Traditional septic systems only work if the soil in the leach area is sufficiently permeable that it can readily absorb the liquid effluent flowing into it. Also, there must be at least a few feet of good soil from the bottom of the leach pipes to the rock or impervious hardpan below, or to the water table. Less commonly, a site can fail because the soil is too permeable, allowing the effluent to reach the groundwater before it is fully treated. Very steep slopes are also unsuitable for a conventional leach field.
The specific standards vary from town to town, but any of these characteristics can prohibit the use of a standard gravity-fed septic system. In some cases, a more expensive alternative septic system may be allowed. To determine is a building site is suitable for a septic system, a percolation test (typically called a “perc test’ or “perk test”) is required.
On rural sites without municipal sewage systems, a failed perc test means that no house can be built — which is why you should make any offer to purchase land contingent on the site passing the soil and perc tests. As prime building sites become increasingly scarce (or prohibitively expensive) in many parts of the country, rural sites that will not pass a percolation or perc test are increasingly common.
In general, soils with high sand and gravel content drain the best and soils with a high clay content or solid rock are the worst. Most soils fall somewhere in the middle with a mix of course sand and gravel particles, small silt particles, and miniscule clay particles – the smallest. To get a rough idea before investing time and money in testing, dig below the top few inches of topsoil (loam) to the lighter soil beneath. If you can take a handful of the damp subsoil and roll it into a thin, flat shape or worm shape that holds together, and it had a sticky firm texture, the soil has a high clay content and will probably fail a standard perc test.
The two main tests used to determine a site’s suitability are a soil evaluation and percolation or perc test. Testing requirements vary greatly from state to state and often from town to town, as most states allow individual towns to establish separate rules within state guidelines. So make sure you talk to your town health officer about what tests are needed, when they can be done, and who should perform them. Whether or not a licensed professional is required, it’s a good idea to hire an seasoned expert with local experience as many of these tests have a bit of wiggle room.
Most tests start with a deep hole test dug by machine to well below the bottom of the proposed leach field – often 7 to 10 feet deep or greater. Soil samples may be taken back to the lab, or visual observations of the soil layers may be sufficient. Soil tests or observations are used to identify the drainage characteristics of the soil, the seasonal high water table, and the depth of the ‘limiting zone,” where the soil is unsuitable for treating sewage due to. The high water table is identified visually by looking for “mottling,” splotches or streaks of color in the soil indicating the occasional presence of water.
The limiting zone is defined by upper layer of the water table or impermeable rock or soil. If the limiting zone is too close to the surface to allow for a conventional leach field, then a mound or other alternative septic system may be required. Typically, the water table or impermeable soil must be at least 3 feet below the bottom of the trenches in the leach field.
While most soil experts believe they have enough information at this point to design an effective septic system, most states today also require perc testing to directly measure the rate at which water percolates through the soil. The test measures how fast water drains into a standard-sized hole in the ground. The results determine whether the town will allow a septic system to be installed, and system designers use the results to size the leach field.
To conduct a perc test, first talk to the local health department official as requirements can vary significantly from town to town as far as who can conduct the test, the minimum number of holes, depth of holes, required absorption rates, and when the tests can be performed. In general, tests cannot be conducted in frozen or disturbed soil, and some areas only allow tests during certain months of the year – so plan ahead.
Test procedure. A typical perc test consists of two or more holes dug about 30 to 40 feet apart in the proposed drain field area (see illustration). The holes are typically 6 to 12 inches in diameter and 2 to 3 feet deep, the typical depth of the absorption trenches in a leach field. Next, you or the technician should place 2 inches of clean gravel in the bottom of the hole.
Scrape the sides of the hole with a sharp tool (nails driven through the end of a 1×2 work well) to loosen soil compacted during digging, and remove any loose soil from the bottom of the hole. Since the perc test is meant to simulate the actual conditions in a working septic system, the soil is then “pre-soaked” for several hours to saturate the soil. Keep adding water to maintain about 12 inches of water in the hole for at least 4 hours. Clay soils should soak for at least 12 hours to fully saturate.
The next day, clean out any loose soil that has fallen to the bottom, and carefully fill each hole with water to a depth of 6 inches above the gravel. Then measure how much the water drops in 30 minutes (or less for highly permeable soil that drains quickly). The times are then carefully documented and used to calculate the percolation rate – the time it takes for the water to fall one inch.
The rate is usually expressed in minutes per inch of drop. A rate of 60 minutes per inch (MPI), meaning the water dropped one inch in 60 minutes, is often the cutoff point for a standard gravity-flow septic system, although the number varies from 30 to 120 minutes depending on local regulations. The cutoff for too-fast percolation is typically 3 to 6 minutes per inch. The worst-performing hole is the one that counts. Some towns require that a town official be present to witness the test.
Test results are usually good for two to five years, and in some cases can be renewed. However, with all things perc, rules vary greatly from town to town so don’t make any assumptions. Always check with the town health department before proceeding.
OTHER SITE CONDITIONS
Septic system regulations vary widely, but most municipalities require that the leach field meet specific requirements above and beyond the perc test. Some common limiting factors are:
- Steep slope. The maximum allowable slope for a conventional system typically ranges from 20% to 30%.
- Filled land. Native soils are typically required, although engineered fill may be acceptable in some cases.
- Wetlands or flood zones. Not acceptable for leach field.
- Site drainage. The leach field should not be in the path of runoff during rain storms, which could cause erosion or flooding of the system.
- Setbacks and clearances. Minimum distances are required from the buildings, property lines, wells, and open water. On small sites, a variance might be required to allow sufficient space.
Even if your site fails a perc or deep-hole test, all is not lost. For sites with high water tables, you may be able to “de-water” the leaching area by strategically placing gravel-filled trenches and subsurface drain pipe to conduct water away from the drain field. You’ll need a highly experienced earthwork contractor, and possibly the help of a civil engineer or geotechnical engineer, to make this work.
Also, a wide range of alternative septic systems have been developed in recent years for use on almost any type of site. Find out which systems are approved for use in your area and which might be suitable for your site. In general, these systems cost more and many require pumps, alarms, and other components that require more monitoring and maintenance than a standard septic system. As these become more common and more widely accepted, formerly unbuildable lots may all of a sudden become approved building lots. As with all new building technology, however, look at products and systems with a proven track record in the field.