CORRESPONDENCE_2007 (1/07 - 6/07)

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is to the northeast (see Fourth Quarter and Annual 2006 Groundwater and Surface Water Monitoring Report, Corral Hollow Sanitary Landfill). 1.6 Groundwater, Surface Water, and Vadose Zone Monitoring Systems Water quality is monitored in accordance with WDR Order No. 5-01-176 and the monitoring program approved by the RWQCB (Figure 2). 1.6.1 Groundwater The present groundwater monitoring network consists of seven monitoring wells. These wells monitor both the deep aquifer and the shallow aquifer. Shallow Aquifer Monitoring Three wells (SB -1, MW4 and MW -5) monitor the shallow, perched aquifer. Well SB -1 is an up -gradient well, whereas wells MW -4 and MW -5 are down -gradient. Deep Aquifer Monitoring Deep aquifer monitoring wells MW -6 and MW -7 were installed in 2003. Well MW -7 is up - gradient to well MW -6. These wells replaced the previously installed deep aquifer wells MW -1, MW -2 and MW -3, which became dry in 1990. The direction of the flow of groundwater in the deep aquifer was determined by monitoring the three original deep aquifer wells. 1.6.2 Surface Water When flowing, Corral Hollow Creek is sampled upstream of the waste management facility at background monitoring point SW -1, and downstream at monitoring point SW -2. Samples are collected after the first storm of the rainy season that produces significant flow and semi- annually thereafter when water is flowing. 1.6.3 Vadose Zone Soil Gas Vadose zone soil gas wells GW -1 through GW -7 are multi -probe wells originally installed to monitor the subsurface concentration of combustible gases near the property boundary. These wells are monitored for combustible gas quarterly, and a selected probe from each well is sampled for VOCs (EPA Method TO -14) semiannually. Corral Hollow EMP WPlan Rev2Mar 22-07.DOC 1-3 March 20, 2007