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C~ • • Methods The soil erosion analysis was conducted using the SEDCAD+, Version 3.0 soft- ware package. Information required to run the program was collected from a variety of sources. These sources included the permit document, field sampling, laborato- ry analysis, published literature, and pro- fessional judgment. This section describes the information that was used in the SEDCAD analyses. Topography of the disturbed mine area was acquired for both the pre-mining and post-miningsituations.Pre-miningtopogra- phy was based on the USGS quad sheets (Figure 1). The post-mining topography was based on a survey after reclamation activities in the Fall of 1989 (Figure 2). The extent of mining disturbance was located on this map during the 1989 sur- vey. Topography from the pre and post-mining base maps was used to divide the water- sheds into subwatersheds containing dis- turbed areas and non-disturbed areas. These sub-watersheds were used to esti- mate time-of-concentration routes, repre- sentative slope lengths, and average slopes. SEDCAD's digitizing capabilities were used to measure vertical and hori- zontal distances and slopes. The area of each watershed was planimetered. The sediment analyses were based on a 10-year, 24hour precipitation event. Ac- cording to "Surface Water Runoff Specifi- cations" in the permit document, a 10 year 24 hour event will produce 3.0 inches of precipitation over the mine site. This single event has been requested by CMLRD during previous studies for sedi- ment analyses . The soil partial distn'bution for pre and post mining came from laboratory test results. Premining soil distributions were based on the Fort Collins and Olney loams soil types. Partial size distributions were weighted, based on the percentage of each soil type present in the individual watersheds. Table 1 indicates the weighted soil distri- butions for individual watersheds used in the sediment analysis. For post mining conditions, several soil samples were collected from the sediment ponds and upland reclaimed so0 and ana- lyzed according to the method described in the SEDCAD manual. The pond partial size distribution data provided a higher percentage in the erodible fraction as compared to the upland soil. For a worst case condition, the pond sediment distribu- tion was used across the entire watershed. To compare the three watershed for pre and post mining, the watershed boundaries remained constant for pre and post-min- ing. This was comparison could not be correctly compared on a watershed basis with out this common reference area. For pre-mining watersheds, soil types, hydrologic groupings, and k factors were determined using the information available and SCS's Procedures for Determining Peak Flows in Colorado (1980). The final Curve Numbers and k factor used in the analyses were determined by weighing the 2