What is a septic tank and leach field? People from non rural areas are often surprized to hear that in the country homes are equiped with Septic Tanks to handle household waste.
The septic tank was invented in France about 1750 and hasn’t changed a great deal. Solid waste goes into the tank and after biological action inside, clear water (called “leachate”) comes out of the tank and is distributed into the ground through the use of a leach field.
For a vacant parcel of land the test to determine what type of system is needed is called the “perc and mantle” test. This test is done to determine the ability of the soil to handle the absorption of the leachate coming from the tank. This test determines the size and placement of the house on a parcel. The cost for a whole perc & mantle process, including county inspections, soil engineer is about $2000. The cost for the back hoe operator is determined on how many holes are needed to acheive a postive result and how long it takes to do that.
A leach field is sized according to the bedrooms in a proposed house, not the bathrooms. It is the number of bedrooms that determine how many people will theoretically live in the house. The results of the testing will define how many feet of leach line must be installed per bedroom.
The first thing you will want is to have a civil engineer who specializes in septic tank and leach field design meet you at the property for a preliminary site evaluation. Your agent should be able to supply you with a list of qualified engineers. The engineer will examin the soil, tree cover and rock outcropping, as well as check o the setback requirements for the leach field such as wells, proposed well location, roadways, creeks, streams, irrigation or drainage, existing easements and property lines.
The “Mantle” test is a 10 to 12 foot long trench dug into the soil with a depth of 6 to 10 feet. This hold exposes the different layers of soil. The county Environmental Health engineer as well as the civil engineer will both look at the soil content. In addition to the type of soil, the mantle test is used to check the depth of any bedrock or ground water. The testing is done in a number of places, the first test is for a primary septic field, while the second test is for the “repair” area. Both the mantle digging and the percolation test need to be done at both locations.
Once the mantles have been dug and examined, the engineers will perform a “percolation” test. They will dig 4 to 6 holes in the vicinity of each mantle test, about 3 feet deep. Then the hole is filled with water and a log is kept noting the rate at which the water drains (percolates), into the soil. The resulting calculations will determine the length of the leach line required for each bedrooms. The length of the lines will dictate the size of the house and the cost of the leach field installation.
A good “perc” rate is usually between 20 to 50 minutes per inch. The rates will determine if a standard system can be installed, or if an engineered system is required.
If an engineered system is needed, it will most likely be either something called a “deep system”, “mound system” “pressure dose”, “sand filtration” or pressure dose and filtration system”. Some advanced engineered systems require county monitoring via phone line and periodic inspection by the county.
Talk to your real estate agent and soil engineer before the test, so you understand the procedures and fees. Make sure you understand the outcome of the test, and what the probable cost are for the design and installation of the system designated by the testing results. Be prepared to spend a bit more on your septuc system than you budgeted for, perhaps 10 – 20%. Be sure you are