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■ A high level sensor, which will activate an alarm and deactivate the sump pump if the leachate level exceeds a set level. This feature prevents the overflow of the tank. ■ A low level sensor, which deactivates the transfer and return pumps, thus preventing either of those pumps from being operated with insufficient water. Leachate is returned to the refuse of Module 3 or Module 4(the portions of the landfill provided with Subtitle D compliant liners) in accordance with the current WDRs. The semi-arid climate of the North County Landfill and the waste shed from which the refuse is collected provides dry refuse in which the leachate can be absorbed. Semi-permanent infusion points include three Deep Infusion Points ("dry" wells), all installed over Module 3 (Figure 5). The highest elevation of the base of Module 3 in this area is approximately 96 NGVD. An additional infusion point installed in Module 4. The Deep Infusion Points (DIPS) are "dry" wells with the bottom of these wells at approximate pp e elevation 140 ft NGVD. Approximately 50 feet of refuse in multiple lifts exists between the point of infusion and the base of the landfill. Normal operation of the leachate system at the North County Landfill provides redundancy in absorption. Leachate normally is pumped from Module 4 to DIP 3. Leachate may also be pumped to the Module 4 leachate storage tank, directly to another infusion point, or to the working landfill face if this face is over the lined area of Module 3 or 4. Volumetric leachate data indicates that the leachate is being absorbed without recirculation. Leachate "seeps" on the sideslopes of Module 3 have not been observed. If any Module 4 leachate returned at DIP 3 percolated to the LCRS of Module 3, it would be returned again to the horizontal collector, 20 feet higher in the fill. When first returning leachate to DIPs 1 and 2 (which were originally drilled to a depth of 45 feet), the flow rate from Module 3 increased greatly, indicating that significant leachate was reaching the LCRS of Module 3. As soon as this condition was discovered, the leachate was directed to DIP 3, and the leachate production for Module 3 returned to normal. The potential for future recirculation at DIPS 1 and 2 was removed by adding neat cement to these DIPs, thereby raising the bottom of the infusion point. Construction and alteration of the DIPS was reported in detail in the First Semester 2005 Groundwater and Surface Water Monitoring Report. However, we are able to continue returning all leachate to DIP 3 without difficulty. Overwhelming of the LCRS system due to excessive recirculation would be indicated by the inability of the Module 3 leachate sump pump to maintain the head on the liner below 12 inches. With the exception of the short period when leachate DIP 2 was used (see above), this has not been observed. Leachate seeps on the side of the fill would be an indication that the refuse has become saturated. This condition has also not been observed. Therefore, we conclude that the relatively small amount of leachate generated at this landfill can be returned to the refuse without negative impact to the LCRS. i North County RCSL 12 Public Works/Solid Waste 1st Semester 2006 Groundwater Monitoring County of San Joaquin—July 31,2006