Direct detection and monitoring of inorganic contaminant plumes is primarily done by using electromagnetic (EM) methods. Lahti and Hoekstra [1991] used ground EM surveys to characterize brine migration at three oil field brine disposal sites in southwest Texas. Frequency-domain EM (terrain conductivity meter or TCM) surveys were used to determine the lateral extent of brine migration; time-domain EM (transient EM or TEM) soundings were used to determine the vertical distribution of brine.
Circumstances in which hydrocarbon and organic
contamination can be detected by noninvasive geophysical methods
at hazardous waste sites are described by Olhoeft
[1992a]. Contaminants that are insoluble in water, such as
tetrachlorethene (perchloroethylene or PCE), a dense nonaqueous
phase liquid (DNAPL) used as a dry cleaning agent, and toluene,
a light nonaqueous phase liquid (LNAPL) used as a component of
gasoline, may be detected with GPR. Toluene can sometimes be detected
with the complex resistivity technique as a result of its chemical
reactions with clay minerals. The best way to identify these
contaminants in large concentrations is by detecting their movement,
through repeated measurements with time. Unfortunately measurements
prior to a spill will rarely exist. Geophysical methods are unable
to detect these chemicals at parts per billion concentrations that
are of regulatory concern.
Daniels et al. [1992] studied the GPR response of LNAPL's. They demonstrated that GPR can detect containers of kerosene and sand-water-kerosene mixtures buried in sand. Containers filled with sand only produced no signal. Their GPR studies of a gasoline spill area indicated that the gasoline produces anomalously low amplitude reflector response above the water table when compared to adjacent areas devoid of hydrocarbons.
Electromagnetic (EM) borehole logging, has been applied to the monitoring of contaminant plumes in glacial sand-and-gravel aquifers at five sites in the northeastern U.S., as reviewed by Williams et al. [1993]. Sources of contamination at these sites include highway deicing salt and leachate from landfills and sewage disposal systems. All these sources produce contaminant plumes that are electrically conductive because they have elevated concentrations of dissolved solids. Detailed lithologic and/or gamma logs are required to identify zones in which elevated EM conductivity is due to silt and clay beds rather than contamination. The monitoring well can be cased with polyvinyl chloride (PVC) to prevent collapse and to allow EM induction logging at regular time intervals to monitor temporal changes in water quality.