Validation of the intrinsic vulnerability to pollution of fractured siliciclastic aquifers using natural background levels

Abstract

Aquifers are important water reserves in water-scarce areas. However, uncontrolled human activities and lack of knowledge of aquifer characteristics increase their vulnerability to pollution. Assessing intrinsic vulnerability is essential for improving groundwater management. However, traditional tools, such as DRASTIC, ignore relevant parameters that, in particular contexts, lead to underestimating vulnerability. This work assesses the intrinsic vulnerability to pollution of a siliciclastic aquifer system (Shallow and Upper) by comparing the DRASTIC and modified DRASTIC methods. Modified DRASTIC included parameters such as Land Use and fracturing patterns relevant in populated areas where hydrogeological configurations involve highly cemented and fractured siliciclastic rocks with secondary porosity that define the main pathways for transporting contaminants from the surface. The intrinsic vulnerability was validated using Natural Background Levels (NBLs) of contaminant indicator ions (NO3−, PO43− and SO₄2-) to identify areas where the concentration of these indicators surpassed the NBLs. These areas were compared with the vulnerability classes from DRASTIC methods. NBLs were: 2.12 mg/l, 0.05 mg/L and 26.77 mg/L (Shallow aquifer), and 3.17 mg/L, 0.17 mg/L and 5.06 ml/L (Upper aquifer) for the ions of NO3−, PO43− and SO₄2-, respectively. The ion concentrations surpassing NBLs were spatially distributed, being more notable in the dry season and showing a greater coincidence with the high vulnerability class of the Modified DRASTIC. Thus, the modified DRASTIC better represented the high vulnerability of the aquifer system to pollution and the negative effect of anthropic activities. Additionally, Pearson's coefficients were calculated between the concentrations of contaminant indicator ions and intrinsic vulnerability indices of DRASTIC methods. However, the coefficients lack statistical significance. These results support the relevance of adding land use and fracturing patterns to assess the intrinsic vulnerability of fractured, siliciclastic aquifers in populated regions and show NBLs as a better validation method than Pearson's coefficients in data-scarce contexts.