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THE RELATIONSHIP OF TOTAL DISSOLVED SOLIDS MEASUREMENTS TO BULK ELECTRICAL CONDUCTIVITY IN AN AQUIFER CONTAMINATED WITH HYDROCARBON
Citation:
Atekwana, E. A., R. Rowe, E. Atekwana, D Werkema, AND F. Legall. THE RELATIONSHIP OF TOTAL DISSOLVED SOLIDS MEASUREMENTS TO BULK ELECTRICAL CONDUCTIVITY IN AN AQUIFER CONTAMINATED WITH HYDROCARBON. JOURNAL OF APPLIED GEOPHYSICS. Elsevier Science BV, Amsterdam, Netherlands, 56:281-294, (2004).
Impact/Purpose:
Research is being conducted to improve and evaluate the resolution of the CR, EM, seismic, and GPR methods over complex geological formations (such as fractured geologies) and to evaluate the capability of these geophysical methods to delineate subsurface organic contaminants.
Description:
A recent conceptual model links high bulk electrical conductivities at hydrocarbon impacted sites to higher total dissolved solids (TDS) resulting from enhanced mineral weathering due to acids produced during biodegradation. In this study, we investigated the vertical distribution of bulk conductivity, and the TDS, and specific conductance in groundwater in order to evaluate the above model. Our results showed steep vertical gradients in bulk conductivity and TDS indicating vertical and spatial heterogeneity at the site. Our results also showed higher TDS in contaminated locations consistent with the above model. In general, increasing groundwater specific conductance (fluid conductivity) resulted in higher bulk conductivity. We observed that at fluid conductivities <40 mS/m), bulk conductivity was inversely related to fluid conductivity. For fluid conductivities greater that 40 mS/m, bulk conductivity increased with increasing fluid conductivity. However, at fluid conductivities greater than 80 mS/m there was significant scatter especially for the contaminated samples. The lack of dependence of bulk conductivity on fluid conductivity at low fluid conductivity and significant scatter at higher fluid conductivity suggest that bulk conductivity could not be entirely accounted for by increase in TDS at the contaminated locations. Although this could simply be related to variation in lithology, we suggest that biodegradational processes in contaminated locations not only alter the fluid conductivity but may also induce changes in the aquifer matrix ( e.g. pitting, etching, and precipitation on mineral surfaces). Such alterations of the aquifer matrix may change the electrical properties of contaminated sediments, and combined with higher fluid conductivities (TDS) can be used to explain spatial and vertical variability in bulk conductivity at contaminated locations. More work is needed to fully understand the impact of biogeochemical changes on the aquifer matrix and the potential influence of such changes on the bulk electrical properties.