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<br />~' <br />In placing the main embankment liner, the splitter embankment liner,, and the <br />lower disposal area liner, work crews experienced difficulty marking the wedge <br />welders perform adequately. A significant percentage of seams were <br />subsequently capped using extrusion welding techniques. The double wedge <br />welding machines periodically melted holes in seams which resulted jn a large <br />number of caps and seams that could not be tested continuously by a{r testing <br />of the channel. All repairs on the seams and interior areas of the liner were <br />vacuum tested for water tightness prior to acceptance and placement of the <br />cover material. <br />UNCONTAINED SEEPAGE <br />Calcuiations of the amount of seepage that will escape the tailings <br />impoundment through the synthetic liner and underlying compacted soils, are <br />based on a number of assumptions. The first assumption being that ten ti al <br />holes in the liner would be .l square centimeters in size with a frequency of <br />one hole per acre. These values correspond to values from EPA docu nt <br />"Design and construction of RCRA/CERCLA Final Covers" for a liner installation <br />implemented with a high level quality assurance program. The second <br />assumption is that the hydraulic head applied to the liner will be minimized <br />by the subdrainage layer and piping and will be about one foot. Wit the <br />known values for permeability of the sub-drainage layer, the compact d soils, <br />and the geomembrane, Darcy's Law predicts total uncontained flow fro the <br />tailings impoundment to be approximately 15,000 cu. ft./yr. or 112,0,0 gallons <br />per year. <br />CONCLUSIONS <br />As it is designed, the San Luis Project tailings impoundment seepage rate is <br />low enough that it will not induce unsaturated flow in the underlying <br />material, hence any pollutants that seep through the liner would not reach the <br />water table. This conclusion is based on the values for hydraulic <br />conductivity calculated or specified for the subdrainage layer, the <br />geomembrane, the compacted soil layer and the liner foundation, as w 11 as the <br />quality assurance program followed during liner installation, and th design <br />performance of the subdrainage layer in limiting the head applied to the <br />liner. This conclusion is further based on moisture content and spe ific <br />retention (5% and lOX respectively) values supplied by the operator or the <br />liner foundation subsoils. <br />ACS/yjb <br />2567E <br />-3- <br />