HEAT TRANSPORT AND THERMAL POLLUTION IN POROUS AND FRACTURED AQUIFERS

Recent availability of simple and reliable temperature measurement devices, and the advancement of 3-D heat transport models have given rise to increased interest in subsurface heat transport at the basin, watershed, and stream reach scale. Here we look at the results from two studies of thermal energy transport. One study considers the potential for thermal pollution from aggregate extraction pits in unconsolidated porous media and the other looks at transport of thermal pollution within fractured sedimentary rock.

An extensive field investigation monitoring thermal plumes emanating from an aggregate pit in Southwestern Ontario has been carried out over a period of eight years. The pit lies within a shallow unconfined aquifer formed by a deposit of glacial fluvial outwash sand and gravel. The hydraulic conductivity, determined using multi-scale sampling methods (grain size, laboratory permeameter tests, insitu slug and pumping tests, and cross-hole GPR), reveals a 3 order of magnitude difference in estimates of hydraulic conductivity depending upon the scale of the investigation. We measured the thermal conductivity of the solid portion of the porous medium in the laboratory, as well as aquifer porosity in the field using crosshole GPR surveys; these data were used to estimate the effective thermal conductivity of the variably saturated porous media.

We simulated thermal energy transport through the aquifer using the three-dimensional heat transport code HEATFLOW (Molson and Frind, 2005). Results show that thermal pulses of alternating warm and cold water persist for up to 200 m in the shallow aquifer and thus can potentially impact the temperature-sensitive ecosystems of nearby streams.

In the second study, transport of thermal plumes, emanating from water filled quarries, is studied within a fractured sedimentary rock aquifer. Although thermal conduction rapidly attenuates temperature differences between the fluid in the matrix and the fractures, the results show that thermal impacts from quarries can again persist to significant distances downgradient.