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 that the Carson River supplies most Hg in the water and upper sediments, with 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 removing the remainder. Losses due to volatilization are negligible. The amount of methylmercury (MeHg) in the reservoir can be accounted for by inputs from the Carson River with minimal methylation occurring in the reservoir. The lack of species conversion and high retention rate appear to be due to the unreactive mineralogy of particulate Hg. Thus, we suggest that Hg dynamics are similar to that of other highly particle-reactive metals where fate is determined by particle movement. Finally, model results suggest an additional source of Hg to the system, which we hypothesize is from deep contaminated sediment that enters the system through sediment mixing caused by seasonal wet and dry cycles and sediment resuspension.