§ 88.188. Hydrologic balance: precipitation event exemption.
(a) To establish the alternative effluent limitations of Group B or C in § 88.187(a) (relating to hydrologic balance: effluent standards), a permittee shall demonstrate to the Department’s satisfaction that a precipitation event has occurred, under the procedures in this section.
(1) The occurrence of a precipitation event greater than a 10-year, 24-hour precipitation event may be demonstrated by meeting the requirements of subsections (b)—(d) for each discharge that exceeds the effluent limits specified in § 88.187, unless the permit specifies a more stringent water quality based effluent limitation, in which case no exemption will be available under this section. If the permittee demonstrates to the Department’s satisfaction that a greater than 10-year, 24-hour precipitation event has occurred, the permittee shall meet the effluent limitation enumerated as Group C in § 88.187(a).
(2) The occurrence of a precipitation event equal to or less than a 10-year, 24-hour precipitation event may be demonstrated by meeting the requirements of subsections (c) and (d) for each discharge that exceeds the effluent limits specified in § 88.187, unless the permit specifies a more stringent water quality based effluent limitation, in which case no exemption will be available under this section. If the permittee demonstrates to the Department’s satisfaction that a precipitation event equal to or less than a 10-year, 24-hour precipitation event has occurred, the permittee shall meet the effluent limitations of Group B in § 88.187(a).
(b) The 10-year, 24-hour precipitation event for specific areas in this Commonwealth shall be determined by reference to data provided by the National Oceanic and Atmospheric Administration or equivalent resources.
(c) For the permittee to demonstrate that the 10-year, 24-hour precipitation event has for the mine area been exceeded, or that dry weather flow conditions did not exist, the permittee shall do one of the following:
(1) Collect 24-hour rainfall information from official United States Weather Bureau Stations within a 25-mile distance (radius) of the site.
(2) Calculate the estimated rainfall event for the site, by appropriate interpolation of the data collected under paragraph (1). Appropriate interpolation shall be accomplished by:
(i) Construction of an isohyetal map in accordance with the guidelines established by the Department.
(ii) Linear interpolation between the isohytes.
(3) Prepare a verified copy of the chart or readout from a Department-approved flow measuring device which continuously records the influent to the permitted treatment facility. The device shall be approved by the Department in writing prior to the event for which the exemption is sought and shall be secure to prevent tampering and acts of third parties.
(4) Prepare an analysis identifying the runoff area tributary to the treatment facility and compare the actual runoff as measured and depicted by the flow measuring device with the runoff expected from the 10-year, 24-hour precipitation event specified for the mine area.
(5) Develop alternative documentation or data concerning the precipitation event. The method or system for developing the documentation or data shall be approved in writing prior to the occurrence of the event for which the exemption is being sought, and shall guarantee the integrity of the information collected.
(d) When the discharge from the site exceeds any effluent limit in the permit, the permittee shall notify the Department within 5 days of the occurrence of the event that he is applying for an exemption from that limit and shall within 30 days thereafter provide the following to the Department:
(1) The data required by subsection (c).
(2) A showing that the facility from which the discharge occurred was designed, maintained and operated during and prior to the event to accommodate or treat a 10-year, 24-hour precipitation event.
(e) The permittee is not entitled to claim a greater than 10-year, 24-hour precipitation event storm exemption unless the permittee has complied with subsections (c) and (d).
(f) Nothing in this section authorizes the Department to grant an exemption for a discharge which the Department finds may have caused or contributed to a violation of general or specific water quality criteria in Chapter 93 (relating to water quality standards).
Authority The provisions of this § 88.188 amended under section 5 of The Clean Streams Law (35 P.S. § 691.5); sections 4(a) and 4.2 of the Surface Mining Conservation and Reclamation Act (52 P.S. § § 1396.4(a) and 1396.4b); section 3.2 of the Coal Refuse Disposal Control Act (52 P.S. § 30.53b); section 7(b) of The Bituminous Mine Subsidence and Land Conservation Act (52 P.S. § 1406.7(b)); and section 1920-A of The Administrative Code of 1929 (71 P.S. § 510-20).
Source The provisions of this § 88.188 adopted December 19, 1980, 10 Pa.B. 4789, effective July 31, 1982, 12 Pa.B. 2382; amended March 30, 1984, 14 Pa.B. 1143, effective August 10, 1985, 15 Pa.B. 2872; amended June 15, 1990, 20 Pa.B. 3383, effective July 27, 1991, 21 Pa.B. 3316; amended March 13, 2020, effective March 14, 2020, 50 Pa.B. 1508. Immediately preceding text appears at serial pages (383995) to (383997).
Cross References This section cited in 25 Pa. Code § 88.187 (relating to hydrologic balance: effluent standards).
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