[1] Substantial research on how hydraulic and geomorphologic factors control hyporheic exchange has resulted in reasonable process understanding; however, the role of fluvial islands on the transient nature of spatial flux patterns remains elusive. We used detailed field observations of the Truckee River, Nevada from 2003 to 2009 to quantify fluid flux between the river and a fluvial island, the streambed, and the adjacent stream bank. We constructed a 3-D numerical flow and heat transport model to further quantify the complex flow paths. Our study expands on previous research typically confined to less comprehensive scales and dimensions, and highlights the transient multidimensionality of the flow field. In fact, 1-D vertical streambed flux estimates indicated that the channel bar tail displayed the highest upward flux throughout the summer; however, 3-D model results indicated that the horizontal contribution was two orders of magnitude higher than the vertical contribution. The channel bar net flux is typically 1.5 orders of magnitude greater than the adjacent stream banks and an order of magnitude less than net streambed fluxes, indicating significant differences in river-aquifer interactions between each of the geomorphic units. Modeling simulations further indicated that the channel bar induces 6 times more fluid flux than an identical location without a fluvial island, consistent with flux estimates from a nearby river restoration location. Moreover, event-based and seasonal transient antecedent moisture and near-stream storage conditions contribute to multidimensional rivergroundwater interactions. These results suggest that fluvial islands are a key driver and significant component of river-groundwater interactions and hyporheic flow.

effects of temperature dependence of the Henry's law equilibrium constant or isotopic fractionation with re-Understanding transport of tritium ( 3 H) in unsaturated zones is spect to 3 H. critical to evaluating options for waste isolation. Tritium typically is a large component of low-level radioactive waste (LLRW). Studies Nevada are investigating 3 H transport from a closed (ADRS) in Nevada investigate 3 H transport from a closed LLRW LLRW facility (Fig. 1). Striegl et al. (1996) attempted to facility. Two boreholes are 100 and 160 m from the nearest waste explain elevated concentrations of tritiated water vapor trench and extend to the water table at 110 m. Soil-water vapor ( 3 HHO g ) found throughout the unsaturated zone at a boresamples from the deep boreholes show elevated levels of 3 H at all hole 160 m from the nearest LLRW trench. They used depths. The objectives of this study were to (i) test source thermal diffusive (Smiles et al., 1995) and advective transport modand gas-advection mechanisms driving 3 H transport and (ii) evaluate els to simulate an isothermal and homogeneous domain. model sensitivity to these mechanisms and to selected physical andThe models were unable to match observed 3 HHO g conhydraulic properties including porosity, tortuosity, and anisotropy. centrations. The diffusive model predicted 3 H migrationA two-dimensional numerical model incorporated a non-isothermal, heterogeneous domain of the unsaturated zone and instantaneous to a maximum distance of approximately 10 m from the isotopic equilibrium. The TOUGH2 code was used; however, it re-source after 30 yr. An order of magnitude increase in the quired modification to account for temperature dependence of both effective diffusion coefficient increased the maximum the Henry's law equilibrium constant and isotopic fractionation with distance by a factor of three. The advective model used respect to tritiated water. Increases in source temperature, pressure, a pressure originating from the source. Striegl et al. (1996) and porosity enhanced 3 H migration, but failed to match measured determined a 1300 Pa source-pressure-difference sus-3 H distributions. All anisotropic simulations with a source pressure tained for 30 yr was required to move the 3 H 100 m, component resembled, in shape, the upper portion of the 3 H distribubut were unable to justify a source-pressure-difference tion of the nearest borehole. Isotopic equilibrium limited migration Ͼ100 Pa. Ultimately, a conceptual model of lateral subof 3 H, while effects of radioactive decay were negligible. A 500 Pa surface liquid transport from LLRW trenches was develpressure increase above ambient pressure in conjunction with a high degree of anisotropy (1:100) was necessary for simulated 3 H transport 3 H distribution. Neither of these studies evaluated the evaluation of simulation results. The numerical model incorporated a non-isothermal, heterogeneous domain.

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radioactive waste. S S : UZIG USGS Soil-plant-atmosphere interac ons strongly infl uence water movement in desert unsaturated zones, but li le is known about how such interac ons aff ect atmospheric release of subsurface water-borne contaminants. This 2-yr study, performed at the U.S. Geological Survey's Amargosa Desert Research Site in southern Nevada, quan fi ed the magnitude and spa otemporal variability of tri um ( 3 H) transport from the shallow unsaturated zone to the atmosphere adjacent to a low-level radioac ve waste (LLRW) facility. Tri um fl uxes were calculated as the product of 3 H concentra ons in water vapor and respec ve evapora on and transpira on water-vapor fl uxes. Quarterly measured 3 H concentra ons in soil water vapor and in leaf water of the dominant creosote-bush [Larrea tridentata (DC.) Coville] were spa ally extrapolated and temporally interpolated to develop daily maps of contamina on across the 0.76-km 2 study area. Maximum plant and root-zone soil concentra ons (4200 and 8700 Bq L -1 , respec vely) were measured 25 m from the LLRW facility boundary. Con nuous evapora on was es mated using a Priestley-Taylor model and transpira on was computed as the diff erence between measured eddy-covariance evapotranspira on and es mated evapora on. The mean evapora on/transpira on ra o was 3:1. Tri um released from the study area ranged from 0.12 to 12 μg d −1 and totaled 1.5 mg (8.2 × 10 10 Bq) over 2 yr. Tri um fl ux variability was driven spa ally by proximity to 3 H source areas and temporally by changes in 3 H concentra ons and in the par oning between evapora on and transpira on. Evapotranspira on removed and limited penetra on of precipita on beneath na ve vegeta on and fostered upward movement and release of 3 H from below the root zone.

Effective isolation of tritium ( 3 H) and other contaminants at waste-burial facilities requires improved understanding of transport processes and pathways. Previous studies documented an anomalously widespread (i.e., theoretically unexpected) distribution of 3 H (>400 m from burial trenches) in a dry, sub-root-zone gravelly layer (1-2-m depth) adjacent to a low-level radioactive waste (LLRW) burial facility in the Amargosa Desert, Nevada, that closed in 1992. The objectives of this study were to: (i) characterize longterm, spatiotemporal variability of 3 H plumes; and (ii) quantify the processes controlling 3 H behavior in the sub-root-zone gravelly layer beneath native vegetation adjacent to the facility. Geostatistical methods, spatial moment analyses, and mass flux calculations were applied to a spatiotemporally comprehensive, 10-yr data set (2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011). Results showed minimal bulk-plume advancement during the study period and limited Fickian spreading of mass. Observed spreading rates were generally consistent with theoretical vapor-phase dispersion. The plume mass diminished more rapidly than would be expected from radioactive decay alone, indicating net efflux from the plume. Estimates of upward 3 H efflux via diffusive-vapor movement were >10´ greater than by dispersive-vapor or total-liquid movement. Total vertical fluxes were >20´ greater than lateral diffusive-vapor fluxes, highlighting the importance of upward migration toward the land surface. Mass-balance calculations showed that radioactive decay and upward diffusive-vapor fluxes contributed the majority of plume loss. Results indicate that plume losses substantially exceeded any continuing 3 H contribution to the plume from the LLRW facility during 2001 to 2011 and suggest that the widespread 3 H distribution resulted from transport before 2001.