A Conceptual Model For The Geoelectrical Response Of Lnapl Plumes In Granular Sediments

Geophysical site characterization investigations at fuel spill sites have been generally guided<br>by a working hypothesis which assumes that the Light Non-Aqueous Phase Liquids (LNAPL) are a<br>fully saturating phase of intrinsically very high electrical resistivity. Using observations from other<br>related sciences, and contrary geophysical observations, a different model is developed which treats<br>these spills as dynamic, changing systems dominated by surprisingly low resistivities. The major<br>geophysical response of a mature or established spill of this type is due to an anomalously low<br>resistivity zone in the lower vadose zone and upper portion of the aquifer. This zone is produced by<br>a high TDS leachate which is aperiodically flushed down from the volume of intimately mixed<br>hydrocarbon, water, oxygen and soil where microbial activity is a maximum. This leachate is a result<br>of acidification of the heterogeneous free/residual product levels by organic and carbonic acids and<br>is produced by the leaching and etching of the native mineral grains and grain coatings. This<br>conductive plume is generally coincident with the uppermost part of the anaerobic dissolved plume<br>as defined by hydrochemical studies, but is thin and most concentrated at the top of the aquifer. It<br>has been best detected and mapped by virtue of the amplitude shadow it causes on GPR profiles, and<br>more recently by direct measurement using Vertical Resistivity Probes (VRP) with readings every two<br>inches from the surface to more than 25 feet in depth. Other electrical geophysical methods function<br>only if conditions are optimal. The conductive zone has been known for some years by hydrochemists<br>and hydrogeologists, especially at sites where water samples are collected from short screens at multilevel<br>wells.