Simulation of mercury transport and fate in the Carson River, Nevada

In 1991 the US EPA designated the Carson River as part of a Superfund site due to its high level of mercury contamination. Water column total mercury concentrations have been observed as high as 61 m gl 1 by the USGS, while some fish have methyl-Hg concentrations four times greater than the human health limit for consumption 1 ppm (1 m gg 1 ). Hg river bank concentrations are extremely high (avg. 64,242 g Kg 1 ) and more than 95% of the total Hg in the water column is associated with particles. The importance of bank erosion on the transport and fate of mercury in this system is therefore apparent. Using the US EPA RIVMOD, WASP5 and MERC4 numeric codes, this study focuses on program modifications that predict bank erosion rates and Hg bank concentrations related to longitudinal slope. Calibration is performed for both Hg and methyl‐Hg water column concentrations using data collected during medium flows. Contribution of MeHg from bank erosion is calibrated with data collected during a high flow event. Subsequent verification is accomplished using data from low and high flow regimes. The model simulates inorganic mercury (n15) with a root mean squared error (RMSE) of 490 ng l 1 . The average error is 21% with a bias of under prediction by 88 ng l 1 . Methylmercury concentrations (n 13) are simulated with a RMSE of 0.90 ng l 1 . This corresponds to an average error of 32% and tendency to over predict MeHg by 0.14 ng l 1 . Post 1997-flood data, collected during a low flow regime, indicate that the system may have significantly changed in terms of mercury concentrations in the channel bottom sediments. © 2000 Elsevier Science B.V. All rights reserved.