A large number of potential light nonaqueous-phase liquid (LNAPL) traps exist in poorly sorted sands underlying the buried valley train surfaces in the lower Mississippi River Valley (LMRV). These traps should be considered during site characterization and remediation activities related to spilled nonaqueous-phase liquids and leaking underground storage tanks on the valley train surfaces of the LMRV. These traps have apparent average dimensions of 0.84 by 0.21 km (0.52 by 0.13 mi), with an average closure height of 2.7 m (8.85 ft). The maximum apparent lateral dimensions are 4.19 by 0.69 km (2.6 by 0.42 mi) and the minimum observed lateral dimensions are 0.06 by 0.02 km (0.03 by 0.01 mi). Apparent closure heights range from less than 1 m (3 ft) to an apparent maximum of 7.6 m (24.93 ft). A depth of about 4.3 m (14.1 ft) is assumed to be a maximum likely excavation depth for underground storage tank installations in the LMRV. The top stratum thickness in 47 of the 100 borings used to construct cross sections in this study was less than 4.3 m (14.1 ft). Thus, it is likely that underground storage tank installations, for example, have or will breach the top stratum at many sites on the valley train surfaces. In order for spilled LNAPLs from underground storage tank sites to migrate into one of these traps, it is probably necessary that the original excavation breach the low-permeability top stratum and that the water table seasonally fluctuates from below the base of the top stratum to above the base of the top stratum.

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