THE EFFECTS OF GROUNDWATER ON THE PHYSICAL LIMNOLOGY OF A FUTURE PIT LAKE PREDICTED USING DYRESM, MARTHA MINE, NEW ZEALAND

The water quality of pit lakes created by open pit mining is an important environmental issue in mountain watersheds. Among the factors that affect pit lake chemistry is the physical limnology of the lake. The 1-D hydrodynamic model DYRESM was used to predict the physical limnology of a proposed pit lake in the Martha epithermal Au-Ag mine, Waihi, New Zealand, as the lake fills over time. The mine will close in 2007, and the pit will be filled with river water over a 5-year period to create a lake approximately 200 m deep. The final surface elevation will be fixed by an outlet to an adjacent stream. DYRESM uses lake morphology, daily climate conditions, and the volume, temperature and salinity of daily inflows to predict the temperature, salinity, and density of a lake with depth and time. The Martha lake prediction considered 5 years of lake filling plus 1 year of steady-state discharge. Groundwater was added from a single point at the bottom of the pit, whereas river water, pit-wall runoff and seepage were added to the lake surface. A pool of river water 15 m deep was used as an initial condition. The predicted limnology was very sensitive to the temperature of the groundwater input. Groundwater temperatures of 17 °C or less produced a permanently stratified pit lake with a strong density gradient at 140 m depth, whereas groundwater temperatures of 20 °C or more produced annual, whole-lake circulation. Predictions are likely to be more sensitive to variations in groundwater salinity than temperature, as small salinity changes have a greater effect on water density than small temperature changes. These findings show it is important to characterize both the temperature and the salinity of groundwater inputs to pit lakes in addition to input rates and discharge points, particularly for pit lakes that are artificially flooded with surface waters, as the density contrast between surface waters and groundwater may induce permanent stratification.