Elucidation of Flow and Transport Processes in a Variably Saturated System of Interlayered Sediment and Fractured Rock Using Tracer Tests

Abstract

The objective of this work was to investigate flow and transport in a layered, variably-saturated system consisting of both fractured rock and sedimentary material during focused infiltration from the surface. Two tracer tests were carried out using the Vadose Zone Research Park (VZRP) at Idaho National Laboratory (INL). The first test was conducted under quasi-steady-state conditions and the second was initiated in a much drier system and thus provided information regarding flow and transport under transient conditions. A one-dimensional analytical model was used to fit breakthrough curves resulting from the two tracer tests. The results of this modeling provide insight into the nature of flow in the fractured basalt, surficial alluvium, and sedimentary interbeds that comprise the vadose zone of the eastern Snake River Plain. Flow through the fractured basalt is focused and preferential in nature and multiple flow paths arise due to numerous fractures functioning as transmissive pathways in addition to flow splitting along geologic contacts. Flow velocities were significantly higher during the test with the wetter flow domain, presumably due to increases in hydraulic conductivity associated with higher water contents of the geologic materials. Perching was observed above the alluvium/basalt contact and above the lower boundary of a locally continuous sedimentary interbed. The perching behavior between the two contacts was fundamentally different; the perched layer above the alluvium/basalt contact was neither laterally extensive nor temporally persistent in the absence of infiltration from the surface. In contrast, the perched layer along the interbed was significantly thicker and gave rise to lateral flow over distances on the order of 100s of meters. Vertical transport is shown to occur predominantly through the main bulk of the sedimentary material of the interbed rather than through preferential pathways; lateral flow appears to occur primarily in the fractured basalt directly above the interbed.ii ACKNOWLEDGEMENTS

Infiltration in Arid Environments: Spatial Patterns between Subsurface Heterogeneity and Water‐Energy Balances

“…Th ere are a number of conclusions that may be derived from these results regarding infi ltration that confi rm the fi ndings of previous studies using more traditional modeling and observational approaches. Th is study also shows that infi ltration into the subsurface is localized and highly transient, with the majority of recharge occurring during short-duration recharge events at a range of temporal scales (e.g., Seyfried and Wilcox, 1995;Seyfried, 1998;Salve, 2005;Sandvig and Phillips, 2006;Scanlon et al, 2006;Zhou et al, 2006;Duke et al, 2007). Th e relatively large recharge rates seen in these results (?20 mm/yr), however, are greater than those presented previously (?0.1 mm/yr) for the same, general, geographic region (Walvoord et al, 2002(Walvoord et al, , 2004, which were arrived at through modeling and observations.…”

“…Subsurface heterogeneity, particularly in fractured systems, further complicates the distribution and movement of moisture in the subsurface, which subsequently aff ects the spatial and temporal distribution of land-atmosphere fl uxes.Individual components of these arid, fractured systems have been studied previously. For example, Duke et al (2007) used fi eld-scale tracer tests to study fl ow and transport in a fractured system consisting of a surfi cial alluvium underlain by fractured basalt. Th ey found fl ow in the fractured basalt to be focused in the fractures, with very low increased matrix saturation.…”

Perched-Water Evaluation for the Deep Vadose Zone Beneath the B, BX, and BY Tank Farms Area of the Hanford Site

“…Wang et al (2010) used a two-dimensional approach to simulate strontium 90 transport in the subsurface of the INEEL, justifying the approach by the large waste sites and the low-permeability features of the perching layers. Duke et al (2007) used a one-dimensional approach to compute contaminant transport in perched water through a laterally extensive low-permeability zone in the INEEL subsurface. In this analysis, it was assumed that most of the waste water had moved downward and perched on top of a low-permeability perching layer and that lateral perched-water flow was minimal.…”

Numerical Evaluation of Unsaturated‐Zone Flow and Transport Pathways at Rainier Mesa, Nevada

“…The analysis of flow and transport pathways near and through perched water at Rainier Mesa can help clarify flow and transport behavior at other existing and potential DOE waste‐storage sites in the arid west where the presence of deep perched water complicates the understanding and monitoring of radionuclide transport. These sites include (i) the Radioactive Waste Management Complex at the Idaho National Laboratory, where lateral flow has been observed in the sedimentary interbeds between lava flows (Duke et al, 2007); (ii) Yucca Mountain, a proposed site for high‐level nuclear waste disposal in volcanic tuffs in southern Nevada, where the stratigraphic and structural setting promotes perching, lateral flow, and possible fault drainage beneath the proposed repository (Bagtzoglou, 2003a, 2003b; Flint et al, 2001, 2002); and (iii) the Los Alamos National Laboratory (LANL) site in northern New Mexico, where perched water above low‐permeability tuff layers beneath wet canyons also creates the potential for lateral flow and complex vadose zone transport paths (Robinson et al, 2005; Birdsell et al, 2005).…”