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f \ J Mr. Thomas M. Robertson 8 June 17, 1981 In order to define the chemical reactions of the tailings water and soils, an assessment of.the major ions and cations in the tailings water and a determination of the major mineral species and the cation exchange C capacity of the soil would be required. Such an evaluation is not considered necessary at the present time. 3.0 SUMMARY AND RECOMMENDATIONS ' Detailed study of the available information at the Lower Tailings dam has shown that there is insufficient information to clearly define the _ seepage path(s) at the present time; however, the results of Phase I do indicate that seepage is concentrated in the east abutment and in the rock. Additional data are needed to confirm or modify these preliminary conclusions and develop realistic remedial measures. The results of Phase I indicate that the probable source of seepage is tailings water seeping into the alluvial deposit in the north end of the pond. The alluvial deposit provides a downstream seepage path under the beached tailings. The cutoff trench which intersects the alluvial deposit restricts further flow of the water downstream through the deposit. The seepage is able to move laterally into the abutments, passing under the cutoff trench there before flowing downstream. It appears that the major seepage is through the east abutment. The lateral extent across the dam and the depth to which water is flowing through the abutments is not defined at this time. Additional information about seepage conditions along the crest of the dam and at the toe of the dam is required in order to clearly define the seepage path(s). The required additional information can be obtained by the subsurface investigation 1 program described below. Subsurface Investigation 1 In order to define the seepage path(s), the primary zones of seepage must be identified by measuring permeabilities and piezometric heads within and beneath the dam. The areas where such information is needed and lacking are beneath the crest of the dam and in the downstream abutments. A subsurface investigation program has been developed which focuses on these areas. The purposes of these investigations will be to: L • Identify pervious zones which could be trans- mittigg significant quantities of seepage. r C • Measure piezometric pressures in order to define . directions of seepage flow. • Provide water quality sampling points to "trace" seepage from the pond. . This will be accomplished by sampling the material encountered, perform- ing hydraulic testing in the boring, and installing piezometers to be used for water level measurement and obtaining water quality samples.