Issues

A century since, Buckingham's 1907 dynamical idealization of soil-moisture movement continues to be the only workable model, though his capillary potential does not satisfy requirements of a field potential. Skepticism of adequacy of Fourier's linear equation for soil moisture likely dissuaded him from writing down the nonlinear equation needed.

Climate-soil-plant-water relation of nonsorted circles in the arctic is not well understood. Modeling these processes revealed surface insulation to cause preferential ice accumulation in noninsulated areas, inhibiting vegetation growth. Lowered freezing rates or increased vegetation on circles reduced water movement and altered the equilibrium.

We present an approach to use the reactant methane as a substitute tracer to quantify methane oxidation in situ during gas push-pull tests with diffusion-dominated gas transport. Under these conditions, carbon stable isotope fractionation was useful only to a limited extent as an indicator for microbial oxidation.

The microgravity environment of space poses new challenges for managing fluid distribution and flow within porous media. This paper presents effects of reduced gravity on hydraulic properties through dynamic measurements and modeling of water retention and saturated flow during parabolic flight-induced variable gravity.

This study presents an intermediate-scale experiment to investigate the applicability of the partitioning tracer technique to heterogeneous sites with complex nonaqueous-phase liquid (NAPL) entrapment architecture. Results of this study demonstrate how the inversion of the tracer breakthrough concentrations provided valuable information about the validity of the equilibrium condition and demonstrate that more accurate NAPL estimates can be obtained using effective partition coefficients in combination with inverse modeling methods

Two dual-permeability models (MACRO and S1D Dual) are compared to predict leaching of selected pesticides in Hawaii conditions to assist the state in future registration of pesticides.

We have characterized the spatial sensitivity of dual-probe heat-pulse sensors for measuring soil heat capacity and water content. The sensitivity is greatest in small areas near the two probes of the sensor. Far from the sensor, contours of equal sensitivity are approximately elliptical in shape.

We examined the temperature sensitivity of one instrument that operates at 50 MHz for a wide variety of soils from the USA. The real component response to a temperature change produced an imaginary component change of 2% per degree Celsius. A high imaginary component is probably responsible for the temperature sensitivity of most soil water sensors.

We present a general framework for relating soil water retention to textural data, which we use to generate improved models and demonstrate relationships to existing models. Developments include a modified Arya-Paris model that eliminates interval-size dependence, simplifies calculations, and improves versatility.

A conceptual model of flow and transport in a ponderosa pine-covered mesa contaminated with high explosives was developed using multiple physical and tracer methods. Chloride-based vadose zone flux estimates are also compared to results from other ponderosa pine forests. These forests occupy areas which may be key recharge areas in semiarid basins.

Analytical water retention models are developed for partial drainage of pores in random mass prefractal (RMP) porous media. The models are tested against drainage curves for two-dimensional RMP structures simulated with an invasion percolation algorithm. The probability that a pore will drain decreases with decreasing pore size and total porosity.

Lysimeters are used within the EU for regulatory assessment of pesticide leaching. This paper shows that lysimeter leachate concentrations may be much lower than field leaching concentrations because (i) the residence time of pesticide in the lysimeter is longer and (ii) a study duration of 2 yr is too short for moderately sorbing pesticides.

U.S. federal agencies use reactive transport models to evaluate contaminant transport and provide guidance to decision makers and regulators for treatment options. Summaries of selected projects are presented to demonstrate the activities in various applications and to present examples of recent advances in subsurface reactive transport modeling.

Increasing stress on freshwater resources worldwide is making us re-evaluate the role of wastewater in the modern water cycle. Is it really waste? Not if it can be used, for example, for irrigation. Wastewater irrigation has been happening since ancient times, but how widespread is the practice today and is it sustainable and safe?

We present findings of a field investigation of the controls on dimensions of wetted soils around lakes in the McMurdo Dry Valleys, Antarctica. These shoreline soils and sediments wick water from adjacent water bodies, a process that provides preferentially consistent moisture (in the austral summer) to these locations in a hyperarid landscape.

A lignitic mine soil profile was instrumented with horizontally distributed soil air pressure sensors and tensiometers and monitored for three weeks. Elevated soil air pressure measurements during infiltration and soil air pressure depression during drainage along narrow columns indicated the occurrence of preferential flow (flow fingers).

This paper presents the results of two field-scale tracer tests carried out at Idaho National Laboratory that provide insights into flow and transport through variably saturated, layered geologic systems consisting of both unconsolidated porous media and fractured rock.

We present a novel application of an Artificial Neural Network (ANN)-based PTF scheme across two spatial scales. ANNs are trained using coarse scale (1:24000) soil properties and used to predict soil water contents at fine scale (1:1). Results show good agreement between ANN predictions after bias correction and field observations at fine scale.

On a steep hillslope, the responses of soil water dynamics did not depend simply on the distance from a tree but were significantly different between the upslope and downslope regions. This was attributed to a great amount of rainwater supplied at the downslope side of the tree stem.

An advective-dispersive solute transport equation that includes plant uptake of water and solute is presented. The model will be useful for estimating transport and uptake of strongly sorbing and persistent contaminants. An example of Cd uptake by wheat is given to illustrate its use and compare it with a one-compartment model.

The special section editors provide an overview the geologic, soil, and vadose zone flow study of the Hanford Site, with an emphasis on transport of organic liquids and geochemical considerations. The 13 articles in the section sample the many questions and approaches involved in dealing with the legacy of nuclear waste production at the site.

Borehole data provide the basis for interpreting the spatial distribution of flow and transport properties of the subsurface region. This paper describes the data-standardization and translation techniques being developed for the Hanford Site to support descriptive architectural and geostatistical analyses of effective flow and transport parameters.

The study is aimed at developing methods to account for the effects of heterogeneity, to construct spatial patterns of the hydraulic properties, and to correctly identify the connectivity of facies that govern flow and transport in Hanford's vadose zone.

The tensorial connectivity-tortuosity (TCT) concept has been extended to unsaturated porous media with immiscible fluids. Numerical experiments of infiltration of two phases, water and a nonaqueous phase liquid (NAPL), were conducted. Results show that the TCT coefficients are functions of both soil heterogeneity and anisotropy.

Numerical models were developed that considered the extent of contamination presently within the vadose zone at the Hanford Site SX tank farm. Predicted groundwater concentrations for technetium-99 at the tank farm fenceline were a function of recharge rates but exceeded the maximum groundwater concentration limit of 900 picocuries per liter.

A surface electrical resistivity survey was conducted over a liquid waste disposal site at Hanford. The results showed that the plume, primarily consisting of nitrate, resides 25 to 44 m below ground surface and is consistent with borehole data.

Clastic dikes cut through sediment layers at high angles and are common occurrences at the Hanford Site. They have been cited as potential fast transport pathways through the vadose zone. We present field data and modeling results suggesting that they may provide fast pathways for vertical transport of water under a restricted range of flux conditions.

Large-scale flow and transport simulations were conducted to improve the understanding of carbon tetrachloride (CT) infiltration and redistribution below the main disposal site at Hanford and to assess the effectiveness of soil vapor extraction in removing residual CT from the vadose zone.

Wet chemical, spectroscopic, and microscopic studies with sediments impacted by tank waste leaks at the Hanford Site have been performed to develop conceptual models of processes governing the transformation, retardation, and transport of tank waste residuals. The work has elucidated key processes that have moderated the impact of tank wastes, and identified others that are unlikely to occur in the field.

Microscale investigations indicate that the conditions of formation for secondary phases control the subsequent mobility and macroscale concerntration of uranium. Microscale and macroscale measurements may be combined to better understand the processes operating at contaminated sites.

Multiple isotopic systems (nitrogen-15 natural abundance and oxygen-18 natural abundance of nitrate; strontium-87/-86; uranium-234, -235, -236, and -238) yield clues to sources, timing, and transport of contaminants in the Hanford Site vadose zone.

Bromide, deuterium oxide, and strontium-87 added to water leaked into unsaturated sediments at the Hanford Site showed strong contrasts in transport. Deuterium oxide and bromide spread out laterally along a thin zone in the sediments, but anion exclusion caused the bromide to move more quickly. Strontium-87 transport was limited due to sorption onto the sediments.

Isotope ratio measurements for Cs and Mo were performed on core samples from a slant borehole drilled near a nuclear waste tank on the USDOE's Hanford Site. The molybdenum-95 isotope makes a good candidate for a tracer of tank leakage because parent isotopes are present during the fission decay chain. Distinct episodes of leakage were indicated.

Laboratory experiments with Hanford Site sediment were conducted to estimate the reductive capacity of permeable reactive barriers under field conditions. The experimental results compared favorably with theoretical estimates based on fundamental flow and transport principles.