Where Does Infiltrated Stormwater Go? Interactions with Vegetation and Sub-surface Anthropogenic Features

The practice of stormwater infiltration is widely used to reduce the amount of urban stormwater runoff delivered to drainage systems and receiving waters. In theory, the practice recharges groundwater, leading to increased urban stream baseflows. However, little is known about the fate of infiltrated stormwater. We tracked the fate of infiltrated stormwater from an infiltration basin using a network of piezometers for three years. We found that groundwater levels downslope of the basin were increased while water levels in an array of reference piezometers lateral to the basin showed no change. In summer, most of the infiltrated stormwater was evapo-transpired by the vegetation downslope of the basin, and thus did not reach the receiving stream. The reverse was true in the colder months, where some infiltrated stormwater did reach the stream as plant water use declined. Particularly during these Summer-Autumn months, anthropogenic disturbances interacted with the plume of infiltrated stormwater: infiltrated stormwater seeped into nearby sewer infrastructure. The study provides evidence that the fate of infiltrated stormwater largely depends on surrounding land use and that urban groundwater pathways are susceptible to disturbance by anthropogenic features. Infiltrated stormwater may not always reach receiving streams as baseflow as often assumed, which has implications for the design and placement of infiltration structures. This experimental data can be used to refine conceptual models of urban catchments and stormwater models.