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Environmental Protection Plan, Schwartzwalder Mine 15-8 <br />o Remove the front and the back of the cofferdam in order to allow groundwater to flow through the <br />gate. <br />The preliminary design assumed that ZVI would be mixed with pea gravel to improve hydraulic <br />conductivity and prevent premature plugging of the gate. A pretreatment ;zone (PTZ) consisting of intert <br />granular media (sand or pea gravel) or coarse reactive media may be constructed directly upgradient of the <br />gate to improve system performance. The high permeability PZT improves hydraulic distribution of the <br />inflowing groundwater across the face of the PRB, and allows a zone for precipitation to occur ahead of the <br />PRB. The need for a PZT would be further investigated and determined in the final design. <br />Periodic maintenance costs may be incurred if precipitates build up to a point where either the reactivity or <br />the hydraulic conductivity of the reactive cell is significantly affected. The reactive cell may have to be <br />flushed with reagents, or the reactive medium may have to be replaced. Rule-of-thumb criteria that have <br />been used for economic analysis at previous sites include a maintenance cost estimate that assumes 25 <br />percent of the reactive metal will be replaced every 10 years for sites with low precipitation potential, and <br />every 5 years for sites with high precipitation potential (ETI, 1996) <br />Monitoring points (wells or piezometers) would be installed in the system during initial construction. <br />Detailed engineering design would be performed to determine the necessary residence time, material <br />volumes, and PRB gate dimensions to achieve optimal performance and longevity. <br />• <br />• <br />4109C.100419 Whetstone Associates