Remediation of Trapped Free‐Phase LNAPL

Abstract

AbstractFree‐phase light nonaqueous phase liquids (LNAPLs) may be trapped in certain stratigraphic and structural features near or at contaminated sites due to seasonal or other variations in the water table elevation. The purpose of this article is to point out particular subsurface conditions that are conducive to trapping of free‐phase LNAPLs and to suggest approaches to remediating LNAPL‐contaminated sites exhibiting similar subsurface geometry and stratigraphy. To trap free‐phase LNAPL, a structure must have, in addition to closed contours, an upper boundary with pores small enough so that the LNAPL will not enter them. This boundary usually consists of clay‐rich sediments. The Lower Mississippi River Valley contains thousands of these potential traps associated with the geomorphic surfaces mapped as outwash or braided stream terraces, which are covered with thin layers of backswamp clays. These traps may have closure heights ranging from about 1 to 7.5 meters or more and have variable lateral extents. Based on surface geomorphic analysis, the potential LNAPL traps in the Lower Mississippi River Valley range in size from about 0.06 by 0.02 km to 4.19 by 0.69 km. The apparent best remediation strategy for LNAPL sites located on these geomorphic surfaces, which contain these trapping structures, is to first determine if free‐phase is present. If it is present, and is contained in one of the stratigraphic traps, the free‐phase can be removed through an extraction well or wells located at the trap apex. Geomorphic analysis and geophysical surveys may be necessary to accurately locate the trap apex. The remaining residual hydrocarbons might best be remediated using an air sparging system, although it may be necessary to install air vents through the clay cap by backfilling augured holes with washed sand. If it is determined that, due to geometry, the dissolved LNAPL plume cannot be adequately remediated using an air sparging system, then groundwater circulation wells or monitored natural attenuation may be alternative technologies. © 2002 Wiley Periodicals, Inc.