dry well

A dry well is an underground chamber, typically a pit or shaft filled with coarse aggregate (gravel, stone, or rubble), that receives stormwater runoff and allows it to percolate into surrounding soil. Instead of directing water to a storm sewer or surface discharge, the dry well holds water temporarily while gravity and soil permeability do the work of dispersal. The water soaks downward through the aggregate and into the groundwater table or surrounding earth, with no outlet pipe or overflow mechanism in normal operation.

Dry wells are commonly used where conventional drainage infrastructure is absent, unaffordable, or undesirable. They absorb runoff from roof gutters, parking lots, driveways, and graded surfaces. The pit itself may be a simple excavated hole, a precast concrete ring or cylinder, or a plastic chamber, typically 4 to 12 feet deep and 2 to 6 feet in diameter depending on the catchment area and soil infiltration rate. The sides and bottom are often lined with landscape fabric to slow sediment migration while permitting water passage. Coarser stone is placed in the upper zones, finer gravel lower down, creating a gradation that balances flow and settling.

Soil, depth, and failure modes

A dry well's effectiveness hinges entirely on soil permeability. Clay-heavy or saturated soils absorb water slowly or not at all; these sites require the well to be deeper, wider, or drained to a sump pump system instead. Sandy and gravelly soils are ideal. Groundwater depth also matters: if the water table is high, the well may flood and become useless. Design standards in most jurisdictions require testing of soil infiltration rate (often expressed in inches per hour) before siting a dry well. Clogging occurs gradually as fine sediment and organic matter accumulate, reducing the effective infiltration rate. Some designs include a sediment sump at the bottom or a pre-filtration catch basin upstream to extend the interval between cleanings.

A poorly sited or maintained dry well can become a stagnant pond, breeding ground for mosquitoes, or a structural hazard if it collapses under load. Surface ponding indicates inadequate infiltration capacity or a blocked inlet. If groundwater is contaminated or sits close to the surface, a dry well can accelerate pollutant migration. For these reasons, many jurisdictions now require a gravel or sand filter layer above the aggregate and specify minimum setbacks from property lines, wells, and septic systems.

The term "dry" reflects the intent: unlike a wet sump or retention pond, water does not accumulate permanently. In practice, a properly designed dry well remains dry or nearly so between storms, with water present only during and briefly after rainfall or irrigation. This distinguishes it from a French drain (which conveys water laterally) or a rain garden (which is shallower and planted). Dry wells remain a simple, low-cost alternative to piped storm drainage in rural and suburban settings, though their reliance on soil conditions makes site investigation non-negotiable.