SITE INFORMATION AND CORRESPONDENCE

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• Lawrence Livermore National Laboratory Site 300 Pits 1 and 7 -3- 24 April 1990 To intercept the eastward ground water flow along the perching horizon, LLNL proposes to install an interceptor drain cut to the bedrock. Located along the northwest margin of the final cap, the drain will divert most intercepted runoff over the rise to the north, where the flows will discharge to a flat percolation area. The ditch will divert additional runoff southward to deeper interceptor trenches which will carry the flows to the open channel to be installed along the east and south margins of Pit 7 to handle surface runoff from the cap. The data indicate most of the ground water recharging the shallow aquifer originates to the south and southwest of Pit 7. Although drawings included with the Subchapter 15 response portray the spur trenches extending up the swales to the west and southwest of Pit 7 to reach down to fresh, dense bedrock, the analysis does not explicitly state that the proposed trench along the Pit 7 margin into which the swale trenches drain shall also be cut to bedrock. Since the interceptor trenches along the southwestern margin of Pit 7 lie directly in the path of the ground water believed to travel along the bedrock/colluvium interface to the recharge zone off the southern end of Pit 7, LLNL should cut this trench to bedrock. In addition to the design information presented in the Subchapter 15 response, the revised CP proposes to monitor the flows diverted by the interceptor drain proposed for the northwestern margin of Pit 7 and to correlate these flows against prior water balance analyses included in the revised CP and the Subchapter 15 response. These correlations will help confirm whether most of the ground water flows along the bedrock/colluvium interface or enters the bedrock before moving to the recharge zone south of Pit 7. LLNL proposes to measure the ground water elevations in monitoring wells weekly . If the ground water rises to within five feet of the Pit 7 bottom, LLNL proposes to deepen the interceptor trenches to 25 feet. The water table rising to within five feet of the pit bottom will demonstrate that the groundwater flows within the upper zone of the bedrock perching horizon below the elevation of the bottom of the shallower interceptor trenches, which shall be cut to the upper bedrock surface. At 25 feet deep, the trenches will reverse the water table gradient away from beneath Pit 7. Conclusions My review of the Subchapter 15 response, the revised CP, and the RI/FS showed the proposed landfill covers comply with the Subchapter 15, Article 8, §2581 requirements and the precipitation and drainage control and ground water monitoring requirements in Articles 4 and 5, respectively. LLNL has demonstrated the proposed drainage ditch and trenches to be installed to bedrock to the west and southwest of Pit 7 will likely intercept most of the shallow ground water flowing along the bedrock perching horizon. These structures will likely eliminate future ground water mounding to the south of Pit 7, and thereby prevent flooding the landfill bottom. If not, LLNL shall deepen the trenches. The final cover and associated drainage control structures proposed for Pits 1 and 7 will likely prevent water from percolating into the landfills in future. By eliminating the flow paths for water to enter the landfills, the closure structures shall likely achieve the performance goal of the Subchapter 15 siting criteria and construction standards for Class I landfills. Therefore, this engineered alternative is acceptable.