Environmental Engineering Reference
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and reduction-oxidation shifts associated with the geochemical condition-
ing of the groundwater from the treatment reactions. Regarding ZVI, geo-
chemical changes to ZVI from the presence of sulfate, nitrate, and oxygen are
widely observed and more research into prevention of performance loss is
an ongoing area of work. Generally, it is now recognized that excess nitrate
can passivate the ZVI corrosion reaction and render ZVI nearly useless as a
treatment material within a matter of months.
Effective remediation of groundwater contaminants using PRBs depends
on achieving appropriate conditions for the degradation reactions to occur
and having a reaction zone (size/thickness) that provides sufficient residence
time for contaminants to degrade to performance objectives. For biological
PRBs, insufficient residence time of the contaminants in the reaction zone
may result in accumulation of regulated intermediate degradation products.
The success of biological PRBs largely depends on the presence of microbes
that are capable of facilitating the requisite degradation reactions.
2.3.3 Monitoring Improvements and Longevity
The most important aspects of monitoring improvements with regard
to PRBs may be the development of alternative compliance monitoring
metrics—including mass discharge and toxicity reduction calculations—
and improved analytical monitoring tools, including compound-specific
isotope analysis (CSIA) and molecular biological tools (MBTs). Analysis of
iron and sulfide mineralogy to evaluate biogeochemical transformation pro-
cesses has become important particularly for organic-media PRBs and the
assessment of precipitation reaction zones within ZVI treatment systems
provides an indication of aging progression with the ZVI PRB. Process mon-
itoring and performance monitoring may require different analytical pro-
tocols, monitoring locations, and monitoring frequencies as the approaches
coincide, such as: (1) baseline characterization for performance comparison
to design considerations; (2) process monitoring to optimize system opera-
tion and performance and to evaluate the need for system modifications;
and (3) performance and compliance monitoring to evaluate and validate
the effectiveness of the system with regard to meeting remedial action
objectives (RAOs).
The improvements in monitoring strategies directly affects evaluation
of PRB longevity, where longevity refers to the ability of a PRB to sustain
hydraulic capture, residence time, and reactivity in the years and decades
following installation. Because PRBs are used to treat plumes that may per-
sist for years or decades, regulators in particular are interested in determin-
ing how long PRBs will continue to retain a desirable minimum level of
hydraulic capture and reactivity without requiring major maintenance or
replacement of the reactive media. Of the several hundred PRBs that have
been installed since the first full-scale PRB application occurred in 1994,
many are reported to be performing acceptably, although the literature does
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