1990-01-09_REVISION - M1988112 (6)

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9E+ i++li0? 1:3:1' $ '~~_ 565 ??47 ER Y. DENVER 03 BATTLE MOUNTAIN RESOURCES, SAN LUIS PROJECT CMI.RD ADEQUACY LETTER RESPONSES Page 6 of 28 The natural moisture content of the 5ubsoil5 is conservatively estimated at above five percent. Using methods outlined in McWhorter and Nelson (1979), the specific retention is estimateo to be about 10 percent. Therefore, the soil moisture content would have to be increased by five percent for unsaturated flow Condit ons to occur and a seepage plume commence tv migrate down to the water table. Based upon the above assumptions the pro,)ected rate of permeation would be 3 x 10-3 cu ft/yr/ft 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 i.he operating life of the pro,)ect. 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 defects, 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 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 evaluat on 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 an<' the applied head will control the volume of seepage. For the purposes of this analysis, permeability of the compacted subsoils is es;imated to be approximately 1 x 10~s cm/sec. The hydraulic head w~lich will be applied to the liner will be minimized by the subdrain.age layer and ,;1p1ng. Given the permeability within the subdrainage layer, the estimated head which could be applied will be about cne ft. Using the assumptions presented the estimated seepage through each hole is approximately 5.4 x 10-~ gpm. Tf~erefore, during final buildout at an area of 16T acres (T.3 x 10" ftz) of impoundment, the total estimated volume of seepage will be equivalent to 63.4 ft3/yr (474 gallons per year). Cissipation of this flow will occur within the subsoil of the site and will resu~t in an increase in moisture content of the site subsoils. Over the entire tatlt~gs area the effect would increase the moisture content of the comp~~cted 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 f*, could be imliacted during the operating life of the project.