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.