Thermal Impact Assessment of Below-Water-Table Aggregate Extraction at the Strada Aggregates Hillsburgh Pit, Hillsburgh, ON.

Strada Aggregates operates a sand and gravel pit located northeast of the Village of Hillsburgh, ON. The Hillsburgh Pit license permits above-water-table extraction of sand and gravel from a 32.7 ha area and below-water-table extraction from a pond with a projected surface area of 9.6 ha when complete. The below-water-table extraction is to progress in four stages, from north to south, to a depth of 10 m.

Extraction of sand and gravel from the Stage 3 area was underway when a thermal plume emanating from the on-site pond was identified and concerns were raised regarding potential thermal impacts to a trout hatchery located on the neighbouring property. Strada ceased the below-water-table extraction but continued monitoring of groundwater levels and temperatures as per license requirements. Earthfx was retained to conduct a detailed review of site conditions and develop a groundwater flow and heat transport model to assess the potential for off-site thermal impacts due to below-water-table aggregate extraction at the Hillsburgh Pit.

Using available on-site and regional borehole data, Earthfx constructed detailed 3-D stratigraphic and hydrostratigraphic models for the study area. Most significantly, the stratigraphic model mapped a buried Catfish Creek Till drumlin that strongly affected groundwater flow east of the site.
Earthfx constructed a distributed hydrologic model for the surrounding watershed using the USGS PRMS code and calibrated it to observed streamflow. The PRMS model simulated 40 years of continuous climate data and determine long-term average groundwater recharge rates. Earthfx constructed an eight-layer MODFLOW-NWT groundwater flow model based on the hydrostratigraphic model. The steady-state groundwater flow was calibrated to match average groundwater levels at on and off-site monitors. The model represented flow in the shallow bedrock and overburden and represented groundwater interaction with all streams, wetlands, and lakes in the study area including the on-site pond.

Groundwater velocities and seepage rates from streams and lakes were used in the MT3DMS transport model. Observed monthly variations in temperatures in recharge and the on-site pond were supplied to represent thermal inputs to the groundwater system. The model was calibrated to observed temperatures in on-site monitors and was able to reproduce the sinusoidal response in the wells and the effects of thermal retardation and attenuation. In particular, the model was able to reproduce the stratification of temperature response to thermal inputs from recharge and the on-site pond.

Once the models were calibrated, the groundwater flow and thermal transport models were applied to predict off-site impacts due to expansion of the on-site pond to mine out remaining aggregate in the Stage 3 area. Thermal modelling confirmed that (1) natural seasonal fluctuations in the temperature of recharging meteoric water, and (2) transient lateral movement of the thermal plume from the on-site pond create two different and interacting temperature signals which are observed in the on-site boreholes. These processes occur at different locations in the vertical profile, with the natural seasonal fluctuations affecting the shallow conditions (as evidenced in the monitoring wells and springs), and the pond creating a deeper thermal plume. The two processes also create offset patterns that reflect different seasonal lag and attenuation patterns. The thermal modelling provides a good match to these processes and helps in the identification and understanding of different effects, particularly at a distance from the pond, where the thermal effects are small and difficult to differentiate. Model results predicted that temperatures at the edge of the property would increase by approximately 0.12°C, with the peak temperature arriving about 1 month earlier than under existing pond conditions. At the second offsite receptor no change in temperature (timing or magnitude) was predicted as a result of the full build-out of the Stage 3 pond.