Percolation vs. Permeation

excerpt from the Soil Report Newsletter of Soilmoisture Equipment Corp.

This is a problem urban dwellers hooked up to municipal sewage systems never heard of. But it is a matter of great concern, including financial, to rural residents using septic systems, waste water ponds and such. It is also a problem the Soilmoisture Equipment Corp. (SEC) has been asked to help solve.

Controlling the effluent from a septic tank or system of septic tanks is obviously a public health concern. The outflow cannot be allowed to return to the surface or contaminate aquifers unless it has been thoroughly cleansed by seeping through the ground.

The problem was solved early in the 20th Century by the Percolation Test, now used by every county government in the nation. A large hole, or holes, is dug, then filled with water. Building inspectors then measure how long it takes the water to seep or percolate into the ground. Determination is made on whether the soil is suitable for a septic system.

There is no doubt the Percolation Test has been a great boon to public health in the US, but there are problems associated with the test, as SEC Soil Scientist Richard White explains: "The Percolation Test is expensive, requiring heavy earth-moving equipment and construction delays while suitable soil is found. Nor is the test an absolute guarantee. In heavy clay soils, found especially in the Western United States, the Percolation Test may give a false reading. In the dry season clay cracks, permitting effluent to disappear into the ground. But in wet seasons, the clay absorbs moisture and binds together. What percolated in June stays on the surface in January, creating a health menace and a most expensive problem for developers."

Richard White explains that a new and better system was developed by scientists at the University of Guelph in Ontario, Canada, then licensed for manufacture by SEC. Called the Permeation Test, it is far cheaper and easier to administer and provides more accurate results. White: "A two-inch hole is drilled into the ground and kept constantly filled with water. The Permeation Test measures not how long it takes for the ground to absorb a fixed amount of water, but how much water the soil is capable of absorbing. A much cheaper and better system."

The problem is that the Percolation Test has been in use so long, written into ordinances and statutes, that it is one of the Ten Commandments of building codes. "If we can just get one county to accept the Permeation Test as an improvement," White says, "the rest will fall like dominoes. It really is cheaper and more accurate."

Toward that end, White was recently invited to Sonora, California, to administer the Permeation Test for a 400-acre housing development.

At the same time the Condor Earth Sciences Co. of Senora performed the Percolation Test. Thus, a side-by-side comparison of the two methods for effectiveness, ease and cost became possible.

As White described it, the Percolation Test, performed by a private contractor, was a considerable undertaking, requiring a backhoe, auguring equipment, truck, pipe, gravel and a water wagon--as well as the services of two men for six hours. They first bored a hole 22 inches into the ground, inserted gravel, then a four-inch plastic pipe, which they filled with water. Nearby they dug a trench three and a half feet deep, then repeated the former operation at that depth.

Meanwhile, White performed the Permeation Test. Using a bucket auger to make a two-inch hole 22 inches deep, he filled it with water from a reservoir that measured the amount of fluid used. He obtained the rate of water absorption by dividing the amount of water used by the elapsed time, quickly producing a graph showing the rate of water absorption every minute. The test continued until the absorption rate reached a steady state, that is, absorption reached a maximum rate where it couldn't go in any faster. The process required one man, no heavy equipment and was finished in two hours. Actually, White ran two tests while the Condor people ran one.

What is most significant is this: By law, the Percolation Test must be performed every l0 acres. On the Senora site, this means 40 different tests, an extensive investment of time and money. The Permeation Test could be performed with considerably less effort.

"The workmen doing the Percolation Test were very interested in my work," White said. "It was certainly easier, quicker and cheaper than what they were doing. The results of both tests are now being compared and evaluated for accuracy." We'll keep you posted.