1. C-40, in: R.R. Sirabian and R. Darlington (Chairs), Bioremediation and Sustainable Environmental Technologies—2013.
Second International Symposium on Bioremediation and Sustainable Environmental Technologies (Jacksonville, FL;
June 10–13, 2013). ISBN 978-0-9819730-7-4, Battelle Memorial Institute, Columbus, OH. www.battelle.org/biosymp
Hydraulic Analysis over a 10-Year Performance Period of a
Zero-Valent Iron Permeable Reactive Barrier
at the Former Carswell Air Force Base, Texas
Marek Ostrowski (mostrowski@brwncald.com), Charles Meyn
(cmeyn@brwncald.com) and Erik McPeek (emcpeek@brwncald.com)
(Brown and Caldwell, Upper Saddle River, New Jersey, USA)
Adria Bodour (adria.bodour.1@us.af.mil) (AFCEE TDV, Lackland AFB, Texas, USA)
Bruce Alleman (bruce.alleman@noblis.org) (Noblis, Columbus, Ohio)
Background/Objectives. A 1,100-foot-long zero-valent iron permeable reactive barrier
(PRB) was installed in 2002 to treat trichloroethene in groundwater originating from two
former landfills located within the former Carswell Air Force Base (“the Site”). The
PRB’s performance was initially investigated under the AFCEE Environmental Remedial
Program Optimization (ERP-O) Phase I review. Although the data available during the
review did not allow a close examination of the hydraulics and/or the contaminant
degradation pathways associated with the barrier, the review did note increasing vinyl
chloride concentrations in groundwater downgradient of the PRB, which was indicative
that the barrier was not performing as designed. The ERP-O Team recommended that a
detailed PRB assessment be conducted to determine the condition of the PRB and to
define future operation/maintenance requirements. Recommended activities included
analyzing the existing data, as well as conducting a more detailed investigation of the
hydraulics and biochemistry issues related to the PRB. The investigation and assessment
were conducted in 2011 and 2012. This presentation focuses on the hydraulic aspect of
the PRB assessment.
Approach/Activities. The phased investigative approach included reviewing long-term
monitoring data and historic documents related to the PRB installation, a water-level
survey of the existing monitoring wells, and installation of new piezometers to allow for
a more precise assessment of the groundwater hydraulic conditions near the PRB. An
assessment of trends in the spacial and temporal aspects of the potentiometric surface
near and within the PRB was performed. A groundwater flow model was used to
investigate the significance of these trends with respect to the hydraulic properties/impact
of the PRB, predominantly to evaluate (1) whether the PRB forms a partial barrier to the
groundwater flow, (2) whether the PRB material has experienced a gradual decrease of
the permeability which may be indicative of the passivation and/or biofouling, and (3)
whether the hydraulic properties of the PRB are relatively uniform. Modeling simulations
was performed to identify the hydraulic signature of the groundwater bypassing the
barrier, and to estimate the dimension of the PRB required to counteract the bypassing
effect as a function of the difference between the aquifer conductivity and the
conductivity of the PRB material. Results of the analyses were used to evaluate the plume
capture and groundwater residence time achieved by the PRB and to compare the
observed hydraulic performance to the design objective.

2. Results/Lessons Learned. Results of assessment showed that the PRB formed a partial
barrier to the ambient groundwater flow in the aquifer. This effect has been present since
the installation; however, it has intensified over the 10-year operation period at a
relatively steady rate, indicating that continual deterioration of the PRB media hydraulic
conductivity is occurring. The deterioration of hydraulic properties of the medium
appears to be nonuniform throughout the PRB volume, indicating that preferential flow
through certain barrier sections occurs, rather than a uniform flow across the entire PRB
volume. The bypass flow and the preferential flow paths impact the effectiveness of the
PRB in controlling the plume, both hydraulically and with respect to the residence time
needed for degradation of the chlorinated VOCs. The change over time (since
installation) in the fraction of the plume that may be bypassing the PRB is being
estimated using a groundwater flow model. Groundwater flow model is also being used
to estimate the magnitude of the PRB extension to the south and to the north, required to
contain the VOC plume. The flow modeling study is being performed and will be
concluded shortly.