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PERMEABLE REACTIVE BARRIER PERFORMANCE MONITORING: LONG-TERM TRENDS IN GEOCHEMICAL PARAMETERS AT TWO SITES
Citation:
Wilkin*, R T. PERMEABLE REACTIVE BARRIER PERFORMANCE MONITORING: LONG-TERM TRENDS IN GEOCHEMICAL PARAMETERS AT TWO SITES. A.C. Gavaskar and A.S.C. Chen (ed.), Remediation of Chlorinated and Recalcitrant Compounds. Battelle Press, Columbus, OH, (2002).
Description:
A major goal of research on the long-term performance of subsurface reactive barriers is to identify standard ground-water monitoring parameters that may be useful indicators of declining performance or impending system failure. Results are presented from studies conducted over the past five years at two Permeable Reactive Barrier sites: U.S. Cost Guard Support Center (Elizabeth City, North Carolina) and the Denver Federal Center (Lakewood, Colorado). Both sites employ zero-valent iron particles to treat dissolved contaminants (Cr(VI) + VOCs at Elizabeth City, VOCs at the Denver Federal Center). Measurement results for ORP and dissolved oxygen are highly dependent on sampling technique and analytical method selection. Upgradient, iron wall, and downgradient regions show non-steady state to quasi-steady state long-term trends in pH, oxidation-reduction potential (ORP), specific conductance, and dissolved oxygen . After five years of operation at the Elizabeth City and Denver Federal Center PRB sites, variability in the standard geochemical parameters is greatest in upgradient regions and least variable in the reactive iron media. There are subtle but consistent changes that occur in downgradient regions, such as increases in pH, iron concentrations, and decreases in ORP. These observations are important, especially at sites where inorganic contaminants are treated or where high natural levels of metals occur, because of the impact to downgradient aquifer geochemistry and potential remobilization or immobilization of previously attenuated species. Trends in the standard geochemical parameters at two sites are discussed in the context of the observed buildup of mineral precipitates and microbial biomass.