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Hydrologic Impact Assessment <br />La Plata Mine, Permit C-87-072 <br />Proposed Release of Liability <br />Prepared by Tom Kaldenbach <br />Environmental Protection Specialist <br />Colorado Division of Minerals and Geology <br />April, 2004 <br />Premining versus Postminin2 Sediment Yield <br />One requirement for release of a reclamation liability is that the reclaimed area not yield more <br />suspended solids to offsite surface water than it did prior to mining (Rule 3.03.1(6)). An alternative <br />to that requirement is that the reclaimed area not yield more suspended solids than adjacent <br />nonmined land. Approximately 10 acres of land surface were disturbed and reclaimed at the La Plata <br />Mine site. The premining sediment yield of this disturbed azea is unknown because the area was <br />disturbed prior to the enactment of the Surface Mining Control and Reclamation Act (SMCRA). <br />(Mining began more than 40 years ago.) <br />A chief cause of soil loss is transport of solids in suspension; therefore, calculated soil loss rates can <br />be valid indicators of the premining and postmining yields of suspended solids. In order to compaze <br />premining and postmining soil loss from the disturbed azea, the Division has calculated premining <br />and postmining soil loss rates using the Universal Soil Loss Equation (LISLE). The LISLE is the <br />product of the following five factors: rainfall-runoff (R), soil erodibility (K),slope-length-steepness <br />(LS), cover-management (C), and erosion control practice (P). Only the cover (C) factor is vazied <br />between the Division's pre- and post-mining calculations; all other variables are held constant. The <br />premining and postmining C-factors used in the calculations both assume cover at the surface is <br />grass, rather than weeds. A premining C-factor of 0.038 is used, based on 50% canopy cover and <br />60% ground cover contacting the ground. A postmining C-factor of 0.014 is used, based on no <br />appreciable canopy, with 69% ground cover contacting the ground. (DMG found grass and litter <br />cover totaled 69% during its 2003 Vegetation Sampling study conducted at the La Plata Mine.) The <br />calculated premining soil loss rate is 1.37 tons/acre/year (40R X 0.30K X 3.O~s X 0.038c X I.OP = <br />1.37 tons/acre/year). The calculated postmining soil loss rate is 0.5 tons/acre/year (40R X 0.30K X <br />3.Os }: 0.014c X O1.OP = 0.5 tons/acre/year). The premining rate is less than the postmining rate, <br />thus indicating postmining soil loss (and sediment yield) is no more than premining conditions. The <br />postmining rate of 0.5 tons/acre/yeaz is a moderate soil loss rate for steep-sloped rangeland in <br />southwest Colorado. <br />The Division's field observations during regular inspections have corroborated the calculated low <br />sediment yield from the La Plata site. The only gullying on the site has been at a head-cut on the <br />steep-sloped drainage, downhill from the former portal bench. This erosional knickpoint appears to <br />be a stable feature as the it does not appear to be migrating up the drainage. DMG inspections have <br />noted the gully as being present for several yeazs. Sediment fans or deltas are not present downslope <br />