Stable carbon isotope fractionation during the reductive dechlorination of chloroethenes by two bacterial strains that dechlorinate to ethene, Dehalococcoides ethenogenes 195 and Dehalococcoides sp. strain BAV1 as well as Sulfurospirillum multivorans and Dehalobacter restrictus strain PER-K23, isolates that do not dechlorinate past DCE, are reported. Fractionation by a Dehalococcoides-containing enrichment culture is also measured for comparison to the isolates. All data adequately fit the Rayleigh model and results are presented as enrichment factors. For strain 195, the measured enrichment factors were -9.6 +/- 0.4, -21.1 +/- 1.8, and -5.8 +/- 0.5 when degrading TCE, cDCE, and 1,1-DCE, respectively. Strain BAV1 exhibited enrichment factors of -16.9 +/- 1.4, -8.4 +/- 0.3, -21.4 +/- 0.9, and -24.0 +/- 2.0 for cDCE, 1,1-DCE, tDCE, and VC, respectively. The surprisingly large differences in enrichment factors caused by individual reductases (RDases) reducing different chloroethenes is likely the result of chemical structure differences among the chloroethenes. For TCE reduction, S. multivorans and D. restrictus strain PER-K23 exhibited enrichment factors of -16.4 +/- 1.5 and -3.3 +/- 0.3, respectively. While all of the organisms studied here utilize RDases that require corrinoid cofactors, the biotic TCE enrichment factors varied widely from those reported for the abiotic cobalamin-catalyzed reaction, indicating that additional factors affect the extent of fractionation in these biological systems. The enrichment factors measured for the Dehalococcoides-containing enrichment culture did not match well with those from any of the isolates, demonstrating the inherent difficulties in predicting fractionation factors of undefined communities. Although compound-specific isotope fractionation is a powerful tool for evaluating the progress of in situ bioremediation in the field, given the wide range of enrichment factors associated with functionally similar and phylogenetically diverse organisms, caution must be exercised when applying enrichment factors for the interpretation of dechlorination data.

SummaryA series of field studies were conducted at the Hanford Site, near Richland, Washington, from FY 2000 through FY 2003 at two different locations to develop data sets to test models of flow and transport in the vadose zone. The field studies were also done to investigate advanced monitoring techniques for evaluating flow-and-transport mechanisms and delineating contaminant plumes in the vadose zone at the Hanford Site. The studies were conducted as part of the Groundwater/Vadose Zone Integration Project Science and Technology Project, now known as the Remediation and Closure Science Project, managed by the Pacific Northwest National Laboratory (PNNL) and supported by the U.S. Department of Energy, Richland Operations Office. This report summarizes the key findings from the field studies and demonstrates how data collected from these studies are being used to improve conceptual models and develop numerical models of flow and transport in Hanford's vadose zone. Results from the field studies and associated analysis have appeared in more than 46 publications generated over the past 4 years. These publications include test plans and status reports in addition to numerous technical notes and papers.Two major field campaigns were performed, one at a well injection test site and a second at a clastic dike test site. Both field studies have resulted in field-scale transport parameters for Hanford conditions, which are useful for improving predictions of subsurface flow and transport at the Hanford Site. In addition, advanced geophysical methods, including high resolution electrical resistivity measurements, were successfully tested and are now being deployed at Hanford for subsurface investigations and tank retrieval detection. The most useful information gained from these studies has been a better understanding of flow and transport in the vadose zone, and this has been specifically related to the impact of small-scale stratigraphic features (e.g., sediment layering) on water flow and contaminant transport. Conceptual models have been developed that take into account the lateral spreading of water and contaminants observed in the field studies. Numerical models of unsaturated flow and transport have been revised to account for lateral spreading of subsurface contaminant plumes.The key results from these studies include: 1) Greater understanding of the complexity of plume migration in the vadose zone at Hanford. Finescale geologic heterogeneities, including grain fabric and lamination, were observed to have a strong effect on the large-scale behavior of contaminant plumes, primarily through increased lateral spreading resulting from anisotropy.2) Observations of anion exclusion in Hanford sediments. Anion exclusion is a mechanism by which negatively charged ions are repelled from the surfaces of negatively charged soil particles, thereby increasing their velocity. Thus, the travel time of ions like pertechnetate, the stable form of 99 Tc found in oxidized environments, may decrease over that of unsaturated water f...

Atmospheric nitrous oxide contributes directly to global warming, yet models of the nitrogen cycle do not account for bedrock, the largest pool of terrestrial nitrogen, as a source of nitrous oxide. Although it is known that release rates of nitrogen from bedrock are large, there is an incomplete understanding of the connection between bedrock-hosted nitrogen and atmospheric nitrous oxide. Here, we quantify nitrogen fluxes and mass balances at a hillslope underlain by marine shale. We found that at this site bedrock weathering contributes 78% of the subsurface reactive nitrogen, while atmospheric sources (commonly regarded as the sole sources of reactive nitrogen in pristine environments) account for only the remaining 22%. About 56% of the total subsurface reactive nitrogen denitrifies, including 14% emitted as nitrous oxide. The remaining reactive nitrogen discharges in porewaters to a floodplain where additional denitrification likely occurs. We also found that the release of bedrock nitrogen occurs primarily within the zone of the seasonally fluctuating water table and suggest that the accumulation of nitrate in the vadoes zone, often attributed to fertilization and soil leaching, may also include contributions from weathered nitrogen-rich bedrock. Our hillslope study suggests that under oxygenated and moisture-rich conditions, weathering of deep, nitrogen-rich bedrock makes an important contribution to the nitrogen cycle.

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