Low water content sediments were treated with NH3 gas to evaluate changes in U mobility as a potential field remediation method for vadose zone contamination. Injection of NH3 gas created high dissolved NH3 concentrations that followed equilibrium behavior. High NH3 concentration led to an increase in pH from 8.0 to 11 to 13, depending on the water content and NH3 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 NH3 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 NH3 treatment, but x‐ray absorption near‐edge structure/extended x‐ray absorption fine structure showed no change in the Na‐boltwoodite concentration. Therefore, NH3 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.

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[1] The Hanford 300 Area is located adjacent to the Columbia River in south-central Washington State, USA, and was a former site for nuclear fuel processing operations. Waste disposal practices resulted in persistent unsaturated zone and groundwater contamination, the primary contaminant of concern being uranium. Uranium behavior at the site is intimately linked with river stage driven groundwater-river water exchange such that understanding the nature of river water intrusion into the 300 Area is critical for predicting uranium desorption and transport. In this paper, we use 2-D surface-based time-lapse electrical resistivity tomography (ERT) to image the inland intrusion of river water during high stage conditions. We inverted approximately 1200 data sets (400 per line over three lines) using high performance computing resources to produce a time-lapse sequence of changes in bulk conductivity caused by river water intrusion during the 2011 spring runoff cycle over approximately 125 days. To invert the data, we use an image differencing approach that does not require regularization in the time dimension, enabling the inversion to accommodate the sharp, time varying contrasts in conductivity imposed by the moving water table. The resulting time series for each mesh element was then analyzed using common time series analysis to reveal the timing and location of river water intrusion beneath each line. The results reveal nonuniform flows characterized by preferred flow zones where river water enters and exits quickly with stage increase and decrease, and low permeability zones with broader bulk conductivity ''break through'' curves and longer river water residence times.Citation: Wallin, E. L., T. C. Johnson, W. J. Greenwood, and J. M. Zachara (2013), Imaging high stage river-water intrusion into a contaminated aquifer along a major river corridor using 2-D time-lapse surface electrical resistivity tomography, Water Resour.

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