1. Introduction
Water resources investigations represent high interest for scientists and environmental authorities of each region. Nowadays, in many regions around the globe, surface waters and groundwater are facing high pressure from climate change, anthropogenic factors, land cover, and related practices [
1]. Therefore, groundwater vulnerability mapping has become an indispensable tool for the delineation of protection zones, reduction of industrial activities, and intense agriculture [
2]. In the last decade, several methods have been developed with respect to groundwater vulnerability assessment, both at a spatial scale and in situ locations.
The negative impacts of climate change on river flow discharge, sea-water intrusion, glacier melting, water table depletion, and poor groundwater quality have been reported in several studies [
3,
4]. The negative pressure of climate warming is reflected in water resources, and also in natural ecosystems, agriculture, and landslide risk areas [
5,
6,
7,
8]. The main impact of climate change on groundwater resources has been divided into the following two types: (i) direct impact with respect to the aquifer recharge and (ii) indirect impact which implies the groundwater demand [
9]. At the same time, the contribution of land use and human impact on groundwater quality and quantity is mentioned [
9]. However, there are many theoretical aspects and several direct and indirect effects of climate change on groundwater that have not already been explored, and they have not happened before [
10,
11].
Water availability is expected to decrease on the European continent in the near future and by mid-century [
12]. Studies regarding these kinds of climate surveys have been based on climate data projections that were performed using different models and sources [
13,
14,
15]. A few examples, based on the high-resolution precipitation data [
16], have analyzed the rainfall erosivity for Europe. In the South Eastern Europe region, the climate impact on groundwater resources has been evaluated considering the high-resolution climate models of precipitations and temperature [
17]. Throughout Europe, many regions have experienced an increase in evapotranspiration due to climate change and land cover practices [
18].
In the Mediterranean areas, due to the negative influence of climate on groundwater resources, springs have been investigated using continuous monitoring of flow discharge, temperature, and electrical conductivity. On the basis of recorded data of temperature, discharge, and electrical conductivity, the VESPA index for groundwater vulnerability in springs was set up by [
19]. They developed and tested the vulnerability of four fractured springs from the Piedmont region considering climate change and pollutants. Spatial analyses using various software are often used for the investigations of karstic and porous aquifers. For instance, a study by [
1] analyzed groundwater vulnerability and pollution risk in the north of Greece using DRASTIC, EPIK, AVI, and DRASTIC-FM methods developed in GIS. Their study indicated the possibility of groundwater quality degradation under sea water intrusion into the karst aquifer and also due to the inadequate delineation of protection zones and agriculture planning. In a study by [
20], the effect of climate including the water availability on the karst aquifers over the world was assessed. In contrast to the porous aquifers, where the changes have been more related to the water table, in the karst regions the effect of climate has been related to the discharge of springs [
20].
An important aspect of groundwater vulnerability is related to intrinsic vulnerability, which refers to the sensitivity of groundwater quality to a certain contaminant and it accounts for the geological, hydrological, and hydrogeological characteristics of the respective area, and also the recharge rate, aquifer properties, and unsaturated zone [
21,
22]. The intrinsic characteristics of the aquifers control groundwater vulnerability to pollution load and contamination [
21]. For an intrinsic vulnerability assessment of aquifers, hydrogeological properties are required. In order to support future plans and strategies regarding water resources, spatial evaluation of groundwater vulnerability could be performed using specific software. Thus, groundwater vulnerability mapping becomes a significant task for planning and decision making [
23]. A study by [
24] determined the intrinsic vulnerability of the quaternary aquifer in Baghdad using DRASTIC and GIS tools. As an example of intrinsic groundwater vulnerability mapping, [
25] used the DRASTIC method in Datong City from China, emphasizing the advantage of the approach in regions with limited data. The DRASTIC method was also used to determine the aquifer vulnerability in western and southern Taiwan [
26,
27].
In the Piedmont region, climate change effects are more intense during dry periods. Because of this aspect, the porous aquifers located at low altitude are facing significant pressure due to water deficit and agriculture practices. A study on long-term analysis including future climate models and land cover scenarios is missing for this region. In addition, [
28] found that DRASTIC, GOD, and TOT methods, applied for the groundwater vulnerability mapping, were not able to identify the most vulnerable areas. The nitrate concentration was used to validate these intrinsic methods, and a good correlation was only partially verified [
28].
Past studies have indicated that the SINTACS and the DRASTIC methods were the two major approaches used for groundwater vulnerability assessment in some subzones of this region. Studies by [
29,
30] used the SINTACS method for groundwater vulnerability mapping. Using a combination of traditional Italian approaches, such as the GNDCI-CNR Basic method and PCSM SINTACS, the authors of [
30,
31] proposed a GIS method for groundwater vulnerability. In the Alessandria district from the east part of Piedmont region, a study by [
31] carried out vulnerability mapping of groundwater using eight parameters at a spatial scale, for example, aquifers, land cover, slope, hydraulic conductivity, etc.
These methods are very useful for groundwater vulnerability mapping, while these approaches are based on multilayers analysis through GIS applications. However, climate data for future groundwater vulnerability prediction were rarely included in the previous methods at a regional scale.
The scope of this study is to determine the vulnerability of groundwater resources at a spatial scale for the Piedmont region, considering climate change factors. This region was chosen as a suitable study area because of its geographical position in the Mediterranean area and its various aquifer types (karst, fractured and fissured rocks aquifers, and porous). In addition, the region presents high variability of climate between the high Alps Mountains and the Po Plain, and also presents biodiversity from a land cover point of view.
2. Study Area
The Piedmont region extends from 44°3′ to 46°27′ N and from 6°38′ to 9°13′ E (
Figure 1). Geographically, its position fits into the south of Europe, in the north-western part of Italy. The northern, western, and southern parts of the Piedmont region are mountainous areas of the Western Alps, while the central and eastern parts are represented by lowlands of the Po Plain. In December 2017, [
32] reported a value of 235 L/person /day for domestic water demand, in the Piedmont region, registered for 2015. This value is higher than 180 L/person /day depicted in Northern Europe and proper management of water resources is needed for the near-future period.
Regarding the orography of the Piedmont region, the relief is distributed similar to an amphitheater, with high elevations in the north, west, and south, whereas the central and east parts are more occupied by plains. Because of these characteristics, the surface water flow converges over the east, where the main river collector is the Po. Groundwater flow and storage, and also groundwater vulnerability are related to the aquifers and climate. The aquifers characteristics are based on geological composition. In the mountain areas, there are predominantly the karstified limestones, marlstones, calystones, and also conglomerates. In the lowlands, the porous aquifers are composed of mainly gravels and sands. In a few locations, aquifers with volcanic and schists can be identified.
Supplementary Material Figure S1 depicts the geological formations of the Piedmont region. The productivity of each aquifer type has been classified into the following six categories: highly productive fissured aquifers, highly productive porous aquifers, low and moderately productive fissured aquifers, low and moderately productive porous aquifers, locally aquiferous rocks (porous or fissured), practically non-aquiferous (porous or fissured) (
Figure 1).
The climate of the region is warm temperate with a Mediterranean influence. According to the Köppen–Geiger climate classification, Cfa and Cfb (warm temperate with hot and warm summers) have been identified in the central, east, west, and north parts of the Piedmont region [
33]. In the south, there is mainly warm temperate with dry summer (Csa), and within the high elevated lands, the Dfb climate (snow fully humid) is depicted [
33]. The mean annual temperature indicated values between –10 °C and 13.1 °C during the 1990s period (
Figure 2a). Future climate models show increases in the mean annual temperature from −7.9 °C to 14.9 °C during the 2020s period (
Figure 2b) and from −7.1 °C to 15.8 °C in the 2050s period (
Figure 2c). In the Piedmont region, values of precipitation have been identified which varied from 617 mm to 2315 mm /year during the 1990s period (
Figure 2d), while in the future, the precipitation values vary from 564 mm to 2190 mm /year (2020s) and from 558 mm to 2137 mm /year (
Figure 2e,f). The annual potential evapotranspiration (ET0) registers values between 0 mm and 576 mm during the 1990s period (
Figure 2g). Significant increases are expected in the following future periods: from 0 mm to 718 mm during the 2020s period and from 220 mm to 760 mm during the 2050s period (
Figure 2h,i).
The vegetation pattern is much diversified in the Piedmont region. In the mountain areas, forest types can be found. Agricultural lands and artificial areas mainly cover the plains. The villages and cities extend mainly in the hills and plains. The bigger urban agglomeration is Turin, located in the west-central side of the Piedmont region. For groundwater vulnerability determination, the importance of land cover study has two main directions which we followed. Firstly, the evapotranspiration and water availability in each type of land cover have a quantitative impact. Secondly, the phosphorous transfer in soils and the load pollution of each type of ecosystem influence direct the quality of the groundwater through an infiltration process.
5. Discussion
The main goal of this work was to determine groundwater vulnerability and risk mapping in one of most important regions of Italy, in the Piedmont region. The applied methodology is based on GIS technology, which is able to combine spatial datasets and generate a groundwater vulnerability map. The input data has significant influences on the results. Climate influences are coming both from the temperature and annual
ET0 values, and also from the annual precipitation, which register high variations at a spatial scale in the Piedmont region. These oscillations of
ET0 and precipitation, together with the land cover, are reflected in the
AETc and water availability amount. Thus, the Po Plain appears to be a very dry land, with high values of
AETc and low values of water availability for all the analyzed periods, for a larger area, and due to this, groundwater recharge is expected to reduce, especially during the future period. In the mountain areas, the high quality of the ecosystems (low PLI) and the high values of water availability contribute to low and very low vulnerability of groundwater. According to [
28], land use does not always control the transport of NO
3, but the groundwater flow. In this sense, our method that includes PIC influences the intrinsic groundwater vulnerability a great deal at the same location as the aquifer. As a limitation of ArcGIS and spatial analysis, in this study, the applied approach does not quantify the groundwater flow and transfer of NO
3 downstream to the measured points. However, the method performs well for the classes of groundwater vulnerability and, additionally, future projections are the strengths of our analysis.
The best of our application, the groundwater vulnerability mapping carried out in this study, also included the effects of climate change and land cover variation. These findings are in line with specific literatures that have indicated high vulnerability of groundwater resources in various regions from Southern and South Eastern Europe. In the south of Europe, the seasonality of dry period induces major changes in the water renewals, especially from May to September. The mean annual precipitation amount is expected to decrease in the Mediterranean area and also in Italy [
39,
40], and the Italian peninsula [
45] as a consequence of climate change. In South Eastern Europe, groundwater vulnerability is also increasing [
40] and the ecosystems are also negatively affected [
7]. A study by [
53] determined groundwater vulnerability under climate change in the Beliş district of the Western Carpathians.
Our study is not without limitations. The applied methodology focused on annual averages of 30 years climate data. This aspect represents a disadvantage because the water stress during drought periods is not considered. The present work considers long-term periods rather than seasonal analysis. At the same time, pollution risks during the short periods are not taken into account.
A comparison of this study with previous investigations of groundwater vulnerability in the Piedmont region shows that our findings are appropriate with the maps generated by [
30,
31]. In their groundwater vulnerability map, generated for the Alessandria district, a high class of vulnerability is predominant for that territory. This detail is in line with the vulnerability map generated in this work. Interestingly, studies by [
30,
31] also found extremely elevated vulnerability in the northern side of the Alessandria district and in some locations from west and south. The differences between our method and the approach by [
30,
31] are related to the input data. They included the hydraulic conductivity, weight strings, and depth to water data in their analysis. Because of the large area of the Piedmont region, we chose to use the PIC layer and PLI instead of hydraulic conductivity and depth to water data, therefore, the findings are slightly different. However, our method generated an accurate groundwater vulnerability map for the entire Piedmont region considering long-term period climate models and land cover projections. The method could be used for other regional studies without issues in performance.
6. Conclusions
This aim of this paper was to determine the groundwater vulnerability mapping over the Piedmont region, in Northern Italy, using spatial analysis in ArcGIS. The approach included high-resolution climate models, aquifers, terrain data, and the dynamic land cover for the past (1990s), present (2020s), and future (2050s) periods. Climate change and land cover have a negative impact on the groundwater resources in the Piedmont region. The groundwater vulnerability map indicates the northern, western, and southern sides of the Piedmont region have low and very low groundwater vulnerability. At the same time, in the Po Plain, high and very high vulnerability occur during the 1990s and 2020s periods. Moreover, during the 2050s period, the areas with high vulnerability are expected to increase in area.
The central and eastern parts of the region are facing climate change problems related to a decrease in water availability, intense agriculture, and highly sensitive aquifers of porous media in the Po Plain. In addition, the areas with high groundwater vulnerability also represent the cultivation lands, which influence water resources quality.
The risk mapping consists of integrating groundwater vulnerability mapping and types of aquifers. Thus, high and very high vulnerability corresponds to porous and karst aquifers because of the high hydraulic conductivity, and also because NO3 and as registered high values in the study area. Medium vulnerability could be identified in the porous aquifers with coarse sediments, but also in the fissured aquifers. Low and very low vulnerability was found in the non-aquiferous media, where the permeability was very low and the capacity of the water storage was reduced.
This application represents a reliable methodology with respect to groundwater vulnerability mapping at a regional scale. The accuracy of mapping carried out here can be improved with in situ measurements for the locations where these are missing. In addition, the groundwater vulnerability maps which were generated in this study, represent important instruments for water management plans in the Piedmont region.