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Electrical Signatures of Ethanol-Liquid Mixtures: Implications for Monitoring Biofuels Migration in the Subsurface
Citation:
Personna, R., L. Slater, D. Ntarlagiannis, D. Werkema, AND Z. Szabo. Electrical Signatures of Ethanol-Liquid Mixtures: Implications for Monitoring Biofuels Migration in the Subsurface. JOURNAL OF CONTAMINANT HYDROLOGY. Elsevier Science Ltd, New York, NY, 144:99-107, (2013).
Impact/Purpose:
In the last two decades, the production of EtOH, one of the most common biofuels in the USA, has substantially increased due to regulations aiming at reducing air pollution and providing a supplement to petroleum. Accidental releases of large volumes of EtOH, particularly during transportation (Spading et al, 2011) and at storage facilities (McDowell et al., 2003), have raised concerns about its environmental fate and potential risks to groundwater (Powers et al, 2001a). Ethanol is currently treated as an emerging contaminant (Gomez and Alvarez, 2010) that may induce substantial adverse effects in the subsurface environment (EPA, 2011). As a powerful disinfectant that has been long used as an antiseptic, EtOH at concentrations as low as 6% v/v is very toxic to soil and aquifer microorganisms (Nelson et al, 2010). Ethanol toxicity can lead to significant alterations in microbial growth, metabolism, viability (Ingram L.O., 1990; Nelson et al., 2010) and community structure (Cápiro et al., 2008; Ma et al., 2011). The persistence of EtOH toxicity in the subsurface may ultimately lead to a significant decrease in microbial population and activity, thus affecting the overall subsurface microbial processes including biodegradation of contaminants.
Description:
Ethanol (EtOH), an emerging contaminant with potential direct and indirect environmental effects, poses threats to water supplies when spilled in large volumes. A series of experiments was directed at understanding the electrical geophysical signatures arising from groundwater contamination by ethanol. Conductivity measurements were performed at the laboratory scale on EtOH-water mixtures (0 to 0.97 v/v EtOH) and EtOH-salt solution mixtures (0 to 0.99 v/v EtOH) with and without a sand matrix using a conductivity probe and a four- electrode electrical measurement over the low frequency range (1-1000 Hz). A Lichtenecker-Rother (L-R) type mixing model was used to simulate electrical conductivity as a function of EtOH concentration in the mixture. We found a decrease in measured conductivity magnitude (|σ|) for all three experimental treatments with increasing EtOH concentration. The applied L-R model fit the experimental data at concentration ≤ 0.4v/v EtOH, presumably due to predominant and symmetric intermolecular (EtOH-water) interaction in the mixture. The deviation of the experimental |σ| data from the model prediction at higher EtOH concentration may be associated with hydrophobic effects of EtOH-EtOH interactions in the mixture. The |σ| data presumably reflected changes in relative strength of the three types of interactions (water-water, EtOH-water, and EtOH-EtOH) occurring simultaneously in EtOH-water mixtures as the ratio of EtOH to water changed. We found no evidence of measurable polarization effects at the EtOH-water and EtOH-water-mineral interfaces over the investigated frequency range. Our results indicate the potential for electrical measurements to characterize and monitor EtOH spills in the subsurface.