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.; . °°: '1-1 P. III IIIIIIIIIIIIIIII 999 kerr coal company ~. box 487 ~ Walden, Colorado 8048[1 ~ (303) 7234721 Date: ~ FiE.GF!~~~D MAY 10 1993 Ms. Anne Ihlenfeldt Colorado Department of Health Division ~~ im~ae:a,o .......,~„yy Water Quality Con[rol Division 4210 Eazt 11th Avenue Denver, Colorado 80220 Re: Kerr Coal Company - Marr Mine (Permit \o. COG8500Z1), Request for Stormwater Exemption Dear Ms. Ihlenfeldt: Kerr Coal Company (Kerr) opera[es the Mazr Mine, located approximately fourteen miles east of the town of Walden, Colorado. Drainage and sediment control for the mining related disturbance is provided bq a number of engineered sedimentation ponds, collection diversion ditches, and associated drainage structures. Discharge from the sedimentation ponds is controlled and regulated under applicable provisions of Permit COG850021. Discharge for the twelve individual ourfalls under this permit comes exclusively from either preapitation or snowmel[ runoff, with no process water discharge. All sedimentation ponds are designed and maintained to provide a minimum detention time of at leazt 24 hours for retained runoff resulting from the 10 year, 24 hour storm event. While the ponds are designed [o provide adequate detention time to allow settlement of ruspended 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 ruspended 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 precipitation runoff, the noted fattors combine to limit the trapping efficiency of the exining sedimentation ponds with the result that Total Suspended Solids (TSS) and Settleable Solids concentrations in pond discharge may exceed app.li ble effluent limitations. The accompanying monitoring repon(s) for pond(s) ~-f-"'~ 9`- ~c' i~TsS}diate eaceedance of TSS and/or Settleable Solids limitations for the corresponding monttoring period. Eaceedance of applicable permi[ effluent limitations resulted from: ^ Discharge from rainfall runoff occurring within 48 hours after measurable precipitation has slopped Discharge from snowmelt runoff occurring within 48 hours after runoff inflows to the pond(s) have stopped Documentation of precipitation and snowmelt occurrence and snowmelt inflows to the ponds is retained a[ the minesite and is available for inspection by authorized agency representatives.