1998-01-10_REVISION - M1988112 - Laserfiche WebLink

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BATTLE MOUNTAIN RESOURCES, SAN LUIS PROJECT CMLRD ADEQUACY LETTER RESPONSES Page 6 of 2B The natural moisture content of the subsoils is conservatively estimated at above five percent. Using methods outlined in McWhorter and Nelson (1979), the specific retention is estimated to be about 10 percent. Therefore, the soil moisture content would have to be I increased by five percent for unsaturated flow conditions to occur and a seepage plume commence to migrate down to the water table. Based upon the above assumptions the projected rate of permeation would be 3 x 10.3 cu ft/yr/ft2 of liner. The effect of this flow rate would increase the moisture content of the compacted subsoil layer beneath the liner by approximately three percent over the operating life of the project. In addition to permeation through the liner, there is a potential for flaws in the liner which could ultimately conduct flow. These flaws could be formed by tears, punctures, seam defect's, etc. The ' estimated seepage volume through these openings will be dependent upon the permeability of the underlying material, the area of the opening(s), and the frequency of the openings. For analysis purposes t it can be assumed that potential holes in the liner would be 10 mm~ (radius = 1.78 mm) in size and would have a frequency of one hole per acre. These values correspond to the EPA's evaluation performed in developing the "Draft Leak Detection Rule", assuming implementation of a high level quality assurance program during liner installation. The permeability of the subsoils beneath the liner and the applied head will control the volume of seepage. For the purposes of this analysis, permeability of the compacted subsoils is estimated to be ' approximately 1 x 10-s cm/sec. The hydraulic head which will be applied to the liner will be minimized by the subdrainage layer and piping. Given the permeability within the subdrainage layer, the estimated head which could be applied will be about one ft. Using the assumptions presented the estimated seepage through each hole is approximately 5.4 x 10-~ gpm. Therefore, during final buildout at an area of 167 acres (7.3 x 10"s ft2) of impoundment, the total I estimated volume of seepage will be equivalent to 63.4 ft3/yr (474 gallons per year). Dissipation of this flow will occur within the subsoil of the site and will result in an increase in moisture content of the site subsoils. Over the entire tailings area the effect would increase the moisture content of the compacted subsoil layer beneath the liner by significantly less than 0.01 percent during operation of the project. At a specific flaw in the liner, a volume of subsoil equivalent to 20 cu ft could be impacted during the operating life of the project.