In situ chemical reduction of aquifer sediments is currently being used for chromate and TCE remediation by forming a permeable reactive barrier. The chemical and physical processes that occur during abiotic reduction of natural sediments during flow by sodium dithionite were investigated. In different aquifer sediments, 10-22% of amorphous and crystalline FeIII-oxides were dissolved/reduced, which produced primarily adsorbed FeII, and some siderite. Sediment oxidation showed predominantly one FeII phase, with a second phase being oxidized more slowly. The sediment reduction rate (3.3 h batch half-life) was chemically controlled (58 kJ mol(-1)), with some additional diffusion control during reduction in sediment columns (8.0 h half-life). It was necessary to maintain neutral to high pH to maintain reduction efficiency and prevent iron mobilization, as reduction generated H+. Sequential extractions on reduced sediment showed that adsorbed ferrous iron controlled TCE reactivity. The mass and rate of field-scale reduction of aquifer sediments were generally predicted with laboratory data using a single reduction reaction.

An in situ redox manipulation (ISRM) method for creating a permeable treatment zone in the subsurface has been developed at the laboratory bench and intermediate scales and deployed at the field scale for reduction/immobilization of chrornate contamination. At other sites, the same redox technology is currently being tested for dechlorination of TCE. The reduced zone is created by injected reagents that reduce iron naturally present in the aquifer sediments from Fe(III) to surface‐bound and structural Fe(II) species. Standard ground water wells are used, allowing treatment of contaminants too deep below the ground surface for conventional treneh‐and‐fill technologies. A proof‐of‐principle field experiment was conducted in September 1995 at a chromate (hexavalent chromium) contaminated ground water site on the Hartford Site in Washington. The test created a 15 m (˜50 feet) diameter cylindrical treatment zone. The three phases of the test consisted of (1) injection of 77, 000 L (20, 500 gallons) of buffered sodium dithionite solution in 17.1 hours, (2) reaction for 18.5 hours, and (3) withdrawal of 375, 000 L (99, 600 gallons) in 83 hours. The withdrawal phase recovered 87% to 90% of the reaction products. Analysis of post‐experimental sediment cores indicated that 60% to 100% of the available reactive iron in the treated zone was reduced. The longevity of the reduced zone is estimated between seven and 12 years based on the post‐experiment core samples. Three and half years after the field test, the treatment zone remains anoxic, and hexavalent chromium levels have been reduced from 0.060 mg/L to below detection limits (0.008 mg/L). Additionally, no significant permeability changes have been detected during any phase of the experiment.

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CH2M HILL Hanford Group, Inc., (CH2M HILL) is designing and assessing the performance of a near-surface disposal facility at Hanford for radioactive and hazardous waste. The waste includes immobilized low-activity waste (ILAW), which consists of vitrified low-level radioactive waste that will be retrieved from Hanford's single-and double-shell tanks, unvitrified low-level radioactive waste, mixed low-level waste, and vitrification melters. The CH2M HILL effort to assess the performance of this disposal facility is known as the Integrated Disposal Facility (IDF) Performance Assessment (PA) activity. The goal of this activity is to provide a reasonable expectation that the disposal of waste will be protective of the general public, groundwater resources, air resources, surface-water resources, and inadvertent intruders. Achieving this goal will require predictions of contaminant migration from the facility. To make such predictions will require estimates of the fluxes of water moving through the sediment within the vadose zone around and beneath the disposal facility. These fluxes, loosely called recharge rates, are the primary mechanism for transporting contaminants to the groundwater. v evaporation. The barrier still performed as expected, but only if the shrub-steppe plant community remained. In essence, the dune sand makes the barrier performance sensitive to vegetation conditions such as fire removal and species replacement. Under the climate change condition most likely to promote recharge (i.e., increased precipitation and decreased temperature), recharge through the barrier remained <0.1 mm/yr in contrast to recharge in Rupert sand, which increased from 2.2 to 27 mm/yr. Land use restrictions are expected to preclude farming at the IDF. To understand the consequences of farming, a simulation was conducted of irrigated potatoes. The results showed that irrigation on the surface barrier significantly increased recharge. Remaining issues concern assumptions about climate change, bioturbation, dune sand deposition, unstable and preferential flow, variability of the properties of the barrier materials and surrounding soil, longevity of the barrier, flaws in the barrier, possible facility deposition of chloride, and the importance of temperature and water vapor flow when recharge rates are lower than 1 mm/yr. The recharge estimates provided in this report were based on a pre-conceptual design of the surface barrier. The final barrier design and the materials that will be used to construct it have not yet been identified. When they are, the final design should be re-evaluated to confirm that its performance is acceptable. In the same vein, the properties of the soil that will surround the final barrier will depend on the plan for reclamation following construction. Once identified, the proposed reclaimed soil should be re-evaluated to confirm that its performance is acceptable. Lastly, the recharge estimates provided in this report were based on a set of assumptions regarding future climate, vegetation, and land us...