Evidence for sites of methylmercury formation in a flowing water system: impact of anthropogenic barriers and water management.

The Truckee River, California-Nevada, USA is impacted by mercury (Hg) contamination associated with legacy gold mining. In this work, we investigated the potential for hot-spots of methylmercury (MeHg) formation in the river. Mercury concentrations in multiple media were also used to assess the impacts of anthropogenic barriers, restoration, and water management in this flowing water ecosystem. Water samples were collected on a seasonal time step over 3 years, and analyzed for total Hg (THg) and MeHg concentrations, along with a variety of other water quality parameters. In addition, we measured THg and MeHg in sediments, THg in macroinvertebrates, and THg and δ(15)N and δ(13)C concentrations in fish. Differences in stable isotopes and Hg concentrations in fish were applied to understand the mobility of fish in the river. Mercury concentrations of specific macroinvertebrate species were used to identify sites of MeHg production. In general, loads of Hg and nutrients in the river reach above the Reno-Sparks metropolitan area were similar to that reported for pristine systems, while within and below the city, water quality impacts were observed. Fish isotope data showed that in the city reach food resources were different than those upriver and downriver. Based on Hg and isotope data, mobility of the fish in the river is impacted by anthropogenic obstructions and water manipulation. Below the city, particle bound Hg, derived from the legacy mining, continues to be input to the Truckee River. This Hg is deposited in riparian habitats and areas of river restoration, where it is methylated and becomes available to biota. During spring, when flows were highest, MeHg produced and stored in the sediments is mobilized and transported downriver. Fish and macroinvertebrate concentrations increased downriver indicating passive uptake from water. The information presented here could be useful for those doing river restoration and water manipulation in mercury contaminated environments.