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Mr. Vern Pfannenstiel <br />February 11, 2010 <br />holes, the development methods used for restoring aquifer hydraulic properties that <br />may have been altered during drilling, the source and quality (including pH, if <br />available) of slug water, and the range and average of the volumes of water used for <br />the slug tests. <br />12. a) Please provide the following information previously requested in Comment 12 <br />in our letter of August 28, 2009: a discussion of how the hydraulic conductivity <br />and transmissivity values obtained in SCCC's wells compare to the regional <br />values reported by Robson and Stewart (1990) for slug tests and pump tests in <br />wells drilled to depths similar to those of SCCC's wells. The submitted pages <br />2.04-49 and 50, including Figure 2.04.7-F7.1 (Revision 12/17/09) compare <br />SCCC's slug test data to Robson and Stewart's laboratory (permeameter) data. <br />Also, please limit each comparison to data from a similar lithology in the lower <br />member of the Williams Fork Formation. <br />b) Please remove from the permit application Figure 2.04.7-F7.2 and its <br />discussion. This figure and its discussion provides data on the hydraulic <br />properties of a marine beach sandstone unit (the Twentymile Sandstone) which <br />diverges from the topic in this part of the permit application, the hydraulic <br />properties of a mud-rich deltaic/back-barrier stratigraphic sequence (the Wadge <br />overburden, coal, and underburden). The contrast in depositional histories <br />between the beach sandstone unit and the Wadge sequence would have created <br />significant differences in these two hydrostratigraphic units' diagenesis and <br />mechanical properties, and these differences would result in contrasting <br />hydraulic properties. <br />c) Please revise Figure 2.04.7-F7.1 so that it compares only slug test data of SCCC <br />and Robson and Stewart (1990), rather than slug test and permeameter data. The <br />Division does not consider comparison of data from well slug tests and <br />permeameter plugs from the lower member of the Williams Fork to be a valid <br />comparison because of the small volume of permeameter plugs compared to the <br />volume of rock tested in slug tests. Also, revise accordingly all other references <br />or uses of the data in Figure 2.04.7-177.1 elsewhere in the permit application. <br />14. Please replace the calculation of vertical bedrock discharge into Grassy and Little <br />Grassy Creek alluvium with a calculation of bedrock discharge into those streams <br />that uses the same bedrock subcrop seepage area of 78 acres as previously used in <br />the vertical discharge calculation, and a hydraulic conductivity value of 2.5 ft/day <br />(the average of 0.03 and 5 feet per day shown for interburden, SCCC's overburden, <br />in Williams and Clark's Table 4). Also add a statement or two describing expected <br />seasonal variation of this bedrock discharge. The calculation of bedrock discharge <br />as only a "vertical" discharge is not acceptable because it assumes that most of the <br />discharge from bedrock occurs normal to the bedrock bedding plane, which would <br />occur only if the alluvium were deposited on a single uneroded bedding plane. The <br />alluvium probably was deposited on an irregular bedrock erosional surface, resulting <br />in most discharge from bedrock occurring parallel to the bedding plane. <br />Page 3