Exfiltration in stormwater management refers to the process by which stored or conveyed stormwater gradually exits a constructed system and infiltrates into the surrounding native soils. It is most commonly associated with engineered practices such as infiltration basins, dry wells, permeable pavement systems, subsurface galleries, and bioretention facilities, all of which are designed to temporarily hold runoff and allow it to soak into the ground rather than being discharged directly to surface waters.
Exfiltration occurs through the bottom and, in some designs, the sidewalls of a stormwater structure. The rate at which it takes place is governed by the permeability of the underlying and adjacent soils, the hydraulic gradient between the stored water and the surrounding soil, and the presence or absence of clogging layers such as sediment buildup or biofilm. Systems intended to promote exfiltration are typically installed in soils with adequate infiltration capacity and are often underlain by uncompacted native soils to maximize performance.
From a water quality perspective, exfiltration provides important treatment benefits. As stormwater passes through soil, physical, chemical, and biological processes remove pollutants. These include filtration of suspended solids, adsorption of nutrients and metals onto soil particles, and microbial degradation of organic contaminants. Because of these processes, exfiltration practices are widely used as best management practices for improving stormwater quality.
From a hydrologic standpoint, exfiltration helps reduce peak runoff rates and total runoff volumes by returning water to the subsurface, thereby mimicking natural groundwater recharge conditions. This can reduce downstream flooding, channel erosion, and thermal impacts to receiving waters.
However, exfiltration is not always appropriate in every setting. It may be limited or prohibited in areas with poorly draining soils, high groundwater tables, or contamination concerns where infiltrating stormwater could mobilize pollutants into groundwater supplies. Proper design, siting, and maintenance are therefore critical to ensure that exfiltration systems function as intended and do not create unintended environmental risks.
In summary, exfiltration is a key mechanism in modern stormwater management that enables both runoff reduction and pollutant removal by allowing captured stormwater to infiltrate into the surrounding soil profile.