Thermal alteration of groundwater caused by seepage from a cooling lake

Abstract

Groundwater temperatures in the vicinity of a 200‐ha power plant cooling lake in central Wisconsin were monitored in the field for 1 year, and the response of subsurface temperatures to seasonal changes in lake and air temperatures was simulated by means of a mathematical model. The cooling lake, which has been in use since May 1975, when a 500‐mw electric generating unit began operation, loses water to the groundwater system at a rate of 2 × 104 m3/d. The zone of thermally altered groundwater is confined to a relatively small area hydraulically downgradient from the cooling lake. However, the lake is situated in a groundwater discharge area, and changes in subsurface temperatures at depths less than 6 m are believed to have affected the vegetation in the thermally altered zone. The model, which couples equations describing groundwater flow with those describing heat transport in the subsurface, was used to simulate the seasonal temperature fluctuations within seven cross sections oriented parallel to the direction of groundwater flow away from the cooling lake. Simulated temperature patterns agreed well with field data but were very sensitive to the distribution of subsurface lithologies. Results from a predictive simulation suggest that when a second 500‐mw generating unit begins operation in 1978, groundwater temperatures will increase less than 5°C at distances greater than 15 m from the cooling lake. The results of this study suggest that the potential for significant thermal alteration of surface water bodies located in groundwater discharge areas is slight.