What Is a Drywell and How Is It Designed for Stormwater?

A drywell is an underground infiltration chamber that disposes of stormwater by percolating it into the surrounding soil. Design requires infiltration testing to confirm the soil rate, sizing the chamber's storage and infiltrative surface for the design storm, providing pretreatment to keep sediment out, and maintaining required separations to groundwater, foundations, and wells. The appeal is obvious: no surface land consumed, no outlet to maintain, and the water recharges the aquifer instead of burdening the storm system. The catch is equally obvious: the entire system depends on the soil's willingness to accept water, forever. Everything in the design — the testing, the safety factors, the pretreatment, the separations — exists to make sure that bet is a safe one.

The infiltration test is the foundation of the design, and it has to be done right: at the actual proposed depth, in the actual soil the drywell will sit in, with the test method the jurisdiction recognizes. I apply the jurisdiction's safety factor to the measured rate — that factor accounts for long-term clogging and test variability — and design to the factored rate. The storage calculation treats the drywell as a small reservoir: inflow from the design storm minus infiltration during the storm equals the required storage volume. The drain-time check then confirms the full volume empties within the allowed window. If it doesn't, the options are more drywells, a bigger chamber, or a different disposal method. Forcing a drywell onto a site that fails the drain-time check is how you get a system that works for one season.

Drywells don't fail loudly — they fail by slowly losing infiltration capacity until a big storm backs water up where it was never supposed to go. The maintenance and protection details are the design. Here's what keeps drywells working for decades.

• Test honestly: infiltration testing at the right depth and location, with the jurisdiction's safety factor applied
• Pretreat everything: catch basin sumps or separators upstream to keep sediment out of the chamber
• Protect during construction: keep construction sediment and equipment loads away from the drywell area
• Provide observation access: a way to check water levels and confirm the system drains between storms
• Plan for the end of life: even maintained systems lose capacity over decades — design access for rehabilitation