Groundwater, acknowledged as a critical part of the hydrological process, is an irreplaceable resource for anthropological activities [1
] and soil humidity preservation in the dry season [2
]. The coastal soil-water-bearing layer includes the active groundwater sources in the coastal areas that allow plants and other organisms to live [3
]. Although fresh water in the soil-water-bearing layer is often not adequate for human consumption, it plays an essential role in sustaining agricultural and natural environments [4
]. Saltwater intrusion into the freshwater surface layer is a natural process that takes place in coastal regions due to changes in the hydrological systems and fluctuations in the groundwater table level [6
]. Concurrent with drought, it severely damages soil layers around coastal regions [8
]. This phenomenon has become a common issue in deltas and coastal lowlands, which are characterized by shallow water tables, insufficient drainage networks, and seawater intrusion [10
]. Salinization is affected by both natural factors (e.g., the topography, climatic conditions, river/canal network, and hydrological regime) and anthropologic drivers (e.g., increased saline/brackish water demand for aquaculture) [5
]. Hence, a consideration of the soil water vulnerability in coastal areas needs to take into account the effects of saltwater intrusion in conjunction with local socioeconomic and physical conditions.
Mapping groundwater vulnerability to salinization in coastal regions provides necessary information for the appropriate management of of groundwater resources [12
]. Different approaches have been applied to estimate groundwater vulnerability, and can be grouped into three categories (i.e., index-based, statistical, and simulation-based approaches) [14
]. With its simple and straightforward method, the DRASTIC model, which was primarily developed by the National Water Well Association and the U.S. Environmental Protection Agency, is among the most widely used approaches to groundwater vulnerability estimation [14
]. However, due to the variation in local conditions (e.g., geological or geo-hydrological settings), removing factors, altering input factors, or adding new parameters such as land use or irrigation types may be conducted with the generic DRASTIC model to achieve a better outcome [18
]. For example, Chenini et al. (2015) [19
] removed two factors (i.e., Aquifer (A) and Hydraulic Conductivity (C)) to quantify the vulnerability index in the saturated aquifer zone of Grombalia. Singh et al. (2015) [20
] added anthropic factors to the DRASTIC index to determine pollutants in an urban area in India. The results of both adjusted models were better than those of the classic model when they were validated using field data. Furthermore, validation has often been implemented because the DRASTIC model is an indirect method that mainly relies on accessible generic data [21
]. A DRASTIC model that is not validated can result in an incorrect vulnerability assessment [12
]. Therefore, this study applied an adjusted DRASTIC model to characterize the water salinity risk in the soil layer. Validation was also conducted to improve the accuracy of the model.
The Mekong River Delta (MRD), the most important region for rice production in Vietnam, is severely affected by the global climate change threats [9
]. The impact is more severe due to the combined effects of upstream water reduction [25
]. It is projected that, with the long-term decline in rainfall and upstream water and increased water consumption during the dry season, the salinization process will be more widespread, and therefore, will significantly threaten the water quality in the region [26
]. Hence, assessing soil water vulnerability to salinization is an essential step in understanding the situation and process of water supply in coastal areas and developing better strategies for groundwater resources management. This achievement is valuable for mitigating land degradation and promoting sustainable agriculture development in the region [28
]. Several studies examining the soil salinity patterns in the MRD revealed that infiltration of saline content into groundwater depends on the land use and river density [8
]. Each land use type will have a different influence on water contamination. For instance, saltwater enters the bottom soil layer more quickly on agricultural land than other land use land cover (LULC) types by the direct use of water for agricultural production from a salted river or canal system in the dry season. Besides, saltwater intrusion will occur more easily in the regions that have a populated river density. Hence, modifying the classic model by combining the river catchment and land use as additional factors is critical to estimating the saltwater intrusion risk in the coastal soil-water-bearing layer in the MRD.
The primary purpose of this study was to model zones vulnerable to saltwater intrusion in the coastal area of MRD using a GIS-based Mod-DRASTIC model. The specific objectives were to (i) quantify the spatial pattern of thematic factors for the Mod-DRASTIC model, (ii) assess the soil water susceptibility to salinization, and (iii) validate the accuracy of the model outcomes. The Ben Tre province, a coastal province located in the MRD, was selected as a case study to implement the research. The findings of this study will support the local government in proposing action strategies for preventing salinization in vulnerable areas and properly managing coastal groundwater resources.
The variations in saline content in the soil-water-bearing layer depend on geo-hydrological settings and anthropogenic activities in a specific study area. Even though the classic DRASTIC model with seven factors has commonly been used for groundwater vulnerability assessment, several studies have pointed out that modeling this without considering the impacts of human activities is a significant limitation [14
]. To overcome this gap, the input of the classic DRASTIC model has been adjusted by adding or eliminating some generic factors, following the local conditions [32
]. In this study, the classic DRASTIC model was adapted to create the Mod-DRASTIC model by adding two factors (i.e., river catchment and land use), based on the important role of these local features in the study area. Seawater intrusion naturally occurs in the dry season with densely populated river and canal networks as it transmits quickly to inland.
Besides, the relationship between soil water vulnerability to salinization and the land use category in the study area was discovered in this study (Table 7
and Figure 6
). Both models revealed that the areas at moderate and high risk of salinization were aquaculture and crop land (over 60%). This demonstrated that most of the areas with high and very high susceptibility to salinity were used for salt production, while forest showed low to moderate vulnerability. In aquaculture land, the different forms of breeding can lead to variations in the salinity category. For instance, high and very high vulnerability to salinization was seen in shrimp farming areas, while low and moderate risk was found in fish farming areas. Obviously, the presence of the river catchment of Ham Luong, crop land, and other land use categories were able to reduce the salinization risk, while the salt production land use category might enhance the vulnerability to saltwater intrusion in the soil-water-bearing layer.
From the results of the Mod-DRASTIC, 69% and 15% of the total land in salt production was discovered to be at high or very high risk of salinization. By contrast, only 22% and 5% of the categories were found in the classic model (Table 6
). Therefore, with a combination of river catchment and land use types, known as anthropogenic activities, an improvement of the map in terms of the soil water susceptibility to salinization can be explored in comparison to the local parameters.
The risk of soil water salinization in the Ben Tre province was evaluated using the generic DRASTIC and Mod-DRASTIC indices. The generic DRASTIC model was modified by incorporating the river catchment and land use layers to analyze the susceptibility to salinization of water in the soil-water-bearing layer in the Ben Tre province, located in the Mekong River Delta. Nine factors were considered for quantifying the vulnerability index, including seven thematic elements of the classic DRASTIC model (i.e., depth of water, net recharge, aquifer media, soil media, topography, vadose zone, and hydraulic conductivity) and two additional parameters (i.e., river catchment and land use pattern), known as the anthropogenic impacts. Weights and ratings of the principal factors were assigned to produce vulnerability maps following the local conditions. Field collection and laboratory analysis of soil water TDS concentration were also used to validate the results.
The results of this study revealed that the coastal districts (i.e., Ba Tri, Thanh Phu, and Binh Dai) were vulnerable areas with high to very high salinity, while around the inland districts (i.e., Cho Lach, Chau Thanh, and Ben Tre City), low to moderate vulnerability to the salinization of the water in the soil layer was recorded. The combination of the river catchment, land use types, and thematic factors of the classic DRASTIC model allowed for the validation of the Mod-DRASTIC model in conjunction with the TDS concentration (R2 = 0.76). Furthermore, the potential of seawater intrusion was assessed for the land use map of the Ben Tre province, and we observed that over 60% of the total aquaculture and crop land area faced a moderate to high salinization risk, whereas 69% and 15% of the total land for salt production was discovered to have high or very high vulnerability to salinization. Therefore, adding the two anthropogenic factors associated with LULC and river catchment in the study area enabled us to model the vulnerability to salinization better. Hence, we can conclude that the additional factors of LU and RC should be involved in the assessment of vulnerability in the MRD.
The methods applied in this study will serve as a valuable reference for future work in land use conversation and hazard analysis, as they can be used worldwide in any other coastal areas, considering any time frames and any biomes. This approach can provide a comprehensive understanding of soil-water-bearing-layer vulnerability to salinization. This will help policymakers and planners to develop appropriate policies in coastal regions to adapt to and reduce the effects of global warming in connection to a water level reduction upstream. In particular, the local government can apply the results to transform land use to mitigate the vulnerability to salinization.