Case History Temporal geophysical signatures from contaminant-mass remediation Vukenkeng Che-Alota 1 , Estella A. Atekwana 1 , Eliot A. Atekwana 1 , William A. Sauck 2 , and D. Dale Werkema Jr. 3 ABSTRACT We have previously documented changes in bulk electrical conductivity, self-potential SP, and ground-penetrating-radar GPR reflections in a field setting caused by biogeochemical transformations of hydrocarbon-contaminated media. These transformations are associated with hydrocarbon biodegrada- tion. The results of surface geophysical surveys acquired in 1996, 2003, and 2007 document changes in geophysical signatures as- sociated with removing hydrocarbon mass in the contaminated zone. Initial investigations in 1996 showed that relative to back- ground, the contaminated area was characterized by higher bulk electrical conductivity, positive SPanomaly, and attenuated GPR reflections. Repeated surveys in 2003 and 2007 over the contami- nated area showed that in 2007, the bulk electrical conductivity had reverted to near-background conditions, the positive SP anomaly became more negative, and the zone of attenuated GPR reflections showed increased signal strength. Removal of hydrocarbon mass in the vadose zone over the plume by a soil vapor-extraction system installed in 2001 was primarily responsible for the changing geophysical responses. Although chemical data from groundwater showed a 3-m-thick con- ductive plume in 2007, the plume was not imaged by elec- trical resistivity. Forward modeling suggests that apparent bulk electrical conductivity of the saturated zone plume has to be three to five times higher than background values to be imaged by electrical resistivity. We suggest that removing hydrocarbon- contaminant-mass reduction by natural or engineered bioreme- diation can be imaged effectively by temporal geophysical surveys. INTRODUCTION Hydrocarbon contamination of groundwater from spills and leaky underground storage tanks threatens groundwater resources. Over the last decade, the use of noninvasive geophysical techniques has been instrumental in detecting and delineating subsurface zones of hydrocarbon contamination. We have documented anomalous in- creases in bulk electrical conductivity over areas of hydrocarbon contamination e.g., Sauck et al., 1998; Atekwana et al., 2000; Werkema et al., 2003; Atekwana et al., 2004a; Atekwana et al., 2004b, Atekwana et al., 2004d; Atekwana et al., 2005. Other geo- physical studies have also characterized subsurface hydrocarbon contamination effectively using electrical resistivity techniques e.g., Benson et al., 1997; Halihan et al., 2005; Kaufmann and De- ceuster, 2007; Yang et al., 2007, ground-penetrating radar GPR e.g., Daniels et al., 1995; Bermejo et al., 1997; Bradford, 2007; Cassidy, 2007, 2008 and self-potential SPe.g., Minsley et al., 2007. The characteristic geophysical response of hydrocarbon-contam- inated media has been attributed to a variety of physical, chemical, and biological mechanisms. For example, Sauck 2000 attributes the increase in the bulk electrical conductivity to higher pore-water conductivity resulting from aquifer solids weathered by the organic and carbonic acids produced during biodegradation. Other geophys- ical studies suggest a variety of mechanisms primarily related to mi- crobial alteration of the hydrocarbon and the host media e.g., Atek- wana et al., 2004c; Atekwana et al., 2004d; Minsley et al., 2007. Re- Manuscript received by the Editor 15 September 2008; revised manuscript received 7 January 2009; published online 26 June 2009. 1 Oklahoma State University, Boone Pickens School of Geology, Stillwater, Oklahoma, U.S.A. E-mail: vukenkeng.chealota@okstate.edu; estella.atekwana@ okstate.edu; eliot.atekwana@okstate.edu. 2 Western Michigan University, Department of Geosciences, Kalamazoo, Michigan, U.S.A. E-mail: bill.sauck@wmich.edu. 3 U. S. Environmental ProtectionAgency, Office of Research and Development, Las Vegas, Nevada, U.S.A. E-mail: werkema.d@epamail.epa.gov. © 2009 Society of Exploration Geophysicists. All rights reserved. GEOPHYSICS, VOL. 74, NO. 4 JULY-AUGUST 2009; P. B113–B123, 8 FIGS. 10.1190/1.3139769 B113 Downloaded 27 Jul 2009 to 139.78.87.30. Redistribution subject to SEG license or copyright; see Terms of Use at http://segdl.org/