Traditional septic systems only work if the soil in the leach field area is sufficiently permeable that it can readily absorb the liquid effluent flowing into it. Otherwise, untreated effluent may back up and pool on the ground surface.
Also, there must be at least a few feet of good soil from the bottom of the perforated leach pipes to the rock or impervious hardpan below, or to the water table. Much of the treatment takes place in this soil layer.
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 and grab a handful. If the soil has a sticky, damp texture, and you can form a small lump of damp subsoil into a long, thin ribbon or worm shape that holds together, then the soil has significant clay content.
If you can form a ribbon of soil 2 in. or longer in the ribbon test, it indicates that the soil has high clay content and may fail a standard perc test.
The two main tests used to determine a site’s suitability for a septic system are a perc test and visual observation of the soil in a test pit, sometimes referred to as a deep hole 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 a seasoned expert with local experience as many of these tests have a bit of wiggle room.
Most evaluations 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. Testers are looking for the high water table, and the presence of rock ledge or impermeable soil that will block absorption of water. In some areas, testers also evaluate the soil for drainage characteristics. Soil samples may be taken back to the lab, or visual observations of the soil layers may be sufficient. In many jurisdictions, the absorption rate of the soil is measured in the field with a perc test.
Visual observations are used to identify the “limiting zone,” where the soil is unsuitable for treating sewage. This is determined by upper layer of the water table, or impermeable soil or rock.
The seasonal high water table is identified visually by looking for “mottling,” splotches or streaks of color in the soil indicating the occasional presence of water. Examiners also look for layers of rock or impermeable soil and may send soil samples to a lab.
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, but the exact distance varies with local regulations.
While most soil experts believe that soil observation can provide enough information 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.
SEASONAL HIGH WATER TABLE
Some municipalities may also require direct testing of the seasonal high water table under certain conditions. For example, this may be required for some types of alternative systems or in areas with a known high water table. The usual approach is to drill small monitoring wells, also called piezometers.
The monitoring wells are essentially plastic pipes stuck into holes drilled in the ground. During the wettest part of the year, the depth of water in the pipes is measured. For a system to be approved, there must be adequate clearance from the bottom of the drain field trenches to groundwater.
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 when soils tend to be wetter – 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 percolation 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 maximum number varies from 30 to 120 MPI depending on local regulations. Some towns require additional “hydraulic” soil tests for sites that test above 30 minutes per inch.
The cutoff for too-fast percolation is typically 1 to 3 minutes per inch. Below that, water absorbs too fast to be effectively treated before reaching the groundwater. The worst-performing hole is the one that counts. Some towns require that a town official be present to witness the test.
Perc test regulations vary a great deal from one town to the next. General ranges for soil permeability are as follows:
- Below 5 MPI: Highly permeable soil. Alternative systems may be allowed, including pre-treatment of effluent, pressurized dosing, or addition of denser soil around trenches.
- 5 – 60 MPI: Conventional leach field permitted.
- 60 – 120 MPI: Low-permeable soil. Alternative systems may be allowed, including pre-treatment of effluent, pressurized dosing, enhanced treatment, mound systems, and other alternative systems.
- Above 120 MPI: Very low-permeable soil. Alternative systems with enhanced wastewater treatment may be allowed under some conditions.
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.
Minimum distances are required from the septic tank and leach field to buildings, property lines, water pipes, wells, and open water. On small sites, a variance might be required to allow sufficient space. You may be required to find suitable space for both the active leach field and a replacement field, for use in 20 or 30 years when the original field is exhausted. Clearances vary from one town to another. Typical minimum distances are:
|SEPTIC SYSTEM MIN. CLEARANCES (typ.)|
|Distance to||Septic Tank||Leach Field|
|House||10 ft.||10-20 ft.|
|Property line||10 ft.||10 ft.|
|Private well||50 ft.||50-100 ft.|
|Potable water piping||10-25 ft.||25 ft.|
|Open water (stream, pond, wetland, etc.)||50-100 ft.||100 ft.|
|Dry gulch/stream bed||10 ft.||25 ft.|
|Subsoil drains||10 ft.||25 ft.|
|Note: Always check with local codes|
Even if your site fails a perc or deep-hole test, all is not lost. On larger sites with more than one potential site for a drain field, you can test a second area. For sites with high water tables, you may be able to “de-water” the drain-field area by strategically placing gravel-filled trenches and subsurface drain pipes 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. Some require annual maintenance contracts to ensure that they continue to work properly.
As alternative systems become more common and more widely accepted, formerly unbuildable lots may become approved building lots. As with any new building technology, however, look for products and systems with a proven track record in the field. Also find a contractor who has first-hand experience installing the particular system you are considering. You don’t want to be the first one on the block to try out an unproven design. A failed septic system is not a pretty sight — or smell!
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