To ensure the sustainable use and protection of groundwater resources and aquifers in the frame of climate change, the drinking water contamination risks related to geogenic sources should be properly assessed. The groundwater pollution from geogenic sources should be duly studied and appreciated by policy-makers for the development of effective water management, through monitoring tools to evaluate the qualitative aquifer status and trends. Particular attention should be dedicated to the understanding of groundwater quality in its association with natural processes and anthropogenic inputs, as well as to aquifer vulnerability [1
], in parallel with the strong need to improve advanced monitoring and early warning systems associated with the transport of pollutants from rocks, soils, and the vadose zone to groundwater. This research will also provide a real-time assessment of radon contamination in water supply and advanced monitoring that will give the evaluation of radiological impacts and trends in the integrated planning of land-uses and the management of groundwater quality. Innovative tools and solutions for decision support on groundwater remediation strategies will be produced, considering the regional water governance constraints. The fundamental principles of the National Water Plan (NWP) related to water planning aim at making the sustainable use of these natural resources in an integrated way through their protection and valorization, as well as the protection of people against extreme phenomena associated with water ingestion (e.g., radiological contamination).
Until 2020, Portugal has a National Strategy for Adaptation to Climate Change (NSACC), where it highlights some specific risks related to the reduction of the flow and recharge of aquifers, especially where these are scarcer. These risks from climate change also modify the water quality, mainly due to the reduction of dilution flow rates causing a deterioration of water bodies [4
]. Following this NSACC report, several strategic objectives were defined from 2021 to 2027: (a) Promote sustainable use based on long-term protection of available water resources, (b) ensure the gradual reduction of groundwater pollution to prevent further contamination, and (c) ensure appropriate supply of good quality groundwater. Thus, these legal impositions also provide some measures to promote efficient management and governance of water, among which a reinforcement and operationalization of a new monitoring system that evaluates the water bodies is implied. Concerns about the overexploitation of water resources are also highlighted in the first inter-municipal planning of the wine-growing Douro region (IPADVT, in Portuguese), and more attention for some regions subject to the disorderly exploitation of water resources for public supply purposes is also reported [5
]. The National Water Resources Information System (SNIRH) provides for the Portuguese territory a total of 22,641 groundwater monitoring points, of which only 779 of them have general information about the quality network of the water bodies related to contamination essentially by nitrates, chlorides, fluorides, and ammoniacal nitrogen [6
]. The economic dimensions of sustainable management of groundwater resources are mainly focused on water pricing and water access for irrigation. Currently, there is not a willingness of policy-makers to ensure financial conditions for the implementation of a Portuguese Action Plan for Radon (PAPR) and active mitigation of water bodies contaminated by geogenic sources. Nevertheless, it is worth noting that the scarcity of water from climate changes could worsen the quality of water resources due to the adverse collateral effects of reduced aquifer recharge and consequently lower dilution of geogenic contaminants.
Rn) is a noble radioactive gas resulting from geogenic sources, with high mobility in natural systems due to its short half-life (3.8 days). Exposure to this colorless, odorless, and tasteless substance can be a serious public health problem, since it is responsible for the radiation dose received by the human population [7
]. According to the World Health Organization (WHO; [12
]), radon is a carcinogenic agent reported as the second leading risk factor of lung cancer after tobacco, which is in accordance to recent epidemiological studies about radon exposure that causes about 20,000 deaths per year [13
]. Regarding exposure to radon, a review of lung cancer mortality in northern Portugal developed by Veloso et al. [14
] related 8514 lung cancer deaths with radon exposure. The primary source of radon in groundwater comes from the successive decays in uranium-bearing minerals of rocks and soils. In the subsoil, when the U-bearing mineral is subject to weathering, radon can easily migrate from the host mineral and precipitate into fractures and microfractures, onto the surfaces of the crystals, or even be mobilized through groundwater circulation under certain pH conditions [15
]. The rate of weathering and hence of radon migration is conditioned by the rock hydraulic diffusivity [24
]. The disintegration of 226
Ra atoms provides the radon product which, being a gas, can easily migrate from the generation site to the intergranular rock spaces (emanation coefficient). In these circumstances and depending on the high amount of water in the rock pores attached to the minerals, radon diffusion can be increased [31
Currently, some scientific limitations are also recognized by the scientific community in the knowledge of the control processes of radon production in rocks, as well as the mechanisms of transport that influence the water–rock interaction dynamics. The innovative goal of this study consists essentially in the evaluation of the ability of radon transfer from the geological substrate to groundwater. The porosity and the potential of radon production in rocks, and the physical and chemical conditions of drinkable water are considered the main contributing features to the radon contamination in water intended for human consumption. The main purpose of this study is to develop a PAPR in groundwater used for public consumption so that in the future, it can be applied by policy-makers in land-use planning and hydrographic region management plans. It is also intended to assess the risk of radon exposure, which includes the annual effective dose from ingestion and inhalation.
3. Results and Discussion
Since it was not possible to sample rocks in areas near the springs, it was decided to use measurements of potential of radon production in rocks (PRn) obtained by Martins [36
], who carried out analyses of porosity and K2
O in these same rocks. A brief description of radon production methodology is portrayed in Pereira et al. [44
]. The total dataset of these three features is depicted in Table S1 from the Supplementary Material
. For a consistent dataset preparation for parallel coordinate visualization (PCV) plot projection, these three measured variables were previously interpolated using the Topo to Raster tool in concomitance with the Extract Multi Values to Points tool from ArcMap [44
]. For these three parameters, there are missing values in Table S1 of the Supplementary Material
because some springs (n = 7) are located in the outside of the raster boundary generated through the performed interpolations (Figure 3
a–c). The summary of the total dataset used for the PCV plot is depicted in Table S2 in the Supplementary Material
. The complete dataset from the Excel worksheet was prepared in the ArcMap [45
] computer package, which is increasingly used in many hydrologic and environmental studies (e.g., [46
]). The PCV plot is a graphical user interface tool (available in Excel using the XLSTAT statistical software) that analyzes the dissimilarities between grouped objects, resulting in a dendrogram, which shows the following data grouping within the suitable number of geological groups. In the PCV procedure, when mean lines are activated, this option lets XLSTAT display for each lithological group a line that corresponds to the mean of the measured variables (222
Rn, altitude, rainfall, electric conductivity, temperature, pH, porosity, K2
O, and PRn) for each nominal variable (geology group). When the rescale option is activated, it allows for comparing how the data are distributed for multivariate measured variables and facilitates the visualization. Thus, the goal of the PCV plot is to comprise what measured features may influence the dissimilarity between each lithological group. According to the data projected in Figure 3
a–c, it is verified that the municipalities of Vila Real and Vila Pouca de Aguiar are composed of a broad cartographic representation of higher values of K2
On the other hand, when crossing these results with the geology of the Vila Real municipality, an overlap is observed with the two-mica granites (Group III and IV) and high K2
O contents, sometimes subjected to the surface weathering due to physical alterations resulting from the adverse temperature and precipitation that occurs in the region. Besides, K2
O content is very noticeable in these type of granites mainly due to the high muscovite amount in comparison to the biotite granites [36
]. On the other hand, it can be verified that in the NE of the VPA territory, there are outcrops of Parautochthonous metasediments with low K2
O content but with higher porosity (Figure 3
a,b). In this way, we can predict that specific groups with high porosity (Groups I and V) have low K2
O contents. The radon production potential is higher in granites than in the metasediments, and as such in both municipalities the granitic rocks can release high radon concentrations into the groundwater (Figure 3
c). In this way, it will be expected for both districts to find high levels of radon in the water supply.
In the first approach, high concentrations of radon in water for human consumption will be expected, and given the substantial amount of analyzed features, the protection planning of drinking water is crucial to ensure public health. For this purpose, the PCV plot was initially used and depicted in Figure 4
The PCV projection displays two typical patterns, highlighting the pH and PRn values (Med = 5.9 and 254.9 Bq m−3
, respectively) for Group V, and remaining high K2
O and 222
Rn values (Med = 5.3% and 5.4% and 441.0 and 588.5 Bq L−1
, respectively) for Groups III and IV (Figure 4
; Table 1
). These results clearly show that although Group V develops a high ability to produce radon (PRn), their release into groundwater will not always be effective if the physical and chemical conditions of water do not facilitate their transport. Therefore, once again the water conditions in the biotite granites group, namely low temperature and high pH (Med = 14.6 °C and 5.9, respectively) did not provide for the release of radon into the water (Figure 4
; Table 1
However, it should also be noted that in the groups of two-mica granites (Group III and IV), there are some differences in several parameters, although not very significant. The features that contributed substantially to a lower concentration of radon in springs of group III were the highest altitude and precipitation (Med = 760.5 m and 1299.7 mm, respectively) compared with lower PRn (Figure 4
, Table 1
). In this particular group (Group III), rainfall rate may have promoted an increase in aquifer recharge resulting from dilution flow in groundwater bodies. Besides, it is verified that Group I present low values in almost all features except for electrical conductivity and porosity (Med = 58.5 μS cm−1
and 3.7%, respectively; Figure 4
, Table 1
). Thus, despite the high porosity displayed by the metasediments, the ability to produce radon is meager, so there is a low amount of radon to be transported to the water used for public consumption.
Therefore, we can anticipate that springs located in metasedimentary areas may represent a low risk of radon exposure. For the efficiency of land-use planning, these considerations should be included in municipality and management planning related to water protection policies (NAP, HRMP, and SWMP) for the promotion of radiation protection in groundwater bodies.
A projection of two maps for the promotion of an effective territory planning and radiological protection of human health is depicted in Figure 5
a,b, to carry out the analysis of the radon risk contamination in drinking water in concomitance with the land-use map.
The data projection about radon contamination in water intended for human consumption shows that the high-risk areas (801–1400 Bq L−1
) overlap the granitic substrates, however with greater cartographic representation in the two-mica granites of the Vila Real municipality and the lower-risk spots (0–200 Bq L−1
) on metasedimentary areas (Figure 5
a). On the other hand, it is also observed that the areas with high population density are located precisely in high-risk regions of radon exposure, and as such, preventive and awareness-raising measures will need to be taken to warn citizens about the hazards of radiological exposure.
A more detailed analysis of radon concentration in the different land-use systems shows that the artificial areas present a higher average radon concentration (506.9 Bq L−1
; Figure 5
b and Figure 6
) because they are mostly implanted in granitic areas (Figure 1
e and Figure 5
). The forest area is quite broad in both municipalities and therefore has a lower average radon concentration (334.6 Bq L−1
) because several lithologies represent it with noticeable differences in radon production from geogenic sources (Figure 5
In the last years, several radiological studies about water intended for human consumption were carried out in some European countries like Portugal (64–9784, 3–2295, and 0.3–938 Bq L−1
), Spain (2–31000 Bq L−1
), Greece (3–26 Bq L−1
), Germany (1–1800 Bq L−1
), and Austria (2–644 Bq L−1
]. The radiological results for this study area show that in several springs, radon concentrations in water (14–1385 Bq L−1
; Table 1
) were higher than the results obtained in other countries.
In general, after a thorough analysis of drinking water (n = 69), it was confirmed that 52 springs have radon concentrations above the limit imposed by international guidelines of 100 Bq L−1
, and 22 of them have radon concentrations above the national guidelines of 500 Bq L−1
. According to European Union requirements, whenever radon concentrations in water exceed the value of 1000 Bq L−1
(n = 4), corrective measures should be considered to implement effective health protection (Table S2
). Since drinking water conditions affect the intake of radionuclides, it is necessary to ascertain that the dose of ingestion and inhalation from this source was properly evaluated for this region. Therefore, we projected in box plots the annual effective dose from ingestion and inhalation of water to ensure the protection from radiological hazards (Figure 7
The global average of total annual effective dose from inhalation of radon is thus 1.1 mSv y−1
(where the decay products present in the air are included), and for ingestion the global average is an annual effective dose of 1.2 mSv y−1
The groups of two-mica granites (Group III and IV) present a higher average in the annual effective dose from ingestion and inhalation of radon compared to the remaining groups. Since these groups have higher concentrations of radon in drinking water (Figure 4
), it is again demonstrated that the annual effective dose from ingestion and inhalation of radon confirms the increased risk if exposure is constant (Figure 7
Regarding the annual effective dose for ingestion, there is only one spring with higher values than the average worldwide. On the contrary, the effective dose by inhalation has a more significant impact in 27 springs (Figure 7
c; Table S2
). Radon inhalation plays the main role in the assessment of radionuclides exposure in water intended for human consumption. As previously mentioned, the highest number of springs with higher effective doses by inhalation (Figure 7
c) is also located in Groups III and IV (Figure 7
d). Several corrective measures can be applied in these areas (e.g., use of aeration diffusers, granular activated carbon filters, ionic and reversed osmosis, and nanofiltration) to minimize the effects of high dose exposure and distinguish the actual risk to human health.
The removal of radon through activated carbon filters is the easiest and low-cost solution for the treatment of consumer water as an individual solution [71
]. Long-term analytical monitoring of at least one hydrological year in areas considered to be at high risk and the protection against the action of stochastic radon effects are also advisable.
According to the Municipality Planning of Vila Pouca de Aguiar, in this territory there are groundwater collection points used for human consumption which have defined some priority guidelines for the preservation of groundwater, including management of water resources to monitor and improve water quality [72
]. Of the total number of sampled springs of the VPA municipality (n = 36), 14% of them present radon concentrations above the limit imposed by Portuguese legislation. On the other hand, the municipality of VR reveals more concern, because of the 33 springs sampled, 48% of them present radon values above 500 Bq L−1
. According to the HRMPs, all groundwater bodies have "Good Condition" resulting from contamination by anthropogenic origin, yet once again, the water contamination from geogenic sources has never been addressed in these plans but should be.
In the groundwater quality disclosure platform (SNIRH), there is not any monitoring water point for the municipality of VPA. On the contrary, for the VR municipality, there are only three monitoring points, with only one single analysis in 2006, concered with conductivity, nitrates, pH, ammoniacal nitrogen, and chlorides. In Portugal, there seems to be remarkable disinvestment in the dissemination and promotion of groundwater quality. However, there is still a great deal of concern about climate changes, with the long-term practice of using groundwater in periods of increased water scarcity for public consumption from deep sources. Due to the water scarcity from climate changes, the Trás-os-Montes and Alto Douro region is particularly affected in regard to groundwater availability and quality used for human consumption [32
Future research will focus on the study of the weights attributed to each of these factors influencing radon contamination in the water intended for human consumption, using Partial Least Squares-Path Modelling.