Section 434.63 - Effluent limitations for precipitation events(a)(1) The alternate limitations specified in paragraph (a)(2) of this section apply with respect to: (i) All discharges of alkaline mine drainage except discharges from underground workings of underground mines that are not commingled with other discharges eligible for these alternate limitations;(ii) All discharges from steep slope areas, (as defined in section 515(d)(4) of the Surface Mining Control and Reclamation Act of 1977, as amended (SMCRA)), and from mountaintop removal operations (conducted pursuant to section 515(c) of SMCRA);(iii) Discharges from coal preparation plants and preparation plant associated areas (excluding acid or ferruginous mine drainage from coal refuse disposal piles).(2) Any discharge or increase in the volume of a discharge caused by precipitation within any 24 hour period less than or equal to the 10-year, 24-hour precipitation event (or snowmelt of equivalent volume) may comply with the following limitations instead of the otherwise applicable limitations: Effluent Limitations During Precipitation
| Pollutant or pollutant property | Effluent limitations |
| Settleable solids | 0.5 ml/1 maximum not to be exceeded. |
| pH | 6.0-9.0 at all times. |
(b) The following alternate limitations apply with respect to acid or ferruginous drainage from coal refuse disposal piles: Any discharge or increase in the volume of a discharge caused by precipitation within any 24 hour period greater than the 1-year, 24-hour precipitation event, but less than or equal to the 10-year, 24-hour precipitation event (or snowmelt of equivalent volume) may comply with the following limitations instead of the otherwise applicable limitations:
Effluent Limitations During Precipitation
| Pollutant or pollutant property | Effluent limitations |
| Settleable solids | 0.5 ml/1 maximum not to be exceeded. |
| pH | 6.0-9.0 at all times. |
(c) The following alternate limitations apply with respect to acid or ferruginous mine drainage, except for discharges addressed in paragraphs (a) (mountaintop removal and steep slope areas), (d) (controlled surface mine discharges) and (f) (discharges from underground workings of underground mines) of this section: (1) Any discharge or increase in the volume of a discharge caused by precipitation within any 24 hour period less than or equal to the 2-year, 24-hour precipitation event (or snowmelt of equivalent volume) may comply with the following limitations instead of the otherwise applicable limitations: Effluent Limitations During Precipitation
| Pollutant or pollutant property | Effluent limitations |
| Iron, total | 7.0 mg/l maximum for any 1 day. |
| Settleable solids | 0.5 ml/l maximum not to be exceeded. |
| pH | 6.0-9.0 at all times. |
(2) Any discharge or increase in the volume of a discharge caused by precipitation within any 24 hour period greater than the 2-year, 24-hour precipitation event, but less than or equal to the 10-year, 24-hour precipitation event (or snowmelt of equivalent volume) may comply with the following limitations instead of the otherwise applicable limitations: Effluent Limitations During Precipitation
| Pollutant or pollutant property | Effluent limitations |
| Settleable solids | 0.5 ml/l maximum not to be exceeded. |
| pH | 6.0-9.0 at all times. |
(d)(1) The alternate limitations specified in paragraph (d)(2) of this section apply with respect to all discharges described in paragraphs (a), (b) and (c) of this section and to:(i) Discharges of acid or ferruginous mine drainage from underground workings of underground mines which are commingled with other discharges eligible for these alternate limitations; and(ii) Controlled acid or ferruginous surface mine discharges; and(iii) Discharges from reclamation areas.(2) Any discharge or increase in the volume of a discharge caused by precipitation within any 24 hour period greater than the 10-year, 24-hour precipitation event (or snowmelt of equivalent volume) may comply with the following limitations instead of the otherwise applicable limitations: Effluent Limitations During Precipitation
| Pollutant or pollutant property | Effluent limitations |
| pH | 6.0-9.0 at all times. |
(e) The operator shall have the burden of proof that the discharge or increase in discharge was caused by the applicable precipitation event described in paragraphs (a), (b), (c), and (d) of this section.(f) Discharges of mine drainage from underground workings of underground mines which are not commingled with discharges eligible for alternate limitations set forth in this section shall in no event be eligible for the alternate limitations set forth in this section.