Life and Death of Building Lots Subject to Perc

Soil experts think many percolation tests prevent contractors from building on suitable lots. Perhaps it’s time to look closer at our soils.

by Paul Fisette © 1990

The ratio of 1 in 30 is a magic number for contractors who build homes in Massachusetts – (Note: perc rate was ammended to 1″/60 minutes in 2004 for Massachusetts). In New Hampshire the number is 1 in 60. Vermont’s number is 1 in 60 too – except when it is 1 in 120. Maine doesn’t have a magic number. Confused? Welcome to perc-rate lotto. Percolation rates, the time it takes 1″ of water to drain from a hole dug near a proposed septic system, must be determined before building permits are issued in most rural communities of the United States. If your rate does not match or beat the minimum legal rate, then you can’t build on the homesite.

Fortunately every state regulates on-site sewage disposal. Hundreds of thousands of lives were claimed by diseases transmitted through contaminated drinking water before the “germ theory” was developed in the late 1800’s. Techniques to keep human waste from entering our drinking water were developed and water-borne diseases caused by misdirected sewage have been greatly reduced. Today’s improved mortality rates and longer life-spans are in many ways attributable to environmental control of sewage.

Regulation of on-site sewage disposal is necessary. About 20 percent of the new homes built in the US use on-site sewage systems, but the percentage of new homes using on-site disposal varies widely according to geographical region. Stephen Dix, director of the EPA National Small Flows Clearinghouse in Morgantown, WV points out, “Some figures indicate that ME, NH, VT, KY, WV and TN are around 40 percent. NC is at 52%. Other states are much lower.”

Regulation is needed, but inconsistent regulation of sewage disposal is confusing. Each state follows a different set of rules. To complicate matters, states regulate septic-design with a minimum acceptable standard. Local boards of health fine-tune state regulation to suit local interests and conditions. The result: local regulations are usually more stringent than state regulations. Experts agree: creating a universal regulation will not work. Local environments must be considered before any ordinance is developed. Dix points out, “We have 23 million on-site systems out there, so we better be careful with the way we handle that waste.” Each community must ask itself: What are we using the aquifers for? And what is the environmental impact of building and disposing sewage near them?

On-Site Disposal

How it works
Most on-site septic designs have three main components: generator(house), partial treatment facility(septic tank) and final treatment facility(leaching field). Approximately 50 gallons of wastewater are generated by each household inhabitant every day. Solid and liquid waste drain from the house into a watertight (usually pre-cast concrete) septic tank. Solids enter and settle to the bottom of the tank while grease and fats rise and are trapped between baffles arranged across the top of the tank. Solids entering the tank displace liquid waste. The displaced liquid runs to the leaching field through an outlet located at the top of the opposite end of the tank. Solids left behind begin to decompose right in the tank.

Liquid sewage carries suspended solids to the disposal field where the goulash percolates through the bed of stones and soil that surround the leaching drains. Waste water is cleansed as it filters through the soil system. An organic layer, or slime mat, forms at the elevation where stones and soil meet. The mat is a virtual feast for millions of bacteria that break-down the sewage. The mat grows as the system receives more use. Infiltration of liquid into the soil is slowed as the mat thickens. And as a result the soil beneath the mat remains unsaturated. Pathogens are scrubbed from the percolating wastewater before they can reach groundwater. All this works perfectly – IF – the leaching field is located, designed and constructed in acceptable soil.

Soils
There are two concerns when it comes to septic discharge: you don’t want your disposal system to back up into your basement and you don’t want your system to dump sewage into the groundwater. If the soil in the disposal area is impermeable you will create a pond of sewage in your yard that will back-up. Conversely, if the soil is too permeable, waste water may rush through the soil too quickly carrying phosphates and nitrates directly into the ground water. This is where a soil scientist comes in handy.

A site assessment specialist or soil scientist can take several soil borings from various locations on a proposed building lot and analyze the soil profile. It will cost a couple of hundred dollars, but it is money well spent. A preliminary soil analysis will provide you with a sense of the lot’s potential before you sink serious money into the test holes required by the local sewage regulators.

Soil scientists can determine the soil’s drainage capability and the average high groundwater mark by inspecting the soil exposed in sample borings. Analysts look at four distinct layers (or horizons) of soil: organic mat, topsoil, subsoil and substratum. Chemical reactions color the soil (mottling) as the water table rises and falls. So the highest point at which soil mottling is observed marks the average seasonal high-water level. Mottles are irregular orange, yellow and gray spots found in the soil profile. For any given year the high-water level may be higher or lower than the average estimated by the height of mottling. But all soil experts agree that soil mottling is a reliable indicator of average high water.

Soils are classified according to the frequency and duration of their saturation. The range extends from excessively-drained soils, where mottles rarely appear within the upper 5 feet of soil, to very poorly-drained soils, where water table may be located at or just below the surface.

The most important soil characteristic used to predict hydraulic behavior is texture. Texture gives designers a way to determine how well the soil can drain. The larger the particle the better the drainage. Smaller particles pack tightly together leaving no room for water to pass. Usually, soils are a made from a combination of sand, loam, silt, and clay. Naturally, water does not drain well through clay because the particles are small and pack solidly together.

Groundwater
One of the most serious considerations affecting septic design is groundwater level. Rain water percolates through the soil filling all the spaces between the particles of soil. The water table rises as the spaces fill and the soil becomes saturated. Groundwater levels rise and fall throughout the year. Typically the water table will be lower during dry seasons and higher during wet seasons. But a very wet period during any season may elevate the water table to unusual heights.

Groundwater may collect in the bottom of an excavation, but this may or may not be the water table. Soil located above the water table draws water up from the water table through capillary action. The soil’s capillary fringe extends from a couple of inches to a couple of feet above the water table, depending on existing soil conditions. If you find water in the bottom of a test hole – just wait a day. The water table will seek its own level and possibly leave the hole.

Regulation and Site Evaluation
All states require you to dig a deep observation hole in the immediate area of any proposed on-site septic system. Most states, but not all, require you to establish soil drainage rates by running a perc test. All states agree in principal that the soil located in the area of a proposed septic system must be evaluated and determined capable of cleansing effluent before a building permit will be issued. But major differences exist between various state regulations.

States do not agree on: the depth and number of observation holes required for each site, the depth of permeable soil located beneath the bottom of the leaching field, the distance between the top of the water table and the bottom of the leaching field, the size of the percolation hole and the rate water must percolate from the hole. Most regulations hold perc-test results as the determining criterion for septic design. Some do not even require a perc test as a screening procedure.

Deep-hole test
On each lot an observation hole is dug so a site evaluator or engineer can look for bedrock, permeable and impermeable material, and groundwater – or signs of the average seasonal high-water. Soils are analyzed and soil horizons inspected. The information is used to locate, size and design the septic system.

In Massachusetts at least two deep-holes are dug and examined on each lot. They must extend 4 feet below the bottom of the leaching area and be at least 10 feet deep, unless bedrock prevents further excavation. You better have 4 feet of naturally occurring permeable soil beneath the bottom of the planned leaching field or you can forget about building a disposal system. And the water table must be at least 4 feet below the system too. But don’t be mislead. You can still construct an acceptable septic system if your permeable soil is 5-feet deep and the water table is 3-feet below the surface. in this case, build a mound system one-foot above grade on clean fill.

New Hampshire’s Water Supply Pollution Control (NHWSPC) regulations require the bottom of leaching fields to rest on permeable soil that is 8-feet deep. Seasonal high-water must be at least 4-feet below the system. The ruling seems rather strict, but surprisingly anyone can conduct their own soil test and design their own septic system in New Hampshire. Most laymen have no real knowledge about soils, especially when it comes to reading test pits to determine the seasonal high-water mark. So as a result many homeowner-designed systems are not approved by local inspectors. And you can’t get a building permit in New Hampshire until you get your septic design approved.

Vermont governs on-site sewage through subdivision regulation. Lots larger than 10 acres are exempt from state regulation, but many towns control the construction of wells and septic systems with local ordinances. Lots smaller than 10 acres are considered single-lot subdivisions. Soils in single-lot subdivisions must be tested and approved before title of ownership is conveyed to a new owner. You can’t break up a 60 acre tract of land into 10 lots without having each septic area approved first. An approved septic permit runs with the land’s title.

Four deep-holes must be evaluated on each single-lot subdivision: two in the proposed leaching area and two more in a reserve area. A reserve area must be available in case the primary system fails. In Vermont, it is assumed that all septic systems will fail, so you have to prove that you have adequate area and soil conditions suitable for two systems. In fact many states have similar provisions for reserve-area testing. Vermont’s observation holes must be 7-feet deep and 4-feet below the bottom of the proposed leaching field. Seasonal high-water must be three feet below the disposal system.

Maine, like most states, requires a licensed site evaluator to inspect soils exposed in the deep-hole test. However, Maine’s law seem relaxed when compared to laws of other states. Here, soil profiles are only logged to a depth of 4-feet, not the 8- or 10-feet commonly required by other states. A soil expert describes each soil horizon in the test report, indicating: color, texture, structure and any restrictive layers of soil that are found. But in Miane they pay particularly close attention to the level of soil mottling, the level of seasonal high-water, because in Maine you can locate your disposal field just 12-inches above the water-table.

Since only 12-inches of permeable soil is required beneath the bottom of the leaching field, most sites need only one or two observation holes to “prove-out” an area. State law does not require local board-of-health departments to inspect the holes – even though the site evaluators are employed directly by the lot owner. But some boards do inspect sites. The Plumbing Code regulates site testing and system design in Maine.

Perc testing
Perhaps the most hotly debated issue related to site evaluation is perc testing. Soil permeability or hydraulic conductivity is one of the most difficult criterion to estimate because there is such a wide margin of variability in results given by perc-tests. Most scientists feel that perc-rates should be used as a design tool not as a go no-go gauge for site approval.

Basically, a perc-test is conducted in the following manner: A small hole, approximately 12″ in diameter (18″ deep in some states, 3′ deep or deeper in others), is dug in the area of the proposed septic field. The perc-hole is kept full of water for a period of time. And then at some point after the hole is “soaked”, the rate at which the water drains from the hole is measured. It is measured as a ratio of 1″ in X minutes. If your test-hole drains slower than the acceptable rate, you’re out of luck. Find another lot to build on. But does this restriction make sense?

A hypothetical landowner who owns a tract of land at the intersection of Massachusetts, New Hampshire and Vermont could get the site approved if the water in a perc-hole drained faster than: 1″/30 min in MA (amended to 1″/60 min in 2004); 1″/60 min in NH.; or 1″/120 min in VT. A similar landowner on the Massachusetts-Connecticut-New York intersection could pass with rates of 1″/30 min (amended to 1″/60 min in 2004), 1″/60 min and 1″/100 min in each respective state. Why would a disposal area work in one corner of these lots but not in another?

Local regulators who require perc-testing agree on one thing: the bottom of the perc-hole should be dug into the least permeable soil found within the region of permeable soil that lies beneath the bottom of the proposed leaching field. But they do not agree on: where or when the holes should be tested, size and depth of the holes, how many tests should be conducted on each lot, how fast water drains from the hole and who should administer and witness the tests. But if you are required to conduct a perc-test, keep in mind — the smaller the diameter of the perc-hole the better chance you have to pass the test. For example: 2,036 cubic inches of water will be exposed to 800 square inches of soil surface in a hole that is 12″ wide by 18″ deep, but only 900 cubic inches of water will be exposed to 500 square inches of soil surface in a hole that is 8″ wide by 18″ deep. So if the regulation offers you an acceptable range of hole sizes, pick the smallest one. The volume to surface area factor is in your favor as the hole diameter gets smaller.

Most engineers feel they can design a perfectly safe disposal area for soils that test-out within a broad range of percolation rates. Scientific literature and the variability of perc-results supports the abolition of perc-testing as the determining criterion for site approval.

Some communities require perc-tests to be conducted in the spring. When you soak a hole, you saturate the ground. Saturated soil is saturated soil, so what sense does a seasonal perc-test make?

Groundwater levels fluctuate. Some years its highest point occurs in March, other years it occurs in October. March of 1989 was one of the driest on record. Many percs done in March passed. If they were taken in the wet month of May, many would have failed miserably because the water table would have been up to the bottom of the test holes in some locations. Perc-tests are conducted to determine the soil’s ability to drain water. You can not determine the soil’s ability to drain water if it is under water. Remember, the height of the water table is determined by the deep hole.

Septic engineers who work straight from perc-rate tables may overlook the soil’s ability to cleanse sewage. A disposal system built on sandy Cape Cod might need only 4 square feet of leaching area according to the perc-test results because the drainage is so good. That might be true if there was only 1 house on every 15 acres of land. But, put a house on every 1/4 acre and you have created a drastically different situation. The sewage in the second case may seriously contaminate the groundwater because the effluent was not exposed to enough surface area of soil to cleanse it.

Observation wells
Have you driven along a country road, seen a white plastic pipe sticking up out of the ground in the middle of a lot and wondered what the tube was used for? It’s called an observation well and it is used to monitor the groundwater level. Before you put money down on the lot ask a few questions.

Many towns require deep-hole tests to be run during the spring season, but allow perc-tests to be conducted at any time during the year. A deep-hole dug in the spring may be full of water, but an above-grade system could still be approved if the soil was permeable. High groundwater would prevent percolation, so some engineers install observation tubes into “wet holes”, fill in the hole around the tube and wait for the water table to drop before conducting a perc-test.

Observation wells are also used to monitor water-table levels while designers try to de-water a site. High groundwater can be lowered on a problem site by using various ditch-and-drain techniques. But, we are talking about a marginal lot here!

Some contractors like to test their lots in the fall so they are ready to build in the spring. They dig their deep hole in the fall, log the soil profile, run a perc-test, install a couple of observation wells and close up the excavations. In the spring the board of health comes back and checks the groundwater level, the only item left to approve.

Trends
Perhaps the future success of on-site sewage disposal rests not on whether we require perc-testing, or whether we have 8-feet of permeable soil beneath our system, but rather on the professional maintenance of these systems. As EPA’s Stephen Dix puts it, “Right now our management strategy stops as soon as we turn the disposal system over to the homeowner. That’s just like running your car without a dipstick and waiting for the engine to die.” There is little doubt that professionally operated systems will be with us in the future. The challenge is left to our communities to educate and regulate proper maintenance. Who knows, maybe we will need operating permits and have maintenance agreements with our local municipalities.

Last updated: November 28, 2007