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<br />It appears that the particle size distribution used in the SEDCAD+ model depicts a Loam <br />material, whereas the permit identifies the material as a Clay Loam material. Colowyo may <br />have derived this particle distribution from only one sample collected on site rather than the <br />norm (as depicted on Table 3). The more commonly noted classification of each soil <br />sample is noted above as fine sand 23%, silt 35% and clay 39%. The particle size <br />distribution should represent the specific soil characteristics within each watershed area. <br />The difference in classification could effect the soil erodibiliry factor as well as the settling <br />velocity in the sediment ponds. <br />Colowyo may wish to address this concern in one of the following ways; <br />1. Sample the various soils within each watershed at the mine to estimate an eroded <br />particle size distribution. This option may provide Colowyo with the most accurate <br />results for the model. <br />2. Revise the particle size distribution used in the SEDCAD+ model to reflect the <br />information located in the permit and Map 6. The particle size distribution should <br />be based on the physical data for the soils located in Table 3 and for each watershed <br />boundary. <br />3. Provide the Division further information and documentation that the particle size <br />distribution used in the SEDCAD+ model accurately reflects the soil characteristics <br />at the Colowyo Mine. As previously noted, the information in the permit does not <br />substantiate the particle size distribution used in the SEDCAD+ model. <br />The Division recommends that this issued be addressed prior to finalizing the modeling on <br />the sediment ponds. Please let me know if you have any questions or need any additional <br />information. <br />Sincerely, <br />,~ <br />~~ <br />Erica S. Crosby <br />Environmental Protection Specialist <br />Enclosure <br />c: \wp51 \colowyo\961205 <br />