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Remediation of Trapped Free‐Phase LNAPL
Author(s) -
Whitworth T. M.,
Elmore Andrew Curtis
Publication year - 2002
Publication title -
remediation journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.762
H-Index - 27
eISSN - 1520-6831
pISSN - 1051-5658
DOI - 10.1002/rem.10023
Subject(s) - outwash plain , geology , environmental remediation , water table , hydrology (agriculture) , trap (plumbing) , phase (matter) , groundwater , trapping , geomorphology , contamination , environmental science , geotechnical engineering , glacier , chemistry , ecology , organic chemistry , environmental engineering , biology
Abstract Free‐phase light nonaqueous phase liquids (LNAPLs) may be trapped in certain stratigraphic andstructural features near or at contaminated sites due to seasonal or other variations in the water tableelevation. The purpose of this article is to point out particular subsurface conditions that are conducive totrapping of free‐phase LNAPLs and to suggest approaches to remediating LNAPL‐contaminated sitesexhibiting 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 containsthousands of these potential traps associated with the geomorphic surfaces mapped as outwash or braided streamterraces, which are covered with thin layers of backswamp clays. These traps may have closure heights ranging fromabout 1 to 7.5 meters or more and have variable lateral extents. Based on surface geomorphic analysis, thepotential LNAPL traps in the Lower Mississippi River Valley range in size from about 0.06 by 0.02 km to 4.19 by0.69 km. The apparent best remediation strategy for LNAPL sites located on these geomorphic surfaces, whichcontain these trapping structures, is to first determine if free‐phase is present. If it is present, and iscontained in one of the stratigraphic traps, the free‐phase can be removed through an extraction well orwells located at the trap apex. Geomorphic analysis and geophysical surveys may be necessary to accurately locatethe trap apex. The remaining residual hydrocarbons might best be remediated using an air sparging system, althoughit may be necessary to install air vents through the clay cap by backfilling augured holes with washed sand. If itis determined that, due to geometry, the dissolved LNAPL plume cannot be adequately remediated using an airsparging system, then groundwater circulation wells or monitored natural attenuation may be alternativetechnologies. © 2002 Wiley Periodicals, Inc.