ABSTRACTS

Rapid fluid content measurement method for fingered flow in an oil-water-sand system using synchrotron X-rays
Alon Rimmer, David A. DiCarlo, Tammo S. Steenhuis, Barnes Bierck, Deanna Durnford, J.-Yves Parlange

The complexity of simultaneous flow of water and non-aqueous phase liquids is largely
unappreciated because few techniques permit accurate quantitative measurement of water and oil contents in rapidly changing flow fields. High intensity X-rays were used at the Cornell High Energy Synchrotron Source CHESS to obtain rapid, accurate, and non-destructive quantitative measurements of the changing fluid contents in a porous medium during infiltration events. Concomitant temporal pressure measurements were obtained for each liquid phase using rapidly responding tensiometers. The system was used for measuring temporal volumetric fluid content changes during a water finger infiltration into sand saturated with a NAPL Soltrol-220 in a two-dimensional chamber. The fluid content distribution of a finger in the oil-water system was found to be similar to air-water systems. The hysteretic constitutive relationship between pressure and the content was developed from the data. The relationship was used to explain why the finger did not widen behind the tip, and why, upon re-infiltration, water followed the previously established path. These findings are relevant for cleanup of oil-contaminated sites because it aids in the understanding of hydrologic control which is an essential component of cost-effective in-situ remediation.

Preferential Flow in Water Repellent Sandy Soils: Principles and Modeling Approaches
Coen J. Ritsema, Jos C. van Dam, John L. Nieber, Louis W. Dekker, K. Oostindie, and Tammo S. Steenhuis

Preferential Flow in Water Repellent Sandy Soils: Principles and Modeling Approaches
Leaching risks of surface-applied agrichemicals in water repellent soils can only be quantified with an acceptable degree of accuracy if knowledge of the underlying principles and an appropriate simulation model are available. The present study aimed to investigate water flow and solute transport processes in a water repellent sandy soil, and to introduce and apply new modeling approaches. Automated TDR measurements revealed that preferential pathways develop rapidly during severe rain storms, causing infiltrating water to be preferentially transported to the deeper subsoil. Furthermore, preferred pathways recurred at the same sites during all rain events. Simulations with a 2-D, numerical finite element flow and transport model indicate that preferential flow paths will only form during infiltration into dry water repellent soils, i.e. in the range below the so-called critical soil water content. Incorporation of hysteresis is essential to generate the formation and recurrence of preferential flow paths with the model. The process of preferential flow and transport has been incorporated in the well-known SWAP model also, and applied to field data of tracer transport through a water repellent sandy soil in the Netherlands. Results indicate early arrival times of bromide in the subsoil in case preferential flow is taken into account.

A simple mixing layer model predicting solute flow to drainage lines under preferential flow
Gil Shalit, Tammo Steenhuis

Concern about the environmental effects of agricultural chemicals through preferential flow has risen in recent years. A simple model is presented describing the processes involved in preferential transport of both soil-adsorbed and non-adsorbed solutes. The model assumes that water and solutes are mixed into an upper soil layer. The water and solutes then flow through macropores to the groundwater. Analysis of the solute breakthrough curves in subsurfact drainage effluent makes it possible to calculate the depth of the mixing layer or the adsorption desorption partition coefficient. Data from a drainage experiment with chloride, 2.4-D and atrazine were used to test the model. The study was performed on a no-till and a conventionally tilled plot. The model and experimental results indicate that only a fraction of the field area participates in transport to the macropores. Differences between breakthrough curves from the conventionally tilled and no-till plots are explained well by the mixing of solutes and water in the upper layer. This simple, physically based model can help us to understand and estimate the environmental threats of herbicides shortly after application.

Modelling Solute Transport in Structured Soils: Performance Evaluation of the ADR and TRM Models
F . Stagnitti, Ling Li, A. Barry, G. Allinson, J.-Y. Parlange, T. Steenhuis, E. Lakshmanan

The movement of chemicals through the soil to the groundwater or discharged to surface waters represents a degradation of these resources. In many cases, serious human and stock health implications are associated with this form of pollution. The chemicals of interest include nutrients, pesticides, salts, and industrial wastes. Recent studies have shown that current models and methods do not adequately describe the leaching of nutrients through soil, often underestimating the risk of groundwater contamination by surface-applied chemicals, and overestimating the concentration of resident solutes. This inaccuracy results primarily from ignoring soil structure and nonequilibrium between soil constituents, water, and solutes. A multiple sample percolation system (MSPS), consisting of 25 individual collection wells, was constructed to study the effects of localized soil heterogeneities on the transport of nutrients (NO3, Cl-, PO 3-) in the vadose zone of an agricultural soil predominantly dominated by clay. Very significant variations in drainage patterns across a small spatial scale were observed (one-way ANOVA, p < 0.001) indicating considerable heterogeneity in water flow patterns and nutrient leaching. Using data collected from the multiple sample percolation experiments, this paper compares the performance of two mathematical models for predicting solute transport, the advective-dispersion model with a reaction term (ADR), and a two-region preferential flow model (TRM) suitable for modelling nonequilibrium transport. These results have implications.

Model for nonreactive solute transport in structured soils with continuous preferential flow paths
R Wallach and TS Steenhuis

A mathematical model for solute movement in a structured soil with well-defined and continuous preferential paths was developed. The model divides the soil profile into one mobile and one "stagnant" pore group. For well-structured soils, the mobile pore group consists of a few well-connected pores that conduct the nonreactive solute downward very rapidly. Only a narrow matrix layer of stagnant solute along the interface between the two pore groups takes part in solute exchange with the preferential paths. Due to differences in time scales between convection and chemical transfer, the rate of chemical exchange between the preferential path and the active matrix layer for short and moderate times after chemical application is controlled mainly by the preferential flow concentration and, to a lesser extent, by the concentration in the matrix active layer. This is expressed mathematically by multiplying the concentration in the active matrix layer in the mass balance equation by a small dimensionless parameter, &epsiv; << 1. A regular small perturbations problem is then obtained whose solution is expressed as the sum of the zeroth- and first-order approximations. The simulated breakthrough curve (BTC) is made up of piecewise linear lines and shows a good fit to the measured solute concentration in the outflow of undisturbed columns that have significant preferential flow. Due to the model's simplicity, the transport parameters are estimated directly by fitting the model output to the measured BTCs.