Chapter 14
Perchlorate removal
BIOREMEDIATION OF PERCHLORATE-CONTAMINATED GROUNDWATER
Perchlorate, an oxyanion extensively used by the aerospace industry in the production of solid rocket fuel, potentially contaminates the drinking water of 12 million people in the United States Even at concentrations in the low part-per-billion range, perchlorate is suspected of affecting hormone production in humans Current methods of remediating perchlorate-contaminated groundwater involve extracting the groundwater and treating it above ground. In situ remediation of perchlorate (that is, remediation that occurs in place, without the need to pump perchlorate-contaminated groundwater to the surface) has the potential for large cost and safety benefits.
In this study, a model for in situ biodegradation of perchlorate in an aquifer in which horizontal flow treatment wells (HFTWs) are used to mix an electron donor into perchlorate-contaminated groundwater to stimulate perchlorate reduction by indigenous microorganisms.
HFTWs have been used effectively for the in situ remediation of chlorinated ethene-contaminated groundwater, and their potential applications have been the subject of a number of studies For example, McCarty et al. (1998) demonstrated that trichloroethene (TCE) could be successfully destroyed in situ using a pair of HFTWs to inject toluene and oxygen into contaminated groundwater at Edwards Air Force Base, CA. The in situ addition of these compounds stimulated cometabolic destruction of TCE by indigenous toluene-oxidizing bacteria.), which depicts an operating concept similar to that applied at the Edwards site, shows a dual screened treatment well pumping in a downflow mode alongside a treatment well pumping in an upflow mode. In the upflow treatment well, water is extracted through the lower screen and reinjected through the upper screen. The flow path is reversed in the downflow well. In the aquifer around the injection screens, bioactive zones form where indigenous bacteria degrade the target contaminant. As shown in Figure 1 (right), the pattern of recirculation created by the HFTW system results in multiple passes of the contaminated groundwater through the bioactive zones. This recirculation, which can be adjusted by modifying the pumping rates in the well pairs, significantly increases the effectiveness of the treatment process. In addition to providing high levels of treatment, HFTWs also reduce risk and costs by treating contaminants in the subsurface, without the need to pump contaminant aboveground.
METHODOLOGY
The model developed during this project simulates advective/dispersive transport of the electron donor (acetate, in this case), perchlorate, and competing electron acceptors (oxygen and nitrate) in the groundwater flow field induced by operation of the HFTW well pair. Microorganisms are assumed to be immobile. The rate of perchlorate reduction is modeled using Monod kinetics, with the rate dependent on both perchlorate and electron donor concentrations. The presence of competing electron acceptors (oxygen and nitrate) serves to decrease the rate of perchlorate reduction. This is modeled using an inhibition coefficient that slows the rate of nitrate reduction if oxygen is present and slows the rate of perchlorate reduction if either oxygen or nitrate are present. The rate of microbial growth is a result of the consumption of the growth substrate (electron donor) less biomass decay, which is modeled as a first-order decay process. The biodegradation model was developed through the Strategic Environmental Research and Development Program (SERDP Project CU-1163). Kinetic parameters for the model were estimated using batch and column studies in the laboratory .A finite difference code was implemented that solves the partial differential equations describing steady-state flow, electron donor and acceptor transport, and reduction of the acceptors by microorganisms that use the electron donor for growth and energy, as described above. By solving these equations for given initial and boundary conditions, the code estimates concentration of donor, acceptors, and biomass at any Groundwater contamination by perchlorate has been recognized as a significant environmental problem across the United States.
In this study, a numerical model was developed to evaluate the potential for an innovative in situ bioremediation technology, horizontal flow treatment wells (HFTWs), to manage perchlorate-contaminated groundwater. The HFTW technology employs paired, dual-screened injection and extraction wells to distribute and mix an electron donor into perchlorate-contaminated groundwater. The HFTW delivery system is designed to promote the reduction of perchlorate by indigenous microorganisms in subsurface bioactive zones, as well as to recirculate the contaminated water between treatment well pairs to achieve multiple passes of contaminated water through the bioactive zones. The numerical model used in this study couples a three-dimensional fate and transport model, which simulates advective/dispersive transport of solutes induced by regional groundwater flow and operation of the HFTWs, with a biodegradation model that simulates perchlorate reduction, and the reduction of competing electron acceptors, by indigenous microorganisms. The model was applied to an example site to demonstrate how in situ perchlorate biotreatment might be implemented in the field. A sensitivity analysis using the model was also conducted to evaluate which engineered and environmental parameters most affect technology performance. Model simulation results demonstrate that this technology is potentially an effective approach for delivering electron donor and stimulating perchlorate biodegradation in contaminated groundwater. The recirculation induced by the HFTW system results in increased treatment efficiency, as compared to treatment that would be achieved by a single pass of contaminated water through a bioactive zone. The model presented in this study is an important tool that may be used to design field evaluations of the technology and ultimately to help transition the technology for commercial application at sites with perchlorate-contaminated groundwater.
REFERENCE
Paper C-07, in: V.S. Magar and M.E. Kelley (Eds.), In Situ and On-Site Bioremediation—2003. Proceedings of the Seventh
International In Situ and On-Site Bioremediation Symposium (Orlando, FL; June 2003). ISBN 1-57477-139-6, published by
Battelle Press, Columbus, OH, www.battelle.org/bookstore.Modeling In Situ Bioremediation OfPerchlorate-Contaminated GroundwaterJeffrey C. Parr (jeffrey.parr@wpafb.af.mil), Mark N. Goltz and Junqi Huang (Air Force Institute of Technology, Wright Patterson Air Force Base, OH, USA)
Paul B. Hatzinger (Envirogen Inc., NJ, USA)
Yassar H. Farhan (Environmental Management Group International, Inc., PA, USA)