Case study: confirmation of TarGOST laser induced 

fluorescence DNAPL delineation with soil boring data

M.B. Okin, S.M. Carroll, W.R. Fisher and R.W. St. Germain

Abstract

Conventional approaches to delineation of dense non-aqueous phase liquids (DNAPL) associated with former manufactured gas plant (MGP) sites frequently over-estimate the thickness and extent of DNAPL. This project used the Tar-specific Green Optical Screening Tool (TarGOST®), developed

and operated by Dakota Technologies, Inc. (DTI), to provide a real-time, semi-quantitative and rapid assessment of the extent of NAPL in the subsurface at an MGP site in Fort Wayne, Indiana. TarGOST probing was conducted both on land using a cone penetrometer test (CPT) rig and from a barge on a river using a Geoprobe rig to assess DNAPL impacts.

Confirmation of the TarGOST probing results was performed using conventional (sonic drilling and Geoprobe) soil borings, and ultraviolet light (UV) fluorescence soil core photography at selected locations. These data supported the TarGOST results very well. Intervals of high fluorescence observed using TarGOST were confirmed in UV photographs and agreed with visual observations during soil borings. Further, there was a strong correlation between the presence of DNAPL as detected by TarGOST and the presence of sandy layers in the alluvium and glacial till as detected by the CPT. Use of TarGOST enabled delineation of the ‘source areas’ where impacts were seen vertically over considerable depths. In contrast, at locations distant from source areas, TarGOST illustrated the presence of DNAPL in thin, high-permeability seams ‘sandwiched’ between lower permeability units. This distinction between ‘source areas’ (thicker deposits) and distal residuals (very thin stringers) significantly affected the evaluation of the feasibility of remedial options considered for the site.

Key words: coal tar, laser, NAPL, PAH, sediments, TarGOST

© 2007 EPP Publications Ltd

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Article code 753