Kessouri et al 2021

There is a growing need to assess long-term impacts of active remediation strategies on treated aquifers. A variety
of biogeochemical alterations can result from interactions of the amendment with the aquifer, conceivably
leading to a geophysical signal associated with the long-term alteration of an aquifer. This concept of postremediation
geophysical assessment was investigated in a shallow, chlorinated solvent-contaminated aquifer
six to eight years after amendment delivery. Surface resistivity imaging and cross-borehole resistivity and
induced polarization (IP) imaging were performed on a transect that spanned treated and untreated zones of the
aquifer. Established relationships between IP parameters and surface electrical conductivity were used to predict
vertical profiles of electrolytic conductivity and surface conductivity from the inverted cross-borehole images.
Aqueous geochemistry data, along with natural gamma and magnetic susceptibility logs, were used to constrain
the interpretation. The electrical conductivity structure determined from surface and borehole imaging was
foremost controlled by the electrolytic conductivity of the interconnected pore space, being linearly related to
fluid specific conductance. The electrolytic conductivity (and thus the conductivity images alone) did not
discriminate between treated and untreated zones of the aquifer. In contrast, inverted phase angles and surface
conductivities did discriminate between treated and untreated zones of the aquifer, with the treated zone being
up to an order of magnitude more polarizable in places. Supporting aqueous chemistry and borehole logging
datasets indicate that this geophysical signal from the long-term impact of the remediation on the aquifer is most
likely associated with the formation of polarizable, dispersed iron sulfide minerals.