According to recent estimates, buildings and the built environment account for about 37 % of global CO2 emissions (Ujma et al., 2025), which makes emission reductions from heating and cooling a central objective of contemporary climate policy. In EU law, the binding framework is primarily provided by the European Climate Law, which makes the ambitious targets of climate neutrality by 2050 and at least a 55 % net reduction in greenhouse-gas emissions by 2030 compared with 1990 legally binding for the EU economy as a whole (Regulation 2021/1119, OJ L 243, 9.7.2021). These objectives are operationalised through the challenging Fit for 55 legislative framework, including the reformed EU ETS and the ETS2 architecture for fuels used, inter alia, in buildings (Directive 2023/959, OJ L 130, 16.5.2023), the Effort Sharing Regulation for non-ETS sectors such as buildings, transport, agriculture and waste, with an EU-wide target of −40 % by 2030 compared with 2005 (Regulation 2023/857, OJ L 111, 26.4.2023), and the LULUCF framework, which provides the net-removals component of the 2030 climate architecture at 310 Mt CO2e (Regulation 2023/839, OJ L 107, 21.4.2023). In parallel, EU energy law promotes the decarbonisation of heating and cooling by strengthening renewable-energy deployment, energy-efficiency obligations and building-performance standards: RED III sets a binding 42.5 % renewable-energy target for 2030 and reinforces renewable shares in buildings and heating/cooling (Directive 2023/2413, OJ L 2023/2413, 31.10.2023); the recast EED requires an 11.7 % reduction in energy consumption by 2030 compared with the 2020 reference scenario (Directive 2023/1791, OJ L 231, 20.9.2023); and the recast EPBD introduces the zero-emission building standard for new public buildings from 2028 and all new buildings from 2030, while ending public support for stand-alone fossil-fuel boilers from 2025 and requiring national pathways towards their phase-out by 2040 (Directive 2024/1275, OJ L 2024/1275, 8.5.2024). In this context, ground-source heat pump (GSHP) systems are considered a well-established and efficient technology for heating and cooling buildings (Kljajić et al., 2020; Sun et al., 2021). GSHP systems also help to reduce greenhouse-gas emissions from buildings, on the assumption that the electricity used to drive them comes from low-carbon or renewable sources. However, as GSHP deployment relies on subsurface components such as vertical borehole heat exchangers, its contribution to building decarbonisation must also be assessed against potential geological and groundwater impacts.
Among different GSHP configurations, systems with vertical borehole heat exchangers (BHEs) are especially common in urban areas where there is not enough space for horizontal ground loops. A typical BHE consists of one or two U-shaped polyethylene pipes placed in a borehole drilled to a depth between 50 and 200 m (in Poland, due to legal restrictions, borehole depths are often limited to 100 m). The space between the pipes and the borehole wall is filled with grout. A heat transfer fluid, usually a mixture of water and glycol, circulates through the pipes and exchanges heat with the surrounding ground (Guz et al., 2024; Hagedorn et al., 2024; Matusevych et al., 2025).
GSHP systems perform better than air-source heat pumps because the ground temperature at depths greater than 10 m – 15 m stays roughly constant throughout the year, usually in the range of 7 °C – 12 °C in central Europe (Savchenko et al., 2015). This is particularly important in winter and during frosty periods, when low outdoor air temperatures strongly reduce the efficiency of air-source units, because the temperature difference between the indoor and outdoor environment becomes very large. This means the seasonal coefficient of performance (SCOP) of a ground-coupled system is higher than that of an air-source unit, which must deal with large temperature changes. According to Forum Energy (2020), the electrification of heating through heat pumps in Poland is expected to require 11 TWh - 13 TWh of electrical energy per year by 2050.
However, as more BHE installations are built, particularly in dense urban areas, their long-term interaction with the subsurface environment becomes an important question. Drilling, grouting and operating vertical boreholes cause changes to the thermal, hydraulic and possibly chemical conditions in the ground and groundwater. A single, properly designed BHE installation usually has a small environmental footprint, but the combined effect of many closely spaced boreholes in one area may be more significant (Attard et al., 2020; Casasso & Sethi, 2019). A separate but practically important issue is the structural and functional difference between geothermal boreholes and water-supply wells, and the risks that appear when one of these is converted into the other; this concerns mainly countries where both markets are growing in parallel.
Classification of environmental impacts of vertical BHE installations by impact category and spatial scale (own research)
The aim of this paper is to review the environmental aspects of vertical closed-loop BHE installations for GSHP systems. The review covers four main topics: (i) thermal changes in the ground, (ii) hydrogeological consequences of borehole construction, with special attention to drilling in karst and other complex geological settings, (iii) risks to groundwater quality, and (iv) the problem of using the same borehole for both geothermal and water supply purposes. This paper tries to identify the factors that determine how large the environmental effects are, and to highlight where better construction practice or clearer regulations are needed. Figure 1 presents a schematic overview of the main impact categories and their spatial scale, which serves as a framework for the discussion that follows.
A GSHP system has three main parts: the ground loop, the heat pump unit with its refrigerant circuit, and the building distribution system. In a vertical closed-loop arrangement, the ground loop consists of one or more BHEs connected to the heat pump through supply and return pipes. The heat pump works on a vapour-compression cycle, in which the refrigerant is evaporated, compressed, condensed and expanded in sequence, moving heat from the ground to the building (Matusevych et al., 2025).
The most common BHE types are the single U-tube, double U-tube and coaxial configurations. In U-tube systems, two or four pipe legs run vertically and are connected at the bottom by a U-bend. In the coaxial type, there is an inner pipe inside an outer pipe. The borehole diameter is usually between 0.12 m and 0.20 m. The annular space around the pipes is filled with grout, which improves the thermal contact between the pipes and the ground, and also provides a hydraulic seal between the geological layers that the borehole passes through (Stauffer et al., 2013). Standard bentonite-based grouts have thermal conductivities of 0.7 W/(m·K) – 1.0 W/(m·K), while thermally enhanced grouts containing additive fillers such as quartz sand can reach 2.0 W/(m·K) or more (Aresti et al., 2021).
The heat transfer fluid in BHE systems is usually a water-glycol mixture or just water. Glycol acts as antifreeze protection when temperatures drop below 0 °C. From an environmental point of view, the type of glycol matters: propylene glycol has low toxicity to aquatic organisms, while ethylene glycol is more harmful if it leaks into the groundwater through pipe damage (Casasso & Sethi, 2019).
When a single BHE is not sufficient to cover the thermal demand of a building, several boreholes are arranged in an area with specified spacing. The PORT PC guidelines used in Poland recommend minimum distances of 6 m for boreholes up to 70 m deep, 8 m for depths of 70 m – 100 m, and at least 8 % of the borehole length for depths over 100 m. Additional setbacks of at least 3 m from property boundaries and 1.5 m from building foundations and underground utilities are also required (according to Polish law). These rules are meant to limit first-order thermal interference between neighbouring boreholes; in larger borehole fields, however, the final spacing should still be verified against the long-term thermal load (PORT PC, 2022).
How much heat can be extracted from a BHE depends largely on the thermal properties of the surrounding geological formation. The key primary properties are thermal conductivity and volumetric heat capacity, while thermal diffusivity, derived from these two parameters, controls the rate at which temperature disturbances propagate through the ground. Rocks and soils with high thermal conductivity, such as water-saturated sandstones, limestones, dolomites and crystalline rocks like granites or gneisses, allow efficient heat transfer and faster recovery of the ground temperature after heat extraction (Stauffer et al., 2013). On the other hand, formations with low conductivity, including dry sands, organic-rich deposits like peats and alluvial muds, and clay-dominated layers, reduce the rate of heat extraction and increase the risk of excessive cooling around the borehole (Luo et al., 2013).
The presence of groundwater in the pore spaces has a major effect on thermal properties, because the thermal conductivity of water (∼0.6 W/(m·K)) is about 25 times higher than that of air. In aquifers with noticeable groundwater flow, the heat is transported not only by conduction but also by advection, which helps to reduce the thermal imbalance near the borehole (Zeng et al., 2021). Another important factor is the vertical variation of the geological layers that a borehole passes through. If the subsurface consists of alternating layers with different properties, the temperature distribution during BHE operation will be non-uniform, which can make it difficult to correctly interpret thermal response tests and to size the installation properly (Guz et al., 2024).
In many BHE design studies, groundwater flow is treated indirectly through hydraulic-conductivity estimates, regional groundwater maps or qualitative descriptions of aquifer conditions. However, for environmental assessment and long-term performance prediction, groundwater flux itself is often the more relevant parameter. Groundwater flux controls the advective component of heat transport, influences the spatial extent and direction of thermal plumes, and may reduce or enhance thermal interference between neighbouring boreholes depending on the orientation of the flow field relative to the BHE layout (Banks, 2015; Diao et al., 2004). While groundwater flow is often inferred from hydraulic-conductivity and hydraulic-gradient data using Darcy’s law, direct or semi-direct methods such as seepage meters, iFLUX sensors, temperature-tracer analysis, passive flux meters, borehole dilution tests and distributed temperature sensing can provide more representative information on the magnitude, direction and variability of advective heat transport (Annable et al., 2005; Simon et al., 2024; Van Putte & Verreydt, under review; Verreydt et al., 2015). These measurements help determine whether groundwater advection should be explicitly included in thermal models, whether borehole spacing or array orientation should be adapted to the prevailing flow direction, and whether long-term monitoring should include groundwater levels, temperature and water quality.
During the heating season, a GSHP system continuously extracts heat from the ground around the BHE. This generates a zone of reduced temperature around the borehole, sometimes called a thermal depression cone. In climates where heat extraction during winter is much larger than heat injection during summer, the ground temperature may gradually decrease over the years. This process is known as thermal depletion or thermal drift (Rybach & Eugster, 2010).
For a single, well-spaced BHE in a formation with moderate thermal conductivity, the long-term temperature change in the near-borehole zone is often on the order of a few degrees Celsius and may stabilise after the first years of operation, although it remains site- and load-dependent (Signorelli et al., 2005). However, when several BHEs are placed close together, their zones of thermal influence may overlap. This leads to a cumulative temperature reduction that is larger than what any single borehole would cause. This effect is especially important in urban areas where space constraints force compact borehole layouts (You et al., 2018).
The consequences of thermal depletion are both technical and environmental. On the technical side, the lower ground temperature means that the system operates with lower coefficient of performance (COP) and increases electricity consumption. In extreme cases, the fluid returning from the BHE may cool below the freezing point of the surrounding ground, which can cause frost heave and damage the borehole (Eslami-Nejad & Bernier, 2012). On the environmental side, the prolonged lowering of subsurface temperatures changes the thermal conditions in the affected geological layers, which may influence geochemical processes (see Sections 6 and 8).
A simple and effective way to prevent thermal drift is to use the system for cooling in summer. During cooling operation, excess heat from the building is rejected to the ground, which helps to rebalance the annual thermal budget. Both passive free-cooling (direct fluid circulation without the heat pump) and active cooling (reversed heat pump cycle) can be used for this purpose. Experience from installations in Switzerland and Germany shows that balanced or nearly balanced systems can maintain stable ground temperatures for more than 30 years (Rybach & Eugster, 2010).
When a BHE borehole is drilled, it creates a vertical conduit through geological layers that may have been hydraulically separated in their natural state. If the grouting of the annular space is incomplete or of poor quality, the borehole may act as a pathway for vertical groundwater flow between aquifer horizons that have different hydraulic heads. This is called cross-flow or inter-aquifer leakage. The possible consequences include mixing of waters with different chemical compositions, migration of contaminants from shallow to deep aquifers or the other way around, and local changes in groundwater levels near the borehole (Attard et al., 2020; Casasso & Sethi, 2019).
Good-quality grouting is therefore very important for groundwater protection. The grout has to meet two requirements at the same time: it must have low enough hydraulic conductivity to prevent water migration along the borehole, and high enough thermal conductivity to allow effective heat transfer between the ground and the BHE pipes. Standard bentonite-cement grouts usually satisfy the hydraulic requirement but have relatively low thermal conductivity. Thermally enhanced grouts with additive fillers like quartz sand have better thermal performance, but their hydraulic integrity needs to be verified (Aresti et al., 2021).
Common problems in borehole construction that can compromise groundwater protection include incomplete filling of the annular space, use of unsuitable grouting materials, failure to seal individual aquifer horizons in multi-aquifer sequences, and damage to the grout from thermal cycling during BHE operation. Specific risks related to drilling in karst terrain and formations with evaporite minerals are discussed in Section 7.
The regulatory framework for BHE installation differs across European countries. In Poland, closed-loop BHE systems as relatively deep boreholes are subject to the provisions of the Geological and Mining Law (consolidated text: Journal of Laws 2026, item 69). Moreover, they are required to comply with building regulations (Construction Law; consolidated text: Journal of Laws 2026, item 524), and in cases where boreholes pass through important aquifer systems, the Water Law (consolidated text: Journal of Laws 2025, item 960) may also apply. The recent amendment of the Act on collective water supply and collective sewage discharge (Journal of Laws 2026, item 605), can also be relevant where boreholes are located in or near sanitary protection zones of public water intakes. In practice, the extent of geological and hydrogeological assessment required before installation varies, and there is a need for more consistent standards (PORT PC, 2022). All consolidated-text references provided above are current as of May 2026.
A problem that is rarely discussed in the academic literature but does occur in practice is the installation of water supply wells in boreholes that were originally drilled for BHE purposes, or the attempt to use a single borehole for both functions at the same time. This practice is sometimes motivated by cost savings or by a lack of understanding of the fundamental differences between the two types of installations. It creates several technical and environmental risks that should be clearly addressed.
A BHE borehole and a water supply well are structurally and functionally different constructions. A BHE borehole is a sealed, fully grouted conduit where the heat transfer fluid circulates in a closed circuit with no contact with the surrounding ground. The grout is specifically chosen to prevent groundwater migration along the borehole. A water supply well, on the other hand, is designed to allow the controlled extraction of groundwater from a specific aquifer. It uses screened intervals, gravel packs and sanitary seals to provide hydraulic access while keeping the water clean (GEO, 2014). These design requirements are in many ways opposite: what is needed for a good BHE (impermeable grouting along the full length) would make a water well useless, and what is needed for a good well (permeable, screened sections) would compromise the sealing function of a BHE.
If someone tries to convert a geothermal borehole into a water well, for example by installing a submersible pump alongside or instead of the U-tube, several problems can arise. The borehole may not provide the sanitary protection needed for a drinking water source. The glycol-water mixture from the BHE circuit may contaminate the extracted water. Removing the grout to create permeable sections would destroy the hydraulic barrier between aquifer layers and bring back the cross-flow risks discussed in Section 4 (Casasso et al., 2020).
The opposite case, installing BHE pipes in an existing or abandoned water well, is also problematic. Water wells typically have screened sections, gravel packs and open annular spaces that do not meet the sealed-borehole requirements of a functioning BHE. Putting U-tube pipes into such a well without first decommissioning it and re-grouting the full annular space would result in a BHE with poor thermal contact, uncontrolled groundwater circulation, and a permanent vertical pathway for interaquifer connection.
Exemplary region of Poland near Częstochowa, showing the locations of vertical borehole heat exchangers (BHEs) constructed within the last three years, indicating growing BHE deployment (own research)
The Geothermal Exchange Organization has stated clearly that geothermal boreholes are not wells, and that treating them as interchangeable leads to avoidable problems (GEO, 2014). In Poland, where both the heat pump market and the private water well sector are growing (Fig. 2), the risk of such confusion is not only theoretical. Clear regulations that distinguish between the two types of boreholes and prohibit conversions without proper engineering redesign would help to prevent incidents that are entirely avoidable.
Apart from the hydraulic risks described above, BHE operation may affect groundwater quality in two additional ways: through the release of heat transfer fluids from leaking pipes, and through changes in geochemical conditions caused by thermal changes. The first risk depends on the integrity of the piping system. Modern high-density polyethylene (HDPE) pipes, if properly installed and protected from mechanical damage, are expected to last more than 50 years. However, defective joints, chemical degradation or accidental damage can lead to fluid losses. Since most BHE systems use water-glycol mixtures, even small leaks introduce glycol compounds into the subsurface (Casasso & Sethi, 2019).
The EU regulation on fluorinated greenhouse gases, revised in 2024, requires a phase-down of synthetic refrigerants with high global warming potential (GWP). Common refrigerants that have been used in heat pump compressor circuits, such as R410A (GWP ≈ 2088) and R407C (GWP ≈ 1774), are being replaced by alternatives with much lower climate impact, including R290 (propane, GWP ≈ 3) and R744 (CO2, GWP = 1). This transition reduces the climate risk from refrigerant leakage in the heat pump itself, but does not change the ground-loop fluid, which in most installations remains a water-glycol mixture.
Regarding geochemical effects caused by temperature changes, the available research suggests that the thermal changes produced by BHE operation, for single, well-spaced boreholes, which typically range of ±1 °C – 2 °C compared to the undisturbed groundwater temperature, are usually too small to cause major changes in groundwater chemistry under most shallow-aquifer conditions. Laboratory and field studies indicate that such small thermal changes, generally do not trigger substantial dissolution of heavy metals, pronounced redox disturbances, or changes in ion concentrations that would be significant from an environmental or health point of view (Bonte et al., 2011; Hähnlein et al., 2013). However, close to boreholes in large BHE fields with small borehole spacing or in aquifer thermal energy storage systems, where temperature changes may be larger and may clearly exceed this range under unbalanced annual loading, more noticeable geochemical effects cannot be ruled out.
Not all geological environments are equally suitable for BHE installation. Karst terrains, developed in soluble carbonate rocks such as limestones, dolomites and marls, present a specific set of challenges. Karst landscapes are characterised by dissolution-enlarged fractures, conduits, cavities and irregular weathering profiles formed by the long-term chemical interaction between groundwater and carbonate minerals. In Poland, karst formations occur in several regions, including the Cracow-Silesian Upland, the Holy Cross Mountains, the Lublin Upland and parts of the Sub-Carpathian zone. These areas are also attractive for housing development, which means that the demand for BHE installations can overlap with geologically sensitive locations.
From a drilling point of view, karst features create unpredictable conditions. A borehole may hit open or clay-filled cavities at various depths, which can cause sudden loss of drilling fluid, uncontrolled movement of the drill string, or collapse of the borehole walls. These conditions make grouting difficult, because the grout may flow into open karst conduits instead of filling the annular space around the BHE pipes. If this happens, the hydraulic seal between geological layers is compromised, and the borehole becomes a potential pathway for vertical groundwater migration or contaminant transport (Goldscheider & Bechtel, 2009).
Another concern, not limited to carbonate rocks, is the possible presence of anhydrite interbeds (CaSO4) in sedimentary successions penetrated by geothermal drilling. When a borehole penetrates an anhydrite layer and introduces water, either from the drilling process or by connecting a water-bearing horizon with a dry anhydrite zone, the mineral transforms to gypsum (CaSO4·2H2O). This reaction is accompanied by a volume increase of about 60 % (Goldscheider & Bechtel, 2009; Sass & Burbaum, 2010), although the actual surface uplift is constrained by the overburden stress (Sass & Burbaum, 2012). The most well-known example of this type of failure is the case of Staufen im Breisgau in Germany, where geothermal boreholes triggered anhydrite swelling and progressive uplift of the town centre. This case has had a strong influence on regulatory discussions across central Europe (Goldscheider & Bechtel, 2009).
It should be noted that karst-related problems are not caused by BHE technology as such, but by the interaction between the drilling process and a specific geological setting. In many karst areas, BHE installations have been completed successfully when a proper geological investigation was carried out beforehand and suitable drilling and grouting methods were used. Pre-drilling site assessment using geological maps, existing borehole data, and where necessary geophysical surveys, can substantially reduce the risk of unexpected subsurface conditions. The use of controlled-pressure grouting, staged grouting in zones of high grout loss, and monitoring of grout return during installation all help to achieve adequate borehole integrity even in challenging formations.
The key point is that the geological and hydrogeological conditions at the installation site are not just a factor affecting thermal performance – they are the main factor determining environmental risk. Sites with relatively uniform, low-permeability formations, such as clay-dominated Quaternary deposits, have a very different risk profile from sites on fractured or karstic bedrock. Not taking this difference into account during planning and permitting can lead to environmental problems that could be prevented by routine geological assessment. This principle applies more broadly to the sustainable construction sector, where proper consideration of site conditions is fundamental to responsible development (Szafranko, 2025).
This review shows that the environmental effects of vertical BHE installations are, in most documented cases, localised and manageable when proper design, construction and operational standards are followed. This does not mean the effects are negligible in all situations. The size of the effects depends on the local geological and hydrogeological conditions, on the quality of borehole construction, and on the long-term operating regime of the system.
Thermal impacts on the subsurface are well understood and follow predictable patterns. For single, correctly sized BHE installations, the temperature change stays within the near-borehole zone. Problems appear mainly when many installations are built close together without coordinated thermal planning, or when systems operate with a permanent yearly imbalance between heat extraction and injection. Both situations can be avoided through proper design, but they require a level of coordination at the municipal level that is currently absent in most places.
The hydrogeological risks related to BHE construction depend more on local conditions. In geologically simple settings, such as uniform Quaternary deposits with moderate permeability, the risk of creating unwanted hydraulic connections between aquifer systems is low, as long as standard grouting procedures are followed. In karst terrains, fractured bedrock, or formations with evaporite minerals, the risk situation is quite different. The examples discussed in this review show that the most serious environmental incidents associated with BHEs have been caused not by the normal thermal operation of the systems, but by problems during the drilling and grouting phase in geologically complex settings. This highlights the importance of proper site investigation before installation. Figure 3 summarises the relationship between geological setting and environmental risk level for the main impact categories identified in this review.
The risk levels shown in Figure 3 also have direct implications for the design and execution of BHE installations. In low-risk settings, standard drilling and grouting practices, together with routine control of grout return during installation, are usually enough to keep the residual risk acceptable. In medium-risk settings, including multi-aquifer Quaternary sequences and transition zones to fractured bedrock, the design should also include a careful choice of grouting materials, sealing of individual aquifer horizons, and post-installation pressure testing where this is feasible. In high-risk settings, such as karst terrains, evaporite formations, and zones with high artesian pressures, detailed pre-drilling site investigation – including borehole-specific hydrogeological assessment and, where useful, geophysical surveys such as ERT – should be treated as a basic condition for installation, not as an optional step.
The conversion of geothermal boreholes to water wells, or the reverse, is a separate but practically important issue, especially in countries like Poland where both markets are growing. The structural incompatibility of the two types of boreholes makes dual-purpose use risky, and clear regulatory guidance would be a reasonable preventive measure.
Environmental risk assessment matrix for BHE installation by geological and hydrogeological setting (own research)
The ecological effects of BHEs, especially the possible sensitivity of groundwater organisms (stygofauna) to temperature changes, have been mentioned in the literature but appear to be a minor concern for well-designed closed-loop systems. The temperature changes at distances greater than a few metres from the borehole are usually within the tolerance range of subterranean species (Hähnlein et al., 2013). Open-loop systems have a larger ecological impact in this respect.
In the context of the Polish energy transition, the main practical priority is to make sure that the regulatory and technical framework keeps pace with the speed of deployment. This includes making site-specific geological assessment stronger, especially in areas with karst or fractured bedrock, creating uniform standards for borehole construction quality, and developing subsurface thermal planning tools at the municipal level. The Directive 2018/2001 (RED II) requires that heat pumps reach a minimum seasonal performance factor to count as a renewable energy source, which provides an additional reason for proper system design. If the ground is depleted because of poor sizing or unbalanced operation, the system performance can fall below this threshold (Matusevych et al., 2025).
The Belgian case provides a useful comparison with Poland because it shows that BHE deployment is shaped not only by geology, but also by governance and spatial pressure. Belgium’s three regions – Flanders, Wallonia and the Brussels-Capital Region – have developed distinct approaches to geothermal permitting, drilling control and groundwater protection, allowing regional adaptation but also creating a more fragmented regulatory landscape (Petitclerc et al., 2025). This is particularly relevant in Brussels, where dense urban development and limited available space make vertical closed-loop systems attractive, while increasing the need to manage cumulative thermal interference between neighbouring installations. In Flanders and Wallonia, contrasting hydrogeological conditions further underline the need for site-specific assessment: unconsolidated and productive aquifer systems are common in parts of Flanders, whereas parts of Wallonia include fractured and carbonate formations where drilling quality, sealing and groundwater protection are critical. The Belgian experience therefore supports the view that BHE regulation should go beyond generic depth and spacing rules and include groundwater conditions, drilling risks and long-term thermal interaction; in high-density settings, groundwater-flux data may become a practical input for permitting, design optimisation and post-installation monitoring.
The growth of GSHP systems with vertical borehole heat exchangers is an important part of the European strategy for reducing carbon emissions from buildings. This review shows that closed-loop BHE installations, when designed, built and operated according to established standards, produce localised and limited environmental effects. The main risks to the subsurface come not from the thermal operation of the systems, but from poor site assessment, low-quality borehole construction, and in some cases from installing boreholes in geological settings that require special precautions.
Thermal changes in the ground are predictable and can be controlled through balanced system operation and proper borehole spacing. Hydrogeological risks, especially those related to drilling in karst terrain or to incomplete grouting, are construction and site-assessment issues that can be solved through geological due diligence and suitable drilling techniques. The conversion or dual use of geothermal boreholes and water wells is an avoidable risk that requires clear regulatory rules. Based on available evidence, groundwater quality impacts from standard closed-loop operation are minor.
For the ongoing energy transition in Poland and across the EU, the following practical priorities can be identified: (i) coordinated subsurface thermal planning at the municipal level, especially in dense urban area (ii) mandatory site-specific geological and hydrogeological assessment for BHE projects in areas with karst, fractured bedrock, sensitive aquifer systems and areas with high geothermal-system density, (iii) improvement of quality assurance for borehole drilling and grouting, including contractor certification and documentation of borehole sealing, (iv) incorporation of groundwater flux measurements in settings where advective heat transport may influence thermal performance or environmental risk, and (v) long-term monitoring of ground temperature and groundwater quality in areas where many BHEs are installed. Such measures would help ensure that the expansion of shallow geothermal energy contributes to decarbonisation without compromising groundwater protection or the sustainable use of the subsurface.