THERMAL GROUND WATER PLUME IMPACTS ON COLD WATER STREAMS FROM AGGREGATE OPERATIONS

The potential environmental impacts of below the water table aggregate extraction are increasingly becoming an issue during the review and approval of these operations. The water filled pits created by these extractions are subject to prolonged exposure to solar radiation in the summer, hence elevated temperatures, and freezing during the winter, hence depressed temperatures. These pits alter the temperature of the down-gradient ground water creating thermal plumes within the ground water flow system. If these pits are near ecologically sensitive streams, wetlands or rivers, there is potential for the discharging ground water to alter the stream temperature. When the impacts are sufficient, adverse effects on thermally sensitive fauna, such as cold-water fish, will result and habitat destruction can occur.

An extensive field investigation monitoring thermal plumes emanating from an aggregate pit in south-western Ontario has been carried out over the past 8 years. Surface water temperatures in the resulting pond are as high as 30 degrees Celsius (15 degrees above background ground water temperature). The resulting cool and warm ground water plumes can be delineated 150 to 200 m down-gradient of the pond. The large network of monitoring wells and thermistors, initially installed and monitored from 1993 to 1997, was updated and expanded in 2001-2002 to accommodate pit expansion and the installation of a pumping well to assess large scale aquifer properties. Our extensive data set is currently being used to calibrate and verify a 3-dimensional numerical model of the site. The new field instrumentation and changed pit geometry will allow a true model validation. Once validated, we will use the model to assess the thermal impacts of aggregate extraction, in a more general sense, on streams, rivers, and other ecologically sensitive areas, and to evaluate the key parameters that govern the movement and behaviour of these thermal plumes.