CORRESPONDENCE_2005-2007 - Laserfiche WebLink

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Hydrogeology To a depth of approximately 80 feet, soil consists of unconsolidated silts, clays, and sands. The measured hydraulic conductivity of the native soil underlying the landfill ranges between 10-6 to 10-8 centimeters/second. Groundwater is first encountered approximately 120 below the ground surface. It is unconfined and fluctuates approximately 4 feet seasonally. Groundwater flow is toward the south with a gradient of approximately 0.002 foot per foot. Groundwater flow velocity has been reported at 67 feet per year. The current groundwater monitoring system is comprised of four monitoring wells, one upgradient well (MW-1) and three down gradient wells (MW-2, MW-3 and MW-3). Environmental Monitoring Volatile organic compounds (VOCs) were first detected in 1991 in MW-2. The cause of the impact was believed to be landfill gas migration. A landfill gas (LFG) collection system was installed in 1996 to mitigate groundwater impacts and control LFG migration. After installation of the LFG collection system monitoring data indicates continuing decreases in VOCs in MW-2. The VOCs (tetrachloroethene (PCE), 1,2 dichloropropane, and dichlorodifluoromethane)have been reported in MW-2. In October 2005 the concentration of PCE, 1,2 dichloropropane, and dichlorodifluoromethane were 1.4 µg/l, 0.67 µg/l, and 1.9 µg/1 respectively. An increasing chloride trend has been developing in MW-2, MW-3 and MW-4. Concentrations do not exceed the recommended secondary maximum contaminant level (MCL) for drinking water(25.0 mg/1) in wells MW-2 and MW-3. Concentrations in MW-2 and MW-3 were reported at 24 mg/1 and 13 mg/l respectively in October 2005. However, concentrations of chloride do exceed the secondary MCL MW-4 (reported at 130 mg/1 in MW-4 in October 2005). Run-off/Run-on facilities Harney Lane Sanitary Landfill utilizes several methods to control run-on and run-off. A perimeter berm/road surrounds the site and prevents water from entering. The facility utilizes drainage ditches along the berm/road to handle storm water runoff from the landfill. The ditches discharge to a stormwater retention pond located at the south end of the site. HYPOTHETICAL RELEASE SCENARIO The Harney Lane Sanitary Landfill is unlined and stopped receiving waste in 1991 and was closed in 1994. Ongoing groundwater monitoring shows some VOC impacts from LFG migration and moderately elevated inorganics in some groundwater monitoring Corrective Action Financial Assurance Cost Estimate 2 May 25,2007 Harney Lane Sanitary Landfill San Joaquin County Public Works Department