Geophysical Detection Of Hydrocarewn And Organic Chemical Contamination

Abstract

Unambiguous detection of hydrocarbon and organic contamination is the most difficult task for noninvasive geophysical methods at hazardous waste sites. The difficulty is two-fold: 1) the low level of geophysical contrast that these contaminants provide against the background soil and rock, and 2) the low level of contaminant concentration considered to be of regulatory concern. Yet, electrical and electromagnetic methods can sometimes detect some hydrocarbon and organic contaminants. The important questions are what levels of contamination can be reliably detected and where? The most important consideration in answering these questions is the level of interaction that occurs between the contaminating chemical and the host matrix materials. The most obvious of contaminants are those which either are insoluble in water or which chemically react with clay minerals. The best understood examples are tetrachlorethene (common dry cleaning solvent) and toluene (common industrial solvent and principle component in gasoline). Both have low solubility in water, high electrical resistivity and low dielectric permittivity. Thus, both are visible to ground penetrating radar (though toluene floats on the water table while tetrachlorethene sinks). Toluene is also catalyzed on the surface of montmorillonite to polymerize into bibenzyl, and the electrochemistry of the polymerization reaction is observable with the complex resistivity technique. Some organic contaminants such as alcohols, which dissolve in water, are essentially without geophysical trace. In between are interactions such as insoluble lenses floating on the water table, suppression of the capillary fringe, change in soil wetting, interference with cation exchange, and others, such as a change in the background geological noise statistics, that sometimes produce a detectable geophysical signature. Few geophysical signatures are unique indicators of hydrocarbon or organic chemical contaminants, but repeated measurements with time can often detect the movement of contaminant fluids, more uniquely indicating their presence. No hydrocarbon or organic chemical contaminant produces a geophysical contrast that is observable at the parts-per-billion concentration level of regulatory concern. Even when the contaminant itself produces no detectable direct geophysical signature, sometimes it may be found by indirect means through the use of geophysical mapping and characterization of geohydrological heterogeneity as a guide to drilling likely traps of the contaminant.