AN INTEGRATED APPROACH TO THE DETERMINATION OF THE QUANTITY, DISTRIBUTION, AND DISPERSAL OF MERCURY IN LAHONTAN RESERVOIR, NEVADA, USA

This paper describes an integrated approach to the assessment of Hg co ntamination in Lahontan Reservoir, Nevada. The study shows that combin ing geomorphic and geochemical information allows a minimal amount of geochemical sampling to achieve a maximum understanding of contaminant quantity, distribution, and movement within the reservoir. Geochemica l data revealed that the chemical variability within the individual st ratigraphic units of the reservoir is much less than the between-unit variability. This allowed the spatial distribution of Hg, Au, and Ag t o be precisely determined by geologic mapping of the individual strati graphic units. A total of nine stratigraphic units were defined from f our sedimentary environments: eolian, coastal, deep-water, and deltaic . The deep-water and deltaic sediments, which comprise approximately 5 5% of the reservoir's area, contain the bulk of the Hg, Au, and Ag. Th e geologic and geochemical data also show that spatial variations in m etal concentration are influenced by a complex set of parameters inclu ding vertical sediment mixing by shrink-swell processes, dilution by n on-contaminated materials, the rate of metal influx to the reservoir, and the sorption potential of the sediments. Mass balance calculations based on cartographic, bulk density, and metal concentration data, sh ow that approximately 3.28 x 10(5) kg of Hg, 2,253 kg (59,600 oz) of A u, and 117,933 kg (3,120,000 oz) of Ag reside within the post-1915 lac ustrine sediments. At current market prices these metals are valued at approximately $ 35 million. Recovery of Au and Ag during site remedia tion would help defray the cost of clean-up operations.