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Heat Under Your Feet Identifies Groundwater Flow
ByWIREs
Authors
In many cases, standard approaches for
analysing temperature-depth profiles to trace groundwater flow are now invalid
because climate change has warmed the land surface and created an inversion in
the geothermal gradient.
Water is one of our most precious resources.
The study of water resources interfaces with many other research fields in
engineering, geoscience, and marine and environmental sciences.
Groundwater discharge (flow from aquifers) to
rivers, lakes, and the ocean impacts their water volumes, thermal regimes, and
biogeochemistry, and thereby alters the functioning, productivity, and
diversity of aquatic ecosystems.
Also, groundwater recharge (downward flow to
aquifers) represents the hydrologic input to groundwater flow systems that
determines how much water can be sustainably pumped from an aquifer without
depleting groundwater resources or causing unacceptable reductions in
groundwater flow to surface water bodies.
Quantifying these groundwater flows is a critical
task facing aquatic scientists and water resources engineers.
Accurately measuring these groundwater fluxes
has been a persistent challenge since water flows can often not be measured
directly.
Groundwater scientists often employ
geochemical methods to estimate groundwater flows, but these are subject to
several limitations and often involve expensive laboratory analyses.
In contrast, heat is a naturally occurring
groundwater tracer that is ubiquitous in the subsurface and relatively
inexpensive to continuously monitor.
When groundwater flows, it causes variations
to groundwater temperatures occur. These variations can be used to quantify
water flows.
Despite the widespread utility of thermal
groundwater tracing methods, their uptake has been limited due to a lack of
understanding of the potential applications of the different methodologies.
Barret Kurylyk (Dalhousie University,
Canada), Dylan Irvine (Flinders University) and Victor Bense (Wageningen
University) provide a detailed of review of the theory, data collection,
and data analysis methods for using temperature-depth profiles to trace
vertical groundwater flows in their WIRES Water review.
They review applications of these methods for
studying groundwater-surface water exchanges, climate reconstructions, deep
groundwater flow systems, and fresh groundwater discharge at the seafloor.
In many cases, standard approaches for
analysing temperature-depth profiles to trace groundwater flow are now invalid
because climate change has warmed the land surface and created an inversion in
the geothermal gradient.
Temperature-depth profile curvature that is
caused by surface warming can be falsely attributed to groundwater flow
impacts.
However, recently developed methods account
for the combined thermal perturbations of groundwater flow and climate change,
and these can be applied in boreholes that have been ‘thermally contaminated’
by climate change.
A key take home message of this paper is that
temperature-depth profiles can be easily measured and analysed, and that these
analyses can yield vertical groundwater fluxes for a range of hydrologic
environments and spatiotemporal scales.
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