GROUND-PENETRATING RADAR FOR THE DETECTION OF LIQUID CONTAMINANTS

The significance of ground penetrating radar (GPR) as a tool to detect near-surface contaminants is illustrated at a site in the Midwest representing petroleum product above the water table. Tests show that ground penetrating radar may provide a means of mapping hydrocarbons in the vadose zone. Results of controlled surveys in a sand test pit at The Ohio State University demonstrate conclusively that there is a clear GPR anomaly over containers of diesel fuel and containers containing the host sand material saturated with diesel fuel. Comparisons of GPR data measured at different times of the year (summer, fall, and winter) at a gasoline spill site in northern Indiana shows direct information on layers and lenses in the vadose zone that tend to accumulate water during times of high moisture and subsequently lose the moisture during dry periods. GPR data collected in the winter over partially frozen ground provided measurements that were more sensitive to the presence of the gasoline than measurements that were made during the summer and fall. The relative propagation transparency of the near-surface zone for GPR measurements from winter data over fozen ground allowed detection of the water table that could not be confidently identified from two of the other data sets. Four important points can be concluded from a comparison of the four data sets: (1) the quality and repeatability of GPR measurements over a clean sand depends on the amount of moisture located in the unsaturated zone above the water table; (2) reflections from sedimentary features can be distinguished from reflections from percolating groundwater; (3) GPR measurements made during the dry month of August are nearly devoid of reflections above the gasoline product, indicating that the water in the unsaturated region may have been displaced by liquid gasoline, or by gasoline vapors; and (4) most importantly, these tests illustrate the resolving power and sensitivity of ground penetrating radar.