SITE CHARACTERIZATION CONSIDERATIONS FOR LNAPL SPILLS IN THE LMRV

A large number of potential LNAPL traps exist underlying the valley train surfaces in the lower Mississippi River Valley (LMRV). These traps should be considered during site characterization activities related to spilled NAPLs 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, with an average closure height of 2.7 m. The maximum apparent lateral dimensions are 4.19 km by 0.69 km and the minimum observed lateral dimensions are 0.06 km by 0.02 km. Apparent closure heights range from less than one meter to an apparent maximum of 7.6 m. The interfluve traps with the largest aerial extent do not always exhibit the largest closure heights. The numbers reported here are undoubtedly affected by the scale of the aerial photographs used to interpret channel and interfluve distributions as well as the lateral spacing of the borings used in making the cross-sections. A depth of about fourteen feet is assumed to be a maximum likely excavation depth for underground storage tank installations in the LMRV. The topstratum thickness in 47 of the 100 borings used to construct cross-sections in this study was less than 14 feet. Thus it is likely that underground storage tank installations have or will breach the topstratum at many sites on the valley train surface in the LMRV. 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 topstratum and that the water table seasonally fluctuates from below the base of the topstratum to above the base of the topstratum. D-10 data suggests that trap closure heights of as much as one to two meters may be required in order for free-phase gasoline to exist in a trap. However, if D-10 diameters do not control capillarity, then smaller closure heights on the order of 40 to 60 cm (or even less) may be all that is required.