(a) Providing for infiltration consistent with the natural hydrologic regime is required. Design of the infiltration facilities shall consider infiltration to compensate for the reduction in the recharge that occurs when the ground surface is disturbed or impervious surface is created.
(b) (1) If it cannot be physically accomplished, then the design professional shall be responsible for demonstrating to the satisfaction of the municipality that this cannot be physically accomplished on the site (e.g., shallow depth to bedrock or limiting zone, open voids, steep slopes and the like versus a financial hardship as defined in § 1043.11). If it can be physically accomplished, the volume of runoff to be infiltrated shall be determined from subsection (c)(2) below depending on demonstrated site conditions, and shall be the greatest volume that can be physically infiltrated.
(2) For example:
A. Any applicant (developer or redeveloper) shall first attempt to infiltrate the volume required in subsection (c)(2)A. below;
B. If the subsection (c)(2)A. below requirement cannot be physically accomplished, then the applicant is required to attempt to infiltrate the volume required in (c)(2)B. below;
C. Finally, if the subsection (c)(2)B. below infiltration volume cannot be physically accomplished, the applicant must, at a minimum, infiltrate the volume required in subsection (c)(2)C. below.
(c) Infiltration BMPs shall meet the following minimum requirements.
(1) Infiltration BMPs intended to receive runoff from developed or redeveloped 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 24 inches between the bottom of the BMP and the top of the limiting zone;
B. An infiltration rate sufficient to accept the additional stormwater volume and dewater completely as determined by field tests conducted by the applicant’s design professional; and
C. The infiltration facility shall be capable of completely draining the retention (infiltration) volume (Rev) within three days (72 hours) from the end of the design storm.
(2) The size of the infiltration facility and Rev shall be based upon the following volume criteria.
A. Modified Control Guideline One (MCG-1) of the Pennsylvania Stormwater Best Management Practices (PA BMP) Manual. The retention (infiltration) volume (Rev) to be captured and infiltrated shall be the net two-year 24-hour volume. The net volume is the difference between the post-development runoff volume and the pre-development runoff volume. The post-development total runoff volume for all storms equal to or less than the two-year 24-hour duration precipitation shall not be increased. For modeling purposes, existing (pre-development) non-forested pervious areas must be considered meadow in good condition or its equivalent, and 20% of existing impervious area, when present, shall be considered meadow in good condition.
B. 1. Infiltrating the entire Rev volume in subsection (c)(2)A. (above) may not be feasible on every site due to site-specific limitations such as shallow depth to bedrock or the water table. If it cannot be physically accomplished, then the following criteria from Modified Control Guideline Two (MCG-2) of the PA BMP Manual must be satisfied.
2. At least the first one-inch of runoff from new or replacement impervious surfaces shall be infiltrated.
Rev= 1 (inch) * impervious area (square feet) + 12 (inches) = cubic feet (cf) |
An asterisk (*) in equations denotes multiplication. |
C. 1. Only if infiltrating the entire Rev volume in subsection (c)(2)B. above cannot be physically accomplished, then the following minimum criteria from Modified Control Guideline Two (MCG-2) of the PA BMP Manual must be satisfied:
2. a. Wherever possible, infiltration facilities should be designed to accommodate infiltration of the entire water quality volume (WQv) (§ 1043.17(a)); however, in all cases at least the first one-half inch of the WQv shall be infiltrated. The minimum infiltration volume (Rev) required would, therefore, be computed as:
Rev= I * impervious area (square feet) + 12 (inches) = cubic feet (cf) |
An asterisk (*) in equations denotes multiplication. |
Where: |
I= The maximum equivalent infiltration amount (inches) that the site can physically accept or 0.50 inch, whichever is greater. |
b. The retention volume values derived from the methods in subsections (c)(2)A., (c)(2)B. or (c)(2)C. above is the minimum volume the applicant must control through an infiltration BMP facility. If site conditions preclude capture of runoff from portions of the impervious area, the infiltration volume for the remaining area should be increased an equivalent amount to offset the loss.
c. Only if the minimum of one-half inch of infiltration requirement cannot be physically accomplished, a waiver from this section is required from the municipality.
(d) Soils: a detailed soils evaluation of the project site shall be required to determine the suitability of infiltration facilities. The evaluation shall be performed by a qualified design professional and at 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 human-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 sub-grade stability; infiltration may not be ruled out without conducting these tests;
(2) 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;
(3) Design the infiltration structure for the required retention (Rev) volume based on field determined capacity at the level of the proposed infiltration surface; and
(4) 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.
(e) Infiltration facilities should, to the greatest extent practicable, be located to avoid introducing contaminants via groundwater, and be in conformance with an approved source water protection assessment or source water protection plan.
(f) Roadway drainage systems should provide an opportunity to capture accidental spills. Road de-icing material storage facilities shall be designed to avoid salt and chloride runoff from entering waterways and infiltration facilities. The qualified design professional shall evaluate the possibility of groundwater contamination from the proposed infiltration facility and perform a hydrogeologic justification study if necessary.
(g) The antidegredation analysis found in Pa. Code Ch. 93.4 et seq. shall be applied in HQ or EV streams.
(h) An impermeable liner will be required in detention basins where the possibility of groundwater contamination exists. The municipality may require a detailed hydrogeologic investigation.
(i) The applicant should provide safeguards against groundwater contamination for land uses that may cause groundwater contamination should there be a mishap or spill.
(Ord. 928, passed 6-18-2014)