Recharge of a high permeability aquifer from an infiltration basin

Infiltration basins are increasingly used worldwide to both mitigate flood risk in urban areas and artificially recharge shallow aquifers. To correctly design and maintain infiltration basins it is important to accurately evaluate the infiltration rate and monitor its temporal evolution to guarantee both flood control and aquifer recharge through time. Moreover, it must be recognized that infiltration of storm water could potentially bring additional contaminant loading into the aquifer. We evaluated the temporal evolution of the infiltration rate of a basin located in the Po plain (Italy), to obtain useful insights about the general recharge behavior of basins overlying highly permeable aquifers that can be used for informed design and maintenance. Four years of groundwater level and rainfall monitoring and field and laboratory tests for the hydrogeological characterization of the soil were made. They supported the development of a calibrated unsaturated-saturated 2-D numerical model simulating the infiltration dynamics occurring below the basin. Results show that clogging by biofilms can progressively diminish the infiltration capacity of an infiltration basin, leading it to become a sort of wetland, but also that a properly working infiltration basin can increase the local recharge amount of highly permeable aquifers more than fifty times with respect to natural conditions. It is important to note that this aspect, along with the fact that the time interval for groundwater quality sampling should be site specific and related to the occurrence of rainfall events, must accurately be taken into account when considering the impact of artificially recharge on groundwater quality. Results also shows that although a numerical approach based on the classical Richards' equation is able to explain most of the observed infiltration, some observations cannot be adequately reproduced for the influence that infiltration preferential paths could have on infiltration dynamics.