Section Env-Wq 1508.07 - Stormwater Treatment Practices: Infiltration Practices Infiltration practices, including but not limited to infiltration trenches, infiltration basins, dry wells, and drip edges, shall comply with the following:
(a) Infiltration practices shall be prohibited in the following areas in addition to those identified in Env-Wq 1507.02:(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) 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;(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;(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 run-off will be discharged to the practice;(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;(e) The volume of the practice, excluding any sediment forebay areas, shall be large enough to contain the WQV without depending on infiltration;(f) The practice shall completely drain the WQV within 72 hours;(g) The design infiltration rate shall be determined in accordance with Env-Wq 1504.14;(h) Subject to (i) and (j), below, the SHWT and bedrock shall be at least 3 feet below the bottom of the practice;(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;(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;(k) If the practice includes an infiltration trench, the following requirements also shall be met: (1) The trench shall be 4 to 10 feet deep;(2) The infiltration media shall be clean, washed, well-graded aggregate with a diameter of 1.5 to 3 inches such that the porosity is 40%;(3) An observation well along the trench center line shall be provided; and(4) The overflow structure shall comply with Env-Wq 1507.02(d);(l) If the practice includes an in-ground infiltration basin, the following requirements also shall be met:(1) The perimeter of the pond shall be curvilinear;(2) Side slopes shall be no steeper than 3:1 and no flatter than 20:1;(3) The basin floor shall be flat, or zero percent slope;(4) The basin floor shall be prepared with one of the following:a. A 6-inch layer of coarse sand or 3/8 inch pea gravel;b. Grass turf that can survive inundation for up to 72 hours and still provide a dense, vigorous turf layer; orc. 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) The total volume of the basin shall: a. Be large enough to infiltrate the 50-year, 24-hour storm without overtopping; orb. Have an overflow structure that complies with Env-Wq 1507.02(d).(m) If the practice includes an underground infiltration basin, the following requirements also shall be met: (1) An observation well or accessible manhole structure shall be provided; and(2) The outfalls shall be designed to discharge the 10-year, 24-hour storm;(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(o) If the practice includes one or more stone drip edges, runoff shall be from roofs only.N.H. Admin. Code § Env-Wq 1508.07
(See Revision Note #1 at chapter heading for Env-Wq 1500) #9343, eff 1-1-09; (See Revision Note #2 at chapter heading for Env-Wq 1500) amd by #9817-A, eff 12-1-10
Amended by Volume XXXVII Number 32, Filed August 10, 2017, Proposed by #12342, Effective 8/15/2017, Expires 8/15/2027.Amended by Number 41, Filed October 12, 2023, Proposed by #13758, Effective 9/28/2023, Expires 9/28/2033 (formerly Env-Wq 1508.06) (see Revision Note #4 at chapter heading for Env-Wq 1500).