MERCURY PATHWAYS IN THE CARSON RIVER-LAHONTAN RESERVOIR SYSTEM, NEVADA, USA

Concentrations of mercury (Hg) were determined in surface waters and a ssociated river bank sediment samples in a river-reservoir system cont aminated by mine wastes. The distribution of total and methyl Hg in su rface waters along the Carson River was similar to that measured in ri ver bank sediments and influenced by how regimes. High levels of Hg (u p to 7,585 and 7.2 ng Hg/L for total and methyl Hg, respectively) dete rmined on surface water samples were in large part discharged from Hg- contaminated tailings, distributed in the river bank sediments. Once i ntroduced into the river during the spring snowmelt runoff, Hg was tra nsported downstream and accumulated in the lacustrine part of the syst em. Elemental Hg (Hg-o) increased from 0.02 ng/L in the noncontaminate d region to about 2 ng/L in the reservoir. The vertical distribution o f total methylmercury (MeHg(T)) in water of the reservoir differs from that observed elsewhere, in both Hg-contaminated and noncontaminated lakes. The highest levels of MeHg(T) (<1 ng/L as Hg) and acid-reactive Hg (4 ng/L) were observed in the alkaline and oxic surface waters. Th e decrease of pH with depth and the absence of oxygen in depth >10 m d id not enhance MeHg production. In the anoxic hypolimnion of the reser voir, the recycling of MeHg(T) was more influenced by the redox cyclin g of Mn. The addition of group VI anions (SeO42-, MoO42-, and WO42-) i n the range of concentrations of oxyanion-forming elements found in th e Carson River system to anoxic sediment slurry spiked with SO42- resu lted in the reduction of rates of MeHg production. Their negative effe ct on MeHg production was enhanced by increasing pH. Group VI anions, analogous to SO42- are inhibitory to sulfate-reducing bacteria, which are known to play a key role in MeHg production in anoxic sediments. A ccordingly, the particular water geochemistry of the Carson River syst em could partly explain the observed low levels of MeHg where one woul d expect higher concentrations.