INFLUENCE OF HYPORHEIC ZONE DYNAMICS ON MERCURY GEOCHEMISTRY OF THE CARSON RIVER, NEVADA

In an on-going investigation of the impact of historic contamination of the Carson River with mercury (Hg) via gold/silver mining activities, we present data related to the potential remobilization of Hg during river-riverbank interaction. We sampled a series of three wells at ~1.5, ~8 and ~30 meters perpendicular to the river and ~40 Km downstream from the initial contamination input. Samples were obtained in July 2001 during relatively low flow and in June 2003 during a higher flow period. Chloride (Cl^-) concentrations were higher in the well (i.e. hyporheic zone) samples than in the river on both sampling occasions, but δD and δ¹⁸O measurements in 2003 indicate that the higher Cl^- values are not due to evapoconcentration, but rather to the leaching of Cl^- from the arid soils. Total Hg is always higher in the hyporheic zone but dissolved Hg and total methyl-Hg are only substantially higher than the river water at low flow conditions. These data suggest that the hyporheic zone is an important component in understanding the biogeochemistry of Hg in the river, and Hg concentrations and speciation there are driven by variations in river dynamics. The data also indicate that the riverbanks may not be as strong a source of methyl-Hg to the river as we had originally hypothesized. However even after ~140 years, the mine wastes incorporated into the floodplain and bank sediment and soils clearly continue to be a source of Hg to downstream aquatic environments.