Filtering practices, including but not limited to surface sand filters, underground sand filters, tree box filters, bioretention systems, pervious asphalt, and pervious concrete, shall comply with the following:

1. (a) Filtering practices shall not be located in:

   1. (1) An area where a filtering practice sidewall is adjacent to hydrologic soil group A soils, unless an impermeable liner is installed along the side wall so as to separate the practice from the group A soils; or
   2. (2) The areas identified in Env-Wq 1507.02(c) unless the practice has an impermeable liner;

2. (b) Pretreatment as described in Env-Wq 1508.12 through Env-Wq 1508.16 shall be provided unless:

   1. (1) The practice is pervious pavement or a tree box filter; or
   2. (2) Roof runoff is the only stormwater discharged to the practice;
3. (c) The practice shall completely drain within 72 hours;
4. (d) All filtering practices that impound stormwater shall have an overflow structure that complies with Env-Wq 1507.02(d);
5. (e) If a sediment forebay is used to meet (b), above, it shall be designed to contain 25% of the WQV and otherwise meet the criteria specified in Env-Wq 1508.12;
6. (f) If the practice has an impermeable liner, (g) through (j), below, shall not apply;
7. (g) The design infiltration rate of the underlying native soil or fill soil shall be determined in accordance with Env-Wq 1504.14;

8. (h) If the infiltration rate, prior to adding a factor of safety, of the underlying soil or proposed fill, as determined pursuant to (g), above, is less than 0.50 inches per hour, an underdrain system shall be provided as follows:

   1. (1) The underdrain shall consist of a 6-inch diameter perforated pipe set in ¾-inch to 2-inch diameter stone or gravel washed free of fines and organic material;
   2. (2) The stone or gravel layer shall extend at least 4 inches above the drainage pipes and 4 inches below the drainage pipes;
   3. (3) The stone or gravel layer shall be separated from the filter media with a 3-inch layer of 3/8-inch pea gravel; and
   4. (4) Underdrains shall be provided at a maximum spacing of 25 feet center-to-center;
9. (i) If the practice is not located within a groundwater protection area or water supply intake protection area, the SHWT and bedrock may be reduced to not less than one foot below the bottom of the filter course material, provided, however, that underdrains may be installed to achieve the minimum one-foot separation between the SHWT and the bottom of the filter course material;

10. (j) If the practice is located within a groundwater protection area or water supply intake protection area, the practice shall have a filter course depth of 18 inches for pervious pavement and 24 inches for all other filtering practices, and have at least one foot of separation between:

    1. (1) The bottom of the filter course material and bedrock; and
    2. (2) The bottom of the filter course material and the SHWT, provided, however, that underdrains may be installed to achieve the minimum one-foot separation between the SHWT and the bottom of the filter course material;

11. (k) If the practice includes a surface filter, the following requirements also shall be met:

    1. (1) The filter, including the storage area above the filter, the filter media voids, and the pretreatment area, shall store at least 75% of the WQV;
    2. (2) The contributing drainage area shall be less than 10 acres;
    3. (3) The filter media shall be a minimum 18 inches deep;

    4. (4) The filter media shall consist of one of the following mixtures volume:

       1. a. 50% to 55% by volume sand that is certified by its producer as meeting the requirements for ASTM C-33 concrete sand, 20% to 30% by volume of loamy sand topsoil with 15% to 25% fines passing the number 200 sieve, and 20% to 30% by volume moderately fine shredded bark or wood fiber mulch with less than 5% passing the number 200 sieve;

       2. b. 20% to 30% by volume of moderately fine shredded bark or wood fiber mulch that has no more than 5% fines passing the number 200 sieve, with 70 to 80% by volume loamy coarse sand used in the mixture meeting the following sieve analysis specification:

          1. 1. From 85% to 100% by weight shall pass the number 10 sieve;
          2. 2. From 70% to 100% by weight shall pass the number 20 sieve;
          3. 3. From 15% to 40% by weight shall pass the number 60 sieve; and
          4. 4. From 8% to 15% by weight shall pass the number 200 sieve;

       3. c. 60% to 85% by volume of sand, 15% to 25% by volume of loam or topsoil, and 3% to 8% by volume of organic matter used in the mixture meeting the following sieve analysis specification:

          1. 1. 100% by weight shall pass the number 4 sieve;
          2. 2. 95% by weight shall pass the number 10 sieve;
          3. 3. From 15% to 40% by weight shall pass the number 40 sieve;
          4. 4. From 10% to 20% by weight shall pass the number 200 sieve; and
          5. 5. From 0% to 5% by weight shall pass a sieve that is finer than the number 200 sieve; and
    5. (5) The filter shall have an access grate;

12. (l) If the practice includes an underground filter, the following requirements also shall be met:

    1. (1) The filter, including the filter media voids and the pretreatment chamber, shall store at least 75% of the WQV;
    2. (2) The contributing drainage area shall be less than 10 acres;
    3. (3) The filter media shall be a minimum 18 inches deep;
    4. (4) The filter media shall consist of 100% of ASTM C-33 concrete sand or one of the mixtures specified in (k)(4), above; and
    5. (5) The filter shall have an access grate;

13. (m) If the practice includes a bioretention system, the following requirements also shall be met:

    1. (1) The ponding area, including the storage area above the filter and the filter media voids, shall store 100% or more of the WQV;
    2. (2) The contributing drainage area shall be less than 5 acres;
    3. (3) The filter media shall be a minimum 18 inches deep;
    4. (4) The filter media shall consist of one of the mixtures specified in (k)(4), above;
    5. (5) Side slopes shall not exceed 3:1;
    6. (6) The surface shall be covered with a minimum of 3-inches of organic material; and

    7. (7) The surface area shall be covered with grasses or have the following planting design plan:

       1. a. Only native, non-invasive species shall be used;
       2. b. Plant layout shall be random and natural;
       3. c. Woody vegetation shall not be used near inflow locations;
       4. d. Vegetation directly over the filter media shall be limited to facultative wetland species as specified in National List of Plant Species that Occur in Wetlands: Northeast (Region 1), May 1988, published by U.S. Fish and Wildlife Service, available as noted in Appendix B, or other species that can withstand periodic inundation, as determined by a certified wetland scientist;
       5. e. Trees or large shrubs shall be planted along the perimeter; and
       6. f. The plan shall establish a perimeter tree canopy with an understory of shrubs and herbaceous plants;

14. (n) If the practice includes pervious asphalt, the following requirements also shall be met:

    1. (1) The practice shall be designed and installed in accordance with UNHSC Design Specifications for Porous Asphalt Pavement and Infiltration Beds, February 2014, revised September 2016, published by University of New Hampshire Stormwater Center, available as noted in Appendix B;
    2. (2) The thickness of the filter course shall be at least 12 inches; and
    3. (3) The ratio of the total contributing area to the area of the pervious surface shall be no more than 5:1;

15. (o) If the practice includes pervious concrete, the following requirements also shall be met:

    1. (1) The filter course material shall consist of NHDOT Standard Specification for Road and Bridge Construction, March 2016, Table 304-1, item number 304.1, available as noted in Appendix B, modified to have 0-6% passing the number 200 sieve;
    2. (2) The practice shall be designed and installed in accordance with American Concrete Institute, ACI SPEC-522.1-13, Specification for Pervious Concrete Pavement, 2013, available as noted in Appendix B;
    3. (3) The practice shall be installed by a contractor certified in pervious concrete installation by the National Ready Mix Concrete Association (NRMCA);
    4. (4) The thickness of the filter course shall be at least 12 inches; and
    5. (5) The ratio of the total contributing area to the area of the pervious surface shall be no more than 5:1; and

16. (p) If the practice includes a non-proprietary tree box filter, then:

    1. (1) The tree box filter shall be not less than 6 feet in diameter and not less than 4 feet deep;
    2. (2) The contributing drainage area shall be less than 0.1 acres;
    3. (3) The filter media shall be 80% by volume sand that is certified by its producer as meeting the requirements for ASTM C-33 concrete sand, 20% by volume of organic material with less than 5% passing the number 200 sieve;
    4. (4) The soil mixture shall be uniform, free of stones, stumps, roots, or similar materials larger than 2 inches; and
    5. (5) The soil mixture shall have a pH between 5.5 and 6.5.

Source. #9343, eff 1-1-09; amd by #9817-A, eff 12-1-10; ss by #12342 (formerly Env-Wq 1508.06); ss by #13758, eff 9-28-23 (formerly Env-Wq 1508.07); ss by #14472, eff 1-19-26, EXPIRES: 1-19-36