Participant: Lee Slater, Department of Earth & Environmental Sciences, Rutgers-Newark
Our previous geophysical research, funded in part by the Meadowlands Environmental Research Institute (MERI) and in part by the New Jersey Water Resources Research Institute (NJWRRI), demonstrated how state-of-the-art electrical geophysical surveys could be employed to investigate shallow water wetland environments, such as Kearny Marsh, from shallow draft survey vessels1. Although we were successful in (a) non-invasively delineating the distribution of junk metal associated with illegal dumping in Kearny Marsh, and (b) identifying a possible groundwater contaminant plume from the 1E Landfill to the north of the marsh, we were unable to conclusively delineate a plume emanating from the Keegan Landfill. This was somewhat surprising, as a 1999 study performed by Langan2, based on geochemical analysis of groundwater and marsh sediment pore water samples, supported the presence of a plume associated with this landfill. The goal of this work is to employ an exciting, innovative and topical geophysical method to the study of the Keegan Landfill in an effort to resolve the ambiguity between the Langan report and the results of our previous geophysical surveys. A revolutionary new field of research, termed “Biogeophysics” has evolved in recent years. Biogeophysics fuses biological and geophysical research to improve knowledge of microbial processes in the subsurface and is a central theme of research conducted by my group at Rutgers-Newark. Naudet et al.3 presented fascinating results whereby a relatively simple geophysical survey known as “self potential” (SP), based on measuring natural electrical currents in the earth, was used to delineate the extent of a contaminant plume emanating from the Entressen Landfill in France. Based on this work, these authors proposed a model for a natural battery in the earth, defined as a “geobattery”, associated with microbial breakdown of contaminants at the plume edge (Fig 1). This battery is hypothesized to exist as (1) the plume boundary is associated with a strong redox gradient between highly reducing conditions with the plume (due to biodegradation and oxygen depletion) and oxidized zones outside of the plume, and (2) the microbial biofilms can grow to link these two zones and provide an electron conductor to complete the circuit required for the geobattery. This work has received a great deal of attention as it implies that a simple geophysical method could be used to detect the edge of contaminant plumes undergoing microbial alteration, Surprisingly, the method has never been authenticated in an independent study. The objectives of this work are to (1) collect a geophysical dataset that may validate the revolutionary Naudet et al. model(2) illustrate that this SP approach can be applied to shallow water wetlands such as Kearny Marsh,(3) define the plume boundary associated with the Keegan Landfill, and (4) reinforce Rutgers-Newark as a center for research in the Biogeophysics arena.
Statement of Research Question:
The research questions posed in this proposal are, (1) “Do microbial communities involved in the biodegradation of organic waste matter in landfills drive a detectable electrical field in the earth that can be observed with electrodes placed at the surface, and (2) “can this phenomenon be used to map a plume front associated with the Keegan Landfill that was undetectable using conventional (resistivity and electromagnetic) geophysical methods?”
The self potential (SP) geophysical method is surprisingly simple: a high-quality, non-polarizing electrode, combined with a precision voltmeter, is used to record the voltages, resulting from natural electrical current flow in the earth, at the earth surface. We will first map the 2D distribution of SP across the landfill and into Keegan marsh within the vicinity of the landfill. The SP measurements on the Keegan landfill will be made in a conventional manner by implanting the electrode into the surficial soil. However, the measurements obtained within the Keegan Landfill will be obtained adopting a novel procedure that we developed as part of our previous MERI and NJWRRI funded research on Kearny Marsh, whereby geophysical measurements are obtained `in survey’ from a shallow-draft paddleboat and spatially georeferenced with precision global positioning by satellite (GPS). In addition to the SP data, point measurements of the redox potential (eH) of the pore fluids will be obtained on a sparser grid across the site. In order to further independently evaluate the contaminant distribution around the landfill, we hope to perform heavy metals analysis of a limited number of sediment samples using the atomic absorption spectrometer available at MERI. We will also collect soil samples from the vicinity of Keegan Landfill and Kearny Marsh to conduct a novel, yet simple, laboratory biogeophysical experiment in conjunction with Dr. Nathan Yee (Rutgers, New Brunswick). In this experiment, native microbial communities within the soil from Kearny Marsh will be placed in columns and continuously fed with a lactate medium such that a strong redox gradient develops across the column. This experiment will test a recently proposed concept that the geobattery exists because microbial biofilms develop a network of electronically conductive nanowires that shuttle electrons between reduced and oxidized zones.
The scientific outcomes of this work include:
- An independent study to test the Naudet et al. geobattery model3 described above
- Further geophysical evaluation of contamination within the vicinity of Keegan Landfill that was undetected with conventional geophysical methods
- Laboratory data to evaluate whether nanowire networks within biofilms explain the presence of the geobattery
This research will form part of the ph.d. dissertation of Chi Zhang, who entered the Rutgers-Newark Environmental Science Ph.D. program in the Fall 2006 semester. Chi was specifically recruited in response to her keen interest, as established in direct communication over the last year, to conduct biogeophysics research. The three-year National Science Foundation (NSF) award EAR 0433739 “Collaborative Research: Investigating the Impact of Microbial Interactions with Geologic Media on Geophysical Properties: Implications for Assessing Geomicrobiology Processes” expires on 09/30/07. This work has been laboratory-based; our planned grant-renewal application will focus on transferring our effort to the field environment. The results obtained in this study will provide critically needed demonstration data to support this renewal effort.
- Mansoor*, N., Slater, L. & Artigas, F. Case history: High-resolution geophysical characterization of shallow-water wetlands. Geophysics 71, B101-B109 (2006).
- Langan Environmental & Engineering Services (LEES), Se diment and water sampling report, Kearny Marsh, Kearny, NJ LEES Report, Elmwood Park, NJ. (1999).
- Naudet, V., Revil, A., Bottero, J.-Y. & Be’gassat, P. Relationship between self-potential (SP) signals and redox conditions in contaminated groundwater. Geophys. Res. Lett. 30, 2091 (2003).