Maximizing the ground water recharge capacity of the development site being developed is required. The applicant shall demonstrate that the design of the infiltration/recharge stormwater management facilities has provided for, to the maximum extent feasible, ground water recharge to compensate for the reduction in the percolation that occurs when the ground surface is disturbed or impervious surface is created. It is recommended that roof runoff be directed to infiltration BMPs which may be designed to compensate for the runoff from parking areas. These measures are required to be consistent with §202, and take advantage of utilizing any existing recharge areas.
A. Infiltration BMPs shall meet the following minimum requirements:
1. Maximum Infiltration Requirements. The applicant shall demonstrate that all regulated activities shall recharge (infiltrate), where practicable, a portion of the runoff created by the development as part of an overall drainage plan. The volume of runoff to be recharged shall be determined from §§224(A)(3)(a) or 224(A)(3)(b), depending upon demonstrated development site conditions.
2. Infiltration BMPs intended to receive runoff from developed areas shall be selected based on suitability of soils and site conditions and shall be constructed on soils that have the following characteristics:
(a) A minimum depth of twenty-four (24) inches between the bottom of the BMP and the limiting zone.
(b) An infiltration and/or percolation rate sufficient to accept the additional stormwater load and drain completely as determined by field tests conducted by the applicant’s design professional.
(c) The recharge facility shall be capable of completely infiltrating the recharge volume within four (4) days.
(d) Pretreatment in accordance with §§221(L) and 221(M) shall be provided prior to infiltration.
3. The size of the recharge facility shall be based upon the following volume criteria:
(a) NRCS curve number equation.
The NRCS runoff shall be utilized to calculate infiltration requirements (P) in inches.
For zero runoff:
P = I(Infiltration)(in.) = (200 CN)- 2
Where: CN=SCS (NRCS) curve number of existing conditions contributing to the recharge facility.
This equation is displayed graphically in, and the infiltration requirement can be determined from, Figure 224.1.
The recharge volume required would therefore be computed as:
Rev = I*% impervious area
Where: I = infiltration requirements (in.)
(b) Annual Recharge - Water Budget Approach. It has been determined that infiltrating six-tenths (0.6) inches of runoff from the impervious areas will aid in maintaining the hydrologic regime of the watershed. If the goals of §224(A)(3)(a) cannot be achieved, then six-tenths (0.6) inches of rainfall shall be infiltrated from all impervious areas, up to an existing site condition curve number of seventy-seven (77). Above a curve number of seventy-seven (77), Equation 224.1 or the curve in Figure 224.1 should be used to determine the infiltration requirement.
The recharge volume (Rev)required would therefore be computed as: Rev = 0.6 or I, whichever is less *% impervious area.
B. Soils. The applicant shall submit a detailed soils evaluation of the development site to determine the suitability of recharge facilities. The evaluation shall be performed by a design professional, and at a minimum, address soil permeability, depth to bedrock and subgrade stability. The general process for designing the infiltration BMP shall be:
1. Analyze hydrologic soil groups as well as natural and man-made features within the watershed to determine general areas of suitability for infiltration practices.
2. Provide site-specific infiltration test results (at the level of the proposed infiltration surface) in accord with ASTM Guide No. D 5126 to determine the appropriate hydraulic conductivity rate.
3. Design the infiltration structure for the required storm volume based on field determined capacity at the level of the proposed infiltration surface.
4. If on-lot infiltration structures are proposed by the design professional, the applicant shall demonstrate to the Township that the soils are conducive to infiltrate on the lots identified.
C. Stormwater Hotspots. A stormwater hotspot is defined as a land use activity that generates higher concentrations of hydrocarbons, trace metals or toxicants than are found in typical stormwater runoff, based on monitoring studies. Table 224.1 provides samples of designated hotspots. If a development site is designated as a hotspot, it has important implications for how stormwater is managed. First and foremost, untreated stormwater run off from hotspots shall not be allowed to infiltrate into groundwater where it may contaminate water supplies. Therefore, the Rev requirement is NOT applied to development sites that fit into the hotspot category (the entire WQv must still be treated). Second, a greater level of stormwater treatment may be needed at hotspot sites to prevent pollutant washoff after construction. EPA’s NPDES stormwater program requires some industrial sites to prepare and implement a stormwater pollution prevention plan.
Table 224.1 - Classification of Stormwater Hotspots
The following land uses and activities are samples of stormwater hotspots:
•Vehicle salvage yards and recycling facilities
•Fleet storage areas (bus, truck, etc.)
•Public works storage areas
•Facilities that generate or store hazardous materials
The applicant shall exercise extreme caution where salt or chloride would be a pollutant since soils do little to filter this pollutant and it may contaminate the groundwater. The design professional shall evaluate the possibility of groundwater contamination from the proposed infiltration/recharge facility and perform a hydrogeologic justification study if necessary. The infiltration requirement in high quality/exceptional value waters shall be subject to DEP Chapter 93 antidegradation regulations. The Township may require the installation of an impermeable liner in detention basins where the possibility of groundwater contamination exists. The Township may require a detailed hydrogeologic investigation.
The applicant shall provide safeguards against groundwater contamination for uses which may cause groundwater contamination, should there be a mishap or spill. (See Appendix E).
D. The applicant shall demonstrate that extreme caution has been exercised where infiltration is proposed in source water protection areas or that may affect a wellhead or surface water intake.
E. Recharge/infiltration facilities shall be used in conjunction with other innovative or traditional BMPs, stormwater control facilities, and nonstructural stormwater management alternatives. (Ord. 181, 10/11/2006, §304)