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Approximately 5.5 x 10(9) g (4.0 x 10(5) 1) of mercury was discharged into the Carson River Drainage Basin of west-central Nevada during processing of the gold- and silver-rich Comstock ore in the late 1800s. For the past 13 decades, mercury has been redistributed throughout 500 km(2) of the basin, and concentrations are some of the highest reported values in North America This article documents the concentration of mercury in the air, water, and substrate at both contaminated and noncontaminated sites within the basin and discusses the implications for risk assessment. At contaminated areas, the range of mercury concentrations are as follows: mill tailings, 3-1610 mu g/g; unfiltered reservoir water, 53-591 ng/l; atmospheric vapor, 2-294 ng/m(3). These values are three to five orders of magnitude greater than natural background. In all media at contaminated sites, concentrations are spatially variable, and air and water mercury concentrations temporally. The study area is situated in a natural mercuriferous belt, and regional background mercury concentrations in all environmental media are higher than values typically cited for natural background. As a mercury-contaminated site in North America, the Carson River Drainage Basin is unusual for a number of reasons, including its location in a natural mercuriferous belt, high and sustained levels of anthropogenic mercury inputs, long exposure time, aridity of the climate, and the riparian setting in an arid landscape, where biological activity is concentrated in the same areas that contain high levels of mercury in multiple media.