The dynamic nature of the annual volume of water discharged down the Carson River over a 10-year period, which included a century flood and drought, was examined in order to gain a better understanding of mercury movement, biological availability, and exposure to waterbirds nesting at Lahontan Reservoir. Total annual water discharge directly influenced total mercury (THg) in unfiltered water above the reservoir and downstream of a mining area, whereas methyl mercury (MeHg) at the same site was negatively related to annual discharge. Annual water storage at Lahontan Reservoir in the spring and early summer, as expected, was directly related to annual Carson River discharge. In contrast to the findings from above the reservoir, annual MeHg concentrations in water sampled below the reservoir were positively correlated with the total discharge and the amount of water stored in the reservoir on 1 July; that is, the reservoir is an important location for mercury methylation, which agrees with earlier findings. However, unfiltered water MeHg concentrations were about 10-fold higher above than below the reservoir, which indicated that much MeHg that entered as well as that produced in the reservoir settled out in the reservoir.

Avian exposure to mercury at Lahontan Reservoir was evaluated in both eggs and blood of young snowy egrets (Egretta thula) and black-crowned night-herons (Nycticorax nycticorax). Annual MeHg concentrations in unfiltered water below the reservoir, during the time period (days 90–190) when birds were present, correlated significantly with mercury concentrations in night-heron blood, snowy egret blood, and night-heron eggs, but not snowy egret eggs. A possible reason for lack of an MeHg water correlation with snowy egret eggs is discussed and relates to potential exposure differences associated with the food habits of both species. THg concentrations in water collected below the reservoir were not related to egg or blood mercury concentrations for either species.

The initial phase of this study was a 2-year evaluation of the effects of Hg on fish-eating birds nesting at Lahontan Reservoir and Carson Lake and at reference areas in northeastern Nevada (Henny et al. 2002). In the first year (1997), reproduction of snowy egrets (Egretta thula) and black-crowned night-herons (Nycticorax nycticorax) was studied via the ‘‘sample egg’’ technique (Blus 1984) (i.e., evaluate reproductive success of each clutch in relationship to the total Hg [THg] concentrations in the sample egg collected from that clutch). Both species laid eggs with comparatively low THg concentrations (Henny et al. 2002). Most eggs had THg concentrations below 0.80 lg/g wet weight (ww), the putative threshold concentration at which reproductive problems might be expected (Heinz 1979; Newton and Haas 1988).

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.