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turnover in the mine pool, the time between maintenance treatments is likely to be 20 -25 years or <br />more. This consideration is important in terms of reducing long -term maintenance burdens to all <br />stake holders (Criterion 2). <br />Secondly, like mine dewatering /active treatment, in -situ treatment is unlikely to significantly <br />reduce potential health risks due to partial reduction of impacts to water quality in Ralston Creek <br />that could be associated with the mine pool. However, because large amounts of contaminants <br />would not be pumped from the mine pool to the active surface environment, and an expanded <br />industrial water treatment facility and evaporation/sludge retention ponds would not be required, <br />new threats to water quality in the creek (Criterion 1), human health risks (Criterion 3), and <br />environmental impacts (Criterion 4) due to the remedial measure itself, would all be lower <br />relative to mine dewatering /active treatment. <br />Initially, there would be increased industrial traffic to the site to administer in -situ treatments and <br />monitor progress, but within 1 -2 years the need for such traffic would be greatly reduced and <br />generally limited to monitoring of conditions in the mine pool. Although in -situ treatment would <br />involve some increase in peripheral offsite health risks to the general public, they would <br />primarily be limited to physical risks associated with transportation accidents, as opposed to <br />mine dewatering which would involve transportation of hazardous materials. The duration and <br />probability of such risks would be relatively small compared to mine dewatering /active treatment <br />and thus, in -situ treatment rates higher with respect to Criterion 3. <br />Considering the preceding conceptual assessments of in -situ treatment of the mine pool with <br />respect to the four remedial Criteria, individual Criterion ratings for this remedial alternative <br />have been conservatively estimated as follows: <br />Criterion <br />1. Effectiveness = <br />2. Sustainability = <br />3. Health Risks = <br />4. Environmental Impacts = <br />Temporary Pipeline Diversion of the Creek past the Mine Site <br />Rating (and summary rationale) <br />6 (partial mitigation, delay in beneficial effects) <br />6 (reasonable cost effectiveness, relatively low O &M) <br />4 (slight increase in health risks due to the method itself) <br />7 (minimal potential for environmental degradation) <br />Wright Water Engineers, Inc. (WWE) has provided Cotter with a technical basis and conceptual <br />design to construct a temporary pipeline diversion to convey Ralston Creek past the <br />Schwartzwalder Mine site between upper and lower groundwater cutoff walls. This strategy will <br />serve several purposes including: 1) completely isolate the creek from all potential sources of <br />water quality impacts, 2) prevent offsite excursions of impacted groundwater, and 3) dewater the <br />alluvial fill so that necessary remedial measures to permanently address all potential sources of <br />impacts to the creek can be identified, assessed and implemented. <br />Because this strategy is expected to provide immediate and complete protection of water quality <br />in Ralston Creek while other mitigation measures are implemented, it rates the highest of all <br />remedial alternatives for preventing any potential impacts from the mine pool in the short term. <br />However, because this measure is temporary, even though the pipeline can remain in place until <br />8 <br />