Because recent legislation has made long-term ground-water monitoring an important issue, the methods by which data at a site are collected are of utmost importance. The traditional method of site assessment, which consists of drilling a number of monitor wells from which water samples can be taken and tested, does not provide enough information on which to base a long-term ground-water monitoring program. In situ, time-series measurements using a variety of geophysical techniques, on the other hand, can provide a wealth of information not available through traditional measurement methods.

Over the past ten years, geophysical measurements made from the surface have been used extensively at waste-disposal sites to assess natural hydrogeologic conditions and map the lateral and vertical distribution of contaminants. In recent years, downhole measurements have been used to produce more-detailed information about vertical contaminant distribution through the use of vertical profiles. Examples illustrating these applications are presented in the text. Made at appropriate intervals over a period of time, surface and downhole measurements can be used to provide highly accurate, three-dimensional maps of contaminant flow in ground water and provide a means of early leak detection at waste-disposal sites.

Because the values measured by geophysical methods are dominated by the specific conductance of pore fluids, they can be related to the specific conductance of ground-water samples, thus correlating geophysics with traditional assessment methods. Correlation between in situ, geophysical measurements and inorganic, ground-water sample analyses have been as good as 0.96 at the 95% confidence level. At one landfill, the correlation with total organic carbon was 0.85 at the 95% confidence level.

Three examples of sites where the spatial extents of contaminant plumes have been mapped using geophysical, time-series measurements are presented in this paper. They consist of a localized spill, a large landfill, and a flowing, abandoned well. Measurements taken over a period of time at each of these sites clearly illustrate how the actual dynamics of a contaminant plume can be assessed as it interacts with natural and man-induced variables.