Introduction

Aquifer thermal energy storage (ATES) is a sustainable and economical space heating and cooling technology. This eco-friendly technique is a great way to balance a mismatch between demand and supply in suitable aquifer system conditions. This open system needs sustainability consideration so the system works permanently and without problems and prevents depletion so it remains available in the future.

Sustainability development and energy resources significantly impact societies (Dincer and Rosen, 2001), so getting an advantage from ATES needs sustainability development to use the ATES system efficiently. Greater utilization efficiency creates a system that lasts a long time (Dincer and Rosen, 2005).

Aquifer thermal energy systems as a backup renewable source of energy have been applied worldwide for heating and cooling, and their environmental advantages have been shown in many studies (Todorov et al. 2020; Turgut et al. 2009; Paksoy et al. 2004; Fleuchaus et al. 2018; Dincer and Rosen 2011; Morofsky 2007; Andersson 2007; Bloemendal and Hartog 2018). Many researchers studied operational principles, such as Lee 2013; Todorov et al. 2020; and Nordell et al. 2015. The feasibility study is one of the most important investigations of ATES in the research (Stemmle et al. 2022; Ramos-Escudero and Bloemendal 2022; Sommer et al. 2015; Courtois et al. 2007, 2008; Paksoy et al. 2004). A Worldwide ATES potential assessment was created by (Tas et al. 2023; Bloemendal et al. 2015; Fleuchaus et al. 2018; and Lu et al. 2019a-b). ATES suitability was studied by Rauf et al. 2023. Bloemendal et al. 2016; Bloemendal et al. 2014; Hahnlein et al., 2013; Dincer and Rosen, 2004 clarified using renewable energy to achieve sustainability. More studies about the design of ATES have been done by (Bridger and Allen 2005; Andersson 2007).

Multi-criteria decision Analysis (MCDA) is a method used to identify criteria with alternative scores, weigh all the criteria, and finally combine them with the rating score, creating a solitary outcome. ArcGIS multi-spatial data is used to interconnect with MCDA to get a tremendous benefit for creating an ATES sustainability map (Rauf et al. 2023; Chen et al. 2010). One of the well-known techniques of MCDA is the Analytic Hierarchy Process (AHP) used in the study of (Saaty 1977, 1980; Saaty and Vargas 1991; Ohta et al. 2007; Marinoni et al. 2009; Drobne and Lisec 2009; Ferretti 2011; Chabuk et al. 2017; Lu et al. 2019b; Alkaradaghi et al. 2020; Eddouibi et al. 2021; Stemmle et al. 2022; Rauf et al. 2023). Integrating the Geographic Information System will improve mapping perspectives and accomplish great work (Sayl et al. 2022).

In this study, the most sustainable places will be shown in the Halabja-Khurmal Sub-basin to ensure the long-lasting usage of ATES. This work has the novelty in sustainability investigation in the Iraqi Kurdistan region, which is very important in aiming to give a guarantee to stakeholders that the application of the aquifer thermal energy storage system in the studied area is necessary and sustainable to get a benefit from the using the renewable aquifer thermal energy storage for heating and cooling. People in this area are suffering from an electricity shortage, so by applying this system and calculating its long-lasting future, people will feel comfortable and help the government find the solution to the electricity shortage. The sustainability indicator in this study is based on six criteria: groundwater transmissivity, groundwater temperature, groundwater discharge, groundwater chemistry, population density, and per capita GDP. These criteria are essential in determining the sustainability of a region.

Study area

The location of the Halabja-Khurmal sub-basin is in Halabja governorate, the northeastern part of Iraq, Fig. 1, with a population of 121,785 in 2023 based on the Sulaimani Statistical Directorate. The studied area is characterized by the Mediterranean-type continental interior climate (Abdullah et al. 2019). It has a hot summer and cold winter with a temperature (3–43) °C with 622-mm average annual precipitation (Rauf et al. 2023). This sub-basin covers an area of 489 square kilometers. Technically, the region is complex and situated within the Western Zagros Fold-Thrust Belt. Geologically, the exposed lithological units are from the Triassic to the Quaternary period, Fig. 2. The initial category of aquifers encompasses intergranular, fissured, karstic, fissure-karstic, aquitard, and aquiclude types.

Fig. 1
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Location map of the Halabja-Khurmal sub-basin

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Fig. 2
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Geological map and testing locations of the Halabja-Khurmal sub-basin. (Modified after Sissakian and Fouad 2014)

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Methodology and data collection

The input criteria

The critical criteria for evaluating the ATES sustainability are six parameters combined for assessing the sustainability of the Halabja-Khurmal sub-basin through groundwater transmissivity, groundwater temperature, groundwater discharge, groundwater chemistry, population density, and per capita GDP. As each parameter's effect on ATES's sustainability differs, the GIS-based multi-criteria decision analysis (MCDA) method approaches an accurate decision analysis for each criterion.

An analytic hierarchy process (AHP) is a well-known technique in the MCDA method due to its flexibility, simplicity, implementation within a GIS environment, and users' ability to derive the weight of criteria in maps (Malczewski 2007; Rauf et al. 2023). AHP is based on a pairwise comparison approach, which compares the criteria based on the degree of importance with each other (Ishizaka and Nemery 2013).

Many field trips in the studied sub-basin were made to investigate the area's geology, soil, lithology, and hydrogeology. Then, thirty boreholes were selected for groundwater sampling. The samples were collected in clean, sterilized 330-ml bottles. Samples were filtered at the time of collection using a preconditioned plastic filtering device with the membrane filter (0.45 µm) and then acidified with nitric acid (1 + 1). In the laboratory, calcium solution (dissolved 630 mg CaCO3 in 50 ml 1 + 5 HCl) and then diluted to 1000 ml) was added to each sample as the standard solution for each metal is 1000 mg/l, and a working standard solution prepared from (1000 mg/l) standard solution for making calibration curve, to each (100 ml) sample added (25 ml calcium solution). Iron and manganese were then analyzed using a graphite furnace. Groundwater temperatures were measured in situ at static water level and were measured monthly between April 2022 and March 2023 in 40 boreholes. Groundwater discharge and transmissivity were derived from the well-pumping test analysis of fifty boreholes in the study area. Figure 2 illustrates the location of the wells that have been tested. Data acquisition is collected from different sources to create criteria for assessing ATES's sustainability, as shown in Table 1 and Fig. 3. ArcGIS was applied to make the required maps using spatial analysis tools and then converted to a raster (30*30) pixel size. All the input criteria are depicted in Fig. 4.

Table 1 Data source for creating criteria used in the ATES sustainability map
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Fig. 3
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Flowchart of creating the ATES sustainability map in the Halabja-Khurmal sub-basin

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Fig. 4
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Input maps for the ATES sustainability: A Groundwater transmissivity, B Groundwater temperature, C Groundwater discharge, D Total iron concentration, E Manganese concentration, F Population density, and G Per capita (GDP$)

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Groundwater transmissivity

Groundwater transmissivity is a crucial parameter in the sustainability of the ATES system. It directly affects the system's efficiency, capacity, and overall performance in storing or retrieving thermal energy in the aquifer.

Groundwater transmissivity measures the speed at which water passes through a unit width of an aquifer under the influence of a hydraulic gradient. Transmissivity is essentially the result of multiplying hydraulic conductivity by the thickness of the reservoir.

In the ATES system, transmissivity is commonly employed to assess the aquifer's ability to supply water to a well, which determines the aquifer's capacity and plays a significant role. ATES systems store thermal energy in aquifers by injecting or extracting water from wells. Transmissivity determines how much water can flow through the aquifer under a given hydraulic gradient. It is essential for ATES systems, as the rate of water movement through the aquifer influences the heat exchange process (Ramos-Escudero and Bloemendal 2022). The heat exchange efficiency in ATES systems depends on the ability of water to move through the aquifer. Higher transmissivity allows for more effective heat exchange between the injected or extracted water and the surrounding rock or sediment. Transmissivity, the aquifer's porosity, and temperature characteristics influence the ATES system's overall capacity (Tas et al. 2023). A higher transmissivity means the aquifer can supply or absorb more water, impacting the system's storage and retrieval capabilities. Knowledge of transmissivity helps in designing wells for ATES systems. The aquifer's transmissivity influences ATES's sustainability to optimize heat exchange and system performance. The Halabja-Khurmal sub-basin has transmissivity ranges between 0.11 and 2254 square meters per day; the most transmissivity is in the north and south of the sub-basin, as illustrated in Fig. 4A.

Groundwater temperature (°C)

Aquifer thermal energy storage systems use the natural properties of ground and groundwater to reserve and restore thermal energy for heating and cooling purposes. These systems leverage that below a certain depth (usually around 10–15 m), the ground temperature remains relatively stable and equals the average annual air temperature (Lee 2013).

The process involves circulating a heat transfer water through wells to exchange heat with the ground. This extraction and storage of heat during one season and its subsequent use during the opposite season effectively creates a thermal energy storage system. Additional storage might be required if the demand for heating or cooling exceeds what is available from the stored thermal energy. Seasonal climate changes naturally facilitate natural storage systems because the ground and groundwater passively absorb thermal energy (Nordell et al. 2007).

Groundwater temperature is recorded monthly at 40 boreholes across the Halabja-Khurmal sub-basin. The groundwater temperature has been almost stable in Celsius from April 2022 to March 2023. The mean groundwater temperature at the individual measuring boreholes was between 16.8 and 23.8 °C, Fig. 5. In the area of interest with the altitude of the sub-basin, the lowest groundwater temperatures occurred in the Byara district, a mountainous area. The reasonable explanation for that is that the mountainous area is colder than the other areas, and due to snow melting, the groundwater temperature in the aquifers beneath the mountain decreases. Temperatures are increasing in the central toward the southwest side of the sub-basin. The highest values were recorded in the areas close to Darbandikhan Lake as the effect of the water body on the groundwater in the surrounding area, Fig. 4B. This vital criterion was used twice with different ratings because the rating for heating purposes differs from that for cooling purposes, as shown in Table 2.

Fig. 5
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Time versus temperature in the study area from April 2022 to March 2023

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Table 2 Criteria class, rating, and weighting used in the ATES sustainability map
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Groundwater discharge

The ATES system relies on groundwater as its primary storage medium. Ensuring a sustainable and appropriately discharged volume of groundwater is imperative for the continued and efficient operation of the ATES system. In the study area, the extraction rates observed from existing wells exhibited a notable range, spanning from less than five meters per second to an impressive over twenty meters per second. This considerable variability underscores the dynamic nature of groundwater availability in the region. Consequently, the productivity of the aquifer assumes heightened importance, emerging as a pivotal parameter that significantly influences the overall sustainability and performance of individual ATES systems within this diverse and variable hydrogeological context (Stemmle et al. 2022). This criterion, as aquifer productivity, becomes essential for optimizing pumping rates, managing water extraction, and ensuring the long-term sustainability of ATES operations tailored to the specific characteristics of the study area Fig. 4C.

Groundwater chemistry

Groundwater chemical composition significantly impacts the functionality of ATES systems and poses potential risks to their components. The most common issue encountered is the gradual clogging of wells, which results in increased water flow resistance and reduced well capacity over time. Scaling, observed as precipitation within the above-ground components and clogging within wells, gravel packs, and adjacent aquifers, diminishes aquifer permeability due to chemical precipitates. This phenomenon is frequently encountered in ATES systems, especially with the precipitation of carbonates in systems operating above 85 °C and Fe and Mn oxides forming in low-temperature systems < 40 °C (Jenne et al. 1992). Changes in water chemistry cause the precipitation of Fe and Mn oxides. Groundwater quality is an essential parameter (Khudair et al. 2021). In the studied area, the groundwater temperature is below 40 °C, so only iron and manganese, which are essential for the sustainability of the ATES system, were analyzed. Hence, in our assessment, only iron and manganese, which are essential for the sustainability of the ATES system, were analyzed. WHO (2008) recommends iron and manganese concentrations below 0.3 mg/L and 0.1 mg/L, respectively (Robey et al. 2014). In the area of interest, the total iron and manganese quantity is lower than the limit provided by WHO, Fig. 4D and E.

Population density (pop/km2)

Population density measures the number of people per unit of land area. This indicator is an exposed measure to assess sustainability, reflecting the potential living spots suitable for applying the Aquifer Thermal Energy Storage system. Typically, higher population density correlates with increased potential of the ATES system, indicating greater sustainability in such scenarios. According to the U.S. Census Bureau, in 2023, Iraq's population density was 94.4 pop/km2. In the study area, the population density of the Halabja-Khurmal sub-basin is 243.8 pop/km2. Figure 4F depicts the population density of the study area's districts population density separately.

Per capita (GDP $)

GDP per capita (current US$) represents the gross domestic product per person, obtained by dividing the GDP by the midyear population. The GDP is the total amount of products and services produced by all resident producers in an economy, accounting for taxes on products and excluding subsidies. This calculation is done without deductions for the depreciation of manufactured assets or the depletion of natural resources. The data is presented in current U.S. dollars. The information is sourced from World Bank national accounts data and OECD National Accounts data documents under a CC BY-4.0 license and aggregates using a weighted average method. This GDP per capita is an annual figure and is part of economic policy and debt, falling under national accounts in U.S. dollars at current prices.

This indicator is chosen for its adaptability in sustainability assessments, reflecting the capacity to respond effectively to unforeseen circumstances. Typically, regions with higher per capita GDP and economically developed areas tend to lean more towards utilizing clean energy than less economically developed regions (Lu et al. 2019b).

As of 2022, Iraq's Gross Domestic Product (GDP) per capita stood at 5937.2 US dollars, Fig. 4G, and based on the Kurdistan Region Statistical Office (KRSO) in 2011, the GDP of the Iraqi Kurdistan Region is 4,452 US dollars. This figure indicates that Iraq's GDP per capita is approximately 4.7 percent of the global average (World Bank 2022). GDP affects renewable energy consumption primarily in high- and low-income countries (Omri and Nguyen 2014). Its role in fostering economic growth, emphasizing its contribution to energy security, especially for countries reliant on imported fossil fuels (Al-Mulali et al. 2013), could drive economic growth more effectively than the reverse (Shahbaz et al. 2015).

Determination of criteria rating and weighting

Determining the criteria rating and weighting involves employing the analytical hierarchy process (AHP) within multi-criteria decision analysis (MCDA) to establish the sustainability of Aquifer Thermal Energy Storage (ATES) areas. AHP, pioneered by Saaty (1980), offers a flexible approach to dissecting complex problems. MCDA permits the consideration of both subjective and objective facets in decision-making (Taherdoost 2017; Manguri and Hamza 2022). Within AHP, experts prioritize criteria based on their importance for the ATES sustainability indicator, utilizing the same scale for comparison (Eddouibi et al., 2021; Rauf et al. 2023). This study integrates six criteria in its analysis.

Initially, the six chosen criteria are juxtaposed to finalize their assigned weights, signifying the significance of ATES in the area of interest. Subsequently, the criteria weights are determined through pairwise comparisons, reflecting their relative importance to ATES. Each criterion is then subdivided, assigning suitable rating values ranging from zero to ten (Saaty 1980, 1977; Malczewski 2000). Higher ratings near ten indicate favorability for ATES sustainability, while ratings closer to zero suggest unsustainability. The criteria's rating and prioritization rely on expert decisions and an extensive review of existing literature.

The weighting for each criterion results from one-on-one pairwise comparisons among individual criterion classes, determining the importance of each class over others, following the AHP method in MCDA. It generates a hierarchical order from one to nine, derived from expert decisions (Stemmle et al. 2022; Saaty 1980) using ArcGIS-spatial MCDA-AHP. The rating and weighting of each criterion, depending on the degree of their importance, was arranged by counting on AHP as depicted in Table 2.

Table 3 illustrates the fifteen pairwise comparisons of the six criteria based on their priority.

Table 3 Pairwise comparison matrix with AHP method for calculating the ATES sustainability
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To validate the AHP outcomes, a consistency ratio (CR) is calculated using the following formula:

$$CR=\frac{CI}{RI}$$
(1)

(where CR is the consistency ratio, RI is the random index based on the number of criteria, Saaty 1980; Lu et al. 2019a, b). The consistency index (CI) is computed via

$$CI=\frac{{\lambda }_{max}-n}{n-1}$$
(2)

with n denoting the number of criteria and \({\lambda }_{max}\) representing the principal eigenvalue of the matrix obtained from the consistency measure calculations (Saaty 1980; Lu et al. 2019a, b).

Calculation of ATES sustainability map

The ATES sustainability in the Halabja-Khurmal sub-basin is determined through a weighted linear combination of the six criteria using the equation:

$${Sa}_{ATES}=\sum_{i=1}^{n}{(w}_{i}{x}_{i}) n=6$$
(3)

where \({Sa}_{ATES}\) represents ATES sustainability, \({w}_{i}\) denotes the weighting factors, \({x}_{i}\) signifies the normalized rates for each criterion, and n denotes the number of criteria.

Result and discussion

The pairwise comparisons of the matrix with AHP have been calculated using formulas (1, 2, and 3). The results indicate a high level of consistency in the fifteen pairwise comparisons, with the CR value being very low (0.3%), as evidenced by the calculated RI (0.013), \({\lambda }_{max}\) (6.019), and CI (0.004) values. The study employs all criteria and their AHP-determined weights to identify the most sustainable location for ATES. Overlaying these layers in ArcGIS generates two final sustainability maps based on AHP-MCDA, one for cooling and the other for heating purposes, portraying sustainability levels in three categories: weak, moderate, and strong.

The sustainability maps highlight areas conducive to ATES heating purposes. Figure 6 is characterized by high groundwater transmissivity, discharge, temperature, good groundwater chemistry, and high population density. On the other hand, the sustainability maps highlight areas conducive to ATES cooling purposes, Fig. 7, which are characterized by high groundwater discharge, transmissivity, and good groundwater chemistry. Conversely, regions lacking these conditions are deemed weak in sustainability for ATES implementation due to temperature variation. Based on Figs. 6 and 7 most of the study sub-basin is moderately sustainable for the ATES system, which has a promising future for applying this environmentally and eco-friendly system.

Fig. 6
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The ATES sustainability map for heating purposes

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Fig. 7
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The ATES sustainability map for cooling purposes

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Conclusion

This research employed the Multi-Criteria Decision Analysis-Analytical Hierarchy Process (MCDA-AHP) technique along with the ArcGIS method to generate sustainability maps for the aquifer thermal energy storage system in the Halabja-Khurmal sub-basin for heating and cooling purposes. Six important data sets were integrated based on their respective influences on the sustainability of (ATES). The spatial analysis combination of the criteria yielded a consistency ratio of 0.3% in the AHP. The resultant map was classified into three categories using pixel calculation in ArcGIS, with each category's specific value dependent on the assigned weighting for individual criteria during ATES sustainability maps calculation.

The ATES sustainability heating map for the Halabja-Khurmal sub-basin delineates that 26.45% of the area is strongly sustainable located in the north and southwestern part of the sub-basin, 73.53% is moderate in the east, central, southeast, and southern regions, 0.02% is weakly sustainable, as a tiny area in the southwestern for aquifer thermal energy storage technology. Favorable areas are characterized by high groundwater temperature, high transmissivity and discharge, good groundwater chemistry, and high population density, whereas regions lacking these conditions receive lower sustainability scores.

The ATES sustainability cooling map for the Halabja-Khurmal sub-basin delineates that 19% of the area is strongly sustainable located in the north and southwestern parts of the sub-basin, 78% is moderate in the northeast, east, central, west, southeast, and southern regions, 3% is weakly sustainable as spots in the south and southwestern areas. Favorable areas are characterized by high transmissivity and good groundwater chemistry. Conversely, regions lacking these conditions are deemed weak in sustainability for ATES implementation.

This study introduces new criteria for assessing ATES sustainability, providing valuable insights for government and decision-makers. ATES's economic benefits and CO2 emission reduction potential in the Halabja governorate underscore the importance of its adoption. Ongoing research aims to explore different data further through comparative analysis and applying diverse indicators. Additionally, future evaluations are expected to benefit from more detailed information about ATES applicability.