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REFUSE P/LE EXPANS/0N <br />RPE. These features will not affect the RPE <br />site stability. <br />~~ • Another important geologic feature is the <br />rockslide/landslideon the eastern edge of the <br />proposed RPE. Due to the size and lack of <br />consolidation of the sandstone boulders <br />within the slide, development of the RPE in <br />this area will be avoided. <br />• Surface hydrologic features within the <br />proposed foundation foot print of [he RPE <br />include six active springs, identified on <br />Figure J-l, and numerous dry ephemeral <br />drainages. Ditches and drains will be <br />established early during the RPE <br />development to convey clear water from the <br />springs and ephemeral drainages away from <br />[he RPE site. <br />5.4 Subsurface Hydrology <br />5.4.1 Subsoil <br />An analysis was performed to determine leachate <br />~~ production within the proposed RPE and rock drain <br />requirements. Exploratory boreholes presented in <br />Appendix E indicate the subsoil in the northern <br />portion of [he RPE consists primarily of gravel. This <br />area has been desienated for Phase I and 2 <br />construction. Analyses of borehole samples reveal a <br />well graded, mixture of silty sand and gravel (ASTM <br />standard classification group GW). These soils <br />typically have a permeability of 2.5 x 10-' cm/sec <br />(Lindeburg, 1992). Beyond the Phase 2 boundary the <br />subsoil is a clay-rich colluvium, characteristic of the <br />area. Permeability of subsoil samples taken from <br />sites adjacent to the RPE, specifically, from [he lower <br />refuse pile and upper storage bench sites, are in [he <br />range of 1.0 x 10"' [0 1.0 x 10"' cm/sec. See Appendix <br />M for further discussion of soil characteristics. <br />The Phase 1 and 2 areas were lined with a HDPE <br />liner. The liner will allow subsurface water to <br />migrate into a system of rock drains where it will be <br />routed to the RPE sedimentation pond. Phase 3 and 4 <br />subsurface will not be lined because of a steeper slope <br />• grade (2.SH:1 V) and greater cla}• content in the <br />colluvium. <br />5.4.2 Leachate Production -HELP Model <br />Estimate <br />Leachate volume is a function of the amount of <br />precipitation, runoff, evapotranspiration, and <br />infiltration into the pile, based upon parameters <br />defining climate, vegetative cover, soil cap, surface <br />grade, and subsurface permeability. An estimate of <br />leachate volume was obtained using the U.S. Army <br />Corps of Engineers (COE) "Hydrologic Evaluation of <br />Landfill Performance (HELP) Model, "Version 3, <br />September, 1994. <br />HELP is a comprehensive quasi-[wo-dimensional <br />hydrologic model of water movement across and <br />through repositories. The model accepts weather, soil <br />and design data, then solves and tabulates surface <br />storage, lateral subsurface drainage, leachate <br />recirculation, unsaturated vertical drainage, and <br />leakage through soil, geomembrane or composite <br />liners. Repository systemsincludingcombinationsof <br />vegetation, cover soils, waste cells, lateral drain <br />layers, barrier soils, and synthetic geomembrane <br />liners may be modeled. The model enables the user <br />to test designs for efficiency and estimate runoff, <br />evapotranspiration, percolation, leachate collection, <br />and leakage. The program was developed by the <br />COE for the U.S. Environmental Protection Agency <br />(EPA), Risk Reduction Engineering Laboratory, <br />Cincinnati, OH. HELP Version 1 was initially <br />released in 1984, and has since become the national <br />standard for landfill and repositorydesignrnodeling. <br />Climate data for the model was obtained from the <br />NOAH National Weather Service station PONIA <br />1SW located in Paonia, Colorado, 15 miles west of <br />Somerset. Monthly averages for years 1957 through <br />1993 were used for the model. To simulate leachate <br />production under worst-case conditions, the monthly <br />precipitation average for May was increased from <br />1.33 to 3.6 inches to create abnormally'moist (with a <br />2.8 inch daily peak, 100-year event) spring runoff <br />conditions for [he model. Although April has a <br />slightly higher probability of a one-day precipitation <br />exceeding I inch, May was used for the 100-year <br />event because temperatures assured precipitation <br />would be in the form of rainfall and consequently <br />result in maximum infiltration. HELP test runs <br />verified that between the tyvo months, May produced <br />more leachate under the 100-year event precipitation <br />conditions. <br />Harding Lawson Associates 7 <br />