Section 258.23 - Explosive gases control(a) Owners or operators of all MSWLF units must ensure that: (1) The concentration of methane gas generated by the facility does not exceed 25 percent of the lower explosive limit for methane in facility structures (excluding gas control or recovery system components); and(2) The concentration of methane gas does not exceed the lower explosive limit for methane at the facility property boundary.(b) Owners or operators of all MSWLF units must implement a routine methane monitoring program to ensure that the standards of paragraph (a) of this section are met. (1) The type and frequency of monitoring must be determined based on the following factors: (ii) The hydrogeologic conditions surrounding the facility;(iii) The hydraulic conditions surrounding the facility; and(iv) The location of facility structures and property boundaries.(2) The minimum frequency of monitoring shall be quarterly.(c) If methane gas levels exceeding the limits specified in paragraph (a) of this section are detected, the owner or operator must:(1) Immediately take all necessary steps to ensure protection of human health and notify the State Director;(2) Within seven days of detection, place in the operating record the methane gas levels detected and a description of the steps taken to protect human health; and(3) Within 60 days of detection, implement a remediation plan for the methane gas releases, place a copy of the plan in the operating record, and notify the State Director that the plan has been implemented. The plan shall describe the nature and extent of the problem and the proposed remedy.(4) The Director of an approved State may establish alternative schedules for demonstrating compliance with paragraphs (c) (2) and (3) of this section.(d) For purposes of this section, lower explosive limit means the lowest percent by volume of a mixture of explosive gases in air that will propagate a flame at 25 °C and atmospheric pressure.(e) The Director of an approved State may establish alternative frequencies for the monitoring requirement of paragraph (b)(2) of this section, after public review and comment, for any owners or operators of MSWLFs that dispose of 20 tons of municipal solid waste per day or less, based on an annual average. Any alternative monitoring frequencies established under this paragraph must:(1) Consider the unique characteristics of small communities;(2) Take into account climatic and hydrogeologic conditions; and(3) Be protective of human health and the environment.56 FR 51016, Oct. 9, 1991, as amended at 62 FR 40713, July 29, 1997