Wednesday, November 15, 2006 - 10:45 AM

Field-scale Reactive Transport in Hydraulically and Lithologically Nonuniform Media.

Anderson Ward and Steve Yabusaki. Pacific NW Nat'l Laboratory, Pacific NW Nat'l Laboratory, Po Box 999 K9-33 3200 Q Ave, Richland, WA 99352

The fate of reactive contaminants in variably saturated sediments is controlled by physical properties as much as geochemical properties. However, most analyses of flow and transport in heterogeneous sediments focus on physical heterogeneity (e.g. permeability and porosity) neglecting the effects of geochemical heterogeneities on concentration distributions.  Those studies that have considered geochemical heterogeneity typically assume that selectivity coefficients and exchange site concentrations derived from flow through column studies can be applied to field-scale problems.  In this study, we explore the ability of lab-measured hydraulic and geochemical properties to describe the field-scale transport behavior of strontium-90 in physically and geochemically heterogeneous sediments at the Hanford site. Two parallel codes, STOMP and CRUNCH, were used to simulate flow and multicomponent ion-exchange resulting from a release of strontium and magnesium during flow from a line source on a 60-m long transect. Results show that a complex interplay of physical and geochemical heterogeneity controls mineral dissolution, precipitation, and access to the reaction sites controlling ion exchange processes. The spatial distribution of tracer and cations and the flux of solute across a horizontal compliance plane could not be described using average hydraulic or geochemical properties derived from cores and laboratory columns. The divergence between field observations and model predictions were significantly reduced by explicitly accounting for hydraulic and geochemical heterogeneity in the sediments.  Information on heterogeneity appears critical for accurate predictions of contaminant spreading and for evaluating the effectiveness of different remediation strategies. The Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle under Contract DE-AC05-76RL01830.