Abstracts

186 - A Determination of the Lower Limit of Vertical Fluxes that
can be Quantified Using Temperature Profiles in Low Permeability
Streambeds
Michael O. McBride 1 & Brewster Conant Jr. 2
1
WorleyParsons Canada, Calgary, Alberta, Canada
2
Department of Earth and Environmental Sciences - University of Waterloo, Waterloo,
Ontario, Canada
Vertical profiles of temperature in streambeds can be modeled to quantify exchanges of
water between groundwater and surface water, but in low permeability and low flux environments
the modeling can yield unreliable or non-unique results. This study was undertaken
to determine the threshold flux below which the influence of advection water (and
heat) can no longer be reliably quantified, because it is so small relative to heat conduction
processes. Thermographs from 1-D vertical arrays of temperature dataloggers deployed in
the top 1.5 m of low permeability streambeds at three different field sites were simulated
using the USGS numerical heat transport model VS2DH and 1DTempPro graphical user
interface. A vertical upward flux of 3.4 x 10 -7 m/s (above the threshold value) was determined
for silty clay till in Logan Drain (Kintore, Ontario). Modeled best fit fluxes of 5 x
10 -8 m/s (marginally above the threshold) and ≤1 x 10 -8 m/s (at or below the threshold),
were determined for the clay streambed deposits of the South and Middle branches of the
Raisin River (near Cornwall, Ontario), respectively. Sensitivity analyses using the South
Branch of the Raisin River data indicated simulated streambed temperature profiles were
practically identical (i.e., non-unique) for specific discharges below 1 x 10 -8 m/s. Although
threshold flux values were site specific and found to be a function of specific discharge,
depth of streambed temperature measurement, and sediment thermal properties, the value
of 1 x 10 -8 m/s was about the minimum reliable flux estimate achievable when using
dataloggers with accuracies of 0.1°C. The threshold flux values had thermal Péclet numbers
(tPe) less than 1 (tPe=1 when contributions to energy transport from advection and
conduction are equal) indicating the Raisin River streambed sites were indeed conduction
dominated. Additional simulations showed that the threshold flux occurs at a flux corresponding
to about tPe=0.1 and so between values of 0.1 and 1.0, advection can still be
quantified. To ensure one can obtain the lowest possible threshold value at a site, high
resolution and accuracy temperature dataloggers should be deployed as deep as feasibly
possible (within the active thermal zone, or at least 1 m deep), both within the zone of
diurnal fluctuations and below to provide optimal transient temperature data to calibrate
the model to. It appears that shallow streambed temperature profiles (≤ 1.5 m) cannot be
used to reliably determine flow directions or magnitudes of fluxes below about 1 x 10 -8 m/s.
IAH-CNC 2015 WATERLOO CONFERENCE
157