Description
Time-domain Reflectometry (TDR) is a geophysical technique which uses an EM wave and parallel waveguides to measure the EM wave velocity of a particular material or medium. TOR calculated EM wave velocity can be used to derive an apparent dielectric constant for a medium of interest. The apparent dielectric constant is sensitive to changes in moisture content. Previous research has demonstrated that the displacement of water by light (less dense than water) non-aqueous phase liquids (LNAPL's) in sand, dramatically lowers the apparent dielectric constant of the medium. The focus of this project was to extend the usefulness of TDR by evaluating how the apparent dielectric constant changes for varying amounts of volumetric LNAPL content. Diesel fuel was chosen as the specific LNAPL. Three LNAPL infiltration experiments were performed with water saturated sand ranging in grain-size from fine-to-coarse. A vertical column was designed and provided the containment for the infiltration events. Waveguides located within the column were used to obtain dielectric data both prior to infiltration (background) and after infiltration (equilibrium). After each infiltration event, the column was placed in horizontal position, opened, and core samples removed from each waveguide location. Volumetric LNAPL content from each core sample was determined by the E.P.A. Method 418.1 and correlated with the equilibrium apparent dielectric constant derived from that waveguide site. The data produced from this experiment demonstrates that the apparent dielectric constant for various sands decreases as LNAPL (diesel fuel) displaces water from soil pore space. An apparent dielectric constant near 5 was measured for LNAPL contents between 0.25 and 0.35 mL/cm3. An apparent dielectric constant near 25 was measured for minute or nonexistent LNAPL concentrations. Due to limited data, an empirical relationship between diesel fuel content and apparent dielectric constant cannot be derived from these data. However, some data do correlate well with Topp et al.'s (1980) air content, which has similar dielectric characteristics. Results from this experiment have also demonstrated that TOR can be used as a qualitative tool in the detection and movement of LNAPL's in a two-phase system.
