Combination Of Downhole Gamma Logging And Soil Vapor Measurements To Track Low Level Chlorinated Solvent Contamination In The Deep Vadose Zone

Downhole gamma logging and a soil vapor investigation were used to identity an<br>unknown source and the subsurface pathway of chlorinated solvents migrating to ground<br>water at 110’ below ground surface (bgs). Thirty (30) borings were installed using Direct<br>Push Technology with 1.75” OD, 1 .OO” ID drive pipe to as deep as 67’ at a New Mexico<br>State Highway and Transportation Department Maintenance Yard in Deming, NM. While<br>the pipe was still in the ground, each hole was gamma logged to establish optimum<br>vertical locations for the installation of permanent soil vapor implant points. Two to four<br>points were then installed in 2-to-11 foot thick sand filter-pack beds, separated by granular<br>bentonite grout, in each boring. The borings were then completed as monitoring wells<br>with traffic covers. Soil gas samples were collected from each vapor implant point and<br>analyzed in a mobile environmental laboratory using EPA method 8010 (modified) for<br>chlorinated volatile organic compounds, including 1,l -dichloroethene (DCE), 1,l<br>dichloroethane (DCA), and 1 , 1, 1 trichloroethane (TCA). Confirmation analyses were run<br>by GC-MS onsite. The resulting three-dimensional representation clearly identified the<br>source area and a plume declining in concentration and descending in depth toward the<br>known area of ground water contamination. The gamma logging was instrumental in the<br>vertical placement of vapor implant points to obtain optimum gas samples, immediately<br>above potential low-permeability layers. Because the Direct Push borings were installed<br>without soil sampling or examination, the gamma logging was very useful in the<br>characterization of subsurface lithology for assessing source location and vapor plume<br>pathways.