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Evaluation Work Plan for 2 1 October 2009 Septic Systems and Percolation Pits Order No. R5-2008 0148 Lawrence Livermore National Laboratory Site 300 San Joaquin County • All four septic systems are located in areas where Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) program remedial actions to restore beneficial uses of groundwater are in progress. Nitrate is included in the remedial actions at these sites as a constituent of concern to be remediated and monitored. • Since issuing the Order, the Building 850 septic system has been scheduled for closure. A CERCLA soil remediation at Building 850 is ongoing and scheduled for completion in November 2009. After the remedial work is completed, the Environmental Protection Department (EPD) staff will proceed with closing the septic system. On 11 August 2009, Central Valley Water Board staff concurred with the 15 May 2009 Building 850 Septic System Closure Plan (Closure Plan). The Closure Plan includes sampling and analysis of soil below the outlet to the leachfield and groundwater downgradient of the leachfield. Upgradient groundwater sampling may be necessary if background concentrations are not available for all constituents of concern. Because this work will be carried out under the Closure Plan, additional monitoring of the Building 850 Septic System under the Work Plan will not be necessary. However any data acquired under the closure of the Building 850 septic system should be used as input to the model for the group of septic systems which includes the Building 850 septic system. In the Work Plan, LLNL proposes the following steps to evaluate the potential for percolation pits and septic systems to negatively impact groundwater beneficial uses: • Percolation pits and septic systems that are no longer in use or planned to be deactivated in the near future will be excluded. • Percolation pits and septic systems that are underlain by a hydrogeologic setting where there are known hydraulic barriers between the pit and regional aquifer will be excluded. • Where adequate data exists, comparisons will be made of upgradient wells to downgradient wells concentrations to assess existing impact from past operations. • Percolation its in close proximity of each p p other and underlain b similar Y y hydrogeologic settings will be grouped and evaluated using one model to represent the group. . 9 p g p Analytes with similar fate and transport parameters will be grouped and an indicator compound will be selected to evaluateotenti p al impact to groundwater. The same approach will be applied to grouping of septic systems. • LLNL proposes to construct a two-part model for each percolation pit and septic system group. will be constructed 9 One art p p ted to evaluate the fate and transport of salts/metals in the unsaturated zone and to the second part will be constructed to evaluate fate and transport in the saturated zone. The analysis will evaluate potential impact to groundwater beneath the percolation pits and septic systems. The saturated zone model may be necessary in areas where there is saturated flow and transport in the subsurface above the regional aquifer due to discharges from the percolation pits and septic sstems. Modeling the saturated zone will be conducted only where unsaturated zone modeling shows a potential for percolation pits or septic sstems to impact groundwater above background