Aquifer recharge by stormwater infiltration basins: Hydrological and vadose zone characteristics control the impacts of basins on groundwater chemistry and microbiology

Stormwater infiltration systems (SIS) are designed to collect and infiltrate urban stormwater runoff into the ground for flood risk mitigation and artificial aquifer recharge. Many studies have demonstrated that infiltration practices can im-pact groundwater chemistry and microbiology. However, quantitative assessments of the hydrogeological factors re-sponsible of these changes remain scarce. Thus, the present study aimed to quantitatively test whether changes of groundwater chemistry and microbiology induced by SIS were linked to two factors associated with vadose zone prop-erties (vadose zone thickness, water transit time from surface to groundwater) and one factor associated with ground-water recharge rate (assessed by groundwater table elevation during rain events). To evaluate changes in chemistry (NO3-, PO43- and dissolved organic carbon concentrations), groundwater samples were collected in wells located in SIS-impacted and non-SIS-impacted zones during experimental periods of 10 days. During the same periods, clay beads were incubated in the same wells to measure changes of groundwater microbial biofilms (microbial biomass, dehydrogenase and hydrolytic activities) induced by SIS. Results showed that changes in PO43- supplied to groundwa-ter during stormwater infiltration was negatively correlated with vadose zone thickness. A short water transit time from surface to groundwater increased dissolved organic carbon concentrations in the aquifer which, in turn, increased biofilm biomasses in groundwater. The groundwater recharge rate during rain events (assessed by groundwater table elevation) diluted NO3- concentrations in the aquifer but also influenced the changes of biofilm activities induced by SIS. Groundwater recharge rate during rain events probably increased the fluxes of water and dissolved organic carbon in groundwater, stimulating the activity of microbial biofilms. Overall, the present study is the first to quantify con-jointly several factors and processes (water transfer, dilution, solute fluxes) that could explain the impact of stormwater infiltration on chemistry and/or microbiology in groundwater.

Automated summary

This study quantitatively assessed the impact of vadose zone properties and groundwater recharge rate on the chemistry and microbiology of groundwater affected by stormwater infiltration. The findings showed that vadose zone thickness was negatively correlated with changes in PO43- concentrations, while a short water transit time from surface to groundwater increased dissolved organic carbon concentrations and biofilm biomasses in groundwater. Groundwater recharge rate during rain events influenced both the dilution of NO3- concentrations and the changes in biofilm activities induced by stormwater infiltration.