Two Decades of Application of Permeable Reactive Barriers to Groundwater Remediation - Permeable Reactive Barrier: Sustainable Groundwater Remediation - page 34

quality surface or waste water, and (3) the treatment media often consists of
recycled or “waste” material such as mulch/compost. The intent of the PRB
is to operate as a hydraulically passive in situ groundwater remedy. Once
the system is constructed, advective groundwater flow is the primary driver
moving groundwater contaminants through the PRB for chemical and/or
biological treatment. The most energy- and carbon footprint-intensive activi-
ties with PRB use are related to construction, creation, and collection of the
materials used in the PRB construction (e.g., reactive media, slurry wall
material), and excavation and disposal of construction spoils.
2.3.5 Construction and Implementation
PRB construction and implementation methods have advanced over the
past 15 years—from conventional trench-and-fill to single-pass trenching,
and from pneumatic injection to large-diameter borehole-filled completions.
Research to develop treatment media with greater reaction rates has been
ongoing for the past 15 years, and the expectation is that reliable, fast-acting,
sustainable materials will continue to be a goal in PRB development. For
PRBs to become more reliable, usable, and sustainable, hydraulic design
improvements must take center stage. Developing reactive media and PRB
systems that can reliably treat mixed plumes and emerging contaminants
will continue to be an important need. Key to the success of this approach
will be sufficient testing to ensure complete treatment of chemicals that may
have different geochemical stability signatures (oxidizing vs. reducing) and
thus require different treatment mechanisms. Biological reactions in PRB
systems that utilize facultative bacteria or cometabolic processes may be par-
ticularly promising in this context. Ensuring that PRBs provide long-term
reliable treatment will be of even greater focus than we have seen over the
past 15 years.
2.3.6 Cost
Representative cost information for PRBs, where cost includes both capital
installation and long-term operation costs, is not widely available in the pub-
lic record. Over the next few years, particularly with a greater emphasis on
implementing green and sustainable remediation methods, the hope is that
there will be less reluctance in providing cost data. For the future, given pos-
sible greater use of PRBs for metals including radioactive constituents, and
as PRBs continue to age, it should be anticipated that closure plans will be
requested more often by regulatory groups.
In some respects, providing cost data for previous installations or as rules
of thumb, may inadvertently bias future design evaluations. This is because
the cost of materials can change greatly due to market conditions (particu-
larly for the use of granular ZVI) and construction methods are dependent
on the unique conditions (location, depth, existing infrastructure). Most