A drywell is an underground structure that collects stormwater runoff and lets it slowly soak into the surrounding soil. Think of it as a buried chamber, usually made of concrete or plastic, that catches rain from your roof, driveway, or yard and feeds it back into the ground instead of letting it flood your property or overwhelm storm drains. Drywells are one of the simplest and most common ways to manage excess water on residential and commercial properties.
How a Drywell Works
The basic concept is gravity. Rainwater flows from a downspout or drainage pipe into the buried chamber. Once inside, the water seeps out through perforated walls or an open bottom into the surrounding soil, where it gradually filters down toward the water table. There are no pumps or moving parts.
Every drywell includes a grit chamber, which is a built-in settling area that traps sand, dirt, and debris before they clog the system. The outlet pipe sits at least 1 inch lower than the inlet pipe so water flows through without backing up. At the bottom, a sump area (at least 18 inches deep) gives sediment a place to settle out of the water before it reaches the surrounding soil.
The soil itself does much of the work. As water percolates through layers of earth between the drywell and the water table (a zone engineers call the vadose zone), natural filtration removes many contaminants. Metals like lead, copper, and zinc tend to bind to soil particles and stay put. Bacteria in the soil break down many organic pollutants. The whole process mimics what happens when rain falls on undeveloped land, just concentrated through a single point.
Concrete vs. Plastic Drywells
Most drywells are built from either precast concrete or high-density plastic, and each material suits different situations.
- Concrete drywells are heavier and more durable, with a lifespan of roughly 40 years or more. Their weight is actually an advantage in areas with high groundwater levels or frequent flooding, because a lighter chamber can shift or even float upward when the surrounding soil is saturated. Concrete is also the better choice if the drywell sits under a driveway or area with vehicle traffic.
- Plastic drywells are cheaper, lighter, and faster to install, lasting 30 years or more with proper maintenance. They work well for new construction and replacement projects where the groundwater table is low and no vehicles will drive over the site. Standard plastic chambers are not rated for vehicle loads unless specifically reinforced.
Because plastic drywells are typically shallower than concrete ones, some jurisdictions allow them to be installed closer to building foundations. In Portland, Oregon, for example, a plastic drywell requires only an 8-foot setback from a foundation compared to the standard 10 feet for concrete.
Why Drywells Matter for Groundwater
When land gets paved over with roads, driveways, and rooftops, rainwater that used to soak into the ground instead rushes across hard surfaces and into storm drains. This creates two problems at once: more flooding during heavy rain and less water recharging underground aquifers.
Drywells reverse that equation. A few major cities in California and Washington get 70 to 100 percent of their groundwater recharge through stormwater drainage wells and drywell infiltration. Fast-growing urban areas in Arizona, Florida, and Texas are expanding their use of stormwater wells as a cost-effective way to manage runoff while replenishing aquifers. Beyond individual properties, drywells are part of a larger strategy to prevent flooding, reduce surface water pollution, and build drought resilience by banking water underground during wet periods for use during dry ones.
Sizing a Drywell for Your Property
A drywell that’s too small will overflow during heavy rain. One that’s too large wastes money. The right size depends on two things: how much water your property generates during a storm and how quickly your soil can absorb it.
The water volume is straightforward to estimate based on the square footage of impervious surface (roof, driveway, patio) draining into the system. Soil absorption rate is trickier and usually requires a percolation test, where water is poured into a test hole and timed to see how fast it drains. Sandy soils absorb water quickly. Clay soils absorb it slowly and may need a larger drywell to compensate.
The general engineering standard is that a drywell should be able to fully drain its design storm volume within 48 hours. This is calculated using a formula that accounts for the facility’s storage volume, the soil’s infiltration rate, and the surface area at mid-depth. Most local stormwater manuals provide specific guidance, and many jurisdictions require a professional to run these calculations before issuing a permit.
Setback Rules and Permits
You can’t put a drywell just anywhere on your property. Local codes require minimum distances from buildings, property lines, septic systems, and wells. As a common example, Portland requires a 5-foot setback from property lines and a 10-foot setback from building foundations. These distances prevent water from seeping into basements or undermining nearby structures. Your jurisdiction’s requirements may differ, so checking local code before installation is essential.
At the federal level, the EPA classifies most drywells as Class V injection wells, a broad category covering any well that puts non-hazardous fluids into the ground. This classification doesn’t necessarily mean you need a federal permit, but it does mean your drywell cannot contaminate underground drinking water sources. Most regulatory requirements are handled at the state and local level, and many residential drywells need only a straightforward building permit.
Contamination Risks
Because drywells channel surface water directly underground, they can also channel pollutants. Stormwater runoff commonly carries metals from roads and parking lots (lead, copper, zinc, cadmium), oil and grease from vehicles, pesticides from lawns, road salt in cold climates, and pathogens from animal waste and even roof surfaces.
Soil filtration handles some of these well. Metals typically bind to soil particles before reaching groundwater. Bacteria in the soil break down many organic compounds. But some pollutants slip through. Nitrate is highly soluble and mobile, posing a real risk to groundwater quality in heavily populated or agricultural areas. Road salt (chloride) moves through soil regardless of pretreatment methods. Certain pesticides have been detected in groundwater below drywell systems in Arizona and Florida.
To reduce these risks, most modern drywell designs include pretreatment features: oil and grit separators, catch basin inserts, or vegetative filter strips that remove sediment and contaminants before water enters the well. These features also prevent clogging, which extends the system’s life.
Signs of a Failing Drywell
Drywells don’t last forever, and they can fail gradually enough that you don’t notice until water is pooling in your yard. The three most common causes of failure are sediment buildup clogging the system, structural cracks or collapse in the chamber walls, and tree roots growing into the pipes or the well itself.
Watch for these warning signs:
- Standing water pooling on top of or around the drywell after rain, instead of draining away
- Slow drainage that takes noticeably longer than it used to
- Foul odors like rotten eggs or sewage near the drywell area, which indicate stagnant water and bacterial growth from a clog
- Visible erosion around pipes or the edges of the well, or sections of ground sinking above the chamber
Regular maintenance, primarily clearing debris from inlet pipes and periodically pumping out accumulated sediment, is the simplest way to prevent these problems. A well-maintained drywell in suitable soil can function reliably for decades.