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migrant labor housing facility operated by San Joaquin County Housing Authority on the western portion of HLSL's property (Figure B-1). 2. Well Spacing The distance between the monitoring gas wells is less than 1000 feet (Table B-1). 3. Well Depths The depth of refuse is not certain, due to the operation of the landfill by others beginning in 1948; however, to the best knowledge of SWD, the perimeter migration monitoring wells extend to an elevation at or below the lowest extent of the waste fill. Shallow Probes allow monitoring from five to ten feet from the surface (Table B-2). Intermediate Probes allow monitoring at or near half the depth of refuse (Table B-2). Deep Probes allow monitoring at or near the depth of refuse (Table B-2). 4. Well Construction Soil gas wells were drilled by a licensed drilling contractor. During drilling, wells were logged and soil cuttings were characterized for visual classification during well installation (Attached). B. DESIGN CONSIDERATIONS HLSL's perimeter monitoring wells were designed based on the following site factors in accordance with Title 27, section 20923. 1. Local Soil and Rock Conditions Local soil and rock conditions did not require special design features for the soil gas wells. The soils immediately underlying the landfill consists of unconsolidated silts, clays, and sands to a depth of approximately 80 feet. The soils are typically, highly impermeable with hydraulic conductivities of approximately 10-6 to 10"6 centimeter/second. 2. Hydrogeologic Conditions The wells were installed in recognition of the hydro geologic conditions at the disposal site, which did not require special design features in the soil gas wells. Groundwater elevation ranges from -30 to -42 feet MSL (120 feet to 130 feet below Harney Sanitary Landfill County of San Joaquin Landfill Gas Monitoring Plan B-2 Department of Public Works Revised July 30,2009 Solid Waste Division