SITE INFORMATION AND CORRESPONDENCE

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• • 1 Lawrence Livermore National Laboratory Site 300 Pits 1 and 7 -2- 24 April 1990 The siting criterion for Class I landfills in Subchapter 15 §2531(b)(1) requires the geologic strata beneath the landfill to have a permeability of 10' cm/sec or less and be of "sufficient thickness. " However, the strata under Pits 1 and 7 exhibit permeabilities on the order of 104 cm/sec. Additionally, the Subchapter 15 construction standards summarized in Table 4.1 require Class I landfills to be underlain with liners and to be fitted with grout curtains or cutoff walls where the potential exists for lateral ground water migration beneath the landfill . The perching horizon underlying Pit 7 both perches and directs the shallow ground water laterally to the south of Pit 7, where mounding occurs. Under Subchapter 15 §2510(b) , alternatives to siting criteria and construction standards may be considered when the discharger demonstrates the standard is infeasible, and there is a specific engineered alternative that is consistent with the performance goal addressed by the standard and affords equivalent protection against water quality impairment. The discharger must demonstrate infeasibility by showing compliance with the standard is either unreasonably and unnecessarily burdensome and will cost substantially more than alternatives complying with the standard, or is impractical and will not promote attainment of the performance standard. The performance goal of closing Pits 1 and 7 according to the siting criteria and construction standards is to isolate the waste and prevent the escape of pollutants from the landfills to surface and ground waters. The landfills do not meet the siting criteria or construction standards, but LLNL has demonstrated that removing all waste from the landfills and installing a liner would be impractical . Preventing ground water from percolating into or flooding the landfill from the bottom, and thereby from releasing waste constituents, would achieve the performance goal . LLNL also has demonstrated that excessive head could develop behind grout curtains or cutoff walls making them impractical as well . Therefore, an engineered alternative can be considered in this case. Bedrock comprised of varying proportions of sandstones, siltstones, and claystones forms a shallow, laterally extensive perching horizon under Pit 7. LLNL has installed numerous ground water monitoring wells (MWs) in proximity to Pit 7 and monitors the ground water elevations in these wells monthly. Ground water elevations in MWs often respond to major storm events by rising relatively rapidly to a peak, then gradually declining. Correlation of the very heavy rainfall on and about 15 February 1986 with later ground water peak elevation dates allowed LLNL to demonstrate the substantial peak elevations in the MWs, in response to the storm, occur progressively later as one moves eastward through the well network. This eastbound ground water wave or pulse confirms that the major recharge zone for the shallow aquifer lies to the west and southwest of Pit 7. Most of the recharge water subsequently flows as ground water from west to east along the bedrock perching horizon before recharging the bedrock at the southern end of Pit 7. The correlation of the MW peak elevations to the 15 February storm also demonstrates the ground water mounding which can occur at the southern end of Pit 7 due to major storm events. LLNL quantitatively estimated that future mounding due to maximum probable precipitation storm events would flood the landfill bottom, demonstrating the need for hydraulic control of the shallow ground water.