Dry well
A dry well is underground storage — a pit of clean rock, or a chamber — that holds runoff while the surrounding soil absorbs it. Two gates decide whether it is a well or a buried bathtub: the perk rate of the soil around it, and the distance between its bottom and the water table. Fail either and the well fills, stays wet, and becomes a mosquito cellar.
The gates
Gate table
| Gate | Pass mark | Why |
|---|---|---|
| Perk rate at well depth | ≥ 0.5 in/hr | The desk's standing gate; many codes say the same, some want more — read yours |
| Bottom above seasonal high water table | ≥ 3 ft | Common minimum separation; a well sitting in groundwater stores nothing |
| Distance from foundation | ≥ 10 ft | Common minimum; more under some codes |
| Drawdown | ≤ 72 h | Same rule as the swale — the 72-hour note |
The sizing arithmetic
Volume first: a 1-inch storm over 800 square feet of roof is about 500 gallons. The well must store what the soil can't absorb while the storm is still falling. Storage comes from the void space in the rock — roughly 30–40% for clean stone — so a 500-gallon need is roughly a 6-ft-deep, 6.5-ft-diameter rock pit, or a pair of chambers. The soil's rate then dictates how long the well takes to empty. Fast soil, small pit. Slow soil, the pit grows until the yard runs out — at which point the answer was never a dry well. That crossover is exactly what the perk test exists to find, before the shovel does.
Failure modes, in order
- Silted in. No pretreatment — roof grit and lawn dirt pour in, the rock voids close, and the well is a planter by year five. A catch basin or filter strip upstream is not optional decoration.
- Placed blind. No perk test, no water-table check. The two gates above are not bureaucracy; they are the only load-bearing parts.
- Fed more than designed. The shed roof gets a gutter “later,” the patio gets extended, and the well drowns. Catchment is the third number for a reason — count it.
Where to go next
- Run a percolation test — the gate exam.
- Catch basin — the pretreatment the well demands.
- French drain — what usually feeds the well.
Sources
- Minnesota Stormwater Manual, infiltration trenches and dry wells — stormwater.pca.state.mn.us
- US EPA NPDES — epa.gov/npdes
- USGS, groundwater basics — usgs.gov