Geochemical and isotopic characteristics associated with high soil conductivities in a shallow hydrocarbon-contaminated aquifer

Data collected from a network of in-situ vertical resistivity probes (VRPs) deployed at a hydrocarbon-contaminated site in SW Michigan showed high conductivities associated with the zone of contamination. Within the contaminated portion of the aquifer, different phases of hydrocarbon impact are recognized, namely, zones with residual and dissolved phase hydrocarbons (RDH) and zones where these phases coexist with free product (RDFH). Bulk soil conductivities were highest (12 to 30 mS/m) in the RDFH zone compared to the RDH zone (10 to 25 mS/m). Geochemical and isotopic data from closely spaced vertical samples within the high conductive zones were used to provide geochemical evidence for biodegradation and to investigate redox processes occurring within the conductive zones. Depth distribution of TEAs and educts showed evidence of reduction of nitrate, iron, manganese, and sulfate across steep vertical gradients. Within the portion of the plume characterized by RDH, SO₄ reduction has supplanted denitrification via dissimilatory nitrate reduction, and the reduction of Fe (III) and Mn(IV) as the major observed redox process. This zone was also characterized by the highest DIC. The δ 13C_DIC values of −16.9 to −9.5‰ suggest that DIC evolution within this zone is controlled by carbonate dissolution through enhanced CO₂ production related to microbial hydrocarbon degradation. Within the portion of the aquifer with RDFH, DIC was lower compared to the RDH location with an associated δ13C_DIC in the range of +6.5 to −4.4‰. Both the DIC and δ 13C_DIC suggest that methanogenesis is the dominant redox process.; With respect to mineral weathering as a possible source of ions contributing to high conductivities, the results show higher concentrations of Na, Ca, and Mg in the contaminated portion of the aquifer compared to uncontaminated parts. This is consistent with the weathering of carbonate and Na and Ca feldspars, the dominant minerals in the aquifer. Higher TDS at the contaminated locations were also coincident with higher DIC, and increased CO₂ production. In general, both TDS and bulk soil conductivity increased with depth at the contaminated and uncontaminated locations. TDS and bulk conductivity were positively correlated at the uncontaminated location, suggesting that TDS was a good predictor of bulk conductivity. However, at the contaminated locations, the correlation between the bulk conductivity and the TDS was poorer, the lack of correlation being more so in portions of the aquifer with free phase hydrocarbons (RDFH). This may suggest that the TDS may not reliably predict the bulk conductivities of hydrocarbon-impacted soils. The present study seeks to provide a likely interrelationship between redox processes, biomineralization of hydrocarbons, and high bulk soil conductivities. Moreover, high conductivities measured at hydrocarbon-contaminated sites may be useful in assessing the potential for natural attenuation and to monitor intrinsic bioremediation at these sites.