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eventually by using junior water rights in priority. For these reasons,.the Division will not <br />require bonding to fill reservoirs for operations along the Front Range. However, the Division <br />will require a statement from the applicant describing their conceptual plan for filling the <br />reservoir. <br />Slurrv Wall Costs <br />Slurry 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 <br />of bentonite and water, mixing of slurry, mixing of soil/bentonite, backfrlling of soinentonite, <br />clean up and demobilization, and testing and quality assurance. Specialized equipment is <br />required to excavate deep slurry trenches, and the cost of excavation increases dramatically for <br />trenches deeper than 35 feet and/or if the bedrock into which the slurry wall will key is hard or <br />otherwise difficult to excavate. However slurry wall costs may or may not be substantially <br />increased by depth to bedrock or by excessive difficulty in excavating the key trench, depending <br />on mine site location. <br />The following slurry wall cost references are 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 foot. <br />• The Federal Remediation Technologies Roundtable "Remediation_ Technologies Screening <br />Matrix and Reference Guide" reports slurry wall installation costs of between $5.00 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 />The costs to install slurry walls at waste containment sites are higher than the costs to line <br />clean water reservoirs using a slurry wall. This is partially due to the need to conduct <br />chemical compatibility testing and the higher degree of quality control used at waste <br />containment sites. <br />In a recent case, the applicant proposed a unit cost of $3.00 per sq. ft for slurry wall installation, <br />and based on our review of recent bids for slurry walls around gravel pits on the Front Range, <br />this is believed to be appropriate at this point in time. The slurry wall bond, whether it is for <br />installation around the entire reservoir perimeter or is a 20 percent contingency bond as <br />discussed above, should not be released until DMG is provided written confirmation from the <br />OSE that the specified leakage criteria has been met. <br />As stated above, it is pertinent to the bond amount that the depth to bedrock used in estimating <br />the extent of the slurry wall required is accurately determined and that nature of the bedrock is <br />investigated. The applicant should provide information from bore holes at the site showing the <br />depth to gravel and the type of bedrock present, any variability in the bedrock encountered, and <br />the depth of weathering in the bedrock. It is implicit to the proposed slurry wall plan that the <br />bedrock is a competent seepage barrier, and this is most likely to be the case. However, if <br />fractured zones, sandy lenses or layers, or deeply weathered bedrock are present, pit floor lining <br />or a deeper bedrock keyway for the slurry wall may be required. Either solution would increase