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Foam delivery of remedial amendments is potentially useful for vadose zone remediation because aqueous amendments are delivered while a low water content is maintained in the treated zones. The transport behavior of foam in the vadose zone has not been well studied. Benchtop-scale column tests were conducted to investigate foam transport, foaming gas and liquid propagation, pressure buildup and distribution, the influence of foam quality and sediment permeability on foam injection pressure, and liquid uptake and distribution during foam flow in unsaturated sediments. Results indicated that under unsaturated conditions, the foaming gas front traveled much faster than the foam front where bubble rupture was significant. A liquid front was formed ahead of the foam flow front. The accumulated liquid in this front was mainly composed of the water injected with the foam. The foam injection pressure was primarily attributed to the resistance to foam flow by the sediment but not to the movement of the water ahead of the foam front. The foam had a high effective viscosity compared with water. A near-linear pressure distribution within the foam-occupied column section was observed. At a fixed injection rate, higher foam quality resulted in lower injection pressure. Higher permeability sediments did not necessarily have a lower foam injection pressure because other factors, such as the foam injection rate, foam bubble diameter, and sediment pore size, also influenced the pressure. In the foam flushed region, the liquid uptake and water content distribution in the sediment was negligibly affected by the initial water content and the foam injection rate within the ranges tested in this study. The implications of these findings to field remediation application were considered.

The feasibility of applying a combination of H2S and NH3 gases for Tc-99 remediation in vadose zone sediments evaluated in laboratory experiments showed a significant mobility decrease from 75 to 95% leachable Tc-99 in untreated sediments to 15 to 48% leachable Tc-99 after gas treatment. Individual H2S or NH3 gas treatment of sediments had little lasting effect. For the combined gas treatment, the H2S gas created reducing conditions at the pore water/mineral interface that pertechnetate was temporarily reduced/precipitated. The NH3 gas created alkaline pore water, which induces some mineral dissolution and subsequent aluminosilicate precipitation as the pH neutralizes, which may coat Tc-99 surface phases and render them less mobile. Surface phase analysis showed Tc-99 associated with weathered basalt clasts and sulfur, possibly from the precipitation of TcSx. Treatment performance was nearly the same at different Tc-99 concentration (34 to 6500 pCi/g), water content (1 to 8%), and gas injection rate, but was sensitive to gas concentrations. Low gas concentrations ( 30%) formed NH4SH precipitate. The 15 - 23% mobile Tc-99 remaining after gas treatment may be caused by limited time for aluminosilicates to precipitate in experiments. Nitrogen species concentrationsmore » showed no nitrate or nitrite production and a significant decrease in the sediment microbial population over three months, suggesting nitrification of added ammonia was inhibited. Overall, this study showed that combined H2S and NH3 gas treatment of low water content sediments was a robust technology to significantly decrease Tc-99 mobility.« less

Low water content sediments were treated with NH 3 gas to evaluate changes in U mobility as a potential field remediation method for vadose zone contamination. Injection of NH 3 gas created high dissolved NH 3 concentrations that followed equilibrium behavior. High NH 3 concentration led to an increase in pH from 8.0 to 11 to 13, depending on the water content and NH 3 concentration. The increase in pore water pH resulted in a large increase in pore water cations and anions from mineral-phase dissolution. Minerals showing the greatest dissolution included montmorillonite, muscovite, and kaolinite. Pore water ion concentrations then decreased with time. Simulations based on initial pore water ion concentrations indicated that quartz, chrysotile, calcite, diaspore, hematite, and Na-boltwoodite (hydrous U silicate) should precipitate. Electrical resistivity and induced polarization tomography (ERT/IP) was able to nonintrusively track these NH 3 partitioning, dissolution, and precipitations processes through changes in conductivity and chargeability. Ammonia treatment significantly decreases the amount of U present as adsorbed and aqueous species in field-contaminated sediments. In contrast, sediments containing a large fraction of U associated with carbonates generally showed little change. Uranium leaching from sediments containing high Na-boltwoodite decreased significantly by NH 3 treatment, but x-ray absorption near-edge structure/extended x-ray absorption fine structure showed no change in the Na-boltwoodite concentration. Therefore, NH 3 treatment of contaminated sediment acts to decrease the highly mobile aqueous and adsorbed U by incorporation into precipitates and appears to decrease mobility of some existing U precipitates (Na-boltwoodite) as a result of mineral coating.