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~' ' III Illlllllllllllll 999 kerr coal company box 487 ~ Walden, Colorado 8CW80 ~ 1303) 7234721 or.,t-.~t7Gr~ Date'~~ 3 MAY 2 4 1993 Ms. Anne Ihlenfeldt Colorado Department of Health !)i~~'""' "• ••• .. _ Water Quality Control Division -"`y~ 4210 East 11th Avenue Denver, Colorado 80220 Re: Kerr Coal Company -Marr Mine (Permit No. COG850021), Request for Stormwater Exemption Dear Ms. llilenfeldt: Rerr Coal Company (Kerr) operates the Marr Mine, located approximately foutteen 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 associated drainage structures. Discharge from the sedimentation ponds is controlled and regula[ed under applicable provisions of Permit COG85002I. Discharge for the twelve individual oudalls under this permit comes exclusively from either predpitation or snowmelt runoff, with no process water discharge. All sedimentation ponds aze designed and maintained to provide a minimum detention time of at least 24 hour for retained runoff resul[ing from the 10 year, 24 hour norm event. While the ponds are designed to provide adequate detention time to allow settlement of suspended solids, due to the charaaeristia of the dominant soil types in this azea both undisturbed and disturbed area runoff typically have relatively high levels of suspended solids. In addition, a relatively high percentage of the ruspended solids in Stormwater runoff are fine colloidal materials which do not readily settle out of suspension. During periods of high flow due to either snowmelt or predpitation runoff, the noted factors combine to limit the trapping efficienry of the eaistiag sedimentation ponds with the result that Toral Suspended Solids (TSS) and Settleable Solids concentrations in pond discharge may exceed applicable effluent limiutions. The accompanying monitoring repon(s) for pond(s) ~y5~/~~ indicate eaceedance of TSS and/or Settleable Solids limitations for the wrresponding monitoring period. Eaceedance of appliable permit effluent limitations resulted from: ^ Discharge from rainfall runoff occurring within 48 hours after measurable predpitation has stopped ~l Discharge from snowmelt runoff occurring within 48 hours after runoff inflows to the pond(s) have stopped Documentation of preapitation and snowmelt omtrrence and snowmelt inflows to the ponds is retained at the minesite and is available for inspection by authorized agency representatives.