The suitability of soil for the subsurface disposal of sewage is determined by its physical characteristics and percolation rates. Physical characteristics include percentage of rock, evidence of groundwater, and depth to rock and/or groundwater. A percolation test measures the rate that clear water seeps into the soil from a standard test hole.
There are three types of conventional percolation tests: Tile field, Deep trench, and Sandmound. The tests are conducted at the depth at which the septic disposal system will be installed, with an additional test conducted four (4) feet below to determine the soil suitability to filter the sewage effluent for tile field and deep trench systems. An additional piece of equipment, an infiltrometer, is used to conduct sand mound tests. Percolation rates for tile field and deep trench systems cannot exceed 30 minutes, and sandmound tests cannot exceed 60 minutes. A percolation rate of less than 2 minutes is not acceptable. To file for a percolation test, an application must be made and fees paid at the Environmental Health Bureau, Carroll County Health Department. After that has been completed, the applicant must arrange for an excavator to dig the test holes. Any expenses associated with this step are the applicant's responsibility. Usually, the excavator will contact the Health Department's Area Sanitarian to schedule a test date. Arrangements for an appointment should be made 3 weeks in advance of the testing. Percolation tests are then conducted and results are mailed to the applicant and filed at the Health Department until a well and/or application is received.
The purpose of a percolation test is to determine the suitability of soils for the installation of a septic system and treatment of sewage effluent. State regulations require that there be at least 4 feet of satisfactory material below the bottom of an on-site sewage disposal system. For this reason, a percolation test is excavated to a depth 4 feet below the depth of the proposed system installation. Physical appearance of the soil is then evaluated. If there is evidence of water or the soils contain excessive rock, the test is considered disapproved for the proposed system. In the case of tile field or deep trench systems, if the soil is not excluded based on appearance, actual tests are conducted at the depth of the proposed system and 4 feet below the depth. These tests are conducted in the following manner:
- 12 inch by 12 inch by 12 inch test holes are dug at the required depths.
- A ruler is placed in an upright position within the test hole.
- Water is poured into the hole to a depth of 7 inches.
- The water is allowed to drop an initial inch to saturate the soil.
- The rate of the second inch is timed to determine the percolation rate.
In some cases, the rate of the third inch is used if it does not appear that the rate has stabilized.
A passing rate for tile field and deep trench tests can be from 2 to 30 minutes.
In the case of sandmound testing, an additional piece of equipment is utilized. An infiltrometer is a simple metal cylinder 12 inches in diameter and two to three feet in length. The test hole is excavated to a depth of 4 feet and any evidence of water or rock in noted. If excessive rock or evidence of water at a depth of 2 feet or shallower, the soils are unsuitable for a sand mound system. Otherwise a test may be conducted. The most restrictive layer of soils within the top 2 feet are identified and a shelf is excavated at this depth. The infiltrometer is placed on this shelf and forced downward into the soil and the soils are tamped around the outside of the infiltrometer to assure an adequate seal. A rate is determined in basically the same manner as outlined above. Acceptable rates are from 5 to 60 minutes.
Back to Top