Per Moldrup

Colloids and Colloid-Facilitated Transport of Contaminants in Soils: An Introduction

Until some two decades ago, it was believed that only the soil liquid and gaseous phases were mobile and could facilitate the transport of chemicals and nutrients through the vadose zone. It is now generally accepted that also part of the soil solid phase is mobile, and that mobile organic and inorganic soil colloids may facilitate chemical transport. However, the magnitude and significance of these colloidal transport processes are yet to be determined. It is essential to examine whether current models for transport and fate of chemicals in soil and groundwater need to be revised. The collection of papers in this special section of Vadose Zone Journal mainly take their origin, but not exclusively, from an international workshop “Colloids and Colloid-Facilitated Transport of Contaminants in Soil and Sediments” held at the Danish Institute of Agricultural Sciences, Denmark, 19–20 Sept. 2002. The workshop was organized to review our present knowledge of colloid behavior and transport in porous media and the possibility of colloid-bound transport of contaminants and nutrients in soil and groundwater. Here we will first give a brief introduction to the topic of mobilization and transport of colloids in the vadose zone, and highlight previous evidence of colloid-facilitated transport. We then introduce the review and technical papers in the special section. We hope that the information provided in this special section will lead to improvements in our understanding and associated conceptual models of contaminant transport and fate in soil.

Relations Between Specific Surface Area and Soil Physical and Chemical Properties

The total specific surface area (SA) is a factor that can relate grain-scale properties to macro-scale physical and chemical properties of a porous medium. It is, therefore, advantageous to establish the relationships between SA and general soil physical properties. In this study we investigated

Particle transport in macropores of undisturbed soil columns

Abstract Particle-facilitated transport may be an important process in the leaching of contaminants such as pesticides, phosphorus and heavy metals. In this work particle transport in macropores through intact soil columns was quantified. Irrigation intensities corresponding to naturally occurring storm events were used. Intact soil columns (18.3 cm inner diameter, 20 cm length) were sampled at two different depths (2–22 cm and 42–62 cm) from a structured sandy loam. Infiltration experiments, consisting of leaching of naturally occurring particles and infiltration with two types of colloidal suspensions were performed on each column. The active macroporosity was estimated in a dye experiment. A significant transport of particles (especially clay and silt) through macropores was observed at both depths. The total amount of mobilized particles at a certain amount of water outflow was found to be higher at depth 42–62 cm than at depth 2–22 cm, but unaffected by irrigation intensity. The particle size in the effluent was found to decrease over time during both the leaching of naturally occurring particles and during the subsequent leaching of colloids from the infiltration with colloidal suspensions, but seemed to stabilize at a particle size 10 μm) into account, was found to describe the leaching of natural particles well using initial particle concentrations on the macropore walls and detachment coefficients for small and large particles as calibration parameters. ©1997 Elsevier Science B.V.

Particle leaching and particle-facilitated transport of phosphorus at field scale

Strongly sorbing compounds such as P, pesticides, and heavy metals can be transported through soils while being adsorbed to mobile colloidal particles. While the rapid leaching of nonadsorbing chemicals is relatively well understood, the particle-facilitated transport of highly sorbing chemicals such as P requires further investigation. The aim of this work was to study spatial variations in particle-facilitated transport of P at the field scale, and investigate which soil-physical or chemical parameters relate to the observed variations. Leaching experiments were performed in the laboratory on 42 undisturbed soil columns sampled in a grid covering 25 by 30 m of an agricultural field. The columns were equilibrated in the laboratory to a pressure head of −20 cm and irrigated at a rate of 10 mm h −1 with an artificial rainwater solution. The experiments exhibited considerable variation among the columns in the accumulated mass of particles and P leached during the 3.5 h of irrigation. Columns taken from the lower part of the field showed the highest mass of leached particles. These columns had higher clay contents and contained more continuous macropores. The mass of particles was negatively correlated to the average electrical conductivity of the effluent, and positively correlated to the macropore flow velocity. The accumulated masses of particulate organic and inorganic P were linearly related to the accumulated mass of particles leached. About 75% of the leached P was transported in a particle-facilitated manner. Overall, soil structure controlled to a large extent the leaching of particles and particle bound P.

Modeling diffusion and reaction in soils : IX. The Buckingham-Burdine-Campbell equation for gas diffusivity in undisturbed soil

Accurate description of gas diffusivity (ratio of gas diffusion coefficients in soil and free air, D s /D 0 ) in undisturbed soils is a prerequisite for predicting in situ transport and fate of volatile organic chemicals and greenhouse gases. Reference point gas diffusivities (R p ) in completely dry soil were estimated for 20 undisturbed soils by assuming a power function relation between gas diffusivity and air-filled porosity (e). Among the classical gas diffusivity models, the Buckingham (1904) expression, equal to the soil total porosity squared, best described R p . Inasmuch as our previous works (Parts III, VII, VIII) implied a soil-type dependency of D s /D 0 (e) in undisturbed soils, the Buckingham R p expression was inserted in two soil- type-dependent D s /D 0 (e) models. One D s /D 0 (e) model is a function of pore-size distribution (the Campbell water retention parameter used in a modified Burdine capillary tube model), and the other is a calibrated, empirical function of soil texture (silt + sand fraction). Both the Buckingham-Burdine-Campbell (BBC) and the Buckingham/soil texture-based D s /D 0 (e) models described well the observed soil type effects on gas diffusivity and gave improved predictions compared with soil type independent models when tested against an independent data set for six undisturbed surface soils (11-46% clay). This study emphasizes that simple but soil-type-dependent power function D s /D 0 (e) models can adequately describe and predict gas diffusivity in undisturbed soil. We recommend the new BBC model as basis for modeling gas transport and reactions in undisturbed soil systems.

Glyphosate sorption in soils of different pH and phosphorus content

The sorption mechanism of glyphosate, one of the most frequently used herbicides in the world, resembles that of phosphate. This study quantifies the variation in glyphosate sorption and desorption to a coarse sandy soil and to a sandy loam soil with varying phosphorus content and pH. Using batch experiments, glyphosate adsorption and desorption isotherms were determined on soil samples taken from long-term field experiments that received different additions of phosphorus and lime during 60-year (coarse sand) and 100-year (sandy loam) periods. Sorption isotherms were non-linear and manifested adsorption desorption non-singularity. The isotherms were best fitted with an extended Freundlich model, which had earlier been shown to describe phosphate sorption data well. The phosphate content in the soils had a significant influence on the sorption of glyphosate. With 0.5 M bicarbonate extractable P (pH 8.5) increasing from 6.2 to 58.7 in the loamy sand and 9.1 to 87.4 in the coarse sand, the extended Freundlich adsorption coefficient (Kf,MF,ads) decreased from 214.7 to 106 and from 154.0 to 83.5, respectively. Liming of the coarse sandy soil resulted in stronger glyphosate sorption because of an increase of reactive amorphous aluminum and iron hydrous oxides with increasing pH values. Glyphosate competes with phosphate for sorption sites, a quality that might result in glyphosate being sorbed more weakly in soils with high phosphorus levels.

Modeling Diffusion and Reaction in Soils: VII. Predicting Gas and Ion Diffusivity in Undisturbed and Sieved Soils

The classical Penman (1940) and Millington-Quirk (1960, 1961) diffusivity models were transformed into general form by introducing a tortuosity parameter, m. Compared with measured diffusivities close to phase saturation (soil-water and soil-air saturation for ion and gas diffusivity, respectively), the Penman (1940) model was superior to the Millington-Quirk models independent of diffusion type. The combined use of the Penman model to predict the diffusivity at phase saturation together with a general Millington-Quirk model to predict relative decrease in diffusivity with decreasing phase content was labeled the Penman-Millington-Quirk (PMQ) model. The best fit of the new PMQ model to measured data was obtained with m = 3 (high tortuosity) and m = 6 (medium tortuosity) for gas diffusivity in undisturbed and sieved soils, respectively, and m = 1 (high tortuosity) for ion diffusivity. Measurements did not suggest a significant difference between ion diffusivity in undisturbed, sieved, or aggregated soils. The differences in m-values between diffusion types are likely caused by different diffusion pathways and geometries for ion and gas diffusivity as well as a large effect of soil heterogeneity and spatial variability on gas diffusivity. The PMQ model predicted gas diffusivity in sieved and undisturbed soil well, but a soil-type dependent model (Part IV ofthis series) was superior for predicting ion diffusivity. The new models seem promising for more accurately predicting gas and ion diffusion and, therefore, for improving simulations of diffusion-constrained chemical and biological reactions in soils.

Gas Permeability in Undisturbed Soils: Measurements and Predictive Models

Accurate prediction of changes in the gas permeability during variable soil-moisture conditions is a prerequisite for improved simulation and design of soil-venting systems for removal of volatile organic chemicals in polluted soils. Air permeability, k, as a function of soil air-filled porosity,

Modeling diffusion and reaction in soils : I. A diffusion and reaction corrected finite difference calculation scheme

Numerically accurate calculation of stimultaneous diffusion and reaction in soil systems is a prerequisite for realistic model simulations of diffusion-controlled chemical fate processes and analysis of experimental data. Recent studies have shown that the inclusion of a first-order reaction term in

Water-Dispersible Colloids: Effects of Measurement Method, Clay Content, Initial Soil Matric Potential, and Wetting Rate

The fraction of clay that disperses in water, water-dispersible clay (WDC), is recognized as an important property with respect to predicting soil erosion and colloid leaching. Using six mineralogically similar soils with 12, 18, 24, 28, 37, and 43% clay, we studied the influence of soil clay content, initial matric potential (IMP; ψ = −2.5, −100, and −15500 hPa), and wetting rate on WDC released in response to infiltration of low–ionic strength rainwater, using a low-energy input measurement of WDC (LE-WDC). These results were referenced by WDC obtained by a conventional, high-energy input measurement based on air-dried soil (HE-WDC). The energy input in the dispersion procedure significantly affected the release of WDC. The amount of HE-WDC increased with clay content, while the amount of LE-WDC decreased with increasing clay content. The decrease in LE-WDC was explained by an increase in cohesive strength, reflected by the increase in water-stable aggregates (≥4 mm). A strong dependency of IMP on LE-WDC was observed, with maximum release of LE-WDC from soils that were at −2.5 hPa before measurement. Decreasing soil matric potential in the period before measurement reduced LE-WDC and also reduced the dependency of soil clay content, with soils incubated at −15500 hPa releasing a low amount of LE-WDC independent of clay content. The content of particulate organic C (POC) in the LE-WDC decreased with increasing clay content, and increased after drying to −15500 hPa. Colloid dispersibility changed as a function of time and moisture status, with the main changes occurring during or immediately after adjustment of the moisture content. Increasing the wetting rate resulted in a doubling of the amount of LE-WDC released from the initially dry soil (−15500 hPa), while no effect of wetting rate was observed at higher initial matric potentials.