Conductivity Profile Rate of Change from Field and Laboratory Data within Biodegrading Petroleum Hydrocarbon

Abstract

We present the results of long term (500 days) measurements of the bulk conductivity in a field and laboratory experiment. Our objective was to determine the rate of change in bulk conductivity and whether this rate of change correlated with the petroleum hydrocarbon degradation. In the field, bulk conductivity was obtained monthly from vertical probes installed at a hydrocarbon contaminated site undergoing biodegradation. In the laboratory, conductivity measurements were made in sand columns simulating the biodegradation of diesel fuel. In general, both the field and laboratory results show increasing bulk conductivity over time from depth zones impacted with petroleum hydrocarbon contamination. The highest increase was observed above the water table where hydrocarbon contamination was in residual and free phase (petroleum smear zone). We fitted the temporal bulk conductivity change in the contaminated sediments and the uncontaminated sediments to an exponential model and then subtracted the two resulting in the bulk conductivity rate of change due to the presence of petroleum hydrocarbons (kdsigma). The highest rate constant, or kdsigma (the slope of the model line), was 0.0061 per day for the field data and 0.0023 per day for the laboratory sand columns. The laboratory data showed a broad conductivity rate increase occurring stratigraphically coincident with the hydrocarbon contamination in the free and residual phase above the saturated zone. The field data showed a general increasing rate of conductivity change within this zone, but also included zones of decreasing change. This is interpreted in part to a dynamic water table and the development of a hydrocarbon smear zone which contains a variable air‐water‐LNAPL (light non‐aqueous phase liquid) mixture. The petroleum hydrocarbon degradation rate of 0.0539 per day was determined for sediments in the saturated zone and 0.107 for sediments in the unsaturated zone. These values are consistent with higher bulk conductivity rates of change in the unsaturated zone except these rates are increasing versus decreasing for the LNAPL concentration rate of change. The bulk conductivity results suggest that the greatest rate of change occurred stratigraphically within hydrocarbon contaminated zones and the magnitude of the rate of change is much less than the magnitude of the hydrocarbon degradation rate. Within the hydrocarbon impacted zone the kdsigma data reveal an inverse relationship with the hydrocarbon concentration rate of change.