Mixing Processes in Wells Tapping Confined Aquifers: Quality and Risks Assessments for Public Drinking Water Supply

In the last decades, scientific attention has grown toward vulnerability assessment of drinking water quality. Groundwater is a crucial resource for drinking water needs, and its vulnerability to contaminations is associated with natural and anthropic processes. The vulnerability of wells tapping confined aquifers is frequently associated with natural contamination by reduced species (e.g., As), while anthropic contamination is considered less relevant. Nevertheless, mixing processes induced by water pumping and related to the well structure can determine the presence of a shallower water fraction in the extracted water, which increases wells vulnerability to anthropic contamination. In this study, redox zonation is applied to water quality data of 16 drinking water wells tapping confined aquifers to assess the main processes affecting water quality. All the considered wells resulted under reducing conditions at different redox states. Three wells showed evidence of mixing processes between shallower and deeper waters. In these cases, mixing processes lower the concentration of As, but increase the vulnerability toward anthropic contamination.