GEOCHEMICAL MODELING OF CONTROLS ON DISSOLVED AS, MN, AND FE IN THE DEXTER PIT LAKE, TUSCARORA, ELKO COUNTY, NEVADA

The Dexter Pit Lake located outside of Tuscarora, Nevada is a mature mine pit lake which began filling in 1990. Lake water is near neutral (pH 7-9) and has low total dissolved solids, however, several elements periodically attain concentrations near or above EPA primary and secondary drinking water standards. Elements of interest include As (primary drinking water standard), Fe and Mn (secondary drinking water standards). The lake is in an open pit resulting from mining of a 40 Ma epithermal gold deposit characterized by a primary mineralogy of quartz and alkali feldspar with accessory sulfide minerals such as galena and chalcopyrite. The pit lake has been the focus of previous hydrologic studies which have investigated and modeled the local hydrodynamics. These studies indicate that the pit lake is a flow-through system, meaning that pit lake water may interact significantly with surrounding ground and surface waters.

Current research is using geochemical data combined with hydrodynamics to model the controls on dissolved As, Mn, and Fe. Preliminary data analysis suggests that the dissolved concentrations of these elements are controlled predominantly by reductive dissolution, oxidative precipitation, and surface adsorption. Seasonal stratification develops during spring and summer and results in the formation of an oxidized epilimnion and a reduced hypolimnion. Annual overturn of lake water in early winter most likely results in the oxidative precipitation of dissolved Mn and Fe from the hypolimnion, which in turn removes As from solution through adsorption. Following this trend of decreasing dissolved concentrations, thermal and chemical stratification is again induced, causing subsequent increases in dissolved concentrations of As, Mn and Fe (from 6 to 17, 150 to 350, and 150 to 300 µg/L respectively) in the hypolimnion. This cycle occurred in both 1999 and 2000. Numerical modeling of the Dexter Pit Lake system will focus on quantifying the controls of adsorption and mineral precipitation reactions on the concentration of dissolved species. Modeling will use either PHREEQC or DYRESM-CAEDYM to link chemical and hydrologic processes and produce predictions of pit lake chemistry. Statistical analyses such as principal component analysis will also be applied to test for subtle trends in the data.