Hydrologic controls on water chemistry and mercury biotransformation in a closed river system: The Carson River, Nevada

The Carson River flows in a closed basin system and the total flow of the river water decreases downstream due to both evaporation and consumptive uses. This river system is fed primarily by snow pack in the Sierra Nevada during the winter, which flows down gradient following melting in spring and summer. Water loss through evaporation in the Carson River results in a downstream buildup of conservative elements such as Cl− and certain oxyanion forming elements including Se, Mo and W, which are known to interfere with the transformation of Hg within the S cycle. In addition to these naturally occurring hydrologic processes and the resulting affects on water chemistry, the Carson River Basin has been historically impacted by Au and Ag mining that used Hg amalgamation techniques. Contamination of Hg in the Carson River system is now well documented and published Hg concentrations in different environmental compartments are extremely high. In this study, hydrologically driven changes in water chemistry of the river system and the resulting effects on Hg cycling were examined. Results show that periods of low water flow correspond to high water pH (up to 8.3), relatively high concentrations of oxyanion forming elements (e.g., As, Se, Mo and W), and low Hg methylation potential in sediment. In contrast, periods of high flow bring about dilution, which results in lower pH (∼7), lower concentrations of oxyanion forming elements, but higher Hg methylation potential. Overall, changes in flow regimes likely affect rates of methyl-Hg (MeHg) production through a combination of factors such as high pH, which favors MeHg demethylation, and the occurrence of relatively high concentrations of Group VI oxyanions that could interfere with microbial SO4 reduction and MeHg production.