Determining In Situ Degradation Rates for Petroleum Compounds via Push-Pull Tests under Natural and Biostimulated Conditions

Eric J. Raes, P.E. (Engineering and Land Planning), Jack Istok, Ph.D. (Oregon State University), Kerry L. Sublette, Ph.D., Jennifer Busch-Harris, and Eleanor Jennings (University of Tulsa), Aaron Peacock, Ph.D., Greg Davis (Microbial Insights, Inc.)

Remedial technologies often are selected based on chemical or geochemical trend data, without specific knowledge regarding the existing site-specific natural attenuation capacity at the site. Push-pull tests are a novel investigatory technique that were utilized to determine site-specific natural attenuation capacity (biodegradation rate kinetics) under both unamended and amended (biostimulation) conditions. The analytical results produced from the push-pull tests allow for the segregation of abiotic versus the biotic (biological) components of the in situ degradation rates, thereby creating a process to determine if the biological component of the degradation rate is significant versus abiotic processes such as dilution and absorption.  A case study utilizing this approach at a large petroleum distribution facility in New Jersey is presented. Baited BioTraps™ were used to screen the response of the indigenous microbial community when exposed to a suite of electron acceptors (nitrate, sulfate and dissolved oxygen). Each BioTrap was amended with a different electron acceptor and a mixture of 12C-benzene and 13C-labeled benzene. Control BioTraps were included and were only amended with benzene (i.e. no terminal electron acceptor). Total benzene loss and incorporation of 13C into microbial biomass were measured for each BioTrap condition.  The relative biodegradation rates under amended conditions (based on total benzene loss) were compared to the relative degradation rates in the unamended controls to determine whether biodegradation is enhanced under certain terminal electron-accepting conditions.  Review of the data from the Bio-Trap study revealed positive responses to the presence of dissolved oxygen and nitrate addition; conversely, the exposure to sulfate produced an inhibition in the uptake of 13C-labeled benzene. Thus, sulfate addition was eliminated from consideration.  A series of single-well, push-pull tests were conducted to determine in situ kinetics in the same wells. Push-pull tests consist of the injection of prepared test solutions into existing site wells. The test solution contains the same electron acceptors (oxygen, nitrate and unamended) used in the Bio-Trap study in addition to a conservative tracer (bromide) to segregate abiotic and biotic in situ degradation rates of the chemicals if interest over time (short term) when exposed to different biostimulation substrates. Review of the push-pull test data supports the conclusion that biodegradation rates increase under biostimulated conditions using both oxygen and nitrate as terminal electron acceptors. Currently, a cost analysis is being performed to determine if the observed increase in rate kinetics under stimulated conditions is economically beneficial when compared to natural attenuation under unamended conditions.

Abstract Q-004, in: Bruce M. Sass (Conference Chair), Remediation of Chlorinated and Recalcitrant Compounds—2008. Proceedings of the Sixth International Conference on Remediation of Chlorinated and Recalcitrant Compounds (Monterey,CA; May 2008). ISBN 1-57477-163-9, published by Battelle, Columbus, OH, www.battelle.org/chlorcon.