Mercury dynamics in the Lahontan Reservoir, Nevada: Application of the QWASI fugacity/aquivalence multispecies model

The Lahontan Reservoir in western Nevada has among the highest mercury (Hg) concentrations recorded in water, sediments and biota. The QWASI fugacity/ aquivalence multispecies model was applied to examine Hg dynamics through a steady-state analysis of high loading conditions. The analysis indicated t hat the Carson River supplies most Hg in the water and upper sediments, wit h minimal inputs from the atmosphere and the Truckee Canal. Model estimates suggest that more than 90% of Hg entering the system from the Carson River at high flow is retained in the sediment of the reservoir, with export rem oving the remainder. Losses due to volatilization are negligible. The amoun t of methylmercury (MeHg) in the reservoir can be accounted for by inputs f rom the Carson River with minimal methylation occurring in the reservoir. T he lack of species conversion and high retention rate appear to be due to t he unreactive mineralogy of particulate Hg. Thus, we suggest that Hg dynami cs are similar to that of other highly particle-reactive metals where fate is determined by particle movement. Finally, model results suggest an addit ional source of Hg to the system, which we hypothesize is from deep contami nated sediment that enters the system through sediment mixing caused by sea sonal wet and dry cycles and sediment resuspension.