WHAT IS A PERC TEST ANYWAY? By Dr. Land

31 Jan

“people like me and the Kennedy’s were born in Massachusetts, so the sanitation code requires that their more volatile tinkle flows through 30 inches more  of soil grade than Connecticut people’s tinkle”.

A “perc test” refers to soils testing to determine conditions and suitability of land for a sewage disposal system (septic system). First to address the spelling many call it a perk test. Dr. Land spells it “perc” because we are talking percolation like as in coffee percolating, not perk as in perks being extra goodies that go along with one’s job. I can’t think of any “extra goodies” I’d be looking for that have anything to do with a septic system. When people say “perc tests”, they are actually referring to two things or functions performed for the testing; deep hole tests and percolation testing. The deep holes (2) are typically dug with the hydraulic bucket of a backhoe or track excavator. The depth can very but they are generally 7 to 10 feet. The board of health sanitarian and/or sometimes civil engineer takes a reading of the soils types. Sometimes the deep holes are dug at an angle so those doing the soils reading can walk right down into the hole. They are reading the soils types relative to their ability to accept/absorb effluent which is the liquid produced from a septic system. I jokingly refer to it as “tinkle” because some of us when we were little kids had to go tinkle. Second is the perc test. This is a little hole dug by hand with a post hole digger (about 8 inches diameter) a few feet down. Water is poured into the hole from a 5 gallon bucket to see at what rate the soils will at that depth  accept liquid or how long it takes the level of water to go down. It is measured by “minutes per inch”. Normally the hole is “pre soaked” first meaning the hole is filled up the first time without the reading being taken (it can be pre soaked before the test appointment) to moisten the soil and if there is clay, the clay can swell and absorb the water. Then the second time water is poured the reading is taken. For example in CT the max or slowest it can be is “60 minutes per inch” meaning the water level must go down at least one inch in an hour. The perc rate will  determine how many square feet of leaching area or “leach fields” the septic system will need. If it is an optimum perc rate and the party is going to build a 2 or 3 bedroom house it may be the sole determining factor in leach field square footage. But more often than not it will be the combination of the perc rate and how many bedrooms that will determine the square foot of leach fields. The number of bedrooms can also be the sole determining factor on the size of the septic tank required. Tank and/or system size is not based on the number  of bathrooms. This is because bathrooms don’t produce waste, people do. So the system requirements are a combination of the soils suitability and the number of anticipated occupancy the system will handle based on the bedrooms. The perc test hole results can vary by location and even sometimes by season or conditions meaning someone extra crafty may actually be able to manipulate the results. The deep hole results are pretty much what’s there is there. Unless something significant occurs that would change the dynamics of the drainage of the land such as a 200 unit condo complex being built next door with parking lots. Otherwise  you could dig the hole in the same spot 50 years later and it would be the same result.

But before I get back to what the deep holes are about, let’s discuss the basics of a septic system. All the waste from the house flows into the septic “tank” which is a non porous concrete tank. These tanks are made by the same concrete companies that make form front steps and basement bulkheads. The tank  size required can vary on the soils, the number of bedrooms and the state code, but it could be say 1,000 gallons or 1,500 gallons. So all the “stuff” is stored in the tank and it sits there, bubbles and brews like a good beer or wine (the tank should be pumped periodically like every 3 years for good maintenance). The tank constantly receives more waste water or effluent so when it gets to a certain level in the tank there is a pipe it goes out (outlet pipe) to be dissipated into the leach fields. To get real fancy, the effluent goes down a pipe to a distribution box that will distributes it to multiple pipes because there are multiple leach fields (number can vary such as 3 more or less). The purpose of the leach fields is to take all that effluent and spread out over a large area of soil so it can interact with the natural enzymes in the soil and be neutralized so it is no longer evil waste. Leach field trenches can be made of different designs and materials, but the most common trenches are crushed stone. Picture a trench being something like the size of a ordinary cardboard box people hoard stuff in except it is long such as 40 feet. The trench consists of crushed stone the entire distance with a plastic pipe with holes on the sides also running the entire distance in the middle of the trench carrying the effluent and dissipating or spreading it out over the entire trench area. In the old days there were “cesspools” rather than septic systems. Cesspools can still work. A cesspool is a lot simpler. It is basically a large tank that is made of stones or brick. So the tank unit itself is actually porous and the effluent leaches out of the brick or stone tank into the soils around it. Not as calculated of an operation and tree roots like what is inside the tank as yummy and can therefore grow right through the cracks into it clogging it up. Also in the old days were “dry wells”. A dry well can be  simply digging a big cavity in the ground, fill it with crushed stone and have a pipe run the effluent or grey water into that in order to dissipate. Many an old lake cottage had small septic tanks in which they didn’t want them filling up with the laundry grey water so they had the laundry water go into a separate dry well.

But back to the deep holes which are important. If the soils are nice gravel, effluent will percolate right down many feet no problem. But most soils are not nice gravel. Gravel and sand is created by water and is often found in the glacial terraces along rivers and streams as it is the grinding action and sedimentation dynamic of the receding glacier and  glacial river which has created the gravel. It is more common in uplands to find other soils conditions that include compact glacial till (hardpan) or even bedrock (ledge). Hardpan is often 2 to 3 feet down in the soil. An example is you could dig and find perhaps the first 4 or 6 inches is decaying vegetation followed by regular brown dirt most of us are used to seeing when using a shovel. But say a few feet farther down one hits hardpan (compact glacial till). This soil is very compact and in the worst water saturation conditions will not accept or absorb water as fast as the water can come in. The most sever period is normally the spring thaw. What happens to the water as it percolates through the first couple feet of soil but when it reaches the hardpan is the water will sit on top of this soil layer and start to travel or sheath along this layer through the ground sideways until it pops out at streams or wetlands. People in the industry use three different words/definition or phases that all are referring to pretty much the same thing:

1. Mottling

2. The restrictive layer

3. Perched water table

While the hardpan is perfectly excavatable and good from in the context of a foundation load bearing capacity, the water traveling through the ground in the above mentioned manner is not considered good as far as effluent percolating and interacting with the enzymes in the soils to neutralize the effluent (it is moving with the gound water). Therefore for example in CT the sanitation code dictates that the bottom of a leach field trench must be at least 18 inches above mottling or this restrictive layer. People like me and the Kennedy’s were born in Massachusetts so the code requires that their more volatile tinkle flows through 30 inches more feet of soil grade (48 inches all together) than CT people’s tinkle. When the mottling is shallower, keeping the trenches above mottling requires gravel fill to raise the leach fields which  increases the cost of the system. That’s why back in the days when they used to build houses you could drive around in MA and sometimes see a big hump area of lawn in the front yard where a lot of fill was needed (perhaps with a big white plastic candy cane pump out pipe for style points) that you really would never see  in CT because there is 30 inches more of fill required for the same soils conditions in MA. So the cost of the septic system will be impacted by how many square feet of leach field one will need determined by the percolation rate, the amount of fill needed determined by the depth to mottling (or it could be depth to ledge which is impervious, 48 inches from the trench bottom needed in CT) and the number of occupants the system will serve based on the bedrooms. Two bedrooms is the minimum size allowed even if you build a one bedroom house. Sand or gravel can be good news for a cheaper septic system and other overall excavation site costs, but keep in mind the better the drainage conditions for land, the worse conditions become for other elements such as if you want a  nice lawn, a nice  garden, timber productivity, crop yield, etc. For most indigenous plants dryer or quickly draining soils means increased drought likely hood, higher seedling mortality rates and lower yielding growth rates.

Dr. Land (Dennis Duga)

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