Issues

The fumigant pesticides 1,3-dichloropropene, methyl isothiocyanate, chloropicrin, and methyl bromide are commonly used in intensive agriculture to control soilborne pathogens. This article provides a review on the degradation mechanisms of these highly volatile pesticides, as their degradation in the soil environment plays a significant role in influencing their transport and effectiveness against soilborne pathogens. An emphasis is placed on the biological degradation mechanisms and responsible bacteria.

Carbon dioxide injected into deep geologic storage sites for carbon sequestration could potentially leak from those sites and seep out at the land surface. Simulations of carbon dioxide transport suggest that secondary trapping mechanisms for carbon dioxide in the unsaturated zone can attenuate small leakage fluxes and reduce surface seepage, but these mechanisms would not significantly attenuate transport to the land surface for large leakage fluxes.

Comparative simulations of a large-scale field infiltration experiment at the Maricopa Agricultural Center (Phoenix, AZ) were conducted using a hierarchy of models based on public, generic, and site data coupled with pedotransfer functions and an inverse procedure. Results showed that models based on public information performed poorly in reproducing observed water contents at the site. Estimating parameters using pedotransfer functions and Bayesian updating did not lead to improved simulations. Only when the parameters were estimated by means of an inverse procedure did one notice a significant improvement in model fit and predictive capability.

The saturation dependence of unsaturated hydraulic conductivity anisotropy is derived with a tensorial connectivity-tortuosity (TCT) concept in which the soil pore connectivity and/or tortuosity are anisotropic and described using a tensor. This tensor quantity was incorporated into relative permeability models and tested on Miller-similar heterogeneous flow domains having anisotropic spatial correlation structure. The results show that the connectivity-tortuosity coefficient is a function of both soil heterogeneity and anisotropy, and that the TCT model can accurately describe the saturation-dependent anisotropy of unsaturated hydraulic conductivity in soils.

Moisture retention relations in Hanford formation gravels were obtained for a very wide energy range. Water films on external grain surfaces were found to be volumetrically insignificant at matric potentials less than about -2 kPa while the residual water in intragranular pores accounted for about 10% of the total porosity. This large volume of intragranular water and the large intragranular specific surface area measured indicate that exchanges of solutes between the intragranular domain of Hanford gravels and their immediate surrounding are significant and diffusion-limited.

Soils and sediments are granular materials, and as such they are neither solid nor fluid but exhibit characteristics of both. A common practice in porous media research is to test hypotheses about flow processes using quasi-two-dimensional Hele Shaw cells or soil columns, presumably packed homogeneously. However, questions arise as to what packing procedures might lead to heterogeneous packing. We present results of experiments, demonstrating how particle size distribution and the rate at which a material is poured will determine the uniformity of the packed sample.

A quantitative method was developed to analyze the spatial distribution of dye tracer infiltration performed on 25 plots located at eight sites. The sequences of layers found in the hierarchical clustering of soil layers having similar dye-stain patterns were interpreted in terms of transport mechanisms and qualitatively compared with the sequence of morphological layers observed in the soil profiles. In most cases there was an agreement between the sequences of layer clusters found in the flow patterns and the observed soil layers. Zones of homogeneous infiltration and preferential flow could be reliably identified from the classification system developed.

Water flow, solute transport, and transformation parameters for the dual-permeability model MACRO were calibrated against numerically generated data representing transient leaching of a tracer and a reactive solute through microlysimeters. The procedure appeared to be promising for estimation of all parameters except for the parameter describing mass exchange between macro- and microporosity, which was estimated with large uncertainties, and reliable estimates for the adsorption coefficient parameter could not be obtained. The use of a modified experimental setup may improve estimations for these two parameters.

The differentiated linearization (DL) and transient single test methods for the analysis of transient flow from a tension infiltrometer were tested in a sandy loam and a clay soil. Applicability of the DL method required large differences in capillary forces between the contact material and the soil. Conducting experiments of relatively long duration reduced the experiment duration effect on the hydraulic conductivity estimates.

To assess the impact of changing water content on NAPL mass transfer during soil vapor extraction, a one-dimensional model of coupled moisture flow and heat transport and transport of a single contaminant component was developed. Simulation results demonstrated that at high water content the transfer of NAPL to the gas phase is limited by the diffusion rate through the water films, while at low water content the NAPL transfer is limited by the slow desorption of NAPL from the solid phase. In particular, at high water content NAPL trapped by water can retard NAPL removal because of a water shielding effect.

Dissolved noble gases offer several potential advantages as groundwater tracers: they are inert, nontoxic, nonbiodegradable, relatively inexpensive, and have low analytical detection limits. The results of laboratory batch and column partitioning tracer tests indicate that dissolved helium and neon may be suitable partitioning tracers for quantifying subsurface DNAPL at field sites. Due to their high Henry's Law constants, these gases partition into trapped air; therefore, they can be used to estimate both the amount of residual NAPL and trapped air when all three phases are present.

In a laboratory evaluation of the dual-probe heat-pulse method, measurements of water content and change in water content were found to be slightly biased, but we have shown that this bias can be removed by employing an empirical calibration relationship. This relationship appears to be universal inasmuch as it was derived from measurements in soil materials with widely differing physical properties. Our results show that excellent precision can be achieved with the method, with water content and change in water content measurements having root mean square errors of 0.022 and 0.012 cubic meters per cubic meter, respectively.

A two-stage laboratory method is presented for rapid estimation of the soil water retention function and the hydraulic conductivity function from near saturation to air dry. The hydraulic conductivity function was estimated inversely using cumulative evaporation amounts and the final water content profile. Agreement with independent conductivity data obtained from a steady-state, direct method confirms the reliability of the method developed.

Using three soils having different textures we conducted a test of the multistep outflow method to determine the unsaturated hydraulic conductivity of the soils. Our results indicated the existence of a hydraulic resistance at the soil-ceramic plate interface, which leads to an underestimation of hydraulic conductivity. We recommend the use of an auxiliary tensiometer near the soil-porous ceramic plate interface to help improve the hydraulic conductivity estimates.

The analytic element method is used to investigate the spatial sensitivity of different electrical resistivity tomography (ERT) arrays. This allows us to define regions of the subsurface where different ERT arrays are most and least sensitive, leading to improved monitoring of vadose zone processes.

An electromagnetic technique called the time domain transmission method is developed and tested for the measurement of spectral dependent dielectric properties within the 0.5- to 1.0-GHz bandwidth. Based on an open transmission line, this technique does not suffer the sample volume limitations or simplifying assumptions of other time domain methods. The method is shown to be a suitable method for measuring soils and vegetation in situ.

Impulse time domain transmission is a novel electromagnetic technique for monitoring water contents. In variably saturated sand, at very low salinities it measures the expected travel time vs. water content relationship. At higher salinities, travel times are faster than expected and have no observable dependence on salinity.