1990-01-09_REVISION - M1988112 (6)

Home Browse Search

5~~ D1. Qa lerlc $ Zi+S 985 5557 SRh DENVER 6' • 6AT'1CE MOUNTAIN RESOURCES, SAN LU[S PROJECT CMLkU ADCQUACY LETTER RFSPONSCS Page 5 of 28 The data presented in Attachment 1 also demonstrate that no changes in metal concentratio~ls and acidity have occurred as a result of the change in processing. 10. Please :ommit to achieving 1x70'6 cm/sec permeability for the underlying colluvfal material after tt has been compacted to 95% maximum Proctor density. It wi )1 be necessary to provide ~~erification to the Division that this permeability has been achieved prior to installation of the VLDP£. ^i.;PG~;Sc: Because of the nature of the underlying colluvfal materials, it is r.ot possible to achieve a uniform 1x10-6 cm/sec permeability after the material has been compacted to a 95% maximum Procl.or density. The combination of the law permeability tailing:• following deposition, the drainage layer's ability to reduce hydraulic head on the geosynthetic liner and the function of both the geosyntfietic liner and compacted soils to limit potential seepage eliminates the potential requirement for a lower permeability of the compacted subsoil. Seepaye potential ;s discussed in the response to guestian 11. BMR can achieve and will commit to achieving a 1110- cm/sec permeability in the colluvfal materials following compa~:tion and to perform material testing during construction to confirm tiese values. Please provide a detailed discussion regarding the pote~ntia) for a detection of leakage from the VLDPE liner to the underlyi,g colluvfal materials. A rationale for the placarnent of the monitcring wells, includfng the potentfal for those wells fo intercept leakage from the tailings faciilty, should be included. Qot^_ntial leakage through the VLDPE liner could occur via two ~Jifferent pathways; seepage via permeation through the liner itself :nd through flaws in the liner. Seepage through the liner itself will :~: controlled by the thickness of the liner, the head cn the liner and the permeability of,~he liner. For the VLDPE liner are equivalent permeability of 1 x 10-` cm/sec (manufacturer's literature) can be asswned. If permeation occurs from the base of the liner, the flu ds will not immediately migrate to the groundwater. The underlying a1'iuvial soils at the project are unsaturated and at a moisture content significantly less than the specific retention of the material. Specific retention is defined as the moisture content above which gravity flow through the soil will occur. In general, the groundwater averages about 150-ft below the surface. r.j __..