Chromium has become an important soil contaminant at many sites, and facilitating in situ reduction of toxic Cr(VI) to nontoxic Cr(III) is becoming an attractive remediation strategy. Acceleration of Cr(VI) reduction in soils by addition of organic carbon was tested in columns pretreated with solutions containing 1000 and 10 000 mg L(-1) Cr(VI) to evaluate potential in situ remediation of highly contaminated soils. Solutions containing 0,800, or 4000 mg L(-1) organic carbon in the form of tryptic soy broth or lactate were diffused into the Cr(VI)-contaminated soils. Changes in Cr oxidation state were monitored through periodic micro-XANES analyses of soil columns. Effective first-order reduction rate constants ranged from 1.4 x 10(-8) to 1.5 x 10(-7) s(-1), with higher values obtained for lower levels of initial Cr(VI) and higher levels of organic carbon. Comparisons with sterile soils showed that microbially dependent processes were largely responsible for Cr(VI) reduction, except in the soils initially exposed to 10 000 mg L(-1) Cr(VI) solutions that receive little (800 mg L(-1)) or no organic carbon. However, the microbial populations (< or = 2.1 x 10(5) g(-1)) in the viable soils are probably too low for direct enzymatic Cr(VI) reduction to be important. Thus, synergistic effects sustained in whole soil systems may have accounted for most of the observed reduction. These results show that acceleration of in situ Cr(VI) reduction with addition of organic carbon is possible in even heavily contaminated soils and suggest that microbially dependent reduction pathways can be dominant.

Although gravels comprise large portions of some vadose zones, their unsaturated hydraulic properties have received relatively little attention. This study examines moisture retention relations in the 2‐ and 6‐mm size fractions of gravels from the Hanford formation vadose zone (Washington State). Understanding flow and transport within this formation is important because parts of it have become contaminated by leakage of radioactive wastes at the Hanford Site. Moisture retention relations were obtained for a very wide energy range, with attention to water retained in intragranular pores and along grain surfaces. External surfaces of these gravels have root mean‐squared roughnesses (rmsr) in the micrometer range, with sparsely distributed deep (hundreds of micrometers) pits. Water films on these external surfaces are volumetrically insignificant at matric potentials less than about −2 kPa. Residual water in these gravels occurs in intragranular pores, accounts for about 10% of the total porosity, and is effectively hydraulically immobile. The intragranular domain in Hanford gravels also has a large specific surface area of about 11 m2 g−1 Thus, exchanges of solutes (including contaminants) between the intragranular domain of Hanford gravels and their immediate surrounding are significant and diffusion limited.

[1] Some environmentally sensitive unsaturated zone sediments, such as those underlying radioactive waste tanks in Hanford (Washington State), contain large fractions of gravels and coarse sands. Coarse, granular media are also included in designs of engineered capillary barriers for subsurface waste isolation. Thus knowledge of the unsaturated hydraulic properties of gravels is needed to understand flow and transport in these critical settings. When standard methods for measuring moisture characteristics or water retention relations are used for gravels, corrections are needed in the near-zero region of matric (pressure) potentials. The need for correction results from gravity stratification of saturation profiles within even short sample columns. Such a correction method was developed and used to determine drainage curves for Hanford gravels having characteristic grain sizes of 8.0-9.5, 4.8-5.3, and 2.0-2.4 mm. In 30 mm tall sample columns, gravity corrections were essential for the 9 and 5 mm gravels and less significant for the 2 mm gravel. Validity of the correction method was demonstrated through accurately reconstructing average column saturation-potential relations from their predicted local saturation-potential relations. The method and results presented here are part of an ongoing study on Hanford gravels and on limits to classical unsaturated hydraulic scaling encountered at large grain sizes.

Although gravels comprise large portions of some vadose zones, their unsaturated hydraulic properties have received relatively little attention. This study examines moisture retention relations in the 2 and 6 mm size fractions of gravels from the Hanford formation vadose zone (Washington State). Understanding flow and transport within this formation is important because parts of it have become contaminated by leakage of radioactive wastes. Moisture retention relations were obtained over a very wide energy range, with attention to water retained in intragranular pores and along grain surfaces. External surfaces of these gravels have root meansquare roughnesses in the µm range, with sparsely distributed deep (hundreds of µm) pits. Water films on these external surfaces are volumetrically insignificant at matric potentials less than about -2 kPa. "Residual" water in these gravels occurs in intragranular pores, accounts for about 10% of the total porosity, and is effectively hydraulically immobile. The intragranular domain in Hanford gravels also has a large specific surface area of about 11 m 2 g -1 . Thus, exchanges of solutes (including contaminants) between the intragranular domain of Hanford gravels and their immediate surrounding are significant and diffusion-limited.

ABSTRACTvariations in chemical (Wilcke and Kaupenjohann, 1998) and microbiological (Seech and Beauchamp, 1988; Dra- (Currie, 1961;Smith, 1977)