- 1). Dig two or three test holes spaced over the area of the proposed drainfield, using a post-hole digger or auger. Make the holes 6 to 8 inches in diameter and as deep as the proposed drainfield, but at least 3 feet above the local water table. If the drainfield includes several different soil types, dig at least two holes in each type of soil.
- 2). Scratch any clay that became compacted or smeared slick during the digging so the walls of the hole will absorb water naturally. Scrape the clay with a large knife or a piece of board with nails driven through it. Remove any loose dirt from the bottom of the hole. Shovel 2 inches of gravel into the bottom of each hole.
- 3). Add 12 inches of water to each hole, using a hose or bucket. Check it 10 minutes later. If the water has drained away, continue with the perc test. If water remains, allow the water to continue soaking in the holes for at least four hours, adding more water regularly to keep it approximately 12 inches deep.
- 4). Add or bail water so each hole contains 6 inches of water at the start of the test. Place a scrap board across the top of each hole, toward one side. This creates a consistent measuring point so you can gauge the depth of the water. Measure the distance from the underside of the board to the surface of the water in inches, using a tape measure or yardstick.
- 5). Measure the distance again in each hole, exactly 30 minutes later. Write down the difference between the two measurements for each hole. If all the water has drained from a hole, ignore the results. Fill the hole again 6 inches deep, measure it, then measure it again 10 minutes later instead of 30 minutes.
- 6). Refill the holes and repeat the tests, measuring the water when you first fill it and again 30 minutes or 10 minutes later. Subtract the second measurement from the first. Repeat for each hole until the result for that hole is no more than 1/4 inch different than the previous result. Write down the final result for each hole.
- 7). Divide 30 (or 10 if you waited 10 minutes) by the final result for each hole to figure the percolation rate in minutes per inch for that hole. Add the results of all the holes together and divide by the number of holes to calculate the average rate for the drainfield area. A rate between five and 90 minutes per inch is usually suitable for regular septic systems. A rate outside those limits indicates you may need to install a special system.
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