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Ctown Jewel Project June 25, 1992 Page 3 current suppositions as to the nature of the emplaced slimes is indexed representative of actual field conditions. Closure - After filling of the impoundment, the area will be capped and graded as necessary to facilitate runoff. Ideally, this will prevent rewet:ting of the slimes thereby minimizing their future liquefaction susceptibility and the volume of leachate that must be treated. Normally, the operator monitors the facility for several years following closure. Thereafter, responsit~ility for maintenance and operation of the facility normally reverts to the State and Federal governments. DSS RATIONALE FOR REJECTING THE PROPOSED UPSTREAM CONSTRUCTION EMBANKMENT SCHEME OVER A MORE CONVENTIONAL EMBANKMENT APPROACH From the DSS's perspective there are three questions that need to ba assessed in determining the appropriateness of a particular embankment construction scheme. What aze the consequences of a failure? Does the proposed design scheme provide a level of reliability that makes a failure appropriately remote given the damage potential? During construction can it be satisfactorily demonstrated by field testing that key parameters related to the stability of the structure equal or exceed design assumptions? f~hat are the consequences of failure? - This project poses a number of difficult issues in estimating the character of the breach, the volume of slime released and the likely extent of slime spreading. A primary issue in this assessment is the motility of the slimes. This will be a function of the degree of saturation of the slimes and the potential for any free water to flow towards the breach in the embankment and entrain slimes. As a minimum, a failure would likely spread slimes several hundreds of feet downstream of the embankment. This would pose an environmental hazard in the form of muddying nearby water courses and concerns of freeing the small residual of cyanide to the environment. The mine operation likely would be halted for a protracted period as it would be a major undertaking to reconstruct the impounding barrier. As a practical matter it woul~. probably be necessary to construct a new dam immediately downstream of the failed section. This would involve an extensive cleanup of the slimes within the footprint of the new dam to properly found it. This cleanup and reconstruction effort would likely be plagued by continuing movements of the slimes from the breached section of the original dam. Furthermore, the likelihood of such a failure is greatest as the impoundment approaches its capacity as the depth of slimes reaches a maximum. In the event of a failure at this time the mine operator would face large costs to restore ttte integrity of the impoundment bu[ with only a minimal prospect for future economic returns. This expenditure would be shortly followed by the costs <essociated with closure of the facility and probably legal fees arising from damage claims. Although a remote possibility, there is the prospect that the operator may not have the resources nor the bonding to cover such costs. State and federal agencies may become, by default, the parties fors:ed to fund large parts of any cleanup and closure activities.