A Borehole Geophysical Method for the Detection and Quantification of DNAPL in Saturated Soils

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DNAPL, Permittivity, CPT, Neutron log, Density log

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Quantifying the volume of dense, non-aqueous phase liquids (DNAPL) in the subsurface is difficult. To date, most studies have estimated DNAPL volume indirectly from dissolved concentrations. DNAPL have very low permittivity (dielectric constant) values (3–4) compared with water (80). Replacing water in pore spaces with DNAPL will cause a decrease in the bulk permittivity. A decrease in porosity will also cause a decrease in the permittivity. In order to use the permittivity to quantify the volume of DNAPL present in saturated soils, it is necessary to measure porosity. In this study, bulk permittivity was measured by a probe on a cone penetrometer tool (CPT). The porosity was calculated from bulk density measurements obtained with an active-gamma logging tool. DNAPL volume is calculated using a modification of Roth's permittivity equation for a three component system, water, DNAPL and matrix. The active-gamma logs were run in small diameter wells adjacent to the CPT locations. Natural gamma and thermal neutron logs were also run in the boreholes.

To test the method, a field site was selected that is known to be heavily contaminated with DNAPL, principally trichloroethylene (TCE). The site is on a large barrier island complex in the southeastern US. The site stratigraphy consists of about 15 m of sands, silts and minor clay, with silt content generally increasing with depth. Boundaries of finer-grained soils delineated by cross plots of CPT variables are present at elevations of − 2.4, − 5.5, − 8.9, and − 9.8 m. Calculated fractional DNAPL, the DNAPL volume over total volume, shows peaks at elevations of − 2.4, − 5.5, − 9.1, and − 9.8 m, near the tops of the finer-grained units. Fractional DNAPL volume averages about 0.12, with maximum values of about 0.25. Porosity ranges from about 0.3 to 0.55, averaging 0.44. DNAPL saturation is the DNAPL volume divided by pore volume, and is a measure of the volume of available pore space occupied by DNAPL. The highest DNAPL saturation values are near the tops of the finer-grained layers. DNAPL saturation averages about 0.25 in the contaminated zone, with maximum values of about 0.45. The method shows significant promise for quantifying DNAPL volume and saturation using cone-penetrometer testing, making it very suitable for saturated, unconsolidated soils contaminated with residual DNAPL.

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Journal of Applied Geophysics, v. 60, issue 2, p. 87-99