Kilometer‐Scale Hydrologic Exchange Flows in a Gravel Bed River Corridor and Their Implications to Solute Migration

Abstract

A well‐characterized field site along a major, gravel bed river corridor was used to investigate the dynamic pathways and impacts of subsurface hydrogeologic structure on kilometer‐scale hydrologic exchange flows between river water and groundwater. An aqueous uranium (Uaq) plume exists within a hyporheic alluvial aquifer at the site that discharges to the Columbia River. We performed temporally intensive monitoring of specific conductance (SpC) and Uaq concentrations within the plume for a 2‐year period at varying distances from the river shoreline, both within and outside a presumed subsurface pathway of lateral hydrologic exchange. SpC and Uaq were utilized as in situ tracers of hydrologic exchange and associated groundwater‐surface water mixing. Seasonal river stage variations by more than 2 m caused distinct events of river water intrusion and retreat from the nearshore, hyporheic alluvial aquifer, resulting in highly dynamic SpC and Uaq patterns in monitoring wells. Simulations of hydrologic exchange and mixing were performed with PFLOTRAN to understand the observed SpC and Uaq behaviors linked to predominant flow directions and velocities in the river corridor as influenced by river stage dynamics and variable aquitard topography. By coupling robust monitoring with numerical flow and transport modeling, we demonstrate complicated multidirectional flow behaviors at the kilometer scale that strongly influenced plume dynamics. Therefore, hyporheic aquifer must be frequently monitored under different flow conditions if water quality is of concern. The resulting hydrologic understanding enables improved interpretation of hydrogeochemical data from this site and other large gravel bed river corridors in the United States and elsewhere.

River Dynamics Control Transit Time Distributions and Biogeochemical Reactions in a Dam‐Regulated River Corridor

“…Although the fluvial deposition on the riverbed plays an important role in controlling the exchange flow, its thickness is highly uncertain and spatially variable (0.5 to~2 m based on field experience). Instead of directly modeling the fluvial layer, a river conductance boundary condition was used at the river-sediment interface following Hammond and Lichtner (2010) and Zachara et al (2020). The conductance coefficient was the main model parameter, which was manually adjusted to fit the water level and SpC measurements in a set of wells.…”

“…We manually adjusted the river conductance coefficient to better match the measured hydraulic heads and SpC in 28 long-term monitoring wells (circled dots in Figure 3a). We started with two conductance values of 1 × 10 −12 and 1 × 10 −13 m based on previous studies (Hammond & Lichtner, 2010;Zachara et al, 2020). Then the conductance values were iteratively refined twice (5 × 10 −13 and 2.5 × 10 −13 m) to improve the match between the simulated and measured groundwater level and SpC.…”

“…More details about our particle tracking scheme are described in the supporting information (Text S2). Williams et al (2008), and the same values have been used in Song et al (2018) and Chen et al (2013) Data originated from Williams et al (2008), and the same values have been used in Shuai et al (2019) and Zachara et al (2020) Data originated from Williams et al (2008), and the same values have been used in Shuai et al (2019) and Zachara et al (2020) 10.1029/2019WR026470…”

Sequential Imputation of Missing Spatio-Temporal Precipitation Data Using Random Forests

“…It serves as an input for various ecohydrological models to determine snowpack, infiltration, surface-water flow, groundwater recharge, and transport of chemicals, sediments, nutrients, and pesticides (Devi et al, 2015). Numerical modeling of surface flow typically requires a complete time series of precipitation along with other meteorological records (e.g., temperature, relative humidity, solar radiation) as inputs for simulations (Dwivedi et al, 2017(Dwivedi et al, , 2018Hubbard et al, 2018Hubbard et al, , 2020Zachara et al, 2020). However, meteorological records often have missing values for various reasons, such as due to malfunctioning of equipment, network interruptions, and natural hazards (Varadharajan et al, 2019).…”

Understanding Contaminant Migration Within a Dynamic River Corridor Through Field Experiments and Reactive Transport Modeling

“…The dynamic interactions between groundwater and river water (GW-RW) plays an important role in the hydrologic and biogeochemical processes in river corridor ecosystems 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 (Ford, 2004;Zachara et al, 2013Zachara et al, , 2016Zachara et al, , 2020Ma et al, 2014a;Shuai et al, 2019). 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).…”