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

Geophysical site characterization investigations at fuel spill sites have b een generally guided by a working hypothesis which assumes that the light n on-aqueous phase liquids (LNAPL) are a fully saturating phase of intrinsica lly very high electrical resistivity. Using observations from other related sciences, and contrary geophysical observations, a different model is deve loped which treats these spills as dynamic, changing systems dominated by s urprisingly low resistivities; The major geophysical response of a mature o r established spill of this type is due to an anomalously low resistivity z one 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 aperiodi cally flushed down from the volume of intimately mixed hydrocarbon, water, oxygen and soil near the base of the vadose zone where microbial activity i s a maximum. This leachate is a result of acidification by organic and carb onic acids of the water-filled capillaries in the heterogeneous mixing zone at the free/residual product level, and is produced by the leaching and et ching of the native mineral grains and grain coatings. This conductive inor ganic plume is generally coincident with the uppermost part of the anaerobi c dissolved hydrocarbon plume as defined by hydrochemical studies, but is t hin and most concentrated at the top of the aquifer. It has been best detec ted and mapped by virtue of the amplitude shadow it causes on ground penetr ating radar (GPR) profiles, and more recently by direct measurement using v ertical resistivity probes (VRP) with readings every 5.08 cm from the surfa ce to more than 7.5 m in depth. Other surface electrical geophysical method s (VES, electromagnetic method (EM), and multi-spaced horizontal resistivit y profiling) can define this zone only if conditions are optimal. The condu ctive zone has been known for some years by hydrochemists and hydrogeologis ts, especially at sites where water samples are collected from short screen s at multi-level wells. (C) 2000 Elsevier Science B.V. All rights reserved.