Geophysical Detection Of Hydrocarewn And Organic Chemical Contamination

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