# Impact of groundwater extraction on subsurface thermal regimes

Maria Klepikova, Victor Bense, Tanguy Le Borgne, Nicolas Guiheneuf, Olivier Bour

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Maria Klepikova, Victor Bense, Tanguy Le Borgne, Nicolas Guiheneuf, Olivier Bour. Impact of groundwater extraction on subsurface thermal regimes. Environmental Research Letters, 2025, 20 (5), pp.054048. 10.1088/1748-9326/adc8bb hal-05108032v2

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LETTER • OPEN ACCESS

# Impact of groundwater extraction on subsurface thermal regimes

To cite this article: Maria Klepikova et al 2025 *Environ. Res. Lett.* 20 054048

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LETTER

# Impact of groundwater extraction on subsurface thermal regimes

Maria Klepikova¹*, Victor Bense³, Tanguy Le Borgne¹, Nicolas Guihéneuf¹ and Olivier Bour¹

¹ Univ. Rennes, CNRS, Géosciences Rennes, UMR 6118, 35000 Rennes, France

² Department of Environmental Sciences, Wageningen University, Wageningen, The Netherlands

* Author to whom any correspondence should be addressed.

E-mail: maria.klepikova@univ-rennes1.fr

**Keywords:** groundwater temperatures, groundwater abstraction, climate, heat transport, temperature anomaly

## Abstract

Groundwater is widely exploited, leading to groundwater depletion, and a reduction of river flows. While the impact of climate change and hydrologic forcings on the hydrogeological cycle has long been demonstrated, a lack of observations of subsurface thermal changes on the scale of decades is hampering the accurate understanding of how human activities change subsurface thermal regimes (i.e. the spatiotemporal evolution of the subsurface temperature). In this paper, we analyze new temperature data illustrating the complex interplay of climate warming and anthropogenically enhanced groundwater flow on subsurface thermal regimes from two sites in different hydrogeological settings. A heuristic numerical model is used for the interpretation of the observed temperature anomalies. We demonstrate that pumping may have a significant impact on the thermal regime of an aquifer, and that depending on hydrogeological conditions and the natural geothermal gradient, this impact might be even more important than that of climate warming.

## 1. Introduction

Recent research has shown the subsurface thermal regime alteration induced by ongoing atmospheric warming with consequent negative effects on groundwater quality (Kolb et al 2017, Koch et al 2021) and groundwater dependent ecosystems (Burns et al 2017, Caldwell et al 2020). Groundwater temperature data issued from the analysis of large databases has revealed local temperature increases in several regions of the globe—such as the Netherlands (Bense and Kurylyk 2017, Bense et al 2020), Austria (Benz et al 2018) and Germany (Hemmerle and Bayer 2020) for ~30 years, and generally elevated groundwater temperature in central Europe (Tissen et al 2019). A more recent study by Benz et al (2024) shows that aquifer temperatures across the globe are projected to rise substantially over this century. Moreover, the published temperature datasets evidenced a high spatial heterogeneity in groundwater warming trends. Available examples, often located in urbanized areas and regions with past changes in land cover, demonstrate that changes in groundwater temperatures, and especially the most extreme of them, are often primarily dominated by local hydrogeological

conditions (Bense and Beltrami 2007, Benz et al 2018, Klepikova 2024).

Indeed, subsurface heat transport is strongly affected by flow processes present over a wide range of spatial and temporal scales (Anderson 2005, Saar 2010). In recharge areas, natural groundwater flow influences subsurface thermal regimes by lowering temperatures as compared to a purely conductive geothermal regime, and raising temperatures in discharge areas (Smith and Chapman 1983, Deming 1993, Taniguchi et al 1999, Kurylyk et al 2019). Being the world's largest freshwater resource, groundwater is under simultaneous threat not only from human induced climate change (Taylor et al 2013) but also from increasing human water consumption (Wada et al 2010). While the impacts of global groundwater pumping and climate change on the hydrogeological cycle, i.e. substantial drops in groundwater levels modifying the recharge/discharge relationships between large-scale hydrogeological units, have long been demonstrated (Ferguson and Maxwell 2012, Graaf et al 2019, de Graaf et al 2024), we still have insufficient knowledge on the combined effect of these forcings on groundwater temperatures.
a. Porous media

<figure><img src="image_9.png" /><figcaption>**Figure 1.** Conceptual model of a temperature depth profile near an abstraction station. (a) In a sedimentary aquifer, a characteristic temperature-depth profile is formed with concave upward profile above the abstraction depth and a convex upward profile below the abstraction depth. In between an increased gradient is found that is visible as a plateau in the profile. (b) In an aquifer in fractured bedrocks, sub-vertical advective heat transfer results in a large-scale temperature anomaly with a peak occurring at the depth of fault-borehole intersection.</figcaption></figure>

b. Fractured media

<figure><img src="image_10.png" /></figure>

In general, at abstraction sites in porous aquifers (figure 1(a)), water flows toward the abstraction well horizontally and vertical flow components usually occur caused by partial penetration of the well, by recharge from leakage through adjacent layers or from a declining water table. Thus, within exploited aquifers, water above the pumping level flows downward, and below the pumping level it flows upward. In theory, if there is no surface warming, due to the background geothermal gradient, water in the shallow subsurface is cooler than water at greater depths. Hence, water that flows downward will cause a negative temperature anomaly (with respect to the geothermal gradient) and water that flows upward from greater depths will cause a positive temperature anomaly (figure 1(a)). In other words, the upper part (above the abstraction depth) shows a concave upward profile and the lower part (below the abstraction depth) shows a convex upward profile with a plateau of increased thermal gradient in between (around the abstraction depth). In fractured aquifers, the abstraction-induced flow is mainly focused in highly fractured zones and/or fault zones, draining a large rock volume beyond the topographic catchment (figure 1(b)). Depending on the orientation of fault zones, the groundwater abstraction-induced groundwater flow (downward or upward) transmits fluid with a contrast temperature (of cooler or warmer origin, respectively) in aquifers. In figure 1(b), pumping induces upward flow of warmer fluid through the sub-vertical fault zone thus resulting in a positive

thermal anomaly, with a peak occurring at the depth of fault-borehole intersection.

Furthermore, surface temperature changes triggered by climate warming and/or land use change, which are currently observed (Benz et al 2024), propagate downward and disturb the upper portion of the profile while leaving lower portions of the profile thermally unchanged (represented by orange lines in figure 1). When groundwater fluxes are combined with surface warming, a conceptual model emerges in which there is an interplay between the downward penetration of the surficial warming signal and the groundwater induced distortion of the temperature field (Irvine et al 2016).

Only very few site-specific studies have characterized the changes in groundwater temperature induced in regions with intensive groundwater exploitation (Uchida et al 2003, Abe et al 2014). A major obstacle preventing the understanding of the impact of these forcings on the subsurface thermal dynamics is a dire lack of field observations, i.e. repeated temperature-depth profiles collected over decades. Differencing the repeated temperature logs for individual boreholes yields real temperature change and eliminates steady-state sources of curvature, thus allowing us to unambiguously detect even relatively subtle changes in subsurface thermal regimes, resulting from transient conditions. In this paper, we analyze such new temperature data illustrating the complex interplay of climate warming and enhanced groundwater flow on subsurface thermal regimes from two sites in different
hydrogeological settings: an aquifer (1) in fractured crystalline bedrock, and (2) in unconsolidated sediments. A simple numerical model is then used for the interpretation of the observed temperature patterns. We first demonstrate that pumping may have a significant impact on the thermal regime of the subsurface, and that depending on hydrogeological conditions and the natural geothermal gradient, this impact might be even more important than that of surface warming.

## 2. Study area, data collection and analysis

### 2.1. La Cabine field site in unconsolidated sediments

The La Cabine field site is located in the southern part of the topographically elevated (maximum 110 m) Veluwe zone close to the city of Arnhem in the Netherlands (figure 2(a)). The upper several hundreds of meters of the Veluwe subsurface are formed by a sequence of sandy and clayey sediments dating from the Holocene and Pleistocene. These overlay marine fine-grained clayey sediments that are of Miocene age, the top of which is regarded as the hydrogeological base below which little groundwater flow occurs. Environmental isotope studies, hydrological analysis, and groundwater flow modeling suggest that the long-term average groundwater recharge across the Veluwe area is ~0.4 m yr⁻¹ which is derived from ~0.9 m yr⁻¹ of precipitation (Gehrels 1999). Drinking water has been extracted since the mid 1960s at an average rate of 10 Mm³ yr⁻¹. Abstraction wells extract at a depth between 60 m and 80 m, which matches one of the sandy aquifers. Figure 2(b) shows the annual average surface temperature and the current surface warming trend for the Veluwe area, obtained from the Royal Netherlands Meteorological Institute.

Between 2016 and 2023, a series of temperature-depth profiles were obtained in the Veluwe area using piezometer tubes installed in boreholes. We assume that these temperature-depth profiles represent an accurate record of the vertical variation in subsurface temperatures in the geological formations surrounding the boreholes. Likely, the small diameter of the piezometer tube (5 · 10⁻² m), in combination with the relatively modest vertical thermal gradients below the zone of seasonal influence (i.e. at depths larger than ~15 m), prevents any vertical convection within the piezometers (Colombani et al 2016). Data were collected using an RBR soloT instrument (https://rbr-global.com/) that was calibrated to an accuracy of 2 · 10⁻³ °C and has a temperature resolution of <5 · 10⁻³ °C and a time constant of 1 s. For the data collection, the stop-go principle (Harris and Chapman 2007) was used to collect temperature measurements each second for 10 s at each 1 m depth interval. A detailed description of the lithologies encountered in

the observation borehole that was recorded at the time of their construction is available. A simplified version of this lithological log is depicted alongside temperature-depth profiles in figure 2(c).

An example of temperature profiles from one of the well (P552) situated directly on the abstraction site La Cabine (approximately 10 m from the extraction well) is provided in figure 2(c). We plot for comparison an unperturbed (i.e. not affected either by climate change or by groundwater flow) temperature profile modeled assuming a steady state climate before pumping and a constant geothermal heat flux. Compared to this profile (gray dashed line), all three (2016, 2019, 2023) temperature profiles show a characteristic S-shape. They first show slight negative gradient (until ~80 m depth) indicative of surface warming followed by a concave upward profile, which is representative for groundwater upwelling (discharge area) (Anderson 2005). The upper ~30 m of temperature profiles are impacted by a decadal surface warming signal propagating downward with groundwater flow, i.e. the concave upward curvature of the profile is enhanced by downward groundwater fluxes (Benz et al 2017), that are in our case induced by the groundwater abstraction. Below the abstraction depth (80–100 m depth), the profile exhibits a convex upward profile characteristic of a discharge area. Below ~190 m, measured profiles follow an unperturbed geothermal gradient. When comparing profiles collected in 2016, 2019 and 2023, the upper part of the profile becomes slightly warmer with time (i.e. temperature increases by ~0.1°C in the uppermost hundred meters in between sampling campaigns).

### 2.2. Ploemeur field site in fractured metamorphic rocks

The Ploemeur aquifer is located on the south coast of Brittany, France, near the city of Lorient in crystalline bedrock terrain characterized by igneous and metamorphic rocks (figure 2(a)). The region consists of tectonic units developed during the Hercynian orogeny. Cores and drill cuttings show very heterogeneous lithological characteristics with an alternation of micaschist enclaves and granitic dykes such as aplites and pegmatites (Touchard 1999). Despite the low permeability of the lithologies encountered, this aquifer is very productive and has been pumped since 1991 for municipal supply at a sustainable rate (1 Mm³ yr⁻¹). Pumping is the single outlet draining a region of at least ~5.5 km² (recharge ~0.2 m yr⁻¹) (Leray et al 2012). Flow is primarily focused in two main geological discontinuities (Touchard 1999). The first one is a highly fractured contact zone between granite and the overlying micaschists (Leray et al 2012). Before dipping quasi vertically at ~200–400 m depth, this contact dips moderately to the north around 30° (Ruelleu et al 2010). The second
<figure><img src="image_11.png" /></figure>

geological discontinuity is a wide fault zone steeply
dipping and striking North 20 (Leray *et al* 2012).
The site has a very dense piezometric coverage, with
25 wells, that are generally 100 m deep and local-
ized in the vicinity of the pumping site (at a dis-
tance <700 m), i.e. close to the aquifer outflow.
Groundwater is mainly extracted from the pumping
well set in the sub-vertical North 20 fault zone. Two
other wells, located a few tens of meters away, are
generally stopped or restarted to meet water needs
on a weekly time scale. The aquifer is monitored
by the Environmental Research Observatory H+
(http://hplus.ore.fr/) with long-term hydrological,

hydrochemical, and geophysical monitoring since
2002. Meteorological records from Lorient, repres-
entative for the study area, show that annual air tem-
perature is currently on average +1 °C higher than in
1990 (figure 2(b)).

Several boreholes were profiled for temperature in
1996, 2005 and in 2020 to obtain a direct measure-
ment of subsurface temperature changes. The tem-
perature precision of the thermometer used in 1996
was on the order of 5 · 10⁻² °C; data from 2005
were obtained with a temperature logging device, the
Idronaut CDT 302 Multi-Parameter Probe with a tool
precision of 5 · 10⁻³ °C; the last measurements were
<figure><img src="image_12.png" /><figcaption>Figure 3. Map showing groundwater temperatures measured in several boreholes at the depth of 50 m at the Ploemeur field site in 2020. Groundwater pumping location is shown by magenta cross. Image © Airbus 2024.</figcaption></figure>

collected with RBRduet loggers, that are similar to RBR soloT loggers while providing both temperature and pressure measurements, with a pressure accuracy of 0.05 m. A simplified version of the lithological log is depicted alongside temperature-depth profiles in figure 2(d).

An example of temperature profiles from one of the wells (F11) situated ~50 m from the abstraction well (figure 3) is provided in figure 2(d). Profiles measured at Ploemeur reveal that over 24 years, a strong increase in groundwater temperature was observed at the site. For example, at a depth of 40 m, a warming of approximately 2.7 °C is observed. At larger depths, this range remains more or less constant, while at shallower depths, it becomes smaller, i.e. ~1.6 °C. Note, that as the pumping started in 1991, the first measurements in 1996 do not represent the natural thermal regime at the site, but already reflect the groundwater extraction-induced changes. We plot for comparison a modeled temperature profile assuming a steady state climate before 1990 and a constant geothermal flux. Patterns similar to the one observed in F11 borehole were observed at several wells situated close to the pumping station, and lower thermal anomalies were observed in boreholes situated further away from the pumping well. At distances larger than a ~500 m from the pumping station we have not encountered significant temperature anomalies (figure 3).

# 3. Data interpretation

## 3.1. Numerical model setup

In order to evaluate whether the hydrogeological scenarios discussed above can indeed explain the general character of the observed temperature trends, we have used a simple numerical modeling approach. Our model incorporates the hydrogeological setting of La Cabine and Ploemeur sites in an idealized way without attempting to reproduce in detail the

geology of each observation well at the places where we compare the modeled temperature-depth profiles with those observed. The equation describing transient heat-transfer by advection through groundwater flow and conduction in a saturated porous and fractured medium is (Bredehoeft and Papaopulos 1965, Stallman 1965):

$$ \rho_b c_b \frac{\partial T}{\partial t} - \kappa_b \nabla^2 T + \rho_w c_w q \nabla T = 0 \quad (1) $$

where T is temperature (°C), ρ is the density (kg m⁻³), κ (W (m·°C)⁻¹) is thermal conductivity, c (J (kg·°C)⁻¹) is the specific heat, q (m s⁻¹) is the Darcy flux, and b and w denote bulk and fresh water, respectively. For both cases, the groundwater flow is considered to be in a steady state, and the flow velocity is independent of variations in fluid viscosity or density that might arise from temperature variations. It is further assumed that the aquifer is fully saturated and that, hence, no unsaturated zone is present near the surface. This is likely to be a reasonable assumption for topographically flat areas with shallow groundwater tables. Equation (1) is solved using COMSOL Multiphysics for hydrogeological and heat flow boundary conditions presented below.

## 3.2. Porous media scenario

We used a 2D model domain of 1000 m long and 200 m deep. A heat flux was imposed at the base of the model representing the background geothermal heat flux (figure 4). Assuming a thermal conductivity of 2.7 W (m·°C)⁻¹ for these fine sediments (Kooi 2008), this geothermal heat flux was estimated to be 60 mW m⁻², in agreement with geothermal heat flux assessments for the Netherlands (Bense and Kurylyk 2017). A fixed hydraulic head and a long-term rise in temperature with a warming rate of 3.8 · 10⁻² °C yr⁻¹ (annual time steps) were set at the top boundary (figure 2(b)). The left- and right-hand sides of the model are assumed to be no flux for both fluid flow
<figure><img src="image_13.png" /><figcaption>**Table 1.** Chosen fluid and rock thermal properties representative of sediments and granite in study areas. The thermal parameters are taken from Kooi (2008), Klepikova et al (2016).</figcaption></figure>

<table><thead><tr><th>Parameter</th><th>Bulk (porous)</th><th>Bulk (fractured)</th><th>Water</th></tr></thead><tbody><tr><td>Density ρ, kg m<sup>-3</sup></td><td>2105</td><td>2500</td><td>1000</td></tr><tr><td>Thermal conductivity κ, W (m·°C)<sup>-1</sup></td><td>2.7</td><td>3.3</td><td>0.59</td></tr><tr><td>Specific heat c, J (kg·°C)<sup>-1</sup></td><td>2700</td><td>740</td><td>4189</td></tr></tbody></table>

and heat. At the left boundary, an abstraction station was implemented between 60 and 80 m depths with a fixed outgoing fluid flux. We have evaluated different abstraction scenarios with the outgoing flux varying in between $1 \cdot 10^{-6}$ and $5 \cdot 10^{-5}$ m s$^{-1}$, resulting in a wide range of thermal Peclet numbers $30 \le Pe \le 1500$, where $Pe = qL\rho_w c_w / \kappa_b$ and $L$ is the characteristic length (m), here the length of the pumping interval. A sand body was assumed for the entire model domain, except for a clay layer between 50 and 60 m depth. In the model, we apply hydraulic conductivity of $25 \text{ m d}^{-1}$ (data obtained from the Geological Survey of the Netherlands, www.dinoloket.nl/). Moreover, constant values representative of sandy sediments where assumed for the different model parameters which are shown in table 1. Results for the transient simulations are examined for $t = 75$ years since the beginning of the groundwater extraction.

Simulated temperature distribution for $t = 75$ years for the outgoing flux of $2.5 \cdot 10^{-5}$ m s$^{-1}$ is shown in figure 4(a). Groundwater abstraction

induces upwelling of water below the pumping level and downwelling above the pumping level. This local groundwater flow disturbs the temperature field in the region of ~500 m around the pumping station. In addition, temperature-depth profiles extracted close to the pumping location are shown in figure 4(b). These results demonstrate (1) that the surface temperature increase propagates downward and (2) that the thermal gradient increases significantly at ~60 m depth before decreasing again below ~90 m of depth. This sudden increase in the thermal gradient around the groundwater abstraction depth is well visible as a 'plateau' in the temperature-depth profile. Above the abstraction depth, we observe a negative thermal anomaly smoothing with time, while below the abstraction depth a positive anomaly of ~1°C is formed. Our modeling results also demonstrate that below the abstraction depth the temperature evolves quickly and the 25 year and 75 year profiles have an almost identical shape. Only the upper part of the temperature profile is impacted by a surface warming signal propagating downward with the
<figure><img src="image_14.png" /></figure>

groundwater flow. Over this 75 year period, climate-
induced groundwater warming at 10 m depth reaches
0.5°C. Overall, the profiles showed strong similarities
with the conceptual model and the temperature pro-
files observed at La Cabine field site. Note, that signi-
ficant altering of the temperature-depth profiles only
occurred at fluxes higher than 2.5 · 10⁻⁶ m s⁻¹. With
increasing Péclet number, the thermal effect of the
groundwater abstraction becomes more pronounced,
and for the outgoing flux of 5 · 10⁻⁵ m s⁻¹ positive
anomalies attain ~2.5°C with the temperature field
affected by the pumping up to 750 m away from the
abstraction well.

**3.3. Fractured media scenario**

For the simulations in a fractured aquifer, we used a
2D model domain of 1000 m long and 400 m deep.
The model domain consists of an impermeable rock
matrix and contains a main fluid conduit, which rep-
resents a permeable fault zone. The fault is dipping
vertically and its thickness is set as 1 m. Groundwater
flow through the fault zone is considered to be in a
steady state, and flow velocity is independent of vari-
ations in fluid viscosity or density that might arise
from temperature variations. Groundwater velocit-
ies are varied over a range between 7 · 10⁻⁷ and 1 ·
10⁻⁴ m s⁻¹, resulting in a wide range of thermal
Péclet numbers 1 ≤ Pe ≤ 120 (here, the character-
istic length was taken as the fault thickness L =
1 m). These values for Péclet number are typical for

fractured aquifers (Klepikova *et al* 2021). A heat flux
of 62.7 mW m⁻² was imposed at the base of the model
(figure 5). The left- and right-hand sides of the model
are assumed to be no flux for both flow and heat.
A fixed hydraulic head and a long-term rise in tem-
perature with a warming rate of 4.7 · 10⁻² °C yr⁻¹
(annual time steps) were set at the top boundary
(fixed in alignment with the observational records in
figure 2(b), except the implemented fixed discharge
from the fault zone. Results for the transient simu-
lations are examined for up to *t* = 30 year. Constant
values representative of granite where assumed for the
different model parameters (table 1).

An example of the calculated temperature dis-
tribution is given in figure 5(a) for Pe = 40 and
for a selected time slice (t = 30 year). Groundwater
abstraction generates upward flow of deep ground-
water along the fault zone. The upward fluid flow
raises temperatures in a zone around the fluid con-
duit. Our results show that after 30 years of ground-
water abstraction, at 100 m depth, the maximum tem-
perature anomaly of 4.9 °C is reached in the imme-
diate vicinity of the fault. For the analyzed range
of Péclet numbers, this temperature varies from 0.8
°C to 5.7 °C. The area around the fault zone where
temperatures are at least 1 °C higher than the back-
ground values is approximately 130 m. For Pe = 120,
this area reaches 150 m. Away from the fault zone,
temperature quickly restores itself to the background
linear conductive profile (figure 5(a)). Furthermore,
the propagation of the surface warming signal, weak in comparison to abstraction-induced warming, can be seen at shallow depths (~20 m).

We also consider temperature profiles for dif-
ferent periods of time (t = 0, 5, 15, 30 year)
since the beginning of the groundwater extraction
(figure 5(b)). Simulated temperature-depth profiles
we find close to the fault along the dashed white line
in figure 5(a) are similar in magnitude and pattern to
those observed at the Ploemeur field site. At this dis-
tance, 25 m away from the fault, the maximum pos-
itive temperature anomaly we find after 30 years of
pumping reaches almost 4 °C.

Moreover, the model results show a strong
dependence of the groundwater warming rate on
the assumed depth of the fault zone. The temperat-
ures observed in Ploemeur are only reached using
fluid conduits that are at least 300 m deep. In addi-
tion, a comparison of the vertical fault zone and the
fault dipping 30° shows a strong difference in tem-
peratures. If the fault zone inclines from the vertical
plane, the flow path from a particular depth is longer
and therefore heat losses towards the surrounding
rocks are higher. Thus, for 400 m deep fault dipping
30°, the resulting temperatures are too low to explain
the observed temperatures in the system and greater
fault depth are needed. These simulation results con-
firm the previously proposed conceptual models of
the Ploemeur aquifer claiming that pumping induces
upwelling of deep and distant water (Ruelleu et al
2010, Leray et al 2012, Roques et al 2018, Osorio-
Leon et al 2023).

4. Discussion and conclusions

Our results reveal that the observed temperature
anomalies are transient features that result from
convective heat transfer induced by groundwater
flow modified by the abstraction. Previous studies
analyzing spatial temperature patterns generated by
topography-driven groundwater (Lampe and Person
2002, Kaiser et al 2011, Kooi 2016, Cherubini et al
2017) suggested that overall groundwater-driven con-
vective cooling exceeds groundwater-driven warm-
ing. Indeed, as permeability is a depth-dependent
parameter, with high values in the first kilometers
(e.g. Roques et al 2016, Achtziger-Zupančič et al
2017), cold meteoric fluids can circulate more eas-
ily and exchange heat more efficiently than hot flu-
ids located in a deeper crust. Thus, according to Kooi
(2016), cooling of the crust from above, by meteoric
fluids, is expected to be more efficient than warm-
ing from below by metamorphic or magmatic flu-
ids and this cooling effect is expected to be small
in unconsolidated sediments and strong in crustal
rocks. Following this logic, intensification of ground-
water flow should cool the underlying crustal rocks
(Kooi 2016). However, this appears to be in disagree-
ment with our results, which show that groundwater

pumping mainly induces positive thermal anomalies. This disagreement may come from the fact that previous works did not directly account for the impacts of groundwater abstraction, i.e. strong changes in local hydraulic gradients influencing the velocities and directions of flow in the groundwater system, and, consequently, they did not consider sufficiently high Péclet numbers (e.g. Ferguson *et al* 2006).

On the one hand, groundwater temperatures are known to lag surface warming induced by a changing climate (Benz et al 2018, 2024, Tissen et al 2019) so that warm heat plumes from the surface penetrate deeper thus increasing groundwater temperatures. On the other hand, abstraction warms the subsurface below the abstraction depth by the induced groundwater flow and the accompanying heat advection. Depending on hydrogeological conditions and the natural geothermal gradient, this impact might be even more important than that of climate change. While relatively small anomalies were observed in a sedimentary aquifer (–0.3 °C above the abstraction depth and +0.6 °C below the abstraction depth, relative to the unperturbed temperature profile), the abstraction-induced warming effect was shown to be important in fractured media (up to +3.8 °C relative to the unperturbed temperature profile in 30 years of pumping and still evolving). The Ploemeur field site is a typical and important example of a crystalline-rock aquifer where high-yield groundwater resources are controlled by the presence of a main transmissive fault zone (Roques et al 2016). When exploited, such faults are known to form fast flow pathways for deep and consequently warmer groundwater (Bense et al 2008).

While providing a physical understanding of the
heat transport phenomena in exploited aquifers,
our heuristic modeling approach limits its ability
to capture site-specific characteristics. The subsur-
face thermal dynamics is also expected to be influ-
enced by hydrogeological heterogeneities, localized
heat transfer effects, and seasonal variations in tem-
perature and/or pumping rate. Moreover, abstraction
wells can also be located in areas where groundwa-
ter discharge or recharge naturally occurs, thus amp-
lifying or reducing the thermal effect of groundwa-
ter abstraction. The consideration of these factors
would require more sophisticated models that will be
developed in future work.

Finally, the pumping-induced changes that
groundwater is experiencing now and will experi-
ence in the near future also occur at a global scale, as
overexploitation occurs at a large scale (Gleeson *et al*
2012, Macdonald *et al* 2016, Jasechko *et al* 2024). The
resulting groundwater warming may have multiple
negative consequences for environmental systems.
Thus, warming generally decreases faunal diversity
and accelerates the cycling of organic and inor-
ganic matter (Koch *et al* 2021) and threaten drinking
water quality (Benz *et al* 2024). In some locations,
groundwater warming may also alter the conditions that sensitive groundwater-dependent ecosystems generally rely on, thus threatening biodiversity and ecosystem services (Burns et al 2017, Benz et al 2024).

## Data availability statement

The data that support the findings of this study are openly available at the following URL/DOI: https://hplus.ore.fr/en/klepikova-et-al-2025-erl-data/.

## Acknowledgments

Funded by the European Union (ERC, CONCRETER, Project Number 101077837). Views and opinions expressed are however those of the authors only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them. The National Observatory for Research in Environment SNO H + supported the field data acquisition and investigation.

## ORCID iD

Maria Klepikova https://orcid.org/0000-0003-4290-2400

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