AN EVALUATION OF THE POTENTIAL OF GROUND PENETRATING RADAR TO DETECT THE PRESENCE AND SPATIAL EXTENT OF SUBSURFACE HYDROPHOBIC SOIL LAYERS

It is well documented that the presence of hydrophobic (water repellant) soil layers in many environments can result in increased rates of storm runoff and erosion. In the case of high energy mountain environments, such as those found in Southern California, USA the combination of steep slopes, relatively erosive soils, infrequent but often intense rainfall events, a landscape subject to fire (both natural and anthropogenic in origin), and the extensive presence of vegetation such as chaparral and related species capable of releasing hydrophobic layer forming compounds during fires, can result in greatly increased rates of runoff and erosion, often resulting in debris and hyperconcentrated flows. One of the challenges involved in evaluating the potential increased levels of runoff and sediment production of fire impacted areas in such settings has been the difficulty of determining the spatial extent, strength, and depth of the hydrophobic layers generated by the fires. In part this is due to the highly spatially variable nature of the fires themselves as well as the variability of the soils and vegetation. To date the primary method of evaluating hydrophobic conditions has involved the use of the labor intensive, point specific, WDPT (Water Drop Penetration Test) often under very challenging field conditions.

A series of laboratory and field tests were undertaken at several fire (burnt watershed) sites in Southern California in an effort to evaluate the potential of ground penetrating radar (GPR) to detect the presence and extent of hydrophobic layers. The laboratory tests, conducted in cooperation with the USGS Petrophysics laboratory in Denver, CO., demonstrated that high frequency GPR was not able to directly detect the presence of hydrophobic materials or layers. However, in a series of field experiments conducted on fire impacted watersheds in Southern California we sought to utilize the ability of GPR to document variation in soil moisture levels to detect the presence and spatial extent of hydrophobic layers. These field experiments have served to demonstrated the potential of this approach. Both the presence and spatial extent of hydrophobic soils were successfully documented. Parallel WDPT’s were used to confirm the success of the GPR approach. Work is ongoing to refine this approach.