Issues

To supplement manuscripts in this issue's special section addressing research at the INEEL, a general overview is given regarding the operations history of the site as well as important soil and groundwater contamination issues. Additionally, the authors briefly introduce the papers in the special section.

The vadose zone at the INEEL is about 200 m thick, with great hydraulic and geochemical heterogeneity in its interbedded basalts and sediments. Several decades of study have identified major influences on contaminant transport within this vadose zone; these influences are related to this heterogeneity.

Field data describing large-scale behavior of water movement in highly heterogeneous vadose zones is difficult to obtain. This paper reviews the results from two large-scale infiltration tests and long-term monitoring data to suggest dominant mechanisms controlling water movement in the vadose zone at the INEEL.

This paper summarizes the tectonic, volcanic, and sedimentary processes that formed the eastern Snake River Plain of Idaho, the host rocks for its aquifer, and the thick vadose zone above the aquifer. The specific effects of each process on the aquifer and vadose zone are presented.

An overview is presented of an applied modeling study of flow and transport through a fractured basalt vadose zone beneath a waste burial site. Conceptual model uncertainty in the simulation results is evaluated, with the main premise being that conceptual uncertainty likely dominates parametric uncertainty.

A series of ponded infiltration tests in variably saturated fractured basalt at Box Canyon, Idaho, provided data for testing of models of flow in fractured rock. A dual-permeability model using TOUGH2 was used to develop a consistent set of hydrogeological parameters by calibration to infiltration front arrival times.

Unsaturated flow in a fracture network is investigated experimentally using a wall of limestone blocks. Results suggest that the interaction of multiple intersections acting in a network creates flow behavior not generally recognized in conventional numerical and conceptual models.

A comparison of several common numerical modeling techniques for water flow in a fractured, porous matrix is presented. The modeling results are compared against detailed experimental data collected under laboratory conditions.

The transport of carbon-14 over large spatial and temporal scales at the RWMC of the INEEL was studied by conducting conservative tracer and carbon-14 transport tests in a large unsaturated soil column. Model parameters derived from bench-scale experiments were used to predict these large-scale results.

This paper surveys recent advances in geophysical methods that are applicable to imaging the heterogeneous properties and processes within the subsurface underlying the INEEL, as well as other waste sites with similar geology and contaminants.

The role of microorganisms in the hydraulic performance of surface barriers was examined in several different prototype barriers installed in the vadose zone of a semiarid, high desert region. A path to integrate biological effects into numerical hydrologic models is considered.

Mixed wastes buried at the RWMC at the Idaho National Engineering and Environmental Laboratory included radionuclides, chlorinated solvents, and lubricating oils. Concentrations, stable carbon isotope, and radiocarbon contents of carbon dioxide in pore gas samples were collected from the subsurface to evaluate rates of natural biodegradation of the lubricating oils.

Treating parameters describing the hydraulic properties of the unsaturated zone as random variables and the flow equations as stochastic, unbiased estimates of the flow state variables of pressure, water content, and fluxes are developed, along with the quantification of uncertainty of such estimates.

A direct numerical simulation approach is advocated toward pore-level flow and transport in porous media, and is implemented using a grid-free, hybrid smoothed particle hydrodynamics numerical method. The model is demonstrated for flow and transport through a simulated porous material.

Temporal and spatial water potential data are presented from 30 advanced tensiometers located within basalt and sediments in the upper 74 m of a thick vadose zone. Results presented suggest that deep portions of the vadose zone are responding to recent (years) changes in surface infiltration.

A well completion method is developed and tested that minimizes the effects of barometric pressure fluctuations on measured water levels. This new design measures water level changes directly, thus eliminating the need to correct for barometric effects during postprocessing.

The vertical and lateral migration of a road dust suppressant, magnesium chloride, was studied at a radioactive waste burial ground. Significant lateral and vertical migrations were found. Results support previous estimates of the spatial distribution of recharge and vadose zone travel times at the site.

Model calculations are presented for the migration of cesium in the presence of high ionic strength fluids resulting from leaks in the S/SX Hanford tank farm. The model is calibrated against field data derived from a leak from one of the tanks.

Pore fluids from throughout a 70-m-thick vadose zone soil section show approximately constant enrichments (shift) in oxygen-18 natural abundance of 3 to 4 per mil relative to winter precipitation. The use of isotopic shift is shown to facilitate the identification of waters originating from other than natural sources.

Flow diversion and focusing caused by a natural flow-barrier system in the unsaturated zone of Yucca Mountain is analyzed using both analytical and three-dimensional numerical solutions. Model results were found to be consistent with observed field data.

The physicochemical properties of soil water and infiltrating water were studied in an undisturbed soil column to examine the effects of rainfall intensity, soil water solution ionic strength, and initial moisture content on the mobilization and transport of natural soil particles.

To predict the preferential transport of Cryptosporidium parvum oocysts through soils an existing model for preferential transport of nonadsorbing solutes was modified to account for adsorption of oocysts to air-water-solid interfaces. Simulation model predictions are compared with measured oocyst breakthrough data.

A generalized function of the normalized root length density distribution vs. normalized root depth was developed using wheat root length density data acquired from the literature. The generalized function can be used to estimate root length density distributions as a function of soil environment, species, and climate.

A method is developed by which a graphical solution is used to invert for electromagnetic wave propagation velocity in soil and subsequently water content from first-arrival travel time data using zero offset profiling ground penetrating radar.

An infiltration experiment on a soil column instrumented with an array of acoustic pulse transmission sensors was performed to test the ability to measure soil water content spatially using the soil water content dependence of acoustic wave velocity in soils.

Two new methods are proposed to estimate gas diffusivity in growing media, directly in greenhouse or nursery pots, and are compared with gas diffusion chamber measurements. The methods used the water desorption curve and the saturated hydraulic conductivity to calculate gas diffusivity. The results are correlated to estimates from the gas diffusion chamber.

Matrix diffusion is an important process for controlling contaminant transport in fractured porous media. Based on field test data, this study demonstrates that the effective matrix-diffusion coefficient may be scale dependent. A fractal-based, preliminary explanation of the scale dependency is presented.