Impact of a Storm-Water Infiltration Basin on the Recharge Dynamics in a Highly Permeable Aquifer

Infiltration basins are increasingly used worldwide to both mitigate flood risk in urban areas and artificially recharge shallow aquifers. Understanding recharge dynamics controlling the quantity and quality of infiltrating water is required to correctly design and maintain these facilities. In this paper, we focus on quantitative aspects and analyze in detail the temporal evolution of infiltration rates in basins overlying highly permeable aquifers. In these settings, recharge is a complex process due to high recharge rate and volume, undetected soil hydraulic heterogeneity and topsoil clogging. A 16-ha infiltration basin in Northern Italy has been intensively characterized and monitored for over four years. Field and laboratory tests were performed to characterize soil hydraulic properties. An unsaturated-saturated numerical model was implemented to obtain additional quantitative information supporting experimental data. Results show a strong impact of the infiltration basin on natural recharge patterns. When properly maintained (no clogging of topsoil), estimated infiltration rates from the bottom of the basin are about fifty times higher than recharge under natural conditions in the same area. When the infiltration basin is not properly maintained, bioclogging progressively diminishes the infiltration capacity of the basin, which turns to have no impact on aquifer recharge. Recharge patterns are highly erratic and difficult to predict. We observed natural recharge rates of the order of 1 m/h and a poor correlation between recharge times and maximum intensity of rainfall events. Due to the complex behavior of the recharge, the numerical model (based on the classical Richards equation) is able to explain many but not all the observed recharge events. Macropores flow and Lisse effects on piezometric measurements may be responsible for the disagreement between model predictions and observations.