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Homestake Mining Company -Bulldog Mine-Lower Tailing Pond ' 3.0 Data/Information/Assessment 3.1 Lower Tailings Impoundment Remediation Work ' Slurry Cutoff Wall and Pump Back System ' The 1982 remedial activities included the construction of a soil/bentonite slurry cut-off wall down gradient from the Lower Tailings Impoundment to intercept surface flow and ' ground water seepage. Seepage trapped by the slurry wall was forced into a concrete collection trench and sump. To facilitate seepage collection around the sump, clean crushed rock was placed below the sump bottom and approximately six feet azound the ' sump circumference. To further aide seepage collection, six inch diameter perforated pipes, surrounded with filter cloth and crushed rock, were connected to the sump and extended 100 feet east and 150 feet west side of the sump, perpendiculaz to the ' collection trench. The seepage collected was then returned to the impoundment via a pump-back system consisting of two- 10 HP turbine pumps with an individual capacity of 175 gpm and a six inch diameter return line, laid along the east side of the stream ' channel, through the embankment to the tailing pond. The performance of the cut-off wall and pumpback system was assessed through the ' evaluation of seepage collection rates and changes in the down-gradient water conditions. Monitoring well WP25 showed a decline in the water level of three feet within three month of the remedial activities. However, adjacent wells did not reflect a similaz ' fluctuation in water levels. At the completion of construction, the pump-back system was returning water to the pond at a rate of 95 gpm; this increased to 130 gpm by October 1983. It was noted in the February 1983 Construction As-Built Report that the ground ' water quality in the monitoring wells below the stony cut-off wall began to improve after the remedial construction. ' Test Grouting Program The 1982 test grouting program was intended to be an investigative tool to determine t whether grouting could significantly reduce seepage beneath the embankment. Grouting was performed in the permeable rock and alluvial fill beneath the embankment. The initial program consisted of six holes spaced approximately 30 feet apart across the ' embankment followed by three additional holes spaced half way between the initial holes. All holes were completed into the Creede Formation. The grout consisted of a ' water and cement mixture varying from a 4:1 to 1: l ratio. In addition, a chemical grout mixture was also tested. Most of the grout take occurred in the alluvial soil at a depth of 80-90 feet below the embankment crest. [n this area the grout take approached 130 cu. ' feet of grout per foot. The average grout take was 12 cubic feet per foot with the rock in the weathered zone neaz the interface with the alluvial soil taking more grout than the more competent rock.