rate water of drop level is in still each slowing hole should 7. be reasonably constant. that clay in If the soil is still swelling, constant. and the In down, this may mean until the rates become more percolation test should be extended last rate observed to calculate number the of minutes sandy soils, use the calculated by dividing the by the num- rate. This rate is the last and the last measurement An example between of inches the next the water to level dropped 3.2. during that time. ber of this calculation is results shown tend in Table to vary widely from time the test. to time, Use from Percolation place to test place, and with the person performing ONSITE WASTEWATER DISPOSAL Measuring Stick Batter Board Reference Point Water Level Gravel Figure 3.14 Measuring drop in water level. the average of the percolation rates of all the test holes to determine the drain field size, if that rate corresponds generally with what you would expect from observing the soils in the drain field area. If the percolation rates vary by more than 20 minutes per inch, there are probably different types of soil in the area. Size the corresponding portions of the drain field accordingly. Table 3.3 shows an approxi- mate relationship between soil type and percolation rate, based on several reported analyses. Table 3.2 Example Percolation Rate Calculation Measurement 12:20 12:30 22 1/2" (refill to 18") 4 1/2" 12:40 22 1/2" (refill to 18") 4 1/2" 12:50 21 1/2" (refill to 18") 3 1/2" 22 1/2" (refill to 18") 2 1/2" 10 minutes 2 inches = 5 minutes per inch SELECTION OF SITE AND SYSTEM Table 3.3 Relationship Between Soil Type and Percolation Rate Soil Type Percolation Rate (min/in.) Loamy sand Sandy loam Silt loam Silty clay loam Silty clay There is one phenomenon which must be considered when using the percolation rate measured in sandy soils to size a drain field. Initial percolation through coarse sand is quite rapid. In fact, doses of sewage effluent are able to move down through the sand as clouds of saturation, and receive little t