which would reduce the risk of eutrophication in the lower Truckee River and the terminal Pyramid Lake.