A dry well in stormwater management is a subsurface infiltration structure designed to receive, temporarily store, and ultimately dispose of stormwater runoff by allowing it to infiltrate into the surrounding soil. Dry wells are typically installed below ground and are commonly used to manage runoff from rooftops, small paved areas, and other relatively clean drainage sources where space is limited and surface infiltration practices are not feasible.
Structurally, a dry well usually consists of a vertical excavation or pit that is backfilled with clean, uniformly graded stone or fitted with a prefabricated chamber, such as a perforated concrete ring, plastic leaching chamber, or similar device. The void spaces within the stone or chamber provide temporary storage for stormwater, while perforations or open bottoms allow water to exfiltrate into the native soils. Many designs include a layer of geotextile fabric to prevent surrounding soils from migrating into the stone and clogging the system.
Hydraulically, dry wells function by capturing runoff and releasing it slowly into the subsurface at a rate controlled by the infiltration capacity of the surrounding soils. The effectiveness of a dry well is therefore highly dependent on soil permeability. They are best suited for well-drained soils and are generally not appropriate in areas with high groundwater tables, shallow bedrock, or poorly draining clay soils, where infiltration rates are insufficient.
From a water quality standpoint, dry wells provide treatment through filtration and soil interaction as water moves through the subsurface. Suspended solids are trapped within the stone or pretreatment components, while dissolved pollutants may be reduced through adsorption and microbial processes in the soil. However, because dry wells discharge directly to the subsurface, they are typically used only for relatively clean runoff and often require pretreatment measures such as leaf screens, sediment traps, or first-flush diversion to prevent clogging and groundwater contamination.
Maintenance is critical to long-term performance. Over time, sediment and debris can accumulate within the system and reduce its infiltration capacity. Routine inspection and cleaning of pretreatment devices, along with occasional rehabilitation or replacement of the stone reservoir, may be necessary to maintain functionality.
A dry well, in summary, is a compact, underground stormwater management practice that promotes exfiltration, reduces surface runoff, and provides limited water quality treatment by directing stormwater into the surrounding soil profile.