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<br />„. <br />Memo to Tom Schreiner 3 May I5, 1999 <br />[he Division will not require bonding to fill Cooley Reservoir. However, the Division will require a <br />statement from the applicant describing their plans for filling the reservoir. <br />Slurry Wall Costs <br />Slurry wall installation costs include mobilization and setup, excavation of regoli[h, excavation of the <br />bedrock key, delivery of bentonite and water, mixing of slurry, mixing of soil/bentonite, backfilling of <br />soil/bentonite, clean up and demobilization, and testing and quality control. Specialized equipment is <br />required to excavate deep slurry trenches, and the cost of excavation increases dramatically for trenches <br />deeper than 35 feet and/or if [he bedrock into which the slurry wall will key is hard or otherwise difficult to <br />excavate. Experience with slurry walls in Adams County and statements in the application indicate that the <br />Cooley Reservoir slurry wall costs will not be increased by substantial depth to bedrock or by excessive <br />difficulty excavating the key trench. Even so, the unit cost of $2.32 per square foot provided in the <br />application is too low to adequately assure that the Division will be able to reclaim [he site in the event of <br />bond forfeiture. <br />The following slurry wall cos[ references are illustrative: <br />The U.S. Army Corps of Engineers installed slurry walls in an extensive levee improvement project in <br />[he Pocket area near Sacramento, California. These slurry walls were 30 feet deep with a reported unit <br />installation cost of $5.00 per square foot. <br />The Federal Remediation Technologies Roundtable "Remediation Technologies Screening Matrix and <br />Reference Guide" reports slurry wall installation costs of between $5.00 and $7.00 per square foot. <br />Environmental Protection Agency document EPA 542-R-98-005, "Evaluation of Subsurface Engineered <br />Barriers at Waste Sites." August 1998 reports slurry wall installation costs of between $5.00 and $15.00 <br />per square foot. <br />The costs to install slurry walls at waste containment sites are higher than the costs to line clean water <br />reservoirs using a slurry wall. This is partially due to [he need to conduct chemical compatibility testing and <br />the higher degree of quality control used a[ waste containment sites. However, unless the applicant can <br />rigorously demonstrate that a lower cost is appropriate, the Division should not accept a slurry wall bond of <br />less than $4.00 per square foot for the Cooley Reservoir site. The slurry wall bond, whether it is for <br />installation around [he entire reservoir perimeter or is a 20 percent contingency bond as discussed above, <br />should not be released until the Division of Minerals and Geology and the Office of the State Engineer are <br />satisfied that the leakage criteria of 0.03 ft;/day/ft'.has been me[. <br />Also pertinent to the bond, particularly if a unit cost per square foot is employed in the reclamation cost <br />estimate, is the depth to bedrock used in estimating the extent of the slurry wall required. The application in <br />Exhibit G lists an average depth to bedrock of 25-35 feet and states [hat the upper 3 feet of bedrock is <br />weathered. It is further stated in Exhibit G that the unweathered bedrock is a competent seepage barrier. In <br />Exhibit L it is stated [hat the slurry wall installation cost estimate assumes an average depth to bedrock of 25 <br />feet and an average penetration of the bedrock key trench of 3 feet. In order to assure that sufficient bond is <br />in place for slurry wall installation, based on the information provided in the application, it should be <br />assumed that the average depth to bedrock is 30 feet and that the key trench must penetrate a minimum of 5 <br />