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<br /> <br />case scenario where the slurry wall has been installed and the pit has been mined out, but it is <br />determined [hat [he slurry wall leaks in excess of 3x0.03 ft'/day/ft'`. Another consideration that <br />enters into bonding for this worst case scenario is the potential for leakage into the pit through the <br />bedrock pit Floor. Unless the applicant can provide a geological evaluation of the proposed pit <br />Floor bedrock that demonstrates [ha[ leakage will not occur, the Division should bond for sealine <br />fractured or sandy bedrock that may be uncovered during mining and that may Peak in excess of <br />es[ablislted criteria (3x0.0015 ft~/day/ft'). <br />Reservoir Fillin <br />Past practices by the Division in permitting lined reservoirs included a requirement to provide <br />bond sufficient to purchase enough water from a reliable source to fill the reservoir one time. <br />Numerous gravel pits have been reclaimed or are proposed to be reclaimed as lined storage <br />reservoirs since the passage of Senate Bill 120 in 1989. It has become clear that there is a great <br />demand for lined storage in over appropriated basins and in particular [he South Platte basin. so it <br />is no longer a substantial concern to the Division that lined reservoirs will not be filled and put to <br />their intended beneficial use. In the worst case, virtually any reservoir could be filled eventually <br />by using junior water rights in priority. For these reasons, the Division will not require bonding <br />to fill the reservoirs. However, the Division will require a statement from the applicant <br />describing their plans for felting the reservoir. <br />Slurry Wal{ Costs <br />Slum wall installation costs include geotechnical investigation and testing, design and quality <br />control, mobilization and setup, excavation of regolith, excavation of the bedrock key, delivery of <br />bentonite and water, mixing of slurry, mixing of soil/bentonite, backfilling of soil/bentonite, <br />clean up and demobilization, and testing and quality assurance. Specialized equipment is <br />required to excavate deep slurry trenches, and [he cost of excavation increases dramatically for <br />trenches deeper than 35 feet and/or if [he bedrock into which the slurry wall will key is hard or <br />otherwise difficult to excavate. Experience with slurry walls in Weld County and statements in <br />the application indicate that the Ft. Lupton Sand and Gravel Mine slurry wall costs will not be <br />substantially increased by depth to bedrock or by excessive difficulty excavating the key trench. <br />The following slurry wall cost references is illustrative <br />The U.S. Army Corps of Engineers installed slurry walls in an extensive levee improvement <br />project in the Pocket area near Sacramento, California. These slurry walls were 30 feet deep <br />with a reported unit installation cost of $5.00 per square toot. <br />The Federal Remedia[ion Technologies Roundtable "Remediation Technologies Screening <br />Matrix and Reference Guide" reports slurry wall installation costs of between $x.00 and <br />$7.00 per square foot. <br />• Environmental Protection Agency document EPA 542-R-98-005, `'Evaluation of Subsurface <br />Engineered Barriers at Waste Sites;' August 1998 reports slurry wall installation costs of <br />between $5.00 and $15.00 per square foot. <br />