Modeling the fate of mercury in the Carson River and Lahontan Reservoir, Nevada

The discovery of the Comstock Lode in western Nevada in 1859 initiated use of mercury to remove gold and silver from crushed ore. Today, Hg is present in the historic mill tailings, deposits adjacent to the Carson River, and in Lahontan Reservoir. Three computer models were used to model the fate of mercury in the Carson River and Lahontan Reservoir. The RIVMOD hydrodynamic model was used to route flows from Carson City, through a 115 km section of the river and reservoir. Modifications to the RIVMOD computer code were necessary to better represent the Carson River. Model predictions show excellent agreement with observed downstream flow and reservoir stage for the 1986 water year. Since most of the Hg in water is associated with particulates, sediment transport was quantified by linking the RIVMOD model with the WASPS5 water quality model. Sediment rating curves were used with sediment transport equations to attain reliable predictions of sediment transport. Model simulations predicted the long term transport of sediment into Lahontan Reservoir and the sediment load into Lahontan Reservoir during the 1986 water year. Mercury methylation and demethylation experiments were done using sediment collected from five sites in the river and reservoir. Experiments show that methylation decreases and demethylation increases in Lahontan Reservoir when compared with the Carson River and that methylation and demethylation are positively correlated with temperature. The MERC4 computer model was used to predict water column Hg concentrations. Predicted methylmercury concentrations were matched to measured Hg water column data collected in May and June, 1994 by adjusting the diffusion rate of Hg from sediment. Inorganic Hg concentrations were not matched through any model calibration. The model was tested using a data set from June, 1995. Methylmercury concentrations were closely predicted in the river and reservoir using all data sets. Inorganic Hg concentrations were low using the mean Hg concentrations from the channel bank but were closer using twice the standard deviation of the mean bank concentrations. The prediction of MeHg concentrations shows that no diffusion of Hg from Lahontan Reservoir bed sediments, and MeHg diffusion only from organic rich bed sediments adjacent to the river channel, appear to be valid assumptions.