Impact of hydrocarbon biodegradation on low frequency electrical properties of unconsolidated sediments

The influence of biodegradation processes and subsequent physicochemical changes on the low frequency electrical properties (e.g., real and imaginary conductivity) of unconsolidated sediments was investigated in laboratory sand columns and core sediments retrieved from a hydrocarbon contaminated site. The low frequency electrical measurements were conducted using induced polarization (IP) method in the frequency range 0.1--1000 Hz. Biological, geochemical, isotopic analyses, scanning electron microscopy images and surface area measurements were conducted to help in the interpretation of the low frequency electrical measurements.; In a laboratory column experiment, the biotic column (nutrient, dissolved diesel and bacteria) showed (a) temporal increase in the real, imaginary, and surface conductivity, and (b) temporal decrease in the formation factor. The abiotic columns (nutrient; and nutrient and dissolved diesel) showed no significant changes. Increase in microbial population numbers, decrease in organic carbon source, nitrate, and sulfate and increase in dissolved inorganic carbon and fluid conductivity were indicative of microbial activity in the biotic column. IP results of core sediments retrieved from the field showed that the magnitude of IP response (e.g., imaginary conductivity) for hydrocarbon contaminated sediments undergoing biodegradation was relatively higher compared to uncontaminated sediments. More specifically, samples from within the smear zone at the site and contaminated with residual hydrocarbon showed a relatively higher magnitude in the IP parameters (e.g., imaginary conductivity) compared to dissolved phase contaminated samples. Previous microbiological study at the site showed a high percentage of oil degrading microorganisms within the smear zone. Further, a laboratory column experiment was conducted to investigate the variations in the temporal changes of IP magnitudes associated with different phases of hydrocarbon contamination (dissolved vs. residual) observed from core sediments from the field. The results showed that the residual diesel contaminated column (residual phase of diesel + nutrients + bacteria) exhibited higher rates of biodegradation compared to the dissolved diesel contaminated column (dissolved phase of diesel + nutrients + bacteria). The higher rate of biodegradation and subsequent microbial-sand-fluid interactions are reflected in a 10--30% higher magnitude of temporal change of measured biological, chemical, isotopic, and electrical parameters in the residual diesel contaminated column. These results suggest that the higher magnitude of IP measurements observed on residual product zone from field cores are due to higher microbial activity.; The findings of this study illustrate the potential utility of low frequency electrical measurements for non-invasive monitoring of microbial activity at sites undergoing natural hydrocarbon degradation. The results of this study further suggest that the interpretation of geoelectrical data from near surface environments should consider effects of microbial processes.