A model for the resistivity structure of LNAPL plumes and their environs in sandy sediments

Geophysical site characterization investigations at fuel spill sites have been generally guided by a working hypothesis which assumes that the light non-aqueous phase liquids (LNAPL) are a fully saturating phase of intrinsically very high electrical resistivity. Using observations from other related sciences, and contrary geophysical observations, a different model is developed which treats these spills as dynamic, changing systems dominated by surprisingly low resistivities; The major geophysical response of a mature or established spill of this type is due to an anomalously low resistivity zone in the lower vadose zone and upper portion of the aquifer. This zone is produced by a high total dissolved solids (TDS) leachate which is aperiodically flushed down from the volume of intimately mixed hydrocarbon, water, oxygen and soil near the base of the vadose zone where microbial activity is a maximum. This leachate is a result of acidification by organic and carbonic acids of the water-filled capillaries in the heterogeneous mixing zone at the free/residual product level, and is produced by the leaching and etching of the native mineral grains and grain coatings. This conductive inorganic plume is generally coincident with the uppermost part of the anaerobic dissolved hydrocarbon plume as defined by hydrochemical studies, but is thin and most concentrated at the top of the aquifer. It has been best detected and mapped by virtue of the amplitude shadow it causes on ground penetrating radar (GPR) profiles, and more recently by direct measurement using vertical resistivity probes (VRP) with readings every 5.08 cm from the surface to more than 7.5 m in depth. Other surface electrical geophysical methods (VES, electromagnetic method (EM), and multi-spaced horizontal resistivity profiling) can define this zone only if conditions are optimal. The conductive zone has been known for some years by hydrochemists and hydrogeologists, especially at sites where water samples are collected from short screens at multi-level wells. (C) 2000 Elsevier Science B.V. All rights reserved.