Thermal plume transport from sand and gravel pits – Potential thermal impacts on cool water streams

Abstract

Summary We investigated the potential thermal impacts from below-water-table aggregate extraction on a cool-water stream supporting Brook trout (Salvelinus fontinalis) and cool-water macroinvertebrates. We monitored thermal plumes emanating from an aggregate pit through an unconfined, glacial-outwash aquifer. Our objectives were to complete a detailed assessment to quantify the persistence of thermal plumes in the subsurface, and establish a framework for guiding these investigations as the growing demand for aggregate increases pressures to pursue extraction in ecologically sensitive areas. During a 10-year period, we measured ground and surface water temperatures in an outwash aquifer and cool-water stream, including two periods of intensive monitoring (22 months and 2.5 years) focusing on plume movement from one aggregate pit. We quantified the aquifer hydraulic conductivity K at the laboratory and field scale, and characterized the effective thermal conductivity λ at an unprecedented level of detail. The mean K’s from the multi-scale tests span two-orders of magnitude, 1.8 × 10−4 to 1.7 × 10−2 m s−1, and are related to the test support volume. The saturated λ has a mean of 2.42 W m−1 K−1, ranges from 2.14 to 2.69 W m−1 K−1, and is correlated to stratigraphic units (gravel, sand, and till). The annual temperature amplitude in the pit is 10 °C above up gradient ground water, and our results show that alternating warm and cool plumes persist in the aquifer for 11 months and migrate up to 250 m down gradient. The observed plume velocity (1.2 m d−1) lags the ground water velocity (2.8 m d−1) due to thermal retardation. Furthermore, hydraulic conductivity is shown to vary with the scale of the test and ground water velocities estimated from pumping tests may overestimate thermal plume velocities. While we focused on plume migration, our results demonstrate that assessing impacts on the aquatic community requires an integrated, multi-disciplinary study. This work can guide such assessments.