doi:10.1016/S0016-7037(00)00482-4 Geochemical evolution of highly alkaline and saline tank waste plumes during seepage through vadose zone sediments JIAMIN WAN, 1, *TETSU K. TOKUNAGA, 1 JOERN T. LARSEN, 1 and R. JEFF SERNE 2 1 Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA 2 Environmental Technology Division Pacific Northwest National Laboratory, Richland, WA 99352, USA (Received April 3, 2003; accepted in revised form July 7, 2003) Abstract—Leakage of highly saline and alkaline radioactive waste from storage tanks into underlying sediments is a serious environmental problem at the Hanford Site in Washington State. This study focuses on geochemical evolution of tank waste plumes resulting from interactions between the waste solution and sediment. A synthetic tank waste solution was infused into unsaturated Hanford sediment columns (0.2, 0.6, and 2 m) maintained at 70°C to simulate the field contamination process. Spatially and temporally resolved geochemical profiles of the waste plume were obtained. Thorough OH - neutralization (from an initial pH 14 down to 6.3) was observed. Three broad zones of pore solutions were identified to categorize the dominant geochemical reactions: the silicate dissolution zone (pH  10), pH-neutralized zone (pH 10 to 6.5), and displaced native sediment pore water (pH 6.5 to 8). Elevated concentrations of Si, Fe, and K in plume fluids and their depleted concentrations in plume sediments reflected dissolution of primary minerals within the silicate dissolution zone. The very high Na concentrations in the waste solution resulted in rapid and complete cation exchange, reflected in high concentrations of Ca and Mg at the plume front. The plume-sediment profiles also showed deposition of hydrated solids and carbonates. Fair correspondence was obtained between these results and analyses of field borehole samples from a waste plume at the Hanford Site. Results of this study provide a well-defined framework for understanding waste plumes in the more complex field setting and for understanding geochemical factors controlling transport of contaminant species carried in waste solutions that leaked from single-shell storage tanks in the past. Copyright © 2004 Elsevier Ltd 1. INTRODUCTION The Hanford Site, located in southeastern Washington, was one of the United States Department of Energy’s major nuclear weapons production facilities from 1940 to 1989 (Riley et al., 1992). About 210 5 m 3 of high-level radioactive and hazard- ous wastes are stored in 177 underground tanks, including 149 single-shell tanks located in the 200 East and West Areas at the center of the Hanford Site. Over time, 67 of the single-shell tanks have leaked or are suspected of having leaked. About 2300 to 3400 m 3 of tank waste solution leaked between the 1950s and 1970s, releasing uranium, cesium, strontium, tri- tium, technetium, iodine, plutonium, chromium, and nitrates (DOE, 1996). Contaminants such as 137 Cs, 99 Tc, U, Cr(VI) and nitrates have been found in elevated concentrations in the vadose zone and all but 137 Cs have been found in groundwater beneath the single-shell tank farms (Ward et al., 1997; CH2M HILL, 2001; McKinley et al., 2001; Zachara et al., 2002). The U.S. Department of Energy has ongoing projects at the Hanford Site to monitor existing contaminant plumes in groundwater and to characterize the subsurface distribution of contaminants in tank farms that have impacted groundwater and sediments. These efforts include the Tank Farm Vadose Zone Character- ization Project and the 200 Area Remedial Action Project. Site characterization activities provide the basis for correc- tive and remedial action decisions. These activities identify scientific questions for resolution through research, and have included collection of many subsurface samples exhibiting varying degrees of contamination and physical/chemical changes. Although much has been learned concerning waste migration, the combination of subsurface heterogeneity and the limitations of field borehole drilling prevent comprehensive, high-resolution inventories of the various contaminants. While a number of laboratory-based studies have made considerable progress toward understanding reactions between tank waste solutions and Hanford sediments, the integration of laboratory and field understanding remains a challenge. Several factors contribute to the gap between field-based observations and many laboratory findings. Most laboratory studies, focused on specific interactions, employ batch equilibration approaches that, in principle, occur somewhere within the contaminated region. However, such approaches by themselves cannot gen- erally identify where within a contaminant plume their isolated reactions are important. Linking such laboratory results to field applications is made even more difficult when recalling that only very small fractions of contaminant plumes are typically sampled or otherwise characterized. The research presented here aims to bridge this gap between field processes and lab- oratory experiments. We developed a laboratory column-pro- filing method that permits measurements that have direct cor- respondence to field contaminant plumes. Besides directly integrating geochemical reactions and transport processes, the method permits direct sampling of pore solutions and sedi- ments within a waste plume with high spatial and temporal resolution. Using this approach, we studied the Hanford SX tank waste-leakage problem, and compared our laboratory re- sults with the geochemical data recently obtained from the field site characterization project. The focus of this study is on evolution of geochemical * Author to whom correspondence should be addressed (jwan@lbl.gov). Pergamon Geochimica et Cosmochimica Acta, Vol. 68, No. 3, pp. 491–502, 2004 Copyright © 2004 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/04 $30.00 + .00 491