(A) Prevention of stormwater runoff is the key objective of Chapter 93 of the DEP regulations, because runoff can change the physical, chemical and biological integrity of waterbodies thereby impacting water quality.
(B) The SWM site plan shall describe how water quality protection requirements will be met. Infiltration BMPs shall be evaluated and utilized to the maximum extent possible to manage the net change in stormwater runoff generated so that post construction discharges do not degrade the physical, chemical or biological characteristics of the receiving waters.
(C) Post construction stormwater infiltration of runoff shall replicate preconstruction infiltration of runoff to the maximum extent possible.
(D) In calculating the volume of runoff that can be infiltrated at a site, the following methodology shall be used:
Rev = [(S)(Rv)(A)]/12 (inches/foot), where:
Rev = Recharge Volume (acre-feet);
S = Soil specific recharge factor (inches);
A = Site area contributing to the recharge facility (acres); and
Rv = Volumetric runoff coefficient, Rv = 0.05 + 0.009 (I), where:
I = Percent impervious area, and S shall be obtained based upon hydrologic soil group from the following table:
Hydrological Soil Group (HSG) | Soil Specific Recharge Factor (S) |
A | 0.38 |
B | 0.25 |
C | 0.13 |
D | 0.06 |
If more than one HSG is present at a site, a composite recharge volume shall be computed based upon the proportion of total site area within each HSG.
(E) In selecting the appropriate infiltration BMPs, the applicant shall consider the following:
(1) Permeability and infiltration rate of the site soils;
(2) Topographic slope and depth to bedrock;
(3) Seasonal high-water table;
(4) Proximity and elevation relative to building foundations, basements and well heads. (Infiltration BMPs shall be located downgrade of these structures);
(5) Erodibility of soils;
(6) Land availability, configuration and topography;
(7) Peak discharge and required volume control;
(8) Streambank erosion;
(9) Efficiency of the BMPs to mitigate potential water quality problems;
(10) Volume of runoff that will be effectively treated;
(11) Nature of the pollutant to be removed;
(12) Maintenance requirements;
(13) Creation/protection of aquatic and wildlife habitat;
(14) Recreational value; and
(15) Enhancement of aesthetic and property value.
(F) A detailed soils evaluation of the project site shall be performed to determine the suitability of infiltration BMPs. The evaluation shall be performed by a qualified professional and, at a minimum, address soil permeability, depth to bedrock, susceptibility to sinkhole formation and subgrade stability. The site testing shall include adequate sampling of all portions of the site, except those areas containing natural resources with total protection as defined by the township zoning ordinance, to determine areas of the property which are suitable for infiltration BMPs. The general process for designing the infiltration BMP shall be:
(1) Analyze hydrologic soil groups as well as natural and human-made features within the site to determine general areas of suitability for infiltration BMPs;
(2) Provide field testing data at the elevation of the proposed infiltration zone (bottom surface of infiltration facilities) to determine appropriate percolation rate and/or hydraulic conductivity;
(3) Design infiltration BMPs for required stormwater volume based on field-determined capacity at the level of the proposed infiltration surface; and
(4) Soil characteristics:
(a) Infiltration BMPs are particularly appropriate in hydrologic soil groups A and B, as described in the Natural Resources Conservation Service Manual TR-55;
(b) Low-erodibility factors (K factors) are preferred for the construction of basins;
(c) There must be a minimum depth of 18 inches between the bottom of any facility and the seasonal high water table and/or bedrock (limiting zones), except for infiltration BMPs receiving only roof runoff which shall be placed in soils having a minimum depth of 12 inches between the bottom of the facility and the limiting zone. The minimum required separation between the limiting zone may be increased, if required by the township, should project specific conditions exist (such as anticipated increased contaminants) which dictate greater prevention of groundwater contamination;
(d) There must be an infiltration and/or percolation rate sufficient to accept the additional stormwater load, and to drain completely as determined by field tests;
(e) A minimum of 30 feet of undisturbed fill shall separate the foundation wall of any building and an infiltration BMP;
(f) The infiltration system shall have positive overflow controls to prevent storage within one foot of the finished surface of grade;
(g) Infiltration rates shall not be used in computing the storage volume of the infiltration system; and
(h) Surface inflows shall be designed to prevent direct discharge of sediment into the infiltration system.
(G) The recharge volume provided at the site shall be directed to the most permeable hydrologic soil group (HSG) available, except where other considerations apply, such as in limestone geology.
(H) Any infiltration BMP shall be capable of completely infiltrating the impounded water within 48 hours from the end of the storm.
(I) Special attention shall be paid to proper installation of infiltration-oriented stormwater management facilities during construction and to careful avoidance of soil compaction during site development.
(J) Caution shall be exercised where salt or chloride would be a pollutant since soils do little to filter this pollutant and it may contaminate the groundwater. Extreme caution shall be exercised where infiltration is proposed in source water protection areas. The qualified 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’s antidegradation regulations, 25 Pa.Code, Chapter 93. The township may require the installation of an impermeable liner in BMP and/or detention basins where the possibility of groundwater contamination exists. A detailed hydrogeologic investigation may be required by the township where the proposed management system is proposed to be located within a source water protection area or when groundwater contamination from the proposed management system is possible.
(K) The SWM site plan must include safeguards against groundwater contamination for uses which may cause groundwater contamination, should there be a mishap or spill.
(L) During the period of land disturbance, runoff shall be controlled prior to entering any proposed infiltration area. Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase, so as to maintain their maximum infiltration capacity.
(M) Infiltration BMPs shall not be constructed nor receive runoff until the entire contributory drainage area to the infiltration BMP has achieved final stabilization.
(N) The requirements for volume control and infiltration are applied to all disturbed areas, even if they are ultimately to be a pervious or permeable land use such as lawn or other landscaped area, given the extent to which development-related disturbance leads to compaction of the soils and reduces their infiltrative capacity.
(Ord. 239, passed 10-14-2020)