Groundwater vulnerability assessment in areas with diverse Quaternary deposits

Groundwater is the most crucial drinking water resource in many areas of the world. In spite of its overall abundance, the resource remains vulnerable to pollution, as groundwater quality may be severely affected by urbanization and growth in industrial activities and agriculture. The most sustainable approach to managing groundwater quality is to ensure its protection, thus avoiding contamination. Therefore, accurate groundwater vulnerability assessment methods are necessary tools for groundwater management and protection.The DRASTIC method is one of the most widely used groundwater vulnerability assessment methods. However, in areas where the main useful aquifers are covered with an extra layer of diverse Quaternary sediments, the original DRASTIC method overestimates the vulnerability of groundwater in overflow areas and in regions where groundwater is occasionally confined. Therefore, the DRASTIC method needs to be modified to increase the accuracy of vulnerability maps in areas with a highly variable Quaternary layer, which remarkably influences the nature of the infiltration conditions.For this, in this study, the depth to water, soil properties, and impact of the vadose zone parameters of the DRASTIC methodology were modified to be suitable in areas with glacial sediments. Originally, the depth to water parameter (D) allows assessing the distance from the ground surface to the aquifer: the deeper the water table, the lower the contamination risk. In the modified DRASTIC method, the water table is as an alternative compared to the bedrock surface beneath the Quaternary sediments layer. When the piezometric head is above the bedrock surface, the aquifer acts as confined, and the movement of the pollutant to the aquifer is hindered. Therefore, the groundwater vulnerability considering the D-parameter is lower in areas where the piezometric head is above the bedrock surface and higher in areas where it is below the bedrock surface.Both the original and the modified DRASTIC methodology were applied in an area with glacial sediments located in Central Estonia. The modified DRASTIC method showed significantly better results than the original DRASTIC method. Furthermore, comparing the maps generated using the modified DRASTIC with a former local groundwater vulnerability assessment method showed considerably more similarities than this by the original DRASTIC method. Thus, the modified DRASTIC method is successfully applicable in areas with an extra layer of diverse Quaternary sediments.The study has been funded by Iceland, Liechtenstein and Norway through the EEA and Norway Grants Fund for Regional Cooperation project No.2018-1-0137 “EU-WATERRES: EU-integrated management system of cross-border groundwater resources and anthropogenic hazards