William A. Jury

SUSCEPTIBILITY OF SOILS TO PREFERENTIAL FLOW OF WATER : A FIELD STUDY

Flow pathways of water and solutes in soils form distinct patterns, which are not a priori predictable. Macropore structure is a prime cause, but other factors, such as differing initial or boundary conditions, may also predispose a soil to produce bypassing of infiltrating water. This study was conducted to assess the flow pathways of water in different soils and to investigate the effect of initial water content on the flow pattern. Dye-tracing experiments were carried out at 14 different field sites. The sites represent a good portion of soils used for agricultural crop production in Switzerland. Each site consisted of two 1.4 by 1.4 m plots, one of which had been covered with a plastic roof for two months before the experiment to achieve different initial water contents. Forty millimeters of water containing the dye Brilliant Blue FCF (C.I. Food Blue 2) were applied within 8 hours onto the plots with a sprinkling apparatus. One day after irrigation the plots were excavated, and the stained pattern was examined on a vertical 1 by l m soil profile. The spatial structure of flow patterns showed remarkable differences. In most soils, water bypassed the soil matrix. In some soils, dye penetrated beyond l m depth, whereas in others it remained in the top 50 cm. Structured soils were more prone to produce bypass flow, deep dye penetration, and pulse splitting than nonstructured soils. The initial water content had a less pronounced effect in some soils and no effect in others.

Transport of chemicals through soil: mechanisms, models, and field applications

Publisher Summary The chapter presents and evaluates current experimental information and theoretical approaches used to represent chemical transport and transformations in unsaturated soil. It focuses on the field regime and discusses the current approaches used for modeling chemical transport in natural media. The field evidence regarding preferential flow is quite consistent in one respect. Preferential flow can occur under a variety of circumstances and is not restricted to clay-rich soils with significant structural voids. Fluid transport through well-defined structural voids is not predictable unless the distributions of voids, aperture sizes and shapes, depths of penetration, and interconnectivity are known. Progress is being made slowly in characterizing transport through rock fractures but there the geometry is much more stable in time than it is in the soil regime. Laboratory studies have demonstrated clearly that soil structure is almost certain to introduce mass transfer limitations to equilibrium between the dissolved and sorbed phases in soil. The chapter concludes that there are fundamental differences between the transport characteristics of the laboratory and field environments.

Transport of Chloride Through an Unsaturated Field Soil

A chloride tracer was applied to the surface of a vegetable field and then leached downward by rainfall and irrigation. Tracer concentrations in a vertical two-dimensional region down to a depth of 2.4 m were monitored with suction cups that, were installed horizontally from a tunnel. The uniformly applied tracer pulse split into a slowly moving main pulse and a series of fast pulses. The first of the fast pulses reached a depth of 2.2 m after an infiltration of just 31 mm of natural rainfall, whereas the peak of the main pulse was still at a depth of 0.84 m by the end of the experiment after an infiltration of 0.853 m. The movement of the main pulse can be described by a convection-dispersion process in a homogeneous medium, provided that time is replaced by cumulative infiltration. However, the values of the parameters that produce a maximum agreement between the model and the observed main pulse have no physical basis, and consequently prediction of solute movement, based on measurements of soil properties, is not possible.

The Emerging Global Water Crisis: Managing Scarcity and Conflict Between Water Users

For the first time in human history, human use and pollution of freshwater have reached a level where water scarcity will potentially limit food production, ecosystem function, and urban supply in the decades to come. The primary reason for this shortage is population growth, which has increased at a faster rate than food production for some years and will add up to 3 billion more people by the middle of the twenty‐first century, mostly in poor and water‐short countries. Water quality degradation has also contributed significantly to a number of problems of global concern, including human drinking water supply and species survival. As of today, some 1.1 billion planetary inhabitants do not have access to clean drinking water, and 2.6 billion do not have sanitation services. Water pollution is a leading cause of death worldwide, and transmits or supports numerous debilitating diseases to populations forced to drink contaminated water. Agriculture is by far the leading user of freshwater worldwide, accounting for almost 85% of global consumption. Because of growing demand, we will need to raise food production by nearly 50% in the next 50 years to maintain our present per capita supply, assuming that the productivity of existing farmland does not decline. Further, we will have to increase it by much more if we are also to alleviate malnutrition among the poorest members of our current population. For a variety of reasons, feasible expansion of irrigated agriculture will be able to accommodate only a portion of this increased demand, and the rest must come from an increase in the productivity of rainfed agriculture. In the absence of coordinated planning and international cooperation at an unprecedented scale, the next half century will be plagued by a host of severe water‐related problems, threatening the well being of many terrestrial ecosystems and drastically impairing human health, particularly in the poorest regions of the world. The latter portion of this chapter discusses ways in which this emerging crisis may be mitigated.

Dependence of pesticide degradation on sorption: nonequilibrium model and application to soil reactors

Abstract The effect of sorption on degradation of the pesticide 2,4-dichlorophenoxyacetic acid (2,4-D) was studied in a soil amended with various amounts of activated carbon (AC). The relationship between sorption and decay of 2,4-D was analyzed using analytical solutions for equilibrium sorption and to a two-site nonequilibrium adsorption model coupled with two first-order degradation terms for the dissolved and sorbed pesticide, respectively. The sorption parameters in the latter model were determined based on data obtained from batch sorption experiments, while those for degradation were obtained from incubation experiments. The adsorption coefficients, ranging from 0.811 to >315 ml g −1 , increased at higher AC, and were negatively related to degradation as measured by the first-order rate constant, implying that degradation is faster from the liquid phase than from the sorbed phase. A nonlinear fit of the decay curves to the nonequilibrium model revealed that degradation rate constants were 0.157 and 0.00243 day −1 for the liquid and sorbed phases, respectively, differing by a factor of 65. Similar results were also obtained using the equilibrium model. A parameter sensitivity analysis of the nonequilibrium model indicates that nonequilibrium sorption will initially favor degradation; however, over the long term, will decrease degradation when desorption kinetics becomes the limiting factor in the degradation process. In the presence of a lag phase that allows appreciable amounts of chemical to diffuse into kinetic sorption sites, nonequilibrium sorption will only impede degradation.

A field study of the effects of soil structure and irrigation method on preferential flow of pesticides in unsaturated soil

Abstract A large number of field plot experiments were performed to characterize the downward flow of three pesticides (atrazine, napropamide and prometryn) and a water tracer (chloride) under various soil water regimes and soil surface conditions. Each experiment consisted of the uniform application of a 0.4-cm pulse of a solution containing a mixture of the four chemicals to the surface of a 1.5 × 1.5-m plot. The plot was then irrigated with 12 cm of water and soil samples were collected and analyzed to a depth of 150 cm. In all, 64 different plots were employed to study individual as well as interactive effects of such variables as irrigation method (continuous or intermittent sprinkling or ponding), pesticide formulation method (technical grade dissolved in water, wettable powder, or emulsifiable concentrate), and tillage (undisturbed or tilled and repacked surface layer) on pesticide transport. While all three pesticides were expected to be retained in the top 10–20 cm, there was considerable movement below this zone. When averaged over all the treatments, 18.8% of the recovered mass of atrazine, 9.4% of the prometryn and 16.4% of the napropamide were found between 30- and 150cm depth. Moreover, all pesticides were highly mobile in the surface 30 cm regardless of their adsorption coefficient. There were occureences of extreme mobility or “preferential flow” of pesticide under every experimental condition except where the pesticides were applied in wettable powder form to plots which had their surface tilled and repacked. This finding implies that there may be fine preferential flow pathways through which solution may move but particulates may not.

A laboratory study of the dispersion scale effect in column outflow experiments.

Abstract A solute transport study was designed to test the validity of the convection-dispersion model in a series of experiments which varied soil column width and length. The studies were performed at three steady-state water flux rates (8, 4, 2 cm d −1 in both undisturbed and packed columns containing Tujunga loamy sand. The series of experiments for both undisturbed and repacked soil began with three 87-cm-long soil columns of inner diameter 10, 15 and 20 cm. Pulses of CaCl 2 were leached through each column at each flow rate, after which the columns were cut in half and the experiments repeated with the upper 43.5 cm of the old columns. Finally, the columns were cut a second time and the upper 21.8-cm columns were used in the final set of experiments. The resulting breakthrough curves were used to test two solute transport models, the one dimensional convection-dispersion equation, CDE, and the mobile-immobile water model, MIM. The breakthrough curves from the undisturbed soil were very skewed, showing both early breakthrough and extensive tailing, whereas the outflow pulses from the repacked columns were symmetric. The CDE dispersion coefficient D or dispersitivity λ significantly increased with increasing column length in the undisturbed soil columns at the higher flux rates, but D was length-independent and considerably smaller in the repacked columns than in the undisturbed soil experiments. Although the MIM model gave a good description of the extensive tailing of the breakthrough curves in any single outflow experiment, its refitted parameters showed no consistent relationship with any of the experimental variables and were generally not invariant with length.

A three-dimensional field study of solute transport through unsaturated, layered, porous media: 1. Methodology, mass recovery, and mean transport

resident concentrations for the three solutes studied. The data were analyzed using the method of moments. In addition to the solute transport experiments, a detailed set of physical properties of the field was obtained by excavating three pits to a depth of 5.0 m and also by taking soil cores throughout the study area. This paper explains the experimental methodology, summarizes the relevant site characteristics, and describes the observed transport based on the zeroth and first order spatial moments. Mass balance varied between 78 and 138%. The field-averaged gravimetric water content and dry bulk density were used to accurately predict the mean vertical plume displacements. The plumes spread relatively little in the horizontal direction.

Contribution of earthworms to PCB bioremediation

Twenty cm deep columns containing Aroclor 1242 contaminated soil were bioaugmented with the PCB-degrading micro-organisms, Ralstonia eutrophus H850 and Rhodococcus sp. strain ACS, each of which were grown on sorbitan trioleate, and induced for PCB degradation by salicylic acid and carvone, respectively. Treatments consisted of soils with and without earthworms. Earthworms were utilized to enhance the dispersal of the bioaugmented PCB-degrading micro-organisms, while simultaneously improving soil aeration, increasing soil carbon and nitrogen content, and modifying the soil microbial community. Bioaugmented soils containing the earthworm Pheretima hawayana achieved 55% removal of total soil PCB as compared to only 39% in identically treated soils without earthworms. Earthworm-treated soils achieved upwards of 65% PCB degradation at subsurface depths, as compared to 44% in soils without earthworms and prior reports of only 10% degradation in soils treated without manual mixing of the inoculum into the soil (McDermott et al., 1989. Two strategies for PCB soil remediation: biodegradation and surfactant extraction. Environmental Progress 8, 46‐51). A methane diffusion study demonstrated that soils containing earthworms attained greater gas diffusion rates. Breakthrough of the methane tracer through the 20-cm column was detected after only 10 min in soils with earthworms, while 340 min was required before breakthrough in soils without earthworms. Using a gas diffusion model, the experimental diffusion coefficients were calculated to be 4.45 £ 10 23 and 5.0 £ 10 24 cm 2 s 21 , respectively. The higher diffusion rate of oxygen into the soil profile provided greater concentrations of the necessary terminal electron acceptor for aerobic PCB degradation. Methane depletion was observed only in soils with earthworms and was attributed to microbial communities unique to the earthworm treated soils. The potential contribution of these communities toward PCB degradation is discussed. q 2001 Elsevier Science Ltd. All rights reserved.

Impact of the plant rhizosphere and augmentation on remediation of polychlorinated biphenyl contaminated soil

This study investigated the interactive effects of bioaugmentation, biostimulation, and the rhizosphere during remediation of Aroclor 1242-contaminated soil. Treatments were repeatedly augmented with polychlorinated bipheny (PCB)-degrading bacteria, inducers (carvone and salicylic acid), surfactant (sorbitan trioleate), minimal salts medium in a 20-cm high soil column, or a combination of these elements. Soils containing a single Brassica nigra plant achieved 61% PCB removal in the 0 to 2 and 2 to 6 cm depths after 9 weeks of bioaugmentation, whereas only 43 and 14% PCB removal, respectively, was achieved in unplanted controls. Gas diffusion coefficients of 13.0 and 5.0 x 10(-7) m2 s(-1) were calculated from a methane diffusion assay for planted and unplanted soils respectively, indicating the positive effect of plant roots on gas diffusion into the soil. A second, modified column study removed 87, 73, 63, and 45% of PCB after 12 weeks in the 0 to 5, 5 to 11, 11 to 26, and 26 to 35 cm depths, respectively, in planted-bioaugmented soils, whereas 65, 54, 53, and 47% of PCB was removed from the planted-minimal salts treatment, respectively. Shifts in the soil microbial community structure were demonstrated by denaturing gradient gel electrophoresis of bacterial 16S ribosomal DNA. Results support that Brassica nigra directly contributed to accelerated PCB removal by increased oxygen diffusion, amendment infiltration, and microbial enrichment.