Abstract
The behavior of a persistent uranium plume within an extended river corridor at the DOE Hanford site is dominantly controlled by river stage fluctuations in the adjacent Columbia River. The plume behavior is further complicated by substantial heterogeneity in physical and geochemical properties of the host aquifer sediments. Multi-scale field and laboratory experiments and reactive transport modeling were integrated to understand the complex plume behavior influenced by highly variable hydrologic and geochemical conditions in time and space. In this paper, we (1) describe multiple data sets from field-scale uranium adsorption and desorption experiments performed at our experimental well-field, (2) develop a reactive transport model that incorporates hydrologic and geochemical heterogeneities characterized from multi-scale and multi-type datasets and a surface complexation reaction network based on laboratory studies, and (3) compare the modeling and observation results to provide insights on how to refine the conceptual model and reduce prediction uncertainties. The experimental results revealed significant spatial variability in uranium adsorption/desorption behavior, while modeling demonstrated that ambient hydrologic and geochemical conditions and heterogeneities in sediment physical and chemical properties both contributed to complex plume behavior and its persistence. This research underscores the great challenges in adequately characterizing this type of site to model the reactive transport processes over scales of 10 m or more. Our analysis provides important insights into the characterization, understanding, modeling, and remediation of groundwater contaminant plumes influenced by dynamic surface water and groundwater interactions.
Highlights
Understanding the spatial and temporal dynamics of hydrologic and geochemical processes in this zone is of critical importance for contaminated site management because approximately 75% of sites regulated under the Resource Conservation and Recovery Act (RCRA) and the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA or Superfund) are located within half a mile of a surface water (Tomassoni, 1999; Biksey et al, 2001), and many of these discharge to surface waters (US-EPA, 1991)
This study investigated contaminant migration within a dynamic groundwater and river water transition zone through both field experiments and reactive transport modeling
The transport of uranium at groundwater and river water (GW-RW) transition zone is affected by the interactions between hydrogeologic and geochemical heterogeneities, as well as hydrodynamic flow and geochemical conditions
Summary
The dynamic interactions between groundwater and river water (GW-RW) plays an important role in the hydrologic and biogeochemical processes in river corridor ecosystems (Stanford and Ward, 1988, 1993; Boulton et al, 2010; Fleckenstein et al, 2010; Harvey and Gooseff, 2015; Ward and Packman, 2019), as well as in contaminant fate and transport within the zone of GW-RW mixingU Migration in River Corridors (Ford, 2004; Zachara et al, 2013, 2016, 2020; Ma et al, 2014a; Shuai et al, 2019). The dynamic interactions between groundwater and river water (GW-RW) plays an important role in the hydrologic and biogeochemical processes in river corridor ecosystems (Stanford and Ward, 1988, 1993; Boulton et al, 2010; Fleckenstein et al, 2010; Harvey and Gooseff, 2015; Ward and Packman, 2019), as well as in contaminant fate and transport within the zone of GW-RW mixing. Complex spatio-temporal plume behaviors can stem from heterogeneities in source distribution and subsurface physical and geochemical properties, the hydrologic connectivity of different elements in the river corridor (e.g., riparian and hyporheic exchange flow paths) (Harvey et al, 2019), the fluxes and transit times of this exchanging water, and the abundance and distribution of aqueous and solid phase reactants along near-shore flow paths (Conant et al, 2004; Zachara et al, 2013). It is challenging to build an accurate conceptual and numerical model that adequately delineates the interactions between fluid flow and chemical processes governing contaminant transformations and mobility, while accounting for the spatial heterogeneities in subsurface properties and dynamic geochemical conditions driven by GWRW interactions (Zachara et al, 2020)
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