A percolation test is a simple test of how fast soil can absorb water. It is performed before installing a dry well to ensure that the dry well can fulfill its purpose of transferring extra surface water deeper into the subsoil.
A dry well is an underground structure used to improve surface drainage. It can be as simple as a hole that is filled with gravel or other materials through which water can drain or can be constructed from concrete. It can be a good choice for clearing up a perennially soggy spot in a lawn or as part of a system providing drainage for a ditch or driveway but requires soil that can absorb water well to work.
To perform a percolation test, use a post hole digger or soil auger to dig a 4-foot-deep hole. If groundwater begins seeping into the hole, the water table is too close to the surface for a dry well to be useful. If no water appears, pour at least five gallons of water into the hole and notice how fast the water drops. If it drops at three minutes per inch or faster, the soil is probably suitable for a dry well. If drainage is extremely fast, fill the hole a second time and recheck the rate of drainage to be sure that the rapid drainage was not due to dry, thirsty soil.