(c) Sizing the system based on seasonal water table data. When a seasonal water table of more than one (1) duration is present in a soil, the loading rate is determined as follows:

1. (1) Determine the depth of each seasonal water table;

2. (2) Adjusting the moderate seasonal water table:

   1. (A) Subtract the depth to the brief SWT from the depth to the moderate SWT and divide by three (3); and
   2. (B) Subtract the result from the depth to the moderate SWT to obtain the adjusted moderate SWT;

3. (3) Adjusting the long seasonal water table:

   1. (A) Subtract the adjusted moderate SWT from the depth to the long SWT and divide by two (2); and
   2. (B) Subtract the above number from the depth to the long SWT to obtain the adjusted long SWT;

4. (4) Adjusting the long seasonal water table where only brief and long seasonal water tables are encountered:

   1. (A) Subtract the depth to the brief SWT from the long SWT and divide by six (6); and
   2. (B) Subtract the above number from the depth to the long SWT to obtain the adjusted long SWT;
5. (5) Compare the loading rates for the brief, adjusted moderate, and adjusted long duration seasonal water table using the soil loading charts;
6. (6) Use the most restrictive loading rate to determine the size of the absorption area; and
7. (7) Soils that only have one (1) duration of seasonal water table are loaded by using the loading rate given in the soil loading charts for the duration of seasonal water table observed.

(d) Sizing the soil absorption area based on percolation data.

1. (1) Procedures of percolation tests.

   1. (A) Number and location of tests.

(i) Three (3) or more test holes spaced uniformly over the primary absorption area and one (1) test hole in the secondary absorption area are required.

1. (ii) Percolation tests shall be performed at the depth of the proposed soil absorption system.

   1. (B) Type of test holes.

      1. (i) The depth of a standard percolation test hole is eighteen inches (18”) and the diameter shall be twelve inches (12”).
      2. (ii) If soil and site characteristics indicate that a deeper or shallower system is desired, then the percolation test shall be run at the depth of the bottom of the proposed absorption trench.

   2. (C) Preparation of test holes.

      1. (i) Carefully scratch the bottom and sides of the holes with a knife blade or sharp-pointed instrument, in order to:

(b) (b) Provide a natural soil interface into which water may percolate.

1. (ii) Remove all loose material from the holes.

   1. (iii) Add two inches (2”) of coarse sand or fine gravel to protect the bottom from scouring and sediment.

   2. (D) (D) Saturation and swelling of the soil.

      1. (i) It is important to distinguish between saturation and swelling.
      2. (ii)

(b) (b) This can be accomplished in a short period of time.

1. (iii)

(b) (b) This is a slow process, especially in clay-type soil, and is the reason for requiring a prolonged soaking period.

1. (iv)

(b) (b) This may be achieved by the use of an automatic siphon.

1. (c) (c) This procedure is to ensure that the soil is given time to swell and to approach the condition it will be in during wet seasons of the year.

   1. (E)

      1. (i) After the saturation period, adjust the depth of water in the holes to six inches (6”) above the gravel.
      2. (ii) From a fixed reference point, measure the drop-in water level at the end of a thirty-minute period, refilling six inches (6”) above the gravel as necessary.
      3. (iii) The drop of water level that occurs during the thirty-minute period is used to calculate the percolation rate.

   2. (F)

      1. (i) Soils which the first six inches (6”) of water seeps away in less than thirty (30) minutes after the twenty-four (24) hours or greater saturation period, the time interval between measurements shall be taken at ten (10) minutes and the test run for one (1) hour.
      2. (ii) The drop that occurs during the final ten (10) minutes is used to calculate the percolation rate.

2. (2)

   1. (A) The size of the absorption area may be determined from the results of the percolation test and the data in Appendix A.
   2. (B) As noted in Appendix A, soil in which the percolation rate is greater than seventy-five minutes per inch (75 mpi) is unsuitable for a standard onsite wastewater system.
3. (3) The size of the absorption area for a system sized using percolation rate data shall not be smaller than that required by seasonal water table data for the same site.