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3-1 3.0 SLURRY WALL CUTOFF AND PUMP -BACK SYSTEM 3.1 GENERAL The primary objective of the slurry wall cutoff and pump -back system was to (a) intercept tailings pond seepage in downstream areas by providing a barrier to surface and subsurface flow, (b) collect intercepted water, and (c) return, by pumping, collected water to the tailings pond. The locations, components, and details of the system are presented in Figures 2 and 3. Components of the slurry wall and pump -back system include: o A bentonite/soil slurry wall and embankment downstream of the toe of the dam to provide a barrier to seeping tailings water, and to contain runoff from precipita- tion in uncontrolled upstream areas. o A collection system upstream of the slurry wall to channel all seepage to a collection trench connected to a concrete sump. o A concrete sump and pump -back system consisting of two turbine pumps which return the tailings water back to the pond. o A new buried discharge line that connects the pump - back system to the tailings pond. conducted from July 11 to July 13, 1982, to delineate the depth to competent rock along the slurry wall axis. Construction of the slurry wall itself, including excavation, soil/bentonite mixing, backfilling, and protective cap placement began on July 14, 1982, and was completed on July 17, 1982. Construction of the drainage collection trench occurred from July 22, 1982, through July 27, 1982. Construction proceeded with final fill placement around the sump along with the pouring of the concrete slab on July 30, 1982. Complete hook-up of the pumps and pump -back system occurred on August Construction of the slurry wall cutoff and pump -back system began on July 8, 1982. Preparation first began for the cross -valley embankment to serve as a working grade for slurry wall excavation as well as a retention embankment for surface flow. Auger drilling from the top of the completed embankment was conducted from July 11 to July 13, 1982, to delineate the depth to competent rock along the slurry wall axis. Construction of the slurry wall itself, including excavation, soil/bentonite mixing, backfilling, and protective cap placement began on July 14, 1982, and was completed on July 17, 1982. Construction of the drainage collection trench occurred from July 22, 1982, through July 27, 1982. Construction proceeded with final fill placement around the sump along with the pouring of the concrete slab on July 30, 1982. Complete hook-up of the pumps and pump -back system occurred on August