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SPATIAL AND TEMPORAL TRENDS IN GROUNDWATER CHEMISTRY AND PRECIPITATE FORMATION AT THE ELIZABETH CITY PERMEABLE REACTIVE BARRIER
Citation:
Wilkin*, R T., R W. Puls*, C J. Paul*, M S. McNeil*, F P. Beck*, AND P J. Clark*. SPATIAL AND TEMPORAL TRENDS IN GROUNDWATER CHEMISTRY AND PRECIPITATE FORMATION AT THE ELIZABETH CITY PERMEABLE REACTIVE BARRIER. Presented at RTDF Meeting, washington, DC, November 06 - 07, 2002.
Impact/Purpose:
To inform the public.
Description:
Accumulation of mineral precipitates and microbial biomass are key factors that impact the long-term performance of PRBs. Both processes can impact remedial performance by affecting zero-valent iron reactivity and permeability. Results will be presented from solid-phase and groundwater monitoring studies conducted at the Permeable Reactive Barrier installed at the U.S. Coast Guard Support Center (Elizabeth City, North Carolina). This PRB was installed in June of 1996. Over the past six years intensive groundwater sampling, subsurface characterization, and solid-phase analysis has been conducted at this site. Groundwater sampling in a detailed network of subsurface monitoring points (approx. 130 wells) has been carried out to determine variability of geochemical parameters in space and time and to evaluate uptake/removal rates of contaminants and other dissolved solutes. Subsurface characterization studies have employed electrical conductivity probes and continuous core sampling to trace the subsurface distribution of the reactive media. Solid-phase characterization studies on core materials have utilized scanning electron microscopy, high-resolution transmission electron microscopy, x-ray diffraction, carbon and sulfur partitioning measurements, sulfur isotope ratio measurements, and analysis of phospholipid fatty acid extracts. These studies reveal the factors that govern the rate of mineral and microbial biomass accumulation in the reactive media. A summary of the data trends and conclusions reached after the first six years of operation of the PRB will be presented.
At this site, six years appears to be too short a period of time to see a clear correlation between change in geochemical parameters indicative of Fe0 reactivity and declining performance with respect to contaminant removal efficiency from the groundwater plume. Nevertheless, pore space is being lost at a measurable rate that is variable (net reduction in porosity <0.005 to 0.060) and clearly tied to the input flux distribution of groundwater solutes.