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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. Presented at 3rd Intl. Conf. on Remediation of Chlorinated and Recalcitrant Compounds, Monterey, CA, 05/20-23/2002.
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 ground water monitoring studies conducted over the past five years at two Permeable Reactive Barrier sites: U.S. Coast Guard Support Center (Elizabeth City, North Carolina) and the Denver Federal Center (Lakewood, Colorado). Both sites employ zero-valent iron to treat dissolved contaminants. Upgradient, iron wall, and downgradient regions show non-steady state to quasi-steady state long-term trends in pH, ORP, specific conductance, and dissolved oxygen. Measurement results for ORP and dissolved oxygen are highly dependent on sampling technique and analytical method selection. After five years of operation at the Elizabeth City and Denver Federal Center PRB sites, variability in the standard geochemical parameters is greatest in the upgradient regions and least variable in the iron. 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 being treated or where high natural levels of metals occur, because of the impact to downgradient aquifer geochemistry and the potential for remobilization of previously attenuated species. Trends in the concentrations of dissolved sulfide and ferrous iron are also discussed in the context of the observed buildup of mineral precipitates and microbial biomass.