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P« LAWRENCE LIVERMORE NATIONAL LABORATORY SUBCHAPTER 15 COMPLIANCE REVIEW -2- 21 September 1989 Pit 17 Shallow Ground Water. During the abnormally wet winter of 1983, shallow ground water flooded the bottoms of Pit 3 and Pit 5 (adjacent to Pit 7) , thereby releasing trichloroethylene and tritium into the ground water. Cross-sections through Pits 3, 5, and 7 show the distances from the three landfill bottoms to the shallow water table to be about the same, implying the potential for the release of Pit 7 wastes during extremely wet winters. In their revised closure plan, LLNL asserted that capping Pit 7 will eliminate aquifer recharge and will ensure the pit bottom remains dry. However, they did not justify this conclusion quantitatively. The April 1987 report entitled "Tritium in Ground Water at Site 300" postulates that the shallow aquifer recharge zone in the vicinity of Pit 7 may be associated with the break in the slope at the base of the hills west of the Pit 7 complex (page 52) . The report also discusses the perching horizon' s role in collecting and directing sub- surface ground water flows. As stated in our 3 February 1989 letter, Subchapter 15, Section 2545 requires cutoff walls or grout curtains are installed at Class I WMUs where the potential exists for lateral fluid migration. A cutoff wall installed at the break in the slope to the west of Pit 7 and tied into the cap would eliminate both the aquifer recharge at this point and the lateral subsurface ground water migration. The Pit 7 cover design should specify either cutoff walls or an equivalent engineered alternative. The Pit 7 cover design specifies .a drainage ditch near the break between the Pit 7 surface and the natural grade. The ditch will divert a substantial portion of the storm water runoff. However, LLNL should account for the fate of the remainder of the runoff by performing a water balance on the drainage basin surrounding Pit 7. In accordance with Subchapter 15, Section 2546, the water balance and the drainage ditch design should employ the maximum probable precipitation as the design rainfall event. Cover Design. LLNL used the Hydrologic Evaluation Landfill Performance (HELP) model to evaluate the landfill cover system design. They used the rainfall data for the years 1980 through 1986 rather than the maximum probable precipitation specified in Subchapter 15, Section 2546. LLNL should reevaluate the landfill design using the appropriate design precipitation. Construction Quality Assurance. LLNL proposes to conduct in-place density and moisture content tests on every 250 cubic yards- (yd') and permeability tests on each 1 ,500 yd' of the compacted low permeability clay layer in the cover. LLNL should propose the means whereby they will randomly locate the field tests. The CQA plan requires LLNL ,personnel to prepare block evaluation reports, the design engineer' s report, and the final CQA report, and specifies that the reports be sent to the Environmental Protection Agency (EPA) and the Department of Health Services (DHS) . CQA reports should also be sent to the Regional Board. MLH:gs