Controlling the effluent from a septic tank or system of septic tanks is obviously a public health concern. The outflow cannot be allowed to return to the surface or contaminate aquifers unless it has been thoroughly cleansed by seeping through the ground. The problem was solved early in the 20th Century by the Percolation Test, now used by every county government in the nation. A large hole, or holes, is dug, then filled with water. Building inspectors then measure how long it takes the water to seep or percolate into the ground.
Determination is made on whether the soil is suitable for a septic system.
Nor is the test an absolute guarantee. In heavy clay soils, found especially in the Western United States, the Percolation Test may give a false reading. In the dry season clay cracks, permitting effluent to disappear into the ground. But in wet seasons, the clay absorbs moisture and binds together. What percolated in June stays on the surface in January, creating a health menace and a most expensive problem for developers.