Introduction

For thousands of years, human populations have settled in mountain regions for several reasons, namely, the search for natural resources, the seek for refuge from persecution, the search for new territories and better livelihoods, the need to adapt to climate modifications, and the demand for spiritual fulfillment (Gardner et al. 2013).

There is a wide variety of mountain environments, depending on some basic physical features like altitude, slope, and latitude, which originate specific temperature and precipitation regimes, and may be very hard for human populations. Mountains are also characterized by high-energy environments, which originate extreme climatic processes and lead to intense rock weathering and soil erosion (Blyth et al. 2002). Climate change is expected to cause major modifications in the hydrological cycle of mountain regions, including in its underground phase, which may impact the recharge and discharge of aquifers and, particularly in systems more vulnerable to anthropogenic pressures, deteriorate the quality of water resources (Barbieri et al. 2021).

In fact, besides historical, social, and economic factors, along with altitude and slope, the geographical distribution of human settlements in mountain regions is determined by the availability of hydrological services, especially the availability of water resources (e.g., Salibi 1997; Okahashi et al. 2018). This environmental framework has driven mountain populations to create productive agroecosystems, with the development of slope terracing and field enclosures, coupled with irrigation systems (Blyth et al. 2002; Feng and Li 2007).

The lack of adequate water supply to mountain villages, especially those located in remote areas, is a factor that aggravates the socio-economic disadvantages of the physical remoteness of these settlements. To overcome this scenario, different solutions may be adopted, according to the hydrological, climatological, and socio-economic conditions, encompassing the use of surface water, groundwater, and even rainwater (Kanji et al. 2012; Komatsu et al. 2020; Kim et al. 2022).

Drave is a mountain village located in North Portugal (Fig. 1), more specifically, in the Gralheira mountain range. The existence of human settlements in the Drave area has been documented since the sixteenth century, although human occupation is probably earlier (Martins 1956; Costa 2013). During the second half of the twentieth century, the population of the Drave was drastically reduced, and the village lost its last permanent inhabitants around the year 2000. Since then, it has only had temporary inhabitants (tourists and members of the Scout Movement).

Fig. 1
figure 1

Some aspects of the Drave village: view of the mountain region (a); Drave settlement (b); water sampling and monitoring (c); circulation of livestock (d); Palhais stream at the beginning of the dry season (e); Palhais stream at the end of the dry season (f); water-filled depression in the Palhais streambed (g)

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The hydrological, hydrogeological, and hydropedological conditions of the mountainous region of Drave have always been a great challenge for the water supply to this locality, whether for agricultural or domestic use. For this reason, the inhabitants had to adapt to very harsh conditions, eventually building a network of hydraulic structures (Costa 2013) to capture and distribute surface water.

This research aims to understand the evolution of water demand and supply to the village of Drave, from the sixteenth century to the present day. Also, the study highlights the fact that Drave is a mountain village, with a specific hydrological framework, and analyses, in more detail, the current situation, proposing a solution to abstract water with quality and volume adequate to the needs to be met in the future.

To characterize the present circumstances regarding the water supply to Drave, chemical analysis (encompassing major ions and Potentially Toxic Elements—PTEs) and microbiological analysis were carried out. The presence of PTEs is of special concern because they are extremely hazardous to human health and may originate severe environmental pollution (e.g., Ricolfi et al. 2020).

Also, the groundwater vulnerability to pollution was assessed using the GVTool, a Geographic Information System (GIS) application (e.g., Duarte et al. 2019). In this case, a DRASTIC index map was created to support recommendations for achieving a more sustainable use of water resources in the Drave territory.

Materials and methods

The hydrological study of the Drave region was carried out in five main steps: (i) compilation of historical information regarding water demand and supply in Drave; (ii) field survey to recognize hydrological conditions, water supply structures (in use and abandoned), as well as to identify water points (hydrogeological inventory) and pollution sources; (iii) water sampling campaigns for chemical and bacteriological quality assessment; (iv) assessment of groundwater vulnerability to pollution; (v) proposal of a solution to the water supply system of Drave, through which the current water resources volume and quality constraints may be overcome.

Study area

Drave (N41°51’; W8°07’) is located in the Gralheira mountain range, North Portugal, at 600 m a.s.l. (Fig. 2), in the municipality of Arouca, and is dominated by a rural territory.

Fig. 2
figure 2

Geographical setting of the Drave region, and water sampling network

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The study area is part of the Central-Iberian Zone of the Iberian Massif (Ribeiro et al. 2007) and is composed of Proterozoic metasedimentary rocks: mostly phyllites and metagreywackes from the Schist-Greywacke Complex (Schermerhorn 1980; Pereira et al. 2007). To the NW, a Variscan muscovite-albite porphyritic granite (the Regoufe granite) also occurs. The intrusion of the Regoufe granite originated low-grade contact metamorphism in the metasedimentary rocks, with biotite blastesis. Quartz veins with W and W-Sn mineralizations, originated by hydrothermal processes during the emplacement of the Regoufe granite, are also common in the region (van Gaans 1995; Pereira et al. 2007). In the surrounding area of Drave, several quartz veins occur in the metasedimentary massif, where small W and W-Sn mines operated in the twentieth century.

The Drave area has mountainous geomorphology (Figs. 1 and 2), with steep slopes, deeply incised valleys, and altitude ranging from 500 m a.s.l. to 977 m a.s.l. (Alto da Drave summit).

The drainage network is controlled by the tectonic structures in the hard-rock massif and consists of the Palhais stream and its tributaries, which are part of the Douro River catchment. The groundwater circulation medium is mostly fractured.

The climatic characteristics of the Drave region are Mediterranean. Drave does not have a meteorological station and climatic information is scarce. The mean annual precipitation is around 1241 mm, with January being the wettest month and July the driest and the mean annual air temperature is 13.3 ℃, with January being the coldest month and July the warmest (normal values for the 1979–2021 period; data from Meteoblue 2022) see Fig. 3. The Drave region Köppen-Geiger climate classification is Csb (the most common in NW Iberia): warm temperate, with dry and warm summers (AEMET-IM 2011; Peel et al. 2007). The air temperature and precipitation values of the last four decades provided by Meteoblue (2022) show an increase in air temperature and a decrease in precipitation in the study area. These trends are in line with the results of research on climate change concerning mainland Portugal (e.g., Pisani et al. 2019; Quinteiro et al. 2019).

Fig. 3
figure 3

Air temperature and precipitation values of the Drave region (1979–2021 climatological normal; data from Meteoblue 2022)

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The prevailing pedological units in the study region are Umbric Leptosols, Umbric Regosols, Cumulic Anthrosols, and Rock outcrops (Agroconsultores and Geometral 1995). The soil texture is mostly sandy loam, loam, or silty loam. The organic matter content is high, and the pH is acidic. The soil profile usually consists of an umbric A horizon, a C horizon, and an R horizon (hard bedrock).

The hydrogeological features of the Drave region are not favorable for the use of groundwater resources: there are few springs located near the village, and the respective flow is low and tends to cease during the dry season; the construction of hand-dug wells is not viable due to the inexistence of a weathering mantle, allied to the relatively high depth of the water table; the construction of drilled wells is not possible because the village is not accessible to drilling equipment. Given these conditions, since earlier times, the Drave communities have mostly used surface water resources.

Water sampling

An inventory of surface water and groundwater points was made to establish the sampling network (Fig. 2). Subsequently, two water sampling campaigns were carried out, according to standard methods (ISO 5667–3:2018 Water quality—sampling—part 3: preservation and handling of water samples). Bearing in mind the chemical and microbiological quality assessment of drinking water, the first campaign took place during the dry season (October 2019), and the second one during the wet season (February 2020).

Water pH, electrical conductivity (EC), and temperature were measured in-situ using a Hanna Instruments HI991300 meter. Water samples were collected minimizing agitation and disturbance. All samples were stored in glass or polyethylene bottles, in the dark, at a temperature below 5 °C, and were delivered to the laboratory within 5 h.

Sterilized sample bottles containing sodium thiosulfate were used for microbiological analyses. Special preservatives were also required for the analysis of some chemical parameters, according to ISO 5667–3:2018 specifications. In this case, additional care was taken not to overfill the bottles to prevent the loss or dilution of preservatives during sampling.

Laboratory analysis

Analysis were performed according to procedures outlined in “Standard Methods for the Examination of Water and Wastewater 23rd edition” (Rice et al. 2017) and “Le Rodier—L’analyse de l’eau 10e édition” (Rodier et al. 2016). The laboratory is accredited under ISO/IEC 17025 since 2007 (Laboratory of the Environmental Health Department of the National Institute of Health Doutor Ricardo Jorge, I.P., Certificate No. L0323-Technical Annex L0323-1, http://www.ipac.pt/pesquisa/acredita.asp). Precision and accuracy were calculated for all analytical methods with values < 10%. Uncertainties were also calculated with results varying from 2 to 15%.

Water turbidity was measured in a Hach 2100 N Laboratory Turbidity Meter, Hach Lange, Düsseldorf, Germany. Electrical conductivity and pH were determined in a Crison MultiMeter MM 41, Hach Lange Spain, S.L.U., Barcelona, Spain. Total alkalinity, bicarbonate (HCO3−), oxidability (O2), and total hardness were analyzed by titration. Phosphate (PO42−), boron (B), total phosphorus (P), and color were analyzed in a Shimadzu UV-1601 spectrophotometer (Shimadzu Corporation, Kyoto, Japan). Chemical oxygen demand (COD) was evaluated in a Hach DR 2800 Spectrophotometer (Hach Company, Loveland, CO, USA). Major inorganic ions (Na+, K+, Mg2+, Ca2+, Cl−, NO3−, F− and SO42−) were analyzed by ion chromatography (DionexTM system DX-120/ICS-1000, Dionex Corporation, Sunnyvale, CA, USA). Potentially Toxic Elements (Cr, Mn, Ni, Cu, Zn, As, Cd, and Pb) were analyzed in a Varian AA240 Atomic Absorption Spectrometer (Varian Inc., Palo Alto, CA, USA). Components, such as aluminium (Al), cyanide (CN), iron (Fe), nitrite (NO2−), ammonia (NH4+), and silica (SiO2) were analyzed in a Continuous Segmented Flow Instrument (San-Plus Skalar, Skalar Analytical, Breda, The Netherlands).

The quality of the Drave drinking water sources was assessed using the Canadian Water Quality Index (CCME-WQI; CCME 2017) which provides a convenient way of summarizing complex water quality data in a single result, and is based on three factors:

  • F1 (scope): the number of parameters not meeting water quality guidelines;

    $$F_{1} = \left( {\frac{{\text{Number of failed variables}}}{{\text{Total number of variables}}}} \right) \times 100$$
    (1)

  • F2 (frequency): the number of times these guidelines are not met;

    $$F_{2} = \left( {\frac{{\text{Number of failed tests}}}{{\text{Total number of tests}}}} \right) \times 100$$
    (2)

  • F3 (amplitude): the amount by which the guidelines are not met. F3 is calculated in three steps:

Step 1: Calculation of Excursion. Excursion is the number of times by which an individual concentration is greater than (or less than, when the objective is a minimum) the objective. When the test value must not exceed the objective:

$${\text{Excursion}}_{i} = \left( {\frac{{{\text{Failed test value}}_{i} }}{{{\text{Objective}}_{i} }}} \right) - 1$$
(3)

For the cases in which the test value must not fall below the objective:

$${\text{Excursion}}_{i} = \left( {\frac{{{\text{Objective}}_{i} }}{{{\text{Failed test value}}_{i} }}} \right) - 1$$
(4)

Step 2: Calculation of Normalized Sum of Excursions (nse). The normalized sum of excursions is the collective amount by which individual tests are out of compliance:

$${\text{nse}} = \frac{{\mathop \sum \nolimits_{i = 1}^{n} e{\text{xcursion}}_{i} }}{{\text{Number of tests}}}$$
(5)

Step 3: Calculation of F3. F3 is calculated by an asymptotic function that scales the normalized sum of the excursions from objectives to yield a range from 0 to 100.

$$F_{3} = \left( {\frac{{{\text{nse}}}}{{0.01{\text{nse}} + 0.01}}} \right)$$
(6)

The CCME-WQI is then calculated as:

$${\text{CCMEWQI}} = 100 - \left( {\frac{{\sqrt {F_{1}^{2} + F_{2}^{2} + F_{3}^{2} } }}{1.732}} \right)$$
(7)

By combining these factors, a value between 0 (worst water quality) and 100 (best water quality) is calculated. The CCME-WQI values are then divided into five descriptive categories (Table 1).

Table 1 CCME-WQI categorization scheme
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The parameters, guidelines, and time period used in the CCME WQI calculation are not specific and could vary from region to region, depending on local conditions, water quality issues and the intended usage of the water (CCME 2017).

Microbiological analyses were performed according to ISO 9308-2:2012 for Escherichia coli; ISO 7899-2:2000 method for intestinal enterococci; ISO 14189:2013 method for Clostridium perfringens and ISO 9308-2:2012 method for coliform bacteria.

Assessment of groundwater vulnerability to pollution

Groundwater vulnerability to pollution is the sensitivity of groundwater quality to a pollutant load and depends on the intrinsic features of the aquifer (van Duijvenbooden and Waegeningh1987).

One of the most widely applied methods to assess groundwater vulnerability to pollution is the DRASTIC index (Aller et al. 1987). This index is computed taking into account seven intrinsic groundwater vulnerability factors: Depth to Groundwater (D), Net Recharge (R), Aquifer Media (A), Soil Media (S), Topography (T), Impact of the vadose zone (I) and Hydraulic Conductivity (C). For this purpose, the following expression is applied (Eq. 8):

$$DRASTIC \, = D_{w} D_{r}^{\prime } \times R_{w} R_{r}^{\prime } \times A_{w} A_{r}^{\prime } \times S_{w} S_{r}^{\prime } \times T_{w} T_{r}^{\prime } \times I_{w} I_{r}^{\prime } \times C_{w} C_{r}$$
(8)

where D, R, A, S, T, I, and C are the factors earlier defined, and w and r are the weight and rating applied to each factor, respectively. The index values range from extremely low groundwater vulnerability (values < 79) to extremely high groundwater vulnerability (values > 200).

Groundwater vulnerability to pollution of the Drave region was assessed through the GVTool application (Duarte et al. 2015, 2019), an open-source application developed in GIS software QGIS which allows the assessment of groundwater vulnerability, through the automatic creation and comparison of groundwater vulnerability maps using the DRASTIC index, among other methods (Duarte et al. 2019). In the DRASTIC component of GVTool, the user can assign the default ratings proposed by Aller et al. (1987) for each parameter, or define specific ratings, adapted to the study area.

Historical overview

One of the earliest historical records mentioning human settlements in Drave is from 1527 and refers to the population register of the Kingdom of Portugal (Costa 2013). Nevertheless, the origin of Drave is most likely earlier.

These communities had to survive in a harsh mountain territory and needed to simultaneously adapt to and change the environment to create better conditions for agriculture. In fact, the soils in the mountains surrounding Drave (especially in the areas dominated by rock outcrops and Leptosols) are hardly suitable for plant growth and pose great difficulties to the practice of agriculture.

The traditional economy which has been developed throughout the history of Drave was founded in a culture of self-sufficiency, and fits the model described for Mediterranean mountains by Batista and Costa (2020): the establishment of a traditional agro-forest-pasture system, in which the soil and water management played a crucial role. To overcome the severe environmental features, the local populations have dealt with the mountain geomorphological, pedological, and hydroclimatological conditions (Ribeiro 2011; Batista and Costa 2020). For this purpose, a system of interconnected fields and plots was created around the village (including a number of agricultural terraces), supported by an irrigation system consisting of a network of weirs (intended to capture water in streams) and channels for transporting water to the irrigated areas (Costa 2013; Batista and Costa 2020). As described earlier, because of the Drave hydrogeological features, the use of groundwater was not a viable choice for water supply.

During the second half of the twentieth century, the population of Drave gradually decreased, and the last permanent inhabitant left the village around the year 2000. Consequently, the type of human occupation, and the traditional socio-economic organization that was associated with it — including the nature of water demand — came to an end.

Yet, Drave did not become an abandoned settlement. Since 1990, tourists have sought out this region for outdoor activities, especially trekking. But the most remarkable form of human occupation of this new phase in the history of Drave is the one carried out by members of the Scout Movement. Since 1995, several major events have taken place in the Drave area, organized by the main Portuguese Scout organization (CNE), which have involved hundreds of Scouts each time, mostly in a camping regime. Finally, in 2003, the Drave Rover Scout Centre was founded, as part of the worldwide network of Goose Scout Centres (DRSC 2022). Since then, the Drave Rover Scout Centre annually welcomes thousands of scouts from Portugal, and many other countries, throughout the year, but particularly during summer.

This recent reality, in which the village is now frequented by temporary inhabitants, many of whom stay overnight in camping sites, for the practice of outdoor sports or civic and spiritual activities, has led to a new type of demand for water resources, no longer for agricultural use, but rather for human consumption and personal hygiene, as well as to supply field toilets.

Current scenario

As the number of permanent inhabitants of Drave decreased, especially during the second half of the twentieth century, the agroforestry system that had been in place for centuries was abandoned. New types of human activities have emerged since the end of the twentieth century, practiced by temporary inhabitants (mainly outdoor enthusiasts and scouts), especially during spring and summer.

This socio-economic transformation has led to a profound change in water demand: the demand for agricultural purposes has ceased, giving way to a more irregular demand and of much lower volume, with emphasis on supplying scouts summer camps, which may have hundreds of participants.

The water supply system currently in place in Drave consists of surface and groundwater points (Fig. 2):

  1. (i)

    Water point A (WPA): spring located in a gallery of an abandoned tungsten mine, approximately 1 km NW of Drave;

  2. (ii)

    Water point B (WPB): fountain located in the centre of the village, supplied through a PVC pipe which transports water from the spring WPA—this is the main water supply point;

  3. (iii)

    Water point C (WPC): Palhais stream, at the village;

  4. (iv)

    Water point D (WPD): fountain located in a campsite, supplied by an adjacent stream;

  5. (v)

    Water point E (WPE): spring located in a valley bottom, at the eastern limit of the village.

Water quality assessment

Water samples were analyzed for thirty-nine physical/chemical/microbiological variables that range from esthetic/technical parameters (e.g., color) to contaminants associated with acute or chronic health effects (e.g., bacteria, arsenic). Twenty-eight variables have permissible limits established by the European Union (Directive (EU) 2020/2184), the U.S. EPA (EPA 2022), or the World Health Organization (WHO 2017) and were used for the calculation of CCME-WQI (Table 2).

Table 2 Values of water quality parameters and corresponding limit values established in the standards
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In all the studied waters (Table 2) the hydrogeochemical facies is Na-Cl, which is common in surface water and groundwater of shallow circulation in the geological context of the Iberian Massif (e.g., Carreira et al. 2011). The relatively high Mg content is due to the presence of biotite and, to a lesser extent, tourmaline, which occur in the metasedimentary rocks due to hydrothermal processes related to the emplacement W-Sn and W-bearing quartz veins, as described in Sect. 2.1.

All physical–chemical parameters except for pH, ammonia, iron, lead, and arsenic are within the maximum acceptable limits required by the EU, WHO, and USEPA. The studied waters are considered acidic, which is in accordance with the geological characteristics of the area. The pH values varied between 5.5 and 6.6, and were lower in the groundwater samples, as in other mountains located in the Iberian Massif (e.g., Espinha Marques et al. 2013; Mansilha et al. 2019). The presence of mineralizations containing sulphides may also contribute to explaining the lower groundwater pH. The highest concentrations of PTEs, namely, Al, Mn, Cd, Pb, Cr, As, and Ni, were detected in WPA. The concentration of arsenic and lead in this water point presents a high-level risk for human consumption.

All samples reveal microbiological contamination (Fig. 4) that was more severe in the rainy season, especially regarding the coliform bacteria and Clostridium perfringens in WPA. The fecal contamination is caused by the circulation of livestock or wildlife and may explain the outbreaks of waterborne diseases reported by tourists and scouts that visit Drave.

Fig. 4
figure 4

Microbiological contamination

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In the studied water samples, the potential sources of fecal contamination are in close proximity to the water points. Microbial contamination of water bodies (streams and aquifers) may be caused by extensive livestock production systems (mainly goats and cows) and by wildlife such as foxes, wild boars, rabbits, wolves, and so on. All these animals have direct access to streams and rivers and defecation into these watercourses is common, with obvious negative implications for water quality because pathogens flow with the current and travel long distances. Regarding groundwater sources, pathogens reach the water table after rainwater infiltration, and percolation through the unsaturated zone.

The Water Quality Indices (WQIs) methodology was applied to categorize groundwater and surface water samples based on rating. The total number of parameters examined was 28, and the total number of individual tests was 56. The number of parameters not meeting the objectives was 8 in WPA (pH, NH4, Fe, Pb, As, intestinal enterococci, Clostridium perfringens, and coliform bacteria), 4 in WPC, WPD, and WPE (pH, E. coli, intestinal enterococci, and coliform bacteria) and 5 in WPB (pH, As, E. coli, intestinal enterococci, and coliform bacteria).

According to the CCME-WQI method, the overall water quality in Drave is classified as “marginal” (Fig. 5), with values of 46.9 for WPA, 51.7 for WPB, 48.1 for WPC, 48.3 for WPD, and 56.2 for WPE. These results indicate that the water resources, both from groundwater and surface water sources, did not meet the drinking water requirements and are not safe for human consumption. Arsenic and microbiological contamination are the most important parameters that determined the rating of water quality, exceeding the standards (objective) of drinking water.

Fig. 5
figure 5

Assessment of water quality in Drave using the CCME-WQI method

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Overall, the water quality assessment results pointed out that the water sources in Drave are contaminated and need proper treatment before human consumption. The information provided by CCME-WQI is a useful tool for describing the state of drinking water quality and can be of great value for water users and managers in the Drave region. To meet the future water demand from tourists and scouts, the possibility of alternative water sources must be considered. Additionally, household or portable water treatment systems can be the last line of protection in preventing waterborne outbreaks in this area.

Groundwater vulnerability to pollution

Groundwater vulnerability to pollution was assessed using the DRASTIC index and the GVTool GIS application, to better describe the hydrological conditions of the Drave region, and to support future decisions regarding the selection of water points, as well as to decide the location of camping sites, infrastructures (particularly sanitation and hygiene facilities) and the circulation of livestock.

The available hydrogeological and climatological information is scarce and most of the required data for computing the DRASTIC index were collected from publicly available geoinformation, namely, topographic, geological, soil maps and databases (Table 3), and complemented by field observations.

Table 3 Dataset used for the GIS-based assessment of groundwater vulnerability to pollution
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All data were processed under QGIS software (QGIS 2022) and in European Terrestrial Reference System 1989—Portugal Transverse Mercator 2006 (ETRS89 PTTM06; EPSG: 3763), the most recent coordinate system adopted in Portugal (DGT 2022).

Since there is no information concerning the depth to the water table in the Drave region, in GVTool the D parameter was estimated using the DEM combined with a simple conceptual hydrogeological model encompassing the relationship between water table depth and geomorphology as described by Duarte et al. (2015, 2019). For this purpose, the following water table depth values were defined: a minimum of 0 m at the drainage network channels (streams), and a maximum value of 50 m at distances greater than 75 m from the stream channels. These values are empirical and result from field observations from the study area as well as from other mountains in Central Portugal (e.g., Duarte et al. 2019).

As for the R parameter — which refers to the Net Recharge, that is, the total amount of water that infiltrates into the soil and reaches the aquifer (in mm/year) — the mean annual precipitation at Drave (1241 mm/year) was adopted, along with a 10% recharge rate, the same calculated by Pisani et al. (2019) for Serra da Estrela mountain, located in Central Portugal. The resulting recharge value was applied to the entire study area.

The A parameter (Aquifer Media) was determined considering that the geology of the study area consists of metamorphic rocks (Schermerhorn 1980; Pereira et al. 2007). The S parameter (Soil Media), which refers to soil features (thickness and texture), was determined according to the pedological information from soil maps as well as from field surveys. Four different soil types were identified: thin or absent soil (areas dominated by rock outcrops), Anthrosols (agricultural terraces), Regosols, and Leptosols. Nevertheless, these four soil units correspond to two DRASTIC ratings: “thin or absent soil” (rating 10), and “sandy loam” (rating 6) encompassing Anthrosols, Regosols, and Leptosols. For this purpose, soil type polygons were directly digitized from the Google Satellite layer in QGIS environment. This possibility was provided with the help of the Quick Map Services plugin. The T parameter, which refers to the terrain slope, was generated from a DEM. The I parameter (Impact of the Vadose Zone) depends on the geological material of the vadose zone below the pedological soil, which, in the Drave region is the same as the aquifer, that is, a metamorphic rock. The C parameter refers to the aquifer hydraulic conductivity. Since there is no hydrodynamic information regarding the Drave aquifer system, a typical value for fractured metamorphic rocks of 10–5 cm/s (Dingman 2015) was adopted for the entire area.

The DRASTIC index map of the Drave region presented in Fig. 6 shows that the groundwater vulnerability to pollution varies from extremely low to moderately low according to the Aller et al. (1987) classification. The lowest computed vulnerability value recorded was 74, and the highest value was 121. Nevertheless, the map highlights some contrasts regarding the spatial variability of the index. The higher vulnerability areas correspond, in general, to valley bottoms, while areas of lower vulnerability correspond to zones with higher slopes, as well as to convex landforms (in which rock outcrops areas are normally dominant).

Fig. 6
figure 6

DRASTIC index map of the Drave region

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The results from the analysis of the DRASTIC index map were then validated during a final phase of field observations, which confirmed that three main factors influence groundwater vulnerability to pollution in the Drave region.

One influencing factor is the slope of the topographic surface. Most of the study area is characterized by steep slopes, which make it difficult for water to infiltrate into the soil. On the contrary, in the vicinity of Drave, several terraces have been built over the last centuries, changing locally the slope of the hillsides, and promoting water infiltration. Infiltration is also greater in soils adjacent to valley bottoms, especially along the Palhais stream and its tributaries.

Another factor is soil thickness because Leptosols and rock outcrops are less favorable to infiltration. In the study area, these conditions are usually present in convex hilltops and in the upper part of slopes, where Leptosols associated with rock outcrops occur. In the lower part of the slopes, where Regosols (from colluvial deposits) and Anthrosols (from terraces) prevail, soil tends to be thicker.

The depth to the water table is also a key factor for understanding the spatial variability of the DRASTIC index. The water table is deeper in convex hilltops and the adjacent part of the slopes and shallower in the base of slopes and valley bottoms.

In summary, the areas of greater vulnerability correspond to the valley bottoms located in the Palhais stream watershed, where the value of the DRASTIC index is greater than 100. The intermediate vulnerability zones correspond to transitional sloping terrain, with Leptosols and Regosols, where DRASTIC index values range from 80 to 99. The areas of lower vulnerability are found in convex hilltops and in steeper slopes, especially those where rock outcrops are dominant. In this case, the DRASTIC index assumes values lower than 79.

Future perspectives

The main issue to be solved regarding water supply to the village of Drave, is the quality of water for human consumption. Other uses, such as supplying field toilets, are less demanding concerning the chemical and microbiological water features.

In the near future, the human occupation will expectably be similar to what has occurred since the last decade of the twentieth century: there will only be temporary inhabitants who come to the village to participate in activities of the Scout Movement, or for leisure activities, especially the practice of outdoor sports. In either case, the main challenge will be to supply good quality drinking water for campers or people who stay in the region for a few hours.

Bearing in mind the research results presented in previous sections, the water points presently used for water supply (WPB and WPD) are not reliable. In the case of WPB, which is connected to WPA, chemical and microbiological quality problems were observed, while in WPD only microbiological issues were observed. The water from the remaining points (WPC and WPE) also bears microbiological quality problems. Additionally, water flow in WPA, WPD, and WPE tends to decrease or even cease during the dry season, especially during drought periods.

A possible solution to meet the water needs of Drave is to keep on using WPD and to establish a new surface water abstraction point upstream from the village (water point F—WPF). The use of water WPA and WPB should be avoided, given their arsenic content. WPE is not a feasible option given its minimal flow and inaccessible location.

In this scenario, in which only surface water would be abstracted (in WPD and WPF), safe drinking water could be provided through the installation of a small-scale treatment system. There are several types of equipment for producing clean freshwater in remote locations where electricity is not available and solar photovoltaic energy is applicable. In the case of Drave, where the greater concern is the microbiological water quality, a water purification system based on ultraviolet sterilization (e.g., Radjabov et al. 2019) could be satisfactory. However, to ensure chemical water quality, other treatment methods could be considered, namely, distillation as well as filtration using membrane technology (e.g., Schäfer et al. 2014). Another possibility is to use individual water purification systems commonly used by trekkers, which consist of pocket-size ceramic filters coupled with sodium or calcium hypochlorite tablets for chemical disinfection (Küpper et al. 2009).

For most of the year, the streams in the Drave region have sufficient flow to meet supply needs. However, during the dry season, especially from July until September, streamflow tends to decrease or even cease at the time of year when water demand is most critical. To deal with this constraint, the location of WPF in the Palhais stream should be carefully determined, after additional hydrological and hydrogeological studies.

Field observations during the present research pointed out that from late October until the beginning of July (that is, during the wet season and part of the dry season), Palhais is a gaining stream. On the contrary, from the middle of the dry season on, Palhais becomes a losing stream. Nevertheless, the hydrological regime is strongly influenced by the presence of alluvial deposits which fill most of the valley bottom upstream from each weir. These deposits can be several meters thick, are very coarse, and can store water that gradually flows downstream, filling depressions caused by fluvial erosion, just downstream of the weirs (Figs. 1e, f, g, and 7).

Fig. 7
figure 7

Conceptual model of water circulation in the Palhais stream alluvial deposits during the late dry season: unsaturated alluvial deposit (A); subsurface water flow in the saturated alluvial deposit (B); weir (C); water flow in fractures in the unsaturated streambed (D); saturated streambed (E); water-filled erosional depression (F). (The weirs are typically around 3.5 m high and 0.7 m thick)

Full size image

The water stored in these depressions could be abstracted, in years of normal precipitation, until the end of the dry season. Therefore, the location of the WPF should be at one of the weirs located upstream of Drave, so that water can be transported to the village by gravity, taking advantage of the ancient water channels network which remains on the mountain slopes. More specifically, as long as Palhais stream flows, surface water may be directly abstracted from the stream channel, by gravity. During the driest part of the year, when streamflow ceases, water may be abstracted from the water-filled depressions which are hydraulically connected to the alluvial deposits (Figs. 1g, and 7). In this case, water could be elevated from the depression to the artificial water channel through a photovoltaic pumping system.

Finally, drinking water quality also depends on preventing pollution in the valley bottoms, where greater groundwater vulnerability to pollution (Fig. 6) coincides with more intense and frequent human activities. In these more sensitive areas, soil pollution will probably result in the circulation of polluted water across the topographic surface (overland flow), in the unsaturated zone (interflow), and in the water table aquifer (groundwater flow), thus transporting pollutants to the streams. Therefore, to prevent pollution of the water abstracted in WPD and WPF, the movement of livestock in the valley bottoms should be avoided, and campsites support infrastructures (especially kitchens and toilets) should be carefully constructed and managed to avoid soil pollution with solid and liquid wastes.

Concluding remarks

For thousands of years, human populations have sought out mountain regions to settle, even though the environmental conditions were often more adverse than in the surrounding lowlands.

The spatial distribution of mountain settlements is controlled by several factors, such as the availability of natural resources or security against hostile populations. Whatever the case, the availability of water resources is a determining factor in this process of population settlement, as water is essential for most agricultural and industrial activities.

The present study focuses on the historical evolution of water demand and supply in Drave, a mountain village located in North Portugal. The analysis highlights the hydrological and hydrogeological particularities of this mountain region and covers the historical period between the sixteenth century and the present day. In this regard, two very distinct historical periods are highlighted. During the first period, from the foundation of the village until the second half of the twentieth century, in which Drave had, mostly, permanent inhabitants, whose subsistence depended, fundamentally, on an agricultural system supplied by surface water, through a network of irrigation channels. In the second period, from the mid-twentieth century on, the type of human occupation changed gradually, and, around the turn of the century, temporary inhabitants (supplied by a network of groundwater and surface water points) became dominant: members of the Scout Movement and tourists, including outdoor sports enthusiasts.

The study of the present scenario of water supply to Drave started with a field survey to inventory the water points currently used. Then, these water points were characterized in hydrogeochemical and quality terms, and chemical and/or microbiological contamination was detected in all cases. The water quality was then classified as “marginal”, according to the CCME-WQI water quality index method.

Groundwater vulnerability to pollution in the Drave region was also assessed. For this purpose, the DRASTIC index was mapped using GVTool, a plugin developed for QGIS software. Groundwater vulnerability ranges from extremely low to moderately low, with the most vulnerable areas corresponding to valley bottoms.

Given the present scenario and the expected water demand in the Drave region, a possible solution for the future water supply was delineated: the use of surface water from the local streams, including abstraction in a new surface water point located upstream of the village by means of a photovoltaic pumping system, and the subsequent water transport by gravity. The installation of a photovoltaic water treatment system is also required to assure water chemical and microbiological quality. Additionally, since groundwater vulnerability to pollution is greater in areas of more intense human activities, preventive measures should be implemented, such as controlling the movement of livestock in the valley bottoms as well as preventing soil pollution in campsites.

The outcomes of this research are applicable to similar situations in other mountain regions around the world with comparable physical frameworks, historical backgrounds, and socio-economic dynamics.