Contrast of aquifer vulnerability and water quality indices between a unconfined aquifer and a deep aquifer in arid zones

The rapid population growth in urban areas stresses the underlying aquifers in both quantity and quality. The identification of aquifer vulnerability and quality characteristics help decision makers in managing groundwater resources and mitigating potential contamination pathways. For this purpose, the units composing complex aquifer systems should be evaluated separately due to their unique hydrogeological characteristics. In this study, two metrics were used to identify zones susceptible to groundwater pollution, these are the following: (1) aquifer vulnerability through DRASTIC; and (2) water quality using the water quality index (WQI) and the contamination index (C_d). These metrics were applied to a case study of an aquifer system in the Valley of San Luis Potosi (VSLP). The system consists of three units: (1) a unconfined aquifer; (2) a granular medium aquifer; and (3) a deep fractured aquifer. In this analysis, the granular medium and the deep fractured aquifer were grouped as one unit. Aquifer vulnerability in the unconfined aquifer has a high vulnerability in the urban area zone. Whereas, the middle-deep aquifer is classified as having a low to medium vulnerability. According to WQI and C_d indices, the unconfined aquifer is highly polluted due to its proximity to the urban area. On the other hand, the middle-deep aquifer shows excellent groundwater quality. These results are consistent with the hypothesis of limited to no hydraulic connection between the middle-deep aquifer and the unconfined aquifer.