1. (a) The method for determining the design infiltration rate of the soils in which the practice will be placed shall be as follows:

   1. (1) Subject to (b), below, for existing natural soils, the applicant shall use the selected default values as presented in (c), below, or the results from the field measurement method described in (d), below;
   2. (2) For existing manmade soils, the applicant shall use the results from the field measurement method described in (d), below;
   3. (3) For proposed imported fill soils containing not more than 10% soil passing the number 200 sieve, the applicant shall use the results from the testing method described in (g), below, as confirmed after the fill is placed but before the practice is installed by results from the field measurement method described in (d), below; or
   4. (4) For proposed imported fill soils containing 10% soil or more passing the number 200 sieve, the applicant shall use a rate determined by a CPESC specialist, certified soil scientist, professional geologist, qualified engineer, or other qualified professional licensed to practice in the state of New Hampshire and confirmed after the fill is placed by using the field measurement method described in (d), below.
2. (b) If an infiltration practice used to meet the stormwater treatment requirements in accordance with Env-Wq 1507.03 infiltrates into soil classified as Abenaki, Adams, Agawam, Boscawen, Caesar, Champlain, Colton, Croghan, Deerfield, Haven, Hinckley, Hoosic, Metallak, Quonset, or Warwick, or any combination thereof, the applicant shall use the results from the field measurement method described in (d), below.

3. (c) The default values for the design infiltration rate shall be as follows:

   1. (1) Using SSSNNE Special Publication No. 5, Ksat Values for New Hampshire Soils, September 2025, available as noted in Appendix B, record the saturated hydraulic conductivity (Ksat) for the lowest Ksat value in the range for the most limiting layer located 0 to 5 feet below the proposed bottom of the practice;
   2. (2) Where more than one soil series is present, compute a weighted Ksat based on the area of each soil series present; and
   3. (3) Multiply the recorded Ksat value by 0.5 and use the result as the default design infiltration rate.

4. (d) The field measurement method for determining the design infiltration rate shall be as follows:

   1. (1) Using one of the methods as described in (e), below, perform the test the specified number of times in a location and, if the specified number is greater than one, average the results, then move to the next test location and repeat; and
   2. (2) After obtaining an average infiltration rate for each of the requisite number of locations as specified in (f), below, average the results and multiply the result by 0.5 to obtain the design rate.

5. (e) The Ksat shall be measured by a CPESC, certified soil scientist, professional geologist, qualified engineer, or other qualified professional licensed to practice in the state of New Hampshire, using one of the following:

   1. (1) A Guelph Permeameter, per the manufacturer’s instructions, which shall be done at least 2 additional times, for a minimum of 3 observations in each location;
   2. (2) A Compact Constant Head Permeameter, per the manufacturer’s instructions, which shall be done at least 2 additional times, for a minimum of 3 observations in each location;
   3. (3) A Double Ring Infiltrometer, in accordance with ASTM 3385 standards and using an inner ring that is at least 12 inches in diameter, which shall be done at least one time, for a minimum of one observation in each location;

   4. (4) A Borehole Infiltration test, in accordance with the following protocol:

      1. a. Install a solid 4- to 6-inch diameter by 30-inch long casing to a depth of 24 inches below the proposed bottom of the practice;
      2. b. Remove any smeared soil surfaces and provide a natural soil interface into which water can percolate;
      3. c. Remove all loose material from inside the casing;
      4. d. Fill the casing with water to a depth of 24 inches and allow to pre-soak for 24 hours;
      5. e. After pre-soaking in accordance with d., refill the casing with 24 inches of water and record the drop in water level from the top of the casing at the end of one hour;
      6. f. Divide the drop in water elevation by one hour to obtain the infiltration rate for the given observation;
      7. g. Repeat e. and f., above, at least 3 additional times, for a minimum of 4 observations in each location; and
      8. h. Use the lower of the average of the calculated infiltration rates or the last observation;
   5. (5) A Modified Phillip Dunne Infiltrometer test, in accordance with ASTM D8152-18 standards, July 2022, available as noted in Appendix B, and using an inner ring that is at least 4 inches in diameter, which shall be done at least 2 additional times, for a minimum of 3 observations in each location; or
   6. (6) A Sealed Single-Ring Infiltrometer and Multiple Pressure Head Procedure test, in accordance with ASTM D8550-25 standards, August 2025, available as noted in Appendix B, which shall be done at least 2 additional times, for a minimum of 3 observations in each location, or in accordance with the manufacturer’s instructions, whichever is greater.

6. (f) The number and location of test locations shall be as follows:

   1. (1) Locate the field tests within the footprint of the proposed practice, sufficiently spaced apart to be representative of the overall conditions;
   2. (2) Conduct the test at the base elevation of the proposed practice;

   3. (3) Subject to (5), below, for infiltration basins, perform:

      1. a. One field test in every 2,500 square feet of infiltration basin area if no manmade soils are present; or
      2. b. One field test in every 1,000 square feet of infiltration basin area for infiltration basins to be located on existing manmade soils; and

   4. (4) Subject to (5), below, for infiltration trenches, perform:

      1. a. One field test in every 100 linear feet of the infiltration trench area if no manmade soils are present; or
      2. b. One field test in every 50 linear feet of the infiltration trench area for infiltration trenches to be located on existing manmade soils.
   5. (5) At least one field test shall be conducted within each distinct soil type present at the location of the proposed infiltration basin or trench.

7. (g) The method for determining the design infiltration rate for proposed imported fill soils shall be as follows:

   1. (1) Determine the Ksat of the proposed fill in accordance with test methods described in ASTM D-2434-22, “Standard Test Method for Measurement for Coarse-Grained Soils”, April 2022, available as noted in Appendix B; and
   2. (2) Apply a minimum factor of safety by multiplying the representative Ksat by 0.5 and use the result as the design infiltration rate.
8. (h) Discharge of stormwater into the ground shall be prohibited or restricted in the areas identified in Env-Wq 1507.02(c).

Source. #9343, eff 1-1-09; amd by #9817-A, eff 12-1-10 (formerly Env-Wq 1504.32); ss by #12342, eff 8-15-17; ss by #14472, eff 1-19-26, EXPIRES: 1-19-36