Quantifying the Effects of Small-Scale Heterogeneities on Flow and Transport in Undisturbed Cores from the Hanford Formation

more than 4 million liters of hazardous and radioactive waste in the vadose zone. Relatively mobile radionuAccelerated migration of contaminants in the vadose zone has clides such as 99Tc, 129I, U, and 3H have traveled further been observed beneath tank farms at the U.S. Department of Energy’s Hanford Reservation, Richland, WA. This paper focuses on quantifythan predicted by current models and have been deing hydrologic processes that control the fate and transport of contamitected in the groundwater, which eventually flows to nants in the unsaturated sediments beneath the Hanford tank farms. the Columbia River (Dirkes and Hanf, 1997; Hartman The experimental approach involved the use of field relevant, longand Dresel, 1997). This suggests the mechanisms of such term unsaturated nonreactive transport experiments in undisturbed accelerated contaminant migration remain unclear. It is sediments from the Hanford Formation. Undisturbed sediment cores thought that vertical preferential flow may occur along were collected from a laminated fine-grained sand unit within the clastic dikes or uncased well boreholes. Finger flow that Hanford Formation in both the vertical direction (flow cross bedding) results from unstable wetting fronts may also increase and the horizontal direction (flow bedding parallel). Laboratory-scale the rate of vertical transport in stratified media (Ritsema saturated and unsaturated flow experiments were conducted using et al., 1993, 1998; Glass et al., 1988, 1989; Sililo and Tellam, multiple nonreactive tracers to investigate hydrologic processes controlling the vertical and lateral spread of contaminants. The nonreac2000). Variation in hydraulic conductivity caused by tive tracers differ in their free-water molecular diffusion coefficients, differences in grain size may also result in lateral flow thus providing a quantitative measure of diffusional processes and and transport along bedding planes (Newman et al., the presence of immobile water. Asymmetric breakthrough curves 1998). Further, diffusion into finer-grained media may (BTCs) and coelution of tracers were observed during saturated flow slow the transport of contaminants and may also act as in both horizontal and vertical cores, indicating advection enhanced a source of contamination as clean water is transported solute dispersion with no accompanying immobile water. Unsaturated through the formation. An enhanced conceptual undertracer transport in the vertical and horizontal cores resulted in earlier standing of the influence of physical heterogeneities breakthrough, asymmetric BTCs, and differential breakthrough of within the Hanford Formation (e.g., variations in grain tracers where the elution of piperazine-1-4-bis(2-ethanesulfonic acid) size, lithologic stratification, and discontinuities) on con(PIPES) preceded that of pentafluorobenzoic acid (PFBA), which preceded that of Br . These results suggest that physical nonequilibtaminant mobility is essential for accurate prediction of rium processes (PNE) such as preferential finger flow coupled with flow and transport processes beneath the tank farms. immobile water may control the unsaturated movement of contamiThe effects of physical heterogeneities in undisturbed nants in the Hanford Formation. media on contaminant flow and transport may be quantified by performing miscible displacement experiments using multiple nonreactive tracers that vary in their T U.S. Department of Energy’s Hanford Reservaaqueous rate of diffusion. This technique has previously tion is a nuclear materials processing facility located been used to quantify the effects of physical nonequilibin the Columbia River Basin in south-central Washingrium (PNE) processes in structured media (Jardine et ton. The area receives an average annual rainfall of 16 al., 1998, 1999; Langner et al., 1999; Becker and Shapiro, cm yr 1. Depth to groundwater averages around 100 m 2000; Moline et al., 2001; Mayes et al., 2003). Physical across the site. The vadose zone is composed of sedinonequilibrium results from fast preferential flow couments from the Hanford Formation, the Cold Creek pled with slow diffusion into less mobile water regimes. Unit, and the Ringold Formation at the groundwater These phenomena result in separation of tracer BTCs interface. Processing waste has historically been disbecause of differential rate of tracer mobility between posed of in cribs, solid waste burial pits, ponds, and/or fast and slow flowing water regimes. underground storage tanks within the Hanford FormaIn an effort to better understand how solutes migrate tion. The most concentrated wastes were stored in sinin semiarid regions, numerous experiments have been gle-shelled tanks in the 200 Area of the Hanford Site conducted in packed columns of silty–sandy media using (Bjornstad, 1990; Agnew et al., 1997). About one-third a single nonreactive tracer (Bond and Wierenga, 1990; of the tanks in the 200 Area have leaked, resulting in Porro and Wierenga, 1993; Padilla et al., 1999; Gamerdinger and Kaplan, 2000). Flow and transport in saturated packed columns typically result in symmetric M.N. Pace, M.A. Mayes, P.M. Jardine, and T.L. Mehlhorn, Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box BTCs, while asymmetric BTCs are observed under un2008, Oak Ridge, TN, 37831-6038; J.M. Zachara and B.N. Bjornstad, saturated conditions. As packed columns become unsatEnvironmental Dynamics and Simulation, Environmental Molecular urated, the larger pores are drained, resulting in areas of Sciences Laboratory, Pacific Northwest Laboratory, Richland, WA. Received 15 July 2003. Original Research Paper. *Corresponding Abbreviations: bp, bubbling pressure; BTC, breakthrough curve; author ([email protected]). CDE, convection–dispersion equation; IC, ion chromatograph; MIM, mobile–immobile model; ORNL, Oak Ridge National Laboratory; Published in Vadose Zone Journal 2:664–676 (2003).  Soil Science Society of America PFBA, pentafluorobenzoic acid; PIPES, piperazine-1-4-bis(2-ethanesulfonic acid); PNE, physical nonequilibrium; PV, pore volume. 677 S. Segoe Rd., Madison, WI 53711 USA