Drywells

A drywell is an underground structure designed to dispose of roof runoff or other surface water by allowing it to infiltrate the surrounding soil. It functions as a subsurface collection chamber that receives water from a downspout or drainage line via a perforated pipe and permits it to percolate into the surrounding earth. In British usage, similar devices are often called soakaways or infiltration wells.

Most drywells consist of a buried chamber or a perforated pipe laid in gravel, sometimes inside modular infiltration chambers, with filter fabric to prevent sediment from entering the soil. Water enters through perforations and slowly percolates away. Older systems use a concrete or masonry pit filled with gravel. Modern designs often employ modular plastic chambers that maximize infiltration capacity while reducing excavation.

A drywell requires soil with adequate permeability, a suitable depth to groundwater or bedrock, and a safe distance from foundations, wells, streams, and potable water supplies. Performance can be limited by freezing conditions, high groundwater, or saturated soils. They are intended for clean or pretreated runoff and are not suitable for the discharge of septic effluent or heavily contaminated water without pretreatment.

Maintenance is typically straightforward but essential; sediment, roots, or oil can clog the system, reducing effectiveness. Infrequent inspections of inlets and filters, and periodic removal of accumulated sediment, are common practice. Local codes often regulate design, installation, and setback requirements, and improper design can cause surface pooling or groundwater issues, so professional planning and permitting may be required.