Multi-indicator assessment of flood impacts on private well water quality

Floods pose significant risks to groundwater quality, particularly for households relying on private wells. This study investigates the vulnerability of private wells completed in fractured crystalline bedrock aquifers in a small watershed near Quebec City (Canada), to surface water intrusion and microbiological contamination during and after flood events. The study hypothesis was that flood events promote these processes in vulnerable private wells, with contrasting responses between non-floodplain and floodplain settings. Thirteen wells and three surface water sites were monitored over 1.5 years, encompassing four flood events. A novel multi-indicator approach was applied, combining hydrostatic and barometric pressure monitoring, stable isotopic composition of water (δ 18 O, δ 2 H), partial pressure of CO 2 (pCO 2 ), pH, and microbiological indicators (total coliforms, Escherichia coli ), to characterize flood-related contamination dynamics in private wells before, during, and after flood events. Results show that nearly all wells were contaminated by bacteria, with concentrations peaking during the 14-day post-flood period. Surprisingly, non-floodplain wells exhibited higher and more episodic contamination than floodplain wells, likely due to their location in recharge zones favoring vertical intrusion of stormwater. In contrast, floodplain wells displayed more constant but moderate contamination, possibly reflecting a hydraulic connection with the river. Stable isotope peaks confirmed rapid intrusion of surface water in several wells after summer storms, whereas the spring flood showed isotopic similarity between recharge water and groundwater, indicating that stable isotopes were not a reliable tracer of spring flood intrusion under snowmelt conditions. Hydrostatic data revealed sharp piezometric peaks indicative of focused recharge pathways, while pCO 2 and pH analyses distinguished open versus closed geochemical system conditions, providing further insight into the vulnerability of the wells. Collectively, these findings highlight diverse contamination pathways and emphasize persistent vulnerability of private wells beyond immediate flood events. This study underscores the importance of proactive monitoring and improved well management to better assess and safeguard private well drinking water quality under increasing hydro-climatic variability.