Absorption Well

An absorption well is a buried pit filled with clean stone. It takes water arriving at one point, most often a roof downspout, and lets it soak away into the surrounding soil. It disposes of water and returns it to the ground rather than keeping any: nothing is drawn back out, and a working one is judged by how fast it empties.

The soil decides whether it can be built at all. The ground must take water at between about 8 and 125 mm an hour (0.3 to 5 in): slower and the pit backs up, faster and water reaches groundwater unfiltered. Clay must stay under about 30 percent of the soil, silt and clay together under 40. Its floor sits at least 0.9 m (3 ft) above rock and above the highest the water table rises, 3 m (10 ft) from a building and 30 m (100 ft) from a drinking well, a septic system or open water. It holds washed stone of 40 to 75 mm (1.5 to 3 in) in filter fabric under at least 30 cm (12 in) of soil, and design guidance limits it to a catchment no larger than 0.4 hectares (1 acre). What goes in should be gone within 48 hours, six to twelve better.

Clogging is what ends one. Sediment, biological growth, trapped air, mineral deposits and swelling clay can all seal the receiving soil, and because that surface is buried, none of it can be raked clean. Two mistakes are made before it is used: assuming a drainage rate the soil cannot deliver, and compacting the infiltration surface with machinery during the build. Water standing past 48 hours means clogging has started. Screened roof water is the one inflow needing no further treatment; anything else wants a settling chamber ahead of it, and runoff carrying pesticides, industrial residue or road salt should not go underground at all. Storms bigger than the design event need a surface overflow, since soaking in is the only outlet. Inspect after every heavy storm for the first few months, then twice a year. Where the inflow is not clean roof water, build it so it can be pumped as well as filled, which is the only way to clean the fill once it fouls.