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Review

The Water–Energy Nexus in 26 European Countries: A Review from a Hydrogeological Perspective

by
Somayeh Rezaei Kalvani
*,
Riccardo Pinardi
and
Fulvio Celico
Department of Chemistry, Life Sciences and Environmental Sustainability, University of Parma, Parco Area Delle Scienze 157/A, 43124 Parma, Italy
*
Author to whom correspondence should be addressed.
Water 2024, 16(20), 2981; https://doi.org/10.3390/w16202981
Submission received: 12 September 2024 / Revised: 11 October 2024 / Accepted: 17 October 2024 / Published: 19 October 2024
(This article belongs to the Special Issue Water and Energy Synergies)
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Abstract

:
The significance of the interconnection between water and energy, known as the water–energy (WE) nexus, is highly regarded in scientific publications. This study used a narrative review method to analyze the existing WE nexus studies performed before 2024 in 26 European countries. The aim of this study is to provide a comprehensive analysis of the existing WE nexus to identify research gaps and to report a conceptual overview of energy consumption related to groundwater use phases, ranging from the tapping to distribution. This information is valuable as a guideline for any future estimates in this field. The results indicate that the WE nexus in 26 European countries comprises a variety of topics, including the water supply system, wastewater treatment, hydropower, desalination, and biofuel production. Most of the focus has been on fossil fuel production, while water supply and desalination were considered rarely. Italy and Portugal had the largest WE nexus. It is highlighted that there have been no studies on the WE nexus focusing on the groundwater supply system that consider the conceptual hydrological model or hydrodynamic processes. In this work, a view of these aspects was provided by taking into account different hydrogeological and hydraulic scenarios that may affect the amount of energy required for groundwater exploitation. Most scientific publications have focused on quantitative analysis. In the future, it will be necessary for WE nexus models to place a greater emphasis on governance and the implications of the WE nexus approach.

1. Introduction

Water and energy are two essential elements for human life and sustainable development that play essential roles in national security, poverty reduction, and economic sustainability [1]. The WE nexus is a novel approach for addressing the interlinkage between these two resources and sustainable resource management [2]. It is claimed that the WE nexus can enhance informed decision makers in water and energy planning and improve understanding of potential alternatives for both policy and technology. It can help resource coordinators and policy makers in energy and water conservation and sustainability [3]. Energy and water are basically interlinked [4]. It was evaluated that approximately 350 bm3 of the water withdrawal was utilized for energy production, including hydropower and bioenergy production. Energy is also required for water production. For example, various stages of water treatment and distribution, including pumping, filtration, and desalination, consume significant amounts of energy [5]. Water scarcity can directly influence energy poverty and vice versa [6]. For example, one study [7] indicated that the reduction in the production of fossil fuel contributes the reduction in water stress in Germany.
The interconnections between water and energy have been initially acknowledged in the United States since at least 1994 [8]. However, over the past years, the interaction between water and energy as the WE nexus has been a hot topic among the scientific community and public [4]. A deep and comprehensive understanding of the WE nexus is crucial to achieve sustainable resource management [7]. There are so many literature reviews about the nexus topic, and most of the existing review papers analyzed the water–energy–food nexus (WEF nexus) at a global scale, such as [4,5,6,7,8,9,10,11,12,13,14,15]. However, only limited review papers focused on two these essential elements (WE nexus), such as [16,17,18,19]. There is one review paper focused on the nexus among water, energy, and food across 26 European countries [20] that demonstrated that water and energy security is a hot topic in European countries because of energy poverty. Thus, this study is significantly different from the previous review [20] because this review considered the interaction between water and energy in different sectors, including wastewater treatment, water supply, hydropower production, biofuel production, and desalination in European countries. In addition, a conceptual outline has been reported based on technical and scientific aspects that can expose the situation of energy consumption related to groundwater exploitation.
The aim of this review is to provide a holistic view on the WE nexus in 26 European countries because despite the high number of publications on the WE nexus, a holistic and comprehensive review on the WE nexus focusing only on European countries does not exist. Therefore, this study focuses exclusively on the WE nexus in European countries to identify the research gaps and provide insights into effective management strategies and policy recommendations with a focused insight into groundwater exploitation.

2. Materials and Methods

To analyze the existing WE nexus, this study used a narrative review approach because it is comprehensive and allows coverage of a different range of topics [21,22]. This study considered papers published before 2024. The following steps have been done:
  • Research was performed using variety of databases that cover all the relevant topics that addressed the interaction between water and energy (WE nexus). Unpublished papers are excluded. The selection criteria focused on published international and English language papers. In this study, Web of Science, Scopus, PubMed, Google Scholar, and Science Direct were used as secure databases.
  • In this step, suitable keywords that provide answers to the research questions were found.
  • All the relevant papers were selected, and non-relevant papers were omitted.
  • In this final step, all of the results and key findings were summarized and written.
In this study, the following keywords were used: water–energy nexus, water energy security, water–energy nexus and desalinate, water–energy nexus and biofuel, water–energy nexus and bioenergy, water–energy nexus and water supply, water–energy nexus and water treatment, water–energy nexus and hydropower, and water–energy nexus and groundwater pumping.
This review classified and analyzed the water–energy nexus based on geography and the types of sectors involved. The ArcGIS software was used to visualize the number of the WE nexus studies in each country and the different study sectors across European countries.

3. Results

3.1. Overview of WE Nexus Studies

3.1.1. The Number of Water–Energy Nexus Studies Across European Countries

Figure 1 illustrates the number of the WE nexus studies across the different European countries. It is indicated that the most WE nexus research has been conducted in Italy and Portugal with nine studies. Both countries rely considerably on hydropower for their electricity generation. This creates a direct interconnection between water availability and energy production and highlights the significance of studying the WE nexus. However, there is a lack of WE nexus studies in some countries like Bulgaria, Estonia, Latvia, and Lithuania. Moreover, it can be concluded that there is lack of studies in some countries characterized by energy poverty, like Bulgaria (the country with the highest level of energy poverty in Europe [23]), Latvia, and Lithuania [23]. Addressing energy security in countries with energy poverty is essential to promote sustainable development and economic growth. It is important for these countries to efficiently manage their limited energy resources and increase resilience to climate change.

3.1.2. Sectors of Water–Energy Nexus Research Across European Countries

Figure 2 illustrates the water–energy nexus across various sectors in European countries. It demonstrates that the WE nexus is explored based on a range of topics, such as household water supply, bioenergy production, hydropower, water treatment, and water desalination. In each European country, the focus varies based on their specific needs. For example, in Italy, the WE nexus approach strongly emphasizes renewable energy sources, particularly hydropower production, to enhance energy efficiency and reduce carbon emissions. However, in other countries like Germany, Spain, and the Netherlands, studies are more focused on the bioenergy sector.
Figure 3 shows the number of WE nexus studies in different sectors in all of Europe. It is indicated that most of the WE nexus studies focused on bioenergy production. Next to bioenergy, the second largest number of studies was allocated to hydropower production. However, desalination has the lowest number of WE nexus publications with one publication in Portugal to overcome seasonal water scarcity and increase water efficiency. Ref. [24] used the WE nexus approach in Portugal to compare two different scenarios for desalination considering economic aspects. There is a lack of studies on the water supply in the context of the WE nexus for the domestic sector, specifically considering the hydrological conceptual model or the type of aquifer for pumping of groundwater. This is significant because pumping groundwater from aquifers with high permeability requires less energy since water moves readily, reducing the resistance against the pump. Understanding the type of aquifer is essential for sustainable groundwater management. Over-extraction from low permeability aquifers can lead to rapid depletion and land subsidence. Only one study in Romania [25] considered the WE nexus for drinking water purposes in the whole life cycle.

3.1.3. Overview of Existing Approaches for the Water–Energy Nexus

Table 1 provides a summary of the WE nexus studies in 26 European countries. It can be seen that during the past several decades, various approaches have been utilized to analyze the interaction between water and energy in various contexts. These approaches include life cycle assessment (LCA), optimization, statistical analysis, hydrological models and economic approaches. Some researchers used scenario analysis to discover the potential tradeoffs of the water–energy–food nexus for the future.
The optimization approach is a decision-making assistance method that helps to find the best possible appropriate solution for the water–energy nexus approach [26,27] by improving the energy efficiency and achieving sustainable water resource management across various scale [28,29,30].
Life cycle assessment integrated with the WE nexus has been used by various scholars, such as [25,31,32,33,34,35]. LCA is considered a standard approach to assess footprint indexes and energy performance [9]. Considering the LCA throughout the whole life cycle of water and energy systems contributes to an understanding of the interlinkage impacts of water and energy at different stages. Moreover, LCA can help to identify the environmental impacts (including greenhouse gas emission, water consumption, and resource depletion) across the different stages of water and energy production and consumption. This approach can help to recognize the stages with powerful negative impacts. This information is important for decision makers for sustainable prioritization. For example, a study by [31] identified that in biofuel production, the crop production stage has the largest environment impact in terms of water resource consumption. Another study by [32] demonstrated that groundwater abstraction for drinking water supply consumed high levels of energy compared to surface water abstraction. Moreover, another study by [36] used LCA to compare the environmental impacts of different alternatives for wastewater treatment technology to find more sustainable alternatives for water and energy use efficiency. However, it is challenging to choose a single functional unit that captures all these aspects because the WE nexus is a multi-functional unit.
Input–output analysis is a common and widely used approach in WE nexus studies to quantify the interlinkage between water and energy by evaluating the monetary and physical flow.
Hydrological models play an important role in the WE nexus by providing information about water availability and water demand. It helps to predict river flow and water revisors for hydropower management. For example, one study [37] used a semi-distributed (CWatM) model to simulate the future water withdrawal and streamflow under climate change scenarios for hydropower production. Another study [38] applied a conceptual semi-lumped hydrological model to assess the impact of climate change and seasonal streamflow on hydropower production. It can also help to predict some events like drought and floods in the future and evaluate the impact of these event on water and energy systems. For example, Refs. [39,40] used the watershed simulation and forecasting system (WSFS) to estimate the impact of severe drought on future hydropower production. Thus, considering hydrological models in the WE nexus benefits decision makers and planers to ensure the sustainable and efficient use of both water and energy resources.
Technological aspects are considered in WE nexus studies. For example, [41] compared different solar panels for freshwater production. However, economic dimensions are considered rarely. Political and social dimensions are not considered. The role of the behavior of the people and culture are not considered in WE studies.
Table 1. Summary of WE nexus studies in European countries.
Table 1. Summary of WE nexus studies in European countries.
Method Key FindingsType of WaterCountryReference
Water footprint + nexus + economic aspectsThere is an essential need to produce a new integrated approach that can manage water for electricity production that considers economic aspects of the water–energy economic Blue waterItaly [42]
LCA + WE nexusEnvironmental impacts of water for energy production and energy for water production (from water withdrawal, water treatment, and distribution) were considered. Moreover, the results demonstrated that water abstraction from surface water is less energy demanding than groundwater.Surface water and groundwaterItaly [32]
Semi-distributed hydrological model CWatM + optimization method
Changes in future prices and climate on hydropower were considered.		An increase in temperature contributes to a slight increase in hydropower performance.Surface water Italy [37]
Urban ecological infrastructure methodIt provides energy for 600 households by improving land usability and environmental sustainability. This mini hydropower plant also reduces the risk of flood.Surface water Italy[43]
Conceptual semi-lumped hydrological mode + bottom-up approach The impact of change in climate, price, and seasonal streamflow on hydropower production was evaluated.Surface waterItaly [38]
WE nexus + LCA + water footprintThe crop cultivation step has the largest impact on water resources. Biogas production in terms of water consumption was unsustainable.Blue waterItaly [31]
Dynamic simulation tool TRNSYS
Energy, economic and environmental model		Two different solar plants sources for freshwater production in domestic sector was compared Surface water Italy[41]
Techno-economic assessment Different scenarios were used to find the best solution to reduce water consumption. Surface waterGermany, Portugal, United Kingdom, and Norway[44]
LCAEnvironmental impacts of different alternatives for wastewater treatment technology were compared to find more energy and water use efficiency alternatives. Grey water (surface water)Germany[36]
InterviewComparison of WE nexus studies in 8 citiesGrey waterGermany[45]
ReviewConsidering the nexus between water and energy for bioenergy production contributes to poverty reduction and food security.Blue waterGermany[46]
Economic cost-benefit analysisUsing green roofs and a photovoltaic system can increase the PV yield by about 0.3%, reduce demand for heating (0.1%), and reduce runoff mitigation (30%).Green waterGermany[47]
ForeseerFuture water, energy, and land demands were estimated.Blue waterGermany[7]
Watershed simulation and forecasting system (WSFS) hydrological model + RCPThe impact of drought on water resources and hydropower production was analyzed. Severe drought can substantially impact on water resources and hydropower production. Surface waterFinland [39]
Hydrological model (WSFS) + Energy PLANThe impact of severe drought on energy production in the future was assessed. The results indicate that stress on energy will be reduced by 2030 because of the development of nuclear energy. Surface waterNordic countries[40]
Interview with water user associationsWUAs can play an important role in water management.Blue waterSpain[48]
Prospective approachBiofuel production had a devastating impact on water resources.Blue water and grey waterSpain[49]
Irrigation efficiency and energy consumption using historical data Irrigation for the agricultural sector required a high amount of energy in Spain.Blue waterSpain [50]
LCAThe environmental impact of carbonization in different energy scenarios was compared.Surface waterSpain[33]
Water footprint + different scenariosThe impact of virtual water imports on water and energy for biofuel production was evaluated. Importing raw materials from a country with abundant water resources can reduce water stress in Spain.Blue waterSpain[51]
Medium-term hydrothermal coordination (MTHC) + unit commitment and dispatch Hydropower generation in different years was compared in terms of cost and the volume of production. Surface waterCroatia[52]
Statistical models, including the ordinary least squares (OLS) + the geographically weighted regressions (GWR) + GIS The results indicated the trend and provided solutions to manage resources.Grey waterFrance[53]
Water–energy–carbon nexus + LCAReducing the use of herbicides and pesticides can improve water quality and reduce the demand for energy to treat raw water. Grey waterNorway, France, Italy, Canada[34]
Water balance + water impact assessmentThe impact of bioenergy production was compared using different scenarios.Blue water France[54]
Water–energy nexusDifferent types of energy production were compared. The result indicated that it is essential to produce a national alternative that can ensure climate resilience in the energy and water sectors.Grey waterGreece[55]
Global macro-econometric model (E3ME)A new model was produced that forecasts future energy demand and carbon emissions. Surface waterThe Netherlands and Latvia[56]
Environmental input–output (EIO) model
+ water efficiency + energy efficiency + carbon emission index		The result indicated that all of these indicators have higher value compared with the global average. Blue waterEU27 countries [57]
Water footprint + carbon footprint + LCAThe water–energy–carbon nexus for wastewater treatment in different industrial sector was evaluated. Grey waterIreland [35]
Statistical analysis (time series)This study evaluates the role of North Atlantic Oscillation (NAO) and East Atlantic pattern (EA) on the WE nexus. Surface water and groundwaterPortugal [58]
EPANET 2.0 (simulation model)Using renewable energy can increase social performance by increasing air quality and prompt eco-efficiency.Surface waterPortugal[59]
Comparison of three scenarios Different scenarios were compared to find the best one for water reuse, including irrigating golf courses with reused water, municipal irrigation with reused water, and irrigating both locations with reused water. Grey waterPortugal[60]
Mathematical modeling
Water–energy–greenhouse gas emissions nexus		Effect of flood on the WEG nexusSurface waterPortugal[61]
Optimization model + cost analysis Two different strategies (centralized and decentralized) were compared.Sea waterPortugal [21]
Top-down and bottom-up approachesThe top-down approach is easy to apply because it does not need a lot of data. It is applicable with minimum data. It can provide accurate evaluation of energy inefficiency. The bottom-up approach can evaluate more details and provides more detail. It also enables the evaluation of disputed energy in pipes.Groundwater and surface waterPortugal[62]
Water and energy efficiency + statistical analysisHigher energy consumption is related to the shower. A device is used to control water and energy consumption. Blue waterPortugal[63]
WAT + RCP
WELC nexus 		Under different forest scenario, no significant change in water flow. Surface waterRomania[64]
LCAThe main consumers of energy in drinking water treatment processes are pumping water and wastewater treatment. Grey waterRomania[22]
Water footprint + gross water consumption, net water consumption, and water balanceThere is a need to provide a method that considers water evaporated from dams. Surface waterRomania[65]
Water–energy nexus using a water footprint approachThe impact of energy production on water resources in different Swedish countries was evaluated.Blue waterSweden [66]

4. Discussion and Conclusions

4.1. Guidelines for WE Nexus Application to Groundwater Resource Use

In relation to the exploitation of groundwater, this work provides an overview of the energy consumption associated, ranging from the extraction to the distribution (see Figure 4). This information is capable of giving guidelines for future estimations in this specific sector. The following proposal scheme is based on technical and scientific knowledge derived from real case studies distinguishing variable scenarios depending on the hydrogeological and hydraulic local settings. The elements outlined in the following sub-chapters can be used as the basis for a solid analysis and models of energy consumption related to groundwater exploitation. The following case studies are mainly derived from works carried out in the Po Valley that, like many other areas of the world, is subject to an increasing demand for groundwater related to multiple uses like drinking, bottling, irrigation, livestock farming, and industry. This territory can be considered an excellent model for the application of the WE nexus as it has heterogeneous features [32] linked to different water-demanding contexts that vary according to geographical location, with lower pressure in the mountainous areas at the edge of the basin and higher pressure in the plain area where the main cities and water-demanding activities are located.

4.1.1. Groundwater Tapping Work Construction

The first energy consumption to be taken into account comes from the implementation of groundwater exploitation works, which are variable depending on the hydrogeological context. With regard to spring tapping works, energy use is closely related to the size, the type of building, and the areal location of the operation. A large catchment work requires more material and thus energy use than a small one, and different types of intakes can be more or less energy-impactful elements (a drainage gallery or an infiltration gallery requires more energy for their construction than a simple artifact to convey outcropping groundwater). The location of the spring may also require higher or lower energy efforts, depending on the dynamics of access to the area. For example, the location of a spring along a slope inaccessible and covered with vegetation requires a high energy effort to build roads and to transport materials, compared to a location on the valley floor that is easily accessible and close to existing utilities. Stepping to contexts where groundwater does not outcrop, it is necessary to realize tapping work construction to drill the subsurface (overall wells [67]). The energy for their construction is directly proportional to the depth of the tapped aquifer (see the tapped depth of well 1 and well 2 in Figure 4) that can vary from a few meters to many hundreds of meters below the surface as observed in the context of the Po Plain [68] or across the entire Italian peninsula (ISPRA water well database [69]) in relation to the different geological settings. In addition, the borehole diameter at the same depth scenario could be an element of energy expenditure during the drilling phase in the same way as the lithology (and in particular mineralogy) of the geological medium greatly affects the energy required to drill the well, in relation to the hardness of the material of which the drilled subsoil is composed (Mohs scale is the main indicator of this character). The drilling rate could increase in an inversely proportional manner to the Mohs hardness, from cm/min to m/min as reported in Hoseinie et al. 2012 [70]. Other technical aspects specific to each well may alter the energy impact associated with their construction, such as the pipe material (the most common are PVC, concrete, and steel). In addition, the following general drilling methodologies could be considered: (i) less impactful methods, in cases of wells drilled by manual digging or with percussion systems; and (ii) more impactful methods, if excavators or well drilling rigs are employed.

4.1.2. Groundwater Tapping

A second and important cause of energy consumption to report is certainly related to groundwater tapping. It could vary based on the function of the hydrogeological context, and it is strictly related to the eventual necessity to uplift groundwater through the use of pumps. Energy consumption related to tapping in the context of springs or for example in the Po Plain “fontanili” context sensu [71] can be considered null or negligible because of the natural outflow of groundwater from the aquifer (see the spring in Figure 4). In contrast, in wells, energy consumption is directly proportional to the installation depth of the pumping system, its flow rate and its efficiency. Groundwater tapping could occur using electric pumps or combustion engine pumps, like Power Take-Off models (PTO) for tractors. The well and the pump efficiency can decrease over time due to possible well clogging phenomena. It mainly depends on the infiltration water’s origin and consequently physical, chemical, and/or biological processes that can deposit significant filling material and/or allow biofilm growth inside the well screen lights and the pumping equipment, decreasing the permeability and the efficiency of the supply system [72,73,74]. The original permeability of the exploited aquifer also affects energy consumption, facilitating withdrawal in locations with greater permeability (see differences in permeability between the shallow [well 2] and the deep [well 1] aquifers in Figure 4). This element is frequently heterogeneous depending on geological setting, for example, as shown in the context of the Parma city area [68], where [75,76] calculated a hydraulic conductivity varying from 1.2 × 10−5 to 4.9 × 10−5 m/s in coarse-grained horizons and from 9.3 × 10−0 to 1.3 × 10−0 m/s in fine-grained horizons.

4.1.3. Groundwater Treatment

As observed in several of the above mentioned studies (see Table 1), a possible subsequent step after extraction that could require energy efforts is groundwater treatment. Its entity could depend on the original condition of the quality of the groundwater and its destination of use. Many aspects can affect water quality. These factors could be related to the natural hydrogeochemical characters of each aquifer, like mineral composition above all, as demonstrated in the Po Plain (e.g., [77]). As an example of these phenomena, numerous testimonies of hexavalent chromium remaining on the Po Valley and precisely in the Northern Apennine have been linked to the mineralogical nature of ophiolitic aquifers. In these cases, peridotites rocks are able to impart non-potable characteristics to groundwater (e.g., [78,79,80]).
To a greater extent, anthropogenic factors can make the aquifer polluted and thus necessitate groundwater treatment. As reported in several studies conducted in the Po Valley, it could be attributed to numerous possible origins, like industry [81,82,83], fertilization and livestock [84,85,86], sewage [87], landfills [88], pharmaceuticals [89], cosmetic products [90], and many others. Saltwater intrusion and upwelling are also human-induced hydrogeological phenomena that occur through pumping and can reduce water quality as schematized for Well 3 in Figure 4 (e.g., [91]). Water quality can be very different even at short distances within the same hydrogeological context, depending on the location of sources of contamination (see the lowland aquifer in Figure 4). Aqueduct or potable water systems may need more operations to purify groundwater considering the human supply, but they also can be extended to various entities for uses other than the tapped water. In cases of pristine groundwater, the treatments, and consequently the energy consumed for this purpose, may be negligible.
Even if characterized by lower consumption, it is also proper to include in this step the energy used for the qualitative and quantitative monitoring actions, which takes place through studies and analyses targeting the groundwater resource by the owners or the public agencies, according to regulations such as the Directive 2000/60/EC of the European Parliament (23 October 2000) and subsequent updates that establish a framework for community action in the field of water policy, including groundwater in the European Union.

4.1.4. Groundwater Distribution System

The last field to be reported that may require energy consumption is that related to the distribution of groundwater resources. As exemplified in Figure 4, the energy to be used in different contexts is closely related to the necessity of contrasting gravity. In a mountain area or where a territory has sufficient topographic gradients, gravity could facilitate the distribution of the resource from origin to delivery without the use of additional energy. On the contrary, in flat settings or in conditions where it is necessary to impart pressure to the distribution network, energy consumption could be drastically higher and related to pumping systems. In these last contexts, there are infrastructural strategies that can reduce the energy used and combine it with gravity using artificial reservoirs or water towers. However, these strategies may still require great efforts in the construction phase. The design of these works is governed by hydraulic projects and principles that can vary substantially depending on a multitude of natural and anthropic factors. The most impactful factors are related to the pressure required to allow the distribution, such as altitude differences, an optimized design of the pipe network, and water losses from the pipe system. In Italy, the National Institute of Statistics (ISTAT, 21 March 2023) revealed that in 2020, 42.2% of pumped groundwater for aqueduct supply was lost due to leaky distribution systems. An exhaustive overview of the abovementioned themes was given by Vilanova and Balestrieri 2014 [92] and Coelho and Andrade-Campos 2014 [93]. Another energy expenditure related to water distribution can be associated with the transport of water resources where local conditions do not allow for a supply system, which often takes place by road, train, or ship. These conditions could be attributed to hydrogeological features combined with the climatic features of an area such as the absence of exploitable aquifers or the presence of drought periods.

4.1.5. The Proposal of a New Application for Hydrogeological Modelling

The combination of the elements in the previous paragraphs influence the energy consumption related to groundwater use. As detailed in this work, they are closely linked to the hydrogeological and hydraulic context. Here, nexus analysis and life cycle assessment methodologies (e.g., for the Po Plain context [32]) are applied to evaluate and compare different process alternatives aimed to support the investigation of new solutions and reducing the energy intensity and the release of greenhouse gas (GHG) emissions while maintaining high-quality services under future pressures resulting from climate change. These methods are mainly focused on the analysis of past or present scenarios, and the use of future-oriented modelling and/or simulations could provide them with greater predictive utility. In environmental science, many kinds of predictive modelling exist. In this case, in relation to energy consumption, the points above have shown the importance of hydrogeology, which is a sector where numerical models are currently used for a variety of purposes. First, they are employed for hydrogeological balance analyses, where they constitute advanced tools useful for quantitative studies mainly aimed at the determination of exploitable groundwater volumes, for tapping work construction planning and for predict climate change scenarios (e.g., ref. [94] as an example for the Northern Apennine context). Numerical models are also employed for environmental hydrogeology that can be used for the prevention and/or remediation of areas contaminated by both soluble (like nitrates) and non-soluble (like hydrocarbon non-aqueous phase) contaminants in groundwater [75,76,95,96]. In conclusion, combining energy consumption data and hydrogeological information, this work offers a new use for numerical modelling of the WE nexus and thus to the evaluation of alternative scenarios useful for energy savings in the field of groundwater.

5. Conclusions

This current review analyzes the existing WE nexus in 26 European countries. It was remarked that the analysis of the WE nexus in European countries was conducted in different types of sectors, including water supply, water desalination, hydropower, and biofuel production. In each European country, the focus varies based on their specific needs. Among the WE studies in Europe, those focused on the interlinkage between water and energy in biofuel production represent 50% of scientific papers, showing the significance of this research topic in European countries. It is indicated that most of the WE nexus studies focused on surface water and blue water resources. The majority of the studies considered groundwater integrated with surface water as blue water. There is lack of studies that separately consider groundwater. Separate evaluations of groundwater would provide deeper insights into its role and vulnerabilities in the context of the WE nexus. Moreover, 7.5% of all studies involved groundwater resources.
The existing bibliography related to the WE nexus in Europe is illustrative of a still limited holistic approach to the field of water (and groundwater). For studies of the WE nexus, there are a variety of approaches that are currently used, but there is not a single and standard approach that can be utilized for the WE nexus globally. The first fundamental challenge for the evaluation of the interaction between water and energy is data availability at regional and global scales. For example, hydrogeological and hydraulic information related to withdrawal are often not made available by public agencies or private companies. Information on this critical theme is essential to standardize data collection approaches, which can facilitate a high degree of estimation accuracy. This work showed in detail how several hydrogeological and hydraulic factors could be extremely influential on energy consumption in the sector of groundwater exploitation. For this reason, a conclusive proposal is to use methods capable of incorporating all groundwater-related energy consumptions with a first step represented by the creation (or consultation) of conceptual and numerical models based on hydrological principles. Numerical models could support and guide the energetic consumption computations related to the entire groundwater use steps (see Section 4.1). From these insights, their application is proposed to be extended to WE nexus estimates to further support the results with scientifically based models and simulations and to make it even more of a useful tool for a predictive look at climate change and related socio-economic and energetic evolutions.

Author Contributions

Conceptualization: S.R.K., R.P. and F.C.; research design: F.C., S.R.K. and R.P.; supervision: F.C.; formal analysis: S.R.K. and R.P.; methodology: S.R.K. and R.P.; project administration: F.C.; writing—original draft preparation: S.R.K., R.P. and F.C.; editing: F.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data used in this analysis are publicly accessible at ISPRA https://www.isprambiente.gov.it/it (accessed on 24 August 2024) ISTAT https://www.istat.it (accessed on 24 August 2024).

Conflicts of Interest

The authors declare no conflicts of interest.

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Water 16 02981 g001
Figure 1. The number of WE nexus studies across the different European countries.
Figure 1. The number of WE nexus studies across the different European countries.
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Figure 2. Sectors of water–energy nexus research across European countries.
Figure 2. Sectors of water–energy nexus research across European countries.
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Figure 3. Number of WE nexus studies in different sectors.
Figure 3. Number of WE nexus studies in different sectors.
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Figure 4. Ideal scheme of the energy consumption rate related to groundwater use and hydrogeological and hydraulic factors.
Figure 4. Ideal scheme of the energy consumption rate related to groundwater use and hydrogeological and hydraulic factors.
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Rezaei Kalvani, S.; Pinardi, R.; Celico, F. The Water–Energy Nexus in 26 European Countries: A Review from a Hydrogeological Perspective. Water 2024, 16, 2981. https://doi.org/10.3390/w16202981

AMA Style

Rezaei Kalvani S, Pinardi R, Celico F. The Water–Energy Nexus in 26 European Countries: A Review from a Hydrogeological Perspective. Water. 2024; 16(20):2981. https://doi.org/10.3390/w16202981

Chicago/Turabian Style

Rezaei Kalvani, Somayeh, Riccardo Pinardi, and Fulvio Celico. 2024. "The Water–Energy Nexus in 26 European Countries: A Review from a Hydrogeological Perspective" Water 16, no. 20: 2981. https://doi.org/10.3390/w16202981

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