Assessment of the vulnerability of groundwater quality with respect to aluminum on crystalline bedrock under temperate climate conditions

Despite meeting legal standards, drinking water quality in sparsely populated areas with scattered groundwater resources in medium mountain ranges may be compromised. This results from inaccurate assessments of human activities, such as forest exploitation, on groundwater intake watersheds or insufficient monitoring frequency to understand their full impact on groundwater quality. This study involved a 3-year monthly monitoring of 18 groundwater intakes for drinking water supply, situated in areas with crystalline bedrocks, a temperate climate, and surrounded by forest exploitation. The quality of groundwater collected from alterites up to 15 m deep was evaluated over time. The correlation between the groundwater physico-chemical parameters (measured through 382 samples between March 2017 and December 2020) and the groundwater intakes’ environment - such as the geological and pedological context, land use, climate, and depth of drains - was investigated to understand their potential impact on groundwater quality. This study revealed a significant degradation in shallow groundwater quality (up to 5 m deep) concerning dissolved aluminum (concentration up to 2 mg L⁻¹), which results from land use and is climate dependent. Indeed, dissolved aluminum concentrations in groundwater can be correlated with the pH of the upper soil horizons, which are mostly impacted by forestry practices, as well as with rainfall events. The results of this study highlight that by integrating efforts to preserve the soil and change forestry practices (e.g., selective logging, avoiding rapid rotations), more efficient protection of groundwater quality can be achieved in the context of acidic soils present on crystalline bedrock.