A. The requirements for groundwater recharge differ for the Schuylkill River watershed and the Tulpehocken Creek watershed. The applicant shall apply the appropriate regulations based on the location of a proposed site within the Schuylkill River or Tulpehocken Creek watersheds.
B. Groundwater recharge for the Schuylkill River watershed.
(1) Maximizing the groundwater recharge capacity of the area being developed is required. Design of the infiltration stormwater management facilities shall give consideration to providing groundwater 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 can be over-designed to compensate for the infiltration losses due to parking areas. 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 § 505-103 and take advantage of utilizing any existing recharge areas.
(2) Infiltration may not be feasible on every site due to site-specific limitations such as soil type. If it cannot be physically accomplished, due to seasonal high water table, soil permeability rate, soil depth or setback distances from special geologic features, then the design professional shall be responsible to show that this cannot be physically accomplished. If it can be physically accomplished, then the volume of runoff to be infiltrated shall be determined from Subsection B(3)(c) depending on demonstrated site conditions and shall be the greater of the two volumes.
(3) Infiltration BMPs within the Schuylkill River Watershed shall meet the following minimum requirements:
(a) Infiltration requirements.
[1] Regulated activities will be required to infiltrate, where site conditions permit, a portion of the runoff created by the development as part of an overall stormwater management plan designed for the site. The volume of runoff to be infiltrated shall be determined from Subsection B(3)(c)[1] or [2], depending upon demonstrated site conditions.
(b) 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:
[1] A minimum depth of 36 inches between the bottom of the BMP and the limiting zone.
[2] 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.
[3] The infiltration facility shall be capable of completely infiltrating the required retention (infiltration) volume within four days (96 hours).
[4] Pretreatment shall be provided prior to infiltration.
(c) The size of the infiltration facility within the Schuylkill River watershed shall be based upon the following volume criteria:
[1] NRCS curve number equation.
[a] The NRCS runoff equation shall be utilized to calculate infiltration requirements (I) in inches.
Where:
CN = SCS (NRCS) curve number of existing conditions contributing to the infiltration facility
[b] This equation is displayed graphically in, and the infiltration requirement can be determined from Figure 505-125.1.
Figure 505-125.1 Infiltration requirement based upon NRCS Curve Number.
The retention (infiltration) volume (Rev) required to meet the infiltration requirement would therefore be computed as:
Rev = I * impervious area (square feet)/(12 in/ft) = Cubic Feet Eqn: 505-125.2
Where:
I = infiltration requirements (in inches)
[2] Annual recharge — water budget approach.
[a] It has been determined that infiltrating 0.46 inch of runoff from the impervious areas will aid in maintaining the hydrologic regime of the Schuylkill River Watershed. If the goals of § 505-125B(3)(c)[1] cannot be achieved, then 0.46 inch of rainfall shall be infiltrated from all impervious areas, up to an existing site conditions curve number of 81. Above a curve number of 81, Equation 505-125.1 or the curve in Figure 505-125.1 should be used to determine the infiltration requirement.
[b] The retention (infiltration) volume (Rev) required again would therefore be computed as:
Rev = I * impervious area (square feet)/(12 in/ft) = Cubic Feet Eqn: 505-125.2
C. Groundwater recharge for the Tulpehocken Creek Watershed.
(1) The ability to retain and maximize the groundwater recharge capacity of the area being developed is encouraged. Design of the infiltration/recharge stormwater management facilities shall give consideration to providing groundwater recharge to compensate for the reduction in the percolation that occurs when the ground surface is paved and roofed over.
(2) These measures are encouraged, particularly in hydrologic soil groups A and B and should be utilized wherever feasible. Soils used for the construction of basins shall have low-erodibility factors ("K" factors).
(3) Infiltration BMPs shall meet the following minimum requirements:
(a) 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:
[1] A minimum depth of 48 inches between the bottom of the facility and the seasonal high water table and/or bedrock (limiting zones).
[2] An infiltration and/or percolation rate sufficient to accept the additional stormwater load and drain completely as determined by field tests conducted by the owner's professional designer.
(b) Infiltration BMPs receiving only roof runoff may be placed in soils having a minimum depth of 24 inches between the bottom of the facility and the limiting zone.
(c) The size of the recharge facility within the Tulpehocken Creek Watershed shall be based upon the following equation:
Rev = [(S)(Rv)(A)]/12 Eqn: 505-125.4
Where:
Rev = Recharge volume (acre-feet)
S = Soil-specific recharge factor (inches)
Rv = Volumetric runoff coefficient
A = Site area contributing to the recharge facility (acres)
And:
Rv = 0.05 + 0.009 (I)
Where:
I = Percent impervious area
And:
S shall be obtained based upon hydrologic soil group based upon the table below:
Hydrologic Soil Group Soil Specific Recharge Factor (S) | |
A | 0.38 inch |
B | 0.25 inch |
C | 0.13 inch |
D | 0.06 inch |
If more than one hydrologic soil group (HSG) is present at a site, a composite recharge volume shall be computed based upon the proportion of total site area within each HSG.
(d) The recharge volume provided at the site shall be directed to the most permeable HSG available.
(e) The recharge facility shall be capable of completely infiltrating the impounded water within 48 hours.
(f) The recharge facility shall be capable of completely infiltrating the impounded water within 48 hours.
D. Groundwater recharge for the Schuylkill River and Tulpehocken Creek Watersheds.
(1) Soils. A detailed soils evaluation of the project site shall be required where practicable to determine the suitability of infiltration facilities. The evaluation shall be performed by a qualified 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:
(a) Analyze hydrologic soil groups as well as natural and man-made features within the site to determine general areas of suitability for infiltration practices. In areas where development on fill material is under consideration, conduct geotechnical investigations of subgrade stability; infiltration is not permitted to be ruled out without conducting these tests.
(b) Provide field tests such as double ring infiltrometer or hydraulic conductivity tests (at the level of the proposed infiltration surface) to determine the appropriate hydraulic conductivity rate. Percolation tests are not recommended for design purposes.
(c) Design the infiltration structure for the required retention (Rev) volume based on field determined capacity at the level of the proposed infiltration surface.
(d) If on-lot infiltration structures are proposed by the applicant's design professional, it must be demonstrated to the municipality that the soils are conducive to infiltrate on the lots identified.
(2) Carbonate areas.
(a) The applicant is required to investigate the ability of all areas on the site which are not underlain by carbonate rock to meet the infiltration requirements of Subsection B(3) or C(3), as applicable. If this investigation proves infeasible, infiltration can occur on areas underlain by carbonate rock by following the recommended procedure below in conjunction with Figure B-2 in Appendix B.
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However, the applicant is not required to use infiltration in carbonate areas even if the site falls into the "Recommended" range on Figure B-2 in Appendix B. If infiltration is not proposed, the calculated infiltration volume [Subsection B(3) or Subsection C(3)] shall be treated by an acceptable BMP by a facility which prevents the required volume of runoff from being discharged off-site. Acceptable methods for treating this volume are storage and reuse, bioretention, wet ponds, and soil composting. Other methods may be acceptable and all methods should generally follow the design guidelines outlined in the Pennsylvania Stormwater Best Management Practice Manual, latest edition.
(b) Infiltration BMP loading rate percentages in Figure B-2 in Appendix B shall be calculated as follows:
(c) The area tributary to the infiltration BMP shall be weighted as follows:
Area Description | Weighting |
All disturbed area to be made impervious | 100% |
All disturbed areas to be made pervious | 50% |
All undisturbed impervious areas | 100% |
All undisturbed pervious areas | 0%
|
(d) Soil thickness is to be measured from the bottom of any proposed infiltration BMP. The effective soil thickness in Figure B-4 in Appendix B is the measured soil thickness multiplied by the thickness factor based on soil permeability, as follows:
Permeability Range | Thickness Factor |
6.0 to 12.0 inches/hour | 0.8 |
2.0 to 6.0 inches/hour | 1.0 |
1.0 to 2.0 inches/hour | 1.4 |
0.75 to 1.0 inch/hour | 1.2 |
0.5 to 0.75 inch/hour | 1.0
|
(e) Whenever a basin will be located in an area underlain by limestone, a geological evaluation of the proposed location shall be conducted to determine susceptibility to sinkhole formations. The design of all facilities over limestone formations shall include measures to prevent groundwater contamination and, where necessary, sinkhole formation.
(f) It shall be the developer's responsibility to verify if the site is underlain by limestone. The following note shall be attached to all drainage plans and signed and sealed by the developer's engineer/surveyor/landscape/architect/geologist:
_________________________, certify that the proposed detention basin (circle one) is/is not underlain by limestone.
(3) Stormwater hotspots:
(a) Following is a list of examples of designated hotspots. If a site is designated as a hotspot, it has important implications for how stormwater is managed. First and foremost, untreated stormwater runoff from hotspots shall not be allowed to recharge into groundwater where it may contaminate water supplies. Therefore, the Rev requirement shall not be applied to development sites that fit into the hotspot category (the entire WQv must still be treated). Second, a greater level of stormwater treatment shall be considered 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.
(b) Examples of hotspots:
[1] Vehicle salvage yards and recycling facilities.
[2] Vehicle fueling stations.
[3] Vehicle service and maintenance facilities.
[4] Vehicle and equipment cleaning facilities.
[5] Fleet storage areas (bus, truck, etc.).
[6] Industrial sites (based on Standard Industrial Codes).
[7] Marinas (service and maintenance).
[8] Outdoor liquid container storage.
[9] Outdoor loading/unloading facilities.
[10] Public works storage areas.
[11] Facilities that generate or store hazardous materials.
[12] Commercial container nursery.
[13] Other land uses and activities as designated by an appropriate review authority.
(c) The following land uses and activities are not normally considered hotspots:
[1] Residential streets and rural highways.
[2] Residential development.
[3] Institutional development.
[4] Office developments.
[5] Nonindustrial rooftops.
[6] Pervious areas, except golf courses and nurseries [which may need an integrated pest management (IPM) plan].
(d) While large highways [average daily traffic volume (ADT) greater than 30,000] are not designated as a stormwater hotspot; however, it is important to ensure that highway stormwater management plans adequately protect groundwater.
(4) Extreme caution shall be exercised where infiltration is proposed in source water protection areas as defined by the local municipality or water authority.
(5) Infiltration facilities shall be used in conjunction with other innovative or traditional BMPs, stormwater control facilities, and nonstructural stormwater management alternatives.
(6) Extreme caution shall be exercised where salt or chloride (municipal salt storage) would be a pollutant, since soils do little to filter this pollutant and it may contaminate the groundwater. The qualified design professional shall evaluate the possibility of groundwater contamination from the proposed infiltration facility and perform a hydrogeologic justification study if necessary.
(7) The infiltration requirement in high-quality or exceptional value waters shall be subject to the Department's Chapter 93 antidegradation regulations.
(8) An impermeable liner will be required in detention basins where the possibility of groundwater contamination exists. A detailed hydrogeologic investigation may be required by the municipality.
(9) The municipality shall require the applicant to provide safeguards against groundwater contamination for land uses that may cause groundwater contamination should there be a mishap or spill.
(10) Unless otherwise specified in Chapter 600, Zoning, the following setbacks for infiltration facilities shall apply:
(a) One hundred feet from water supply wells.
(b) Ten feet downslope or 100 feet upslope from building foundations.
(c) Fifty feet from septic system drainfields.
(d) Fifty feet from a geologic contact with carbonate bedrock, unless a preliminary site investigation is done in the carbonate bedrock to show the absence of special geologic features within 50 feet of the proposed infiltration area.
(e) One hundred feet from the property line unless documentation is provided to show all setbacks from wells, foundations and drainfields on the neighboring property will be met.
(11) Where pervious pavement is permitted for parking lots, recreational facilities, nondedicated streets, or other areas, pavement construction specifications shall be noted on the plan.
10. Editor's Note: Appendix B is included at the end of this chapter.