1. (a) Except as required by (b) through (e), below, runoff from all proposed impervious areas, including but not limited to roadways, parking areas, driveways, roofs, and redeveloped impervious areas, shall be treated with one or more of the stormwater treatment practices described in Env-Wq 1508.03 through Env-Wq 1508.11, provided that the requirements of this section are met along with all method-specific criteria.

2. (b) Except as required by (c), below, runoff from proposed impervious area that would result in a discharge of stormwater within one-quarter mile of a surface water of the state impaired for phosphorus, or within one-quarter mile of a lake or pond, shall be treated with one or more of the following stormwater treatment practices:

   1. (1) A bioretention system with an internal storage reservoir that meets the requirements of Env-Wq 1508.06, provided the filter media is amended with 5% water treatment residuals or 0.5% iron filings;
   2. (2) An infiltration practice that meets the requirements of Env-Wq 1508.07;

   3. (3) A filtering practice that meets the requirements of Env-Wq 1508.08, provided one or more of the following is also met:

      1. a. The practice infiltrates the WQV; or
      2. b. The filter media is amended with 5% water treatment residuals or 0.5% iron filings;
   4. (4) A gravel wetland that meets the requirements of Env-Wq 1508.05, provided that the installation of an infiltration practice pursuant to (2), above, is not feasible;
   5. (5) A stormwater treatment practice that has pollutant removal efficiencies equal to or greater than (1) through (3), above; or
   6. (6) Treatment practices supported by a pollutant loading analysis completed in accordance with (j), below, that demonstrates there will be no increase in phosphorus loading to the surface water of the state as a result of the project.

3. (c) Runoff from proposed impervious areas that would result in a discharge of stormwater within one-quarter-mile of a class A surface water or within the watershed of a surface water that is an outstanding resource water (ORW) as defined in Env-Wq 1708.04(a), shall be treated with one or more of the following stormwater treatment practices:

   1. (1) A gravel wetland that meets the requirements of Env-Wq 1508.05;
   2. (2) A bioretention system with an internal storage reservoir that meet the requirements of Env-Wq 1508.06, provided that the filter media is amended with 5% water treatment residuals or 0.5% iron filings;
   3. (3) A stormwater treatment practice that has pollutant removal efficiencies equal to or greater than (1) or (2), above, and that incorporate a denitrification process; or

   4. (4) Treatment practices supported by a pollutant loading analysis completed in accordance with (j), below, that demonstrates:

      1. a. There will be no increase in nitrogen and phosphorus loading to the surface waters of the state as a result of the project; and
      2. b. For projects with a net increase in impervious surfaces, utilize stormwater BMPs listed in Env-Wq 1508.05 or Env-Wq 1508.06 for denitrification.

4. (d) Runoff from proposed impervious areas that would result in a discharge of stormwater within one-quarter mile of a surface water of the state impaired for nitrogen shall be treated with one or more of the following stormwater treatment practice:

   1. (1) A gravel wetland that meets the requirements of Env-Wq 1508.05; or
   2. (2) A bioretention system with an internal storage reservoir that meets the requirements of Env-Wq 1508.06.

5. (e) Runoff from the following areas shall be presumed to meet water quality standards without the need for stormwater treatment practices:

   1. (1) Impervious areas for the purposes of lot development, provided that:

      1. a. The individual lot is a single family or duplex residential lot;
      2. b. The individual lot is one acre or greater;
      3. c. The individual lot is not in a watershed impaired for nitrogen or phosphorus;
      4. d. The total disturbance for each individual lot does not exceed 100,000 square feet or 50,000 square feet if any portion in the protected shoreland; and
      5. e. The individual lot does not drain to one or more proposed stormwater treatment practices;
   2. (2) Access roads that are vegetated;
   3. (3) Utility infrastructure, including substations pads and access roads that will receive fewer than 52 trips per year; and
   4. (4) Solar panels.
6. (f) Stormwater treatment practices shall be accessible for proposed maintenance activities.
7. (g) Infiltration rates for designing stormwater treatment practices shall be determined in accordance Env-Wq 1504.14.
8. (h) Stormwater treatment practices shall meet the water supply well setback criteria identified in Env-Wq 1508.02.
9. (i) Subject to (j), below, stormwater treatment practices shall be designed for the WQV or WQF, as applicable, calculated in accordance with Env-Wq 1504.10 and Env-Wq 1504.11, respectively.

10. (j) If a pollutant loading analysis is conducted pursuant to (b)(6) or (c)(4), above, it shall be performed as follows:

    1. (1) Load export rates for total nitrogen and total phosphorus shall be as provided in Appendix F of the General Permits for Stormwater Discharges from Small Municipal Separate Storm Sewer Systems in New Hampshire, December 7, 2020, available as noted in Appendix B; and
    2. (2) Pollutant removal efficiencies for total nitrogen and total phosphorus shall be as provided in Appendix F of the General Permits for Stormwater Discharges from Small Municipal Separate Storm Sewer Systems in New Hampshire, December 7, 2020, available as noted in Appendix B.

Source. #9343, eff 1-1-09; amd by #9817-A, eff 12-1-10; ss by #12342, eff 8-15-17; ss by #13758, eff 9-28-23; ss by #14079, eff 9-24-24; ss by #14472, eff 1-19-26, EXPIRES: 1-19-36