Appendix F Previous Geophysical Characterization and Monitoring at the Orfrc

Hydrogeological and Geochemical Characterization Both surface and crosshole tomographic geophysical data have been used in the saprolite and carbonate gravel (Areas 3 and 2, respectively) to delineate large scale geologic structure, nitrate plume boundaries, and localscale fracture zonation. Surface electri seismic data were originally used in a reconnaissance mode to choose the location for the Plot 2 biostimulation experime Figure F1 illustrates electrical resistivity variations along the A-A’ transect adjacent to the S-3 Ponds area. Comparison with measured nitrate concentrations (obtai from wellbore locations) shows that at this site, zones of low electrical resistivity (o high electrical conductivity) are correlated with zones having higher nitrate concentration. This observation is consistent with our other studies at the site, which suggest that electrical methods are more sensitive to variations in pore fluid ionic strength at the FRC than to hydrogeological heterogeneity. Superimposed on the electrical profile are contour lines indicating the seismic velocity gradients obtained from surface seismic data. These velocity contours can be interpreted in terms of gross lithological structure. The results of this surface geophysical study, which is further described by Watson et al. (2005), were used to define the Plot 2 biostimulation study site location, highlighted by the black rectangle in Figure F1. A