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W. Michael Carroll North County Recycling Center and Sanitary Landfill and Foothill Sanitary Landfill San Joaquin County also maintain a high hydraulic conductivity over time and resist plugging or clogging. The hydraulic conductivity of drainage materials depends primarily on the grain size of the finest particles present in the soil.,.The most essential requirement for a landfill leachate drainage layer is that it has adequate drainage,capacity to handle the maximum leachate flow produced during landfill operations. Furtheni6re,we have concerns regarding bioclogging, especially when the geofabric is thick; therefore less pe *A able to viscous leachate flow. Subtitle D requires the gravel drains leachate that enters the drainage layer to prevent 30 cm (12 inches) or more accumulation on top of the liner during the active life.of the unit's leachate collection system. The soil material used for the drainage layer should be investigated at the borrow pit prior to use at the landfill. Typical borrow pit characterization testing would include laboratory hydraulic conductivity and grain size distribution. Coarse granular drainage materials, unlike low-permeability soils, can be placed dry and do not need to be heavily compacted. Compacting granular soils tends to grind the soil particles together, which increases the fine material and reduces hydraulic conductivity. To minimize settlement following material placement, the granular material may be compacted with a vibratory roller. The County proposes that the contractor manufacture the LCRS drainage layer from on-site material, rather than importing it from off-site sources. The proposal should include the amount of material required for the LCRS at each site, whether there is sufficient amount of material available to borrow on- site, and how the material is to be crushed, sorted and graded to meet the proposed standards. If you choose to propose this alternative,we request that the above issues be addressed in a revised application. Furthermore,we request that the following specification be changed, if applicable: 2.10 ALTERNATIVE CRUSHED ROCK DRAINAGE LAYER Change Specification C to read: "Containing no limestone, gypsum or other material which will adversely react with the leachate or dissolve away upon contact with the leachate and potentially interact with the bentonite in the GCL." If you have any questions or comments,please call me at(916)255-3137. KIM A. SCHWAB, R.G. Engineering Geologist cc: Robert McClellon, San Joaquin County Environmental Health Division, Stockton Keith Kennedy, Permitting and Enforcement Division, CIWMB, Sacramento kas:NC FH Us Gravel