Infiltration practices, including but not limited to infiltration trenches, infiltration basins, dry wells, and drip edges, shall comply with the following:

1. (a) Infiltration practices shall be prohibited in the following areas in addition to those identified in Env-Wq 1507.02(c):

   1. (1) Into soils where the infiltration rate, prior to adding a factor of safety, is less than 0.5 inches per hour, as calculated per Env-Wq 1504.14; and
   2. (2) Subject to (b), below, into soils where the infiltration rate, prior to adding a factor of safety, is more than 10 inches per hour, unless the stormwater directed to the infiltration practice has been treated in accordance with a practice described in Env-Wq 1508.03 through Env-Wq 1508.11 prior to entering the infiltration practice;
2. (b) Soils may be amended to reduce the infiltration rate to less than or equal to 10 inches per hour, prior to adding a factor of safety, as confirmed by results from field measurement methods described in Env-Wq 1504.14(e), provided that if soils are amended, they shall be amended to 24 inches or deeper;
3. (c) Pretreatment as described in Env-Wq 1508.12 through Env-Wq 1508.16 shall be provided if stormwater other than or in addition to roof runoff will be discharged to the practice;
4. (d) If a sediment forebay is used to meet (c), above, it shall be designed to contain 25% of the WQV and otherwise meet the criteria specified in Env-Wq 1508.12;
5. (e) The volume of the practice, excluding any sediment forebay areas, shall be large enough to contain the WQV without depending on infiltration;
6. (f) The practice shall completely drain within 72 hours;
7. (g) The design infiltration rate shall be determined in accordance with Env-Wq 1504.14;
8. (h) Subject to (i) and (j), below, the SHWT and bedrock shall be at least 3 feet below the bottom of the practice;
9. (i) If the practice is located within a groundwater protection area or a water supply intake protection area, the SHWT and bedrock shall be at least 4 feet below the bottom of the practice;
10. (j) If the stormwater directed to the infiltration practice has been treated in accordance with a practice described in Env-Wq 1508.03 through Env-Wq 1508.11 prior to entering the practice, the SHWT and bedrock may be reduced to not less than one foot below the bottom of the practice;

11. (k) If the practice includes an infiltration trench, the following requirements also shall be met:

    1. (1) The trench shall be 4 to 10 feet deep;
    2. (2) The trench depth shall be greater than the trench width;
    3. (3) The infiltration media shall be clean, washed, and well-graded aggregate with a diameter of 1.5 to 3 inches such that the porosity is 40%;
    4. (4) An observation well along the trench center line shall be provided; and
    5. (5) The overflow structure shall comply with Env-Wq 1507.02(d);

12. (l) If the practice includes an in-ground infiltration basin, the following requirements also shall be met:

    1. (1) The perimeter of the pond shall be curvilinear;
    2. (2) Side slopes shall be no steeper than 3:1 and no flatter than 20:1;
    3. (3) The basin floor shall be flat, or zero percent slope;

    4. (4) The basin floor shall be prepared with one of the following:

       1. a. A 6-inch layer of coarse sand or 3/8 inch pea gravel;
       2. b. Grass turf that can survive inundation for up to 72 hours and still provide a dense, vigorous turf layer; or
       3. c. A layer of coarse organic material, such as erosion control mix or composted mulch, that is tilled into the soil, soaked, and allowed to dry; and

    5. (5) The total volume of the basin shall:

       1. a. Be large enough to infiltrate the 50-year, 24-hour storm without overtopping; or
       2. b. Have an overflow structure that complies with Env-Wq 1507.02(d).

13. (m) If the practice includes an underground infiltration basin, the following requirements also shall be met:

    1. (1) An observation well or accessible manhole structure shall be provided; and
    2. (2) The outfalls shall be designed to discharge the 10-year, 24-hour storm;
14. (n) If the practice includes one or more dry wells, each dry well shall be equipped with an overflow structure that complies with Env-Wq 1507.02(d); and
15. (o) If the practice includes one or more stone drip edges, runoff shall be from roofs only.

Source. #9343, eff 1-1-09; amd by #9817-A, eff 12-1-10; ss by #12342 (formerly Env-Wq 1508.05); ss by #13758, eff 9-28-23 (formerly Env-Wq 1508.06); ss by #14472, eff 1-19-26, EXPIRES: 1-19-36