MODELING PHYSICAL AND BIOGEOCHEMICAL PROCESSES IN DEXTER PIT LAKE, TUSCARORA, NEVADA

The physical and biogeochemical characteristics of a mine pit lake in arid northern Nevada were examined during a two year study. These data are used to calibrate a model that describes water quality in pit lake systems. Once the model is calibrated, it will be used to predict water quality for future time periods or assess the effect of changes in the system. Dexter Pit Lake is neutral to slightly basic with conductivities of 200-350 µS/cm. A seasonal creek provides snowmelt to the pit lake in spring. Elevated temperatures and high concentrations of Cl and NO₃ in the bottom water during periods of ice cover suggest groundwater inputs. The water column of the pit lake stratifies during summer and late fall. Concentrations of dissolved O₂ decrease, whereas concentrations of dissolved Mn, Fe, and As increase in the bottom water during the stratification period. Depending on the particular year, bottom water either becomes anoxic and sulfidic or approaches anoxia during the latter stages of stratification.

The Dynamic Reservoir Simulation Model (DYRESM) and Computational Aquatic Ecosystems Dynamics Model (CAEDYM) are used to describe coupled physical and biogeochemical processes in the pit lake. Information on bathymetry, meteorological forcing parameters, spatial and temporal distributions of temperature and salinity, and compositions of surface inflow and groundwater are used in DYRESM to model mixing processes (including lake turn over), to constrain the hydrologic balance, and to quantify the relative importance of surface and groundwater inflow, precipitation, and evaporation in controlling the concentrations and distributions of conservative elements (major ions, B, Li, Sr, and U). DYRESM-CAEDYM describes observations of geochemical changes in dissolved O₂, Mn, and Fe in the bottom water during stratification. In addition to physical processes, O₂ concentrations are modeled by considering exchange across the air-water interface, demand by the sediments, and consumption during organic matter diagenesis in the water column. The model considers physical processes, release from the sediments, and transitions between oxidized and reduced forms to describe observed concentrations of Mn and Fe. Future work will focus on modeling spatial and temporal distributions of dissolved As.