BOREHOLE AND SURFACE GEOPHYSICAL INVESTIGATIONS: SAVANNAH, GEORGIA

Geophysical investigations conducted at a military installation in Georgia were used to characterize the subsurface geology controlling the migration and entrapment of DNAPL contaminants. The geology consists of coastal plain sediments, of which the presence or absence of clay layers is probably the greatest control for the vertical migration of DNAPL. Geophysical methods consisted of two-dimensional electrical resistivity imaging (2D-ERI), borehole electrical and natural-gamma logging, and direct-push electric logging.

Results from modeling the 2D-ERI data provide gross spatial distributions and trends of electrical properties of the subsurface, which can be directly correlated with the underlying geology. The 2D-ERI data have mapped an electrically resistive layer ranging from 7 to 15m in thickness interpreted to be sands and silts of Pliocene to Holocene in age, which represent a potential contaminant storage and/or migration zone. The base of this zone coincides with an upper clay unit where the resistive layer is thinner (<9 m), and with a lower clay unit where it is thickest. Borehole logging was used to provide vertical control on the position and extent of clay horizons. A thin clay horizon was consistently detected at an average depth of 8m, and most likely acts as the primary barrier against vertical migration of the contamination as contaminant levels are consistently the greatest immediately above.

Comparison between the borehole and surface geophysical data indicate that the 2D-ERI models do not fully resolve all of the clay horizons, and that greater vertical resolution is afforded by borehole logging. Combined use of borehole and surface data allows for a more consistent interpretation of the 2D-ERI models, and by extension, provides better information for placing monitor/sampling wells. A topographic depression on the clay surface was mapped using both the resistivity and borehole data.