Gravity Septic Systems in Extremely Gravelly Soils
In the past, good drainage in a septic system was a big plus because the drainfield could be reduced in size and construction costs were saved. Lately however, excessive drainage found in extremely gravelly soils is not so good. Shallow drinking water aquifers are more at risk from contamination by septic systems when water containing sewage effluent drains too quickly away from the drainfield. Bacterial action in the soil below and surrounding the drainfield is where treatment takes place in septic systems. This biological action takes several hours, usually at least a day or two to happen in the top 36 inches of soil due to oxygen availability, but not if drainage is excessive.
The solution to this problem is to provide a sand lining below the drainfield. A layer of treatment medium, a special washed and graded sand is placed below the drainfield during construction to support slower drainage and to encourage populations of beneficial bacteria. This module shows how to specify and detail the construction of a lined gravity type drainfield for a three-bedroom house in extremely gravelly soil. Included are construction notes and specifications for all materials used in this type of drainfield construction.