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Aquifers contain the largest store of unfrozen freshwater, making groundwater critical for life on Earth. Groundwater temperatures infl uence stream thermal regimes, groundwater-dependent ecosystems, aquatic biogeochemical processes, water quality, and the geothermal potential. Yet little is known about how groundwater responds to surface warming across spatial and temporal scales. We simulate current and projected groundwater temperatures at the global scale and show that groundwater at the depth of the water table is projected to warm on average by 3.3 ° C between 2000 and 2099 (RCP 8.5). However, regional groundwater warming patterns vary substantially due to spatial variability in climate and water table depth. The highest warming rates are projected in Central Russia, Northern China, and parts of North America and the Amazon rainforest. Results also show that by 2099, 234 million people are projected to live in areas where groundwater exceeds the highest threshold for drinking water temperatures set by any country.

Aquitard hydraulic properties are notoriously difficult to assess, yet accurate aquitard hydraulic conductivity estimates are critical to quantify recharge and discharge to and from semi-confined aquifer systems via hydraulic head gradients. Such flux quantification is required to evaluate the risks of aquifer exploitation by groundwater abstraction and aquifer vulnerability to surface contamination. In this study, we consider a regionally important aquitard and compare existing hydraulic conductivity estimates obtained through traditional methods to those inferred from long-term hydraulic head monitoring and thermally-derived vertical groundwater fluxes (0.04--0.25 m/y). We estimate the fluxes using numerical modeling to analyse the propagation of decadal climate signals into temperature-depth profiles and fitting the simulated and observed inflection point depths (minimum temperature). Results reveal that climate-disturbed temperature-depth profiles paired with multi-level head data can yield accurate vertical fluxes and aquitard hydraulic conductivities. This approach for characterizing groundwater systems and quantifying flows to and from sedimentary aquifers is more efficient but yields results that are comparable to conventional methods.

In inland settings, groundwater discharge is known to thermally modulate receiving surface water bodies and provide localized thermal refuges; however, the thermal influence of intertidal springs on coastal waters and the thermal sensitivity of these springs to climate change are not well studied. We addressed this knowledge gap with a field- and model-based study of a threatened coastal lagoon ecosystem in south-eastern Canada. We paired in-situ thermal and hydrologic monitoring with analyses of drone-based thermal imagery to estimate the discharge to the lagoon from intertidal springs and groundwater-dominated streams in summer 2020. Results, which were generally supported by independent radon-based groundwater discharge estimates, revealed that the combined summertime spring inflows (0.047 m3 s-1) were comparable to the combined stream inflows (0.050 m3 s-1). Heat flux analyses indicated that the net advection for the streams and springs were also comparable to each other but were two orders of magnitude less than the downwelling shortwave radiation across the lagoon. Although the lagoon-scale thermal effects of groundwater inflows were small compared to atmospheric forcing, spring discharge dominated heat transfer at a local scale, creating pronounced cold-water plumes along the shoreline. A numerical model was used to investigate seasonal and multi-decadal groundwater temperature patterns to relate measured spring temperatures to their respective aquifer source depths, and to consider long-term groundwater warming. Based on the different climate scenarios used for 2020 to 2100 (5-year averaged air temperature increase up to 4.32 °C), modelled 5-year averaged subsurface temperatures increased 0.08 to 2.23 °C in shallow groundwater (4.2 m depth) and 0.32 to 1.42 °C in the deeper portion of the aquifer (13.9 m), indicating the depth-dependency of warming. This study presents the first analysis of the thermal sensitivity of groundwater-dependent coastal ecosystems to climate change and indicates that coastal ecosystem management should consider the potential impacts of groundwater warming.

The National Hydrometric Program, operated by the Water Survey of Canada, is the primary source of surface water quantity data in Canada. The network is cost-shared between the federal and provincial governments, and decisions relating to station placement are made according to both federal and provincial interests. Nova Scotia is a small maritime province in Atlantic Canada that is roughly one third the size of England and Wales and has a diverse climate and geology. The Nova Scotia hydrometric monitoring network currently consists of just 31 stations. The overall objective of this study was to determine how well the current network captures the level of hydrologic variability expected in the province using a regional catchment classification scheme. To accomplish this, we developed a combined inductive-deductive catchment classification system and applied it to the province’s active monitoring network and ungauged major watersheds. Initially, hydrologic signatures were used to quantify the catchment function of 47 long-term gauged catchments and to cluster similarly behaving catchments. We identified five generalized flow classes and then attempted to replicate this classification using a deductive-based decision tree framework with physiographic and meteorological explanatory variables. The validated decision tree was used to classify the active hydrometric network and 250+ major watersheds in the province. The network was assessed to determine how well it covered the expected hydrologic variability in the major watersheds across the province. The decision tree proved to be a useful tool for understanding the current network’s coverage and could also be easily applied by practitioners to identify appropriate donor catchments for ungauged watersheds.