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'"'` i III IIIIIIIIIIIII III 999 kerr coal company L6ox 987 ~ Walden, Colorado 8048D ~ 13D31 723-4727 RECEIVED Date• JUN 10 1993 Ms. Anne Ihlenfeldt Colorado Department of Health Division vi Minerals b Geoloyy Water Quality Control Division 4210 East 11th Avenue Denver, Colorado 80220 Re: Y,err Coal Company - Mazr Mine (Pe :snit I~ro. COG850021), Request fer Stormwater Exemption Deaz Ms. Ihlenfeldt: Kerr Coal Company (Kerr) operates the Marr Mine, looted approximately fourteen miles east of the town of Walden, Colorado. Drainage and sediment control for the mining related disturbance is provided by a number of engineered sedimentation ponds, collection diversion ditches, and assodated drainage structures. Discharge from the sedimentation ponds is controlled and regulated under applicable provisions of Permit COG850021. Discharge for the twelve individual outfalls under this permit comes eadusively from either predpitatioa or snowmelt runoff, with no process water discharge. All sedimentation ponds are designed and maintained to provide a minimum detention time of at least 24 hours for retained runoff resulting from the 10 year, 24 hour storm event. While the ponds aze designed to provide adequate detention time to allow settlement of suspended solids, due to the characteristic of the dominant soil types in this area both undisturbed and disturbed area runoff typically have relatively high levels of suspended solids. In addition, a relatively high percentage of the suspended solids in stormwater ntnoff are fine colloidal materials whirh do not readily settle out of suspension. During periods of high flow due to either snowmelt or predpitation runoff, the noted factors combine to fimit the trapping effidency of the existing sedimeatadon ponds with the result that Total Suspended Solids (TSS) and Settleable Solids concentrations in pond discharge may~ea appliable effluent limitations. The accompanying monitoring report(s) for pond(s) -~~v , l„} indicate eaceedance of TSS and/or Settleable Solids limitations for the corresponding monitoring period. Ezceedance of applicable permit effluent limitadons resulted from: ^ Dischazge from rainfall runoff oaurring within 48 hours after measurable precipitation has stopped Dischazge from snowmelt runoff occurring within 48 hours after runoff inflows to the pond(sj have stopped Documentation of precipitation and snowmelt occurrence and snowmelt inflows to the ponds is retained at the minesite and is available for inspection by authorized agenry representatives.