The goal of in situ redox manipulation (ISRM) is to create a permeable treatment zone capable of removing redox‐sensitive contaminants from ground water. The objective of this study was to evaluate the effectiveness of one promising ISRM technology: chemical reduction of aquifer sediments by sodium dithionite (Na 2S2O4) injection. The technology was evaluated in intermediate‐scale laboratory experiments designed to investigate the kinetics of Fe(III)‐reduction and dithionite‐disproportionation reactions in a radial flow field over similar transport distances (∼ 7 m) and time scales (∼ 72 hours) as those used in a field trial for remediation of chromate contaminated ground water at the Department of Energy Hanford site in Washington state. Four hundred liters (∼ 1 pore volume) of 0.1 M Na2S2O4 in a 0.4 M K2CO3/0.04 M KHCO3 buffer were injected at a rate scaled to field values. Dithionite breakthrough curves at sampling ports were approximately described by the advection‐dispersion equation with a two‐part reaction model containing first‐order rate coefficients for dithionite reaction with sediment Fe(III) (k1= 0.13 hr −1) and dithionite disproportionation (k2= 0.05 hr −1). Analyses on sediment cores collected from the physical model indicated that substantial Fe(III) was reduced to Fe(II) and that the dithionite‐treated sediment was capable of removing 2 mg/L chromate from ∼ 100 column pore volumes of synthetic ground water. These results indicate that the ISRM technology is a potentially feasible method for removing chromate from Hanford ground water.

, the Pacific Northwest Laboratory has been conducting a geohydrologic in

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No abstract

Two soil sample collection and preservation methods for volatile organic compounds, used during site characterization activities, were evaluated using standard U.S. Environmental Protection Agency analytical methods. A conventional bulk method recommends completely filling a sample container with soil; a less commonly used methanol method recommends placing a soil aliquot into methanol. Analytical results showed large negative biases associated with the bulk samples as compared to the methanol samples for aromatic compounds. Order of magnitude differences in concentrations measured between the methods were observed for benzene and toluene. Lesser differences were noted for xylenes and ethylbenzene. Limited data for chlorinated compounds suggest behavior similar to the aromatic species. A limited spike recovery study was conducted using the methanol method on laboratory and field samples. Samples were analyzed 82 days after spike addition. Poorer spike recoveries were noted from spiked methanol vials transported to the field and used for collection of soil samples. Differences between mean recovery values for the laboratory and field samples appear to be the result of losses during sample collection and transport. Despite the 82-day holding time, spike recoveries were within 70% of initial spike concentrations. These results demonstrate the stability of using methanol as a preservative for soil samples.