The Carson River is a vital water resource for local municipalities and migratory birds travelling the Pacific Flyway. Historic mining practices that used mercury (Hg) to extract gold from Comstock Lode ore has left much of the river system heavily contaminated with Hg, a practice that continues in many parts of the world today.
Between 1997 and 2013, the United States Geological Survey (USGS) collected and analyzed Carson River water for Hg and methylmercury (MeHg) concentrations resulting in a sixteen-year record of unfiltered total mercury (uf.THg), filtered (dissolved) Hg (f.THg), total methylmercury(uf.MeHg), filtered MeHg (f.MeHg), and particulate-bound THg (p.THg) and MeHg (p.MeHg) concentrations. This represents one of the longest continuous records of Hg speciation data for any riverine system, thereby providing a unique opportunity to evaluate long-term trends in concentrations and annual loads. During the period of analysis, uf.THg concentration and load trended downward at rates of 0.85% and 1.8% per year, respectively. Conversely, the f.THg concentration increased at a rate of 1.7% per year between 1998 and 2005, and 4.9% per year between 2005 and 2013. Trends inflow-normalized partition coefficients for both Hg and MeHg suggest a statistically significant shift from the particulate to thefiltered phase. The upwardly accelerating f.THg concentration and observed shift from the solid phase tothe aqueous phase among the pools of Hg and MeHg within the river water column signals an increased risk of deteriorating ecological conditions in the lower basin with respect to Hg contamination.
More broadly, the 16-year trend analysis, completed 140 years after the commencement of major Hg releases to the Carson River, provides a poignant example of the ongoing legacy left behind by gold and silver mining techniques that relied on Hg amalgamation, and a cautionary tale for regions still pursuing the practice in other countries. EPA funded work from 1997 through 2013.
Samples were collected at the Weeks Bridge location from April 1997 through February 1999, and at seven other Carson River locations from April 1998 through February 1999, and at Weeks Bridge and below Lahontan Reservoir from 2000 through 2013. Assist EPA in studying the transport of mercury-contaminated water and sediment in the Carson River system in western Nevada. The study included periodic measurements of streamflow, and the collection and analysis of water samples for total mercury, methylmercury, and suspended sediment at two Carson River locations. The study also included field measurements and the collection and analysis of samples from the north basin of Lahontan Reservoir.
EPA used the data to determine loadings of the dissolved and particulate fractions of total mercury and methylmercury into and from Lahontan Reservoir, to better understand sources and transformations of mercury in the Carson River system, evaluate the effects of reservoir stratification on methylmercury production, and ultimately to help determine whether remedial action is warranted to reduce the levels of mercury contamination in and near the Carson River. A journal article from Eric Morway, Carl Thodal and Mark Marvin-DiPascuale, USGS, summarizing the findings is entitled "Morway_Thodal_MMD_HgMeHg_2017.pdf". This study, a component of the Remedial Investigation for the Carson River Superfund Site, included collection and analysis of water samples for total mercury and methylmercury from Weeks Bridge above Lahontan Reservoir, located on the Carson River. The USGS Water Science Center has collected and analyzed water samples in the Carson River system through an Interagency Agreement with EPA. The EPA is publishing this data in support of the Carson River Mercury NPL Site in Nevada.
Data was compiled and evaluated for the OU2 Remedial Investigation Report (EPA, 2017), which describes the nature and extent of contamination from the Site. The report contains the Human Health Risk Assessment and Ecological Risk Assessment. Literature and other source Hg data are summarized in the RI for surface waters, sediments, and biological tissues.
