An integrated approach to the determination of the quantity, distribution, and dispersal of mercury in Lahontan Reservoir, Nevada, USA

Abstract

This paper describes an integrated approach to the assessment of Hg contamination in Lahontan Reservoir, Nevada. The study shows that combining 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. Geochemical data revealed that the chemical variability within the individual stratigraphic units of the reservoir is much less than the between-unit variability. This allowed the spatial distribution of Hg, Au, and Ag to be precisely determined by geologic mapping of the individual stratigraphic units. A total of nine stratigraphic units were defined from four sedimentary environments: eolian, coastal, deep-water, and deltaic. The deep-water and deltaic sediments, which comprise approximately 55% of the reservoir's area, contain the bulk of the Hg, Au, and Ag. The geologic and geochemical data also show that spatial variations in metal concentration are influenced by a complex set of parameters including vertical sediment mixing by shrink-swell processes, dilution by non-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, show that approximately 3.28 × 10 5 kg of Hg. 2.253 kg (59,600 oz) of Au, and 117,933 kg (3,120,000 oz) of Ag reside within the post-1915 lacustrine sediments. At current market prices these metals are valued at approximately $ 35 million. Recovery of Au and Ag during site remediation would help defray the cost of clean-up operations.