Jirka Simunek

Preferential transport of nitrate to a tile drain in an intermittent-flood-irrigated field: Model development and experimental evaluation

A comprehensive field experiment was conducted near Las Nutrias, New Mexico, to study field-scale flow and transport in the vadose zone. The field data were analyzed in terms of a two-dimensional numerical model based on the Richards equation for variably saturated water flow, convection-dispersion equations with first-order chemical decay chains for solute transport, and bimodal piecewise-continuous unsaturated hydraulic functions to account for preferential flow of water and nitrate-nitrogen (NO3-N; loosely used as NO3 ) following flood irrigation events at the experimental site. The model was tested against measured NO3 flux concentrations in a subsurface tile drain, several monitoring wells and nested piezometers, and against resident NO3 concentrations in the soil profile (obtained at 52 spatial locations and four depths along a transect). NO3 transport at the field site could be described better with the bimodal hydraulic functions than using the conventional approach with unimodal van Genuchten-Mualem type hydraulic functions. Average resident nitrate concentrations measured across the soil profile were predicted reasonably well. However, NO3 flux concentrations in the subsurface tile drain and piezometers at the field site were occasionally underestimated or overestimated depending upon the irrigation sequence in three field benches, probably reflecting unrepresented three-dimensional regional flow/transport processes. Limiting the capture zone to a region closer to the tile drain did lead to a better match with observed sharp increases and decreases in predicted NO3 flux concentrations during the irrigation events. On the basis of this result we inferred that the preferential flow intercepted by the tile drain was generated in close proximity of the drain and essentially oriented vertically. In summary, our study suggests that irrigation scheduling in adjacent field plots, drainage design (e.g., spacing between tiles, drain depth, drain diameter) and effectiveness (e.g., drain blockage), preferential flow in (horizontal) surface-opened shallow cracks and (vertical) macropores, and transient regional groundwater flow can add significant uncertainty to the predictions of (local-scale) flow and transport to a tile drain.

SIMULTANEOUS INVERSE ESTIMATION OF SOIL HYDRAULIC AND SOLUTE TRANSPORT PARAMETERS FROM TRANSIENT FIELD EXPERIMENTS: HOMOGENEOUS SOIL

Inverse estimation of unsaturated soil hydraulic and solute transport properties has thus far been limited mostly to analyses of one–dimensional experiments in the laboratory, often assuming steady–state conditions. This is partly because of the high cost and difficulties in accurately measuring and collecting adequate field–scale data sets, and partly because of difficulties in describing spatial and temporal variabilities in the soil hydraulic properties. In this study, we estimated soil hydraulic and solute transport parameters from several two–dimensional furrow irrigation experiments under transient conditions. Three blocked–end furrow irrigation experiments were carried out, each of the same duration but with different amounts of infiltrating water and solutes resulting from water depths of 6, 10, and 14 cm in the furrows. Two more experiments were carried out with the same amounts of applied water and solute, and hence for different durations, on furrows with water depths of 6 and 10 cm. The saturated hydraulic conductivity (Ks) and solute transport parameters in the physical equilibrium convection–dispersion (CDE) and physical nonequilibrium mobile immobile (MIM) transport models were inversely estimated using the Levenberg–Marquardt optimization algorithm in combination with the HYDRUS–2D numerical code. Soil water content readings, cumulative infiltration data, and solute concentrations were used in the objective function during the optimization process. Estimated Ks values ranged from 0.0389 to 0.0996 cm min–1, with a coefficient of variation of 48%. Estimated immobile water contents (.im) were more or less constant at a relatively low average value of 0.025 cm3 cm–3, whereas the first–order exchange coefficient (.) varied between 0.10 and 19.52 min–1. The longitudinal dispersivity (DL) ranged from 2.6 to 32.8 cm, and the transverse dispersivity (DT) ranged from 0.03 to 2.20 cm. DL showed some dependency on water level and irrigation/solute application time in the furrows, but no obvious effect was found on Ks and other transport parameters, most likely because of spatial variability in the soil hydraulic properties. Agreement between measured and predicted infiltration rates was satisfactory, whereas soil water contents were somewhat overestimated, and solute concentrations were underestimated. Differences between predicted solute distributions obtained with the CDE and MIM transport models were relatively small. This and the value of optimized parameters indicate that observed data were sufficiently well described using the simpler CDE model, and that immobile water did not play a major role in the transport process.

HYDRUS: Model Use, Calibration, and Validation

The HYDRUS numerical models are widely used for simulating water flow and solute transport in variably saturated soils and groundwater. Applications involve a broad range of steady-state or transient water flow, solute transport, and/or heat transfer problems. They include both short-term, one-dimensional laboratory column flow or transport simulations, as well as more complex, long-duration, multi-dimensional field studies. The HYDRUS models can be used for both direct problems when the initial and boundary conditions for all involved processes and corresponding model parameters are known, as well as inverse problems when some of the parameters need to be calibrated or estimated from observed data. The approach to model calibration and validation may vary widely depending upon the complexity of the application. Model calibration and inverse parameter estimation can be carried out using a relatively simple, gradient-based, local optimization approach based on the Marquardt-Levenberg method, which is directly implemented into the HYDRUS codes, or more complex global optimization methods, including genetic algorithms, which need to be run separately from HYDRUS. In this article, we provide a brief overview of the HYDRUS codes, discuss which HYDRUS parameters can be estimated using internally built optimization routines and which type of experimental data can be used for this, and review various calibration approaches that have been used in the literature in combination with the HYDRUS codes.

INVERSE ESTIMATION OF SOIL HYDRAULIC AND SOLUTE TRANSPORT PARAMETERS FROM TRANSIENT FIELD EXPERIMENTS: HETEROGENEOUS SOIL

While inverse parameter estimation techniques for determining key parameters affecting water flow and solute transport are becoming increasingly common in saturated and unsaturated zone studies, their application to practical problems, such as irrigation, have received relatively little attention. In this article, we used the Levenberg–Marquardt optimization algorithm in combination with the HYDRUS–2D numerical code to estimate soil hydraulic and solute transport parameters of several soil horizons below experimental furrows. Three experiments were carried out, each of the same duration but with different amounts of water and solutes resulting from 6, 10, and 14 cm water depths in the furrows. Two more experiments were performed with the same amounts of applied water and solute and, consequently, for different durations, on furrows with depths of 6 and 10 cm of water. We first used a scaling method to characterize spatial variability in the soil hydraulic properties, and then simultaneously estimated the saturated hydraulic conductivity (Ks) and the longitudinal dispersivity (DL) for the different horizons. Model predictions showed only minor improvements over those previously obtained assuming homogeneous soil profiles. In an effort to improve the predictions, we also carried out a two–step, sequential optimization in which we first estimated the soil hydraulic parameters followed by estimation of the solute transport parameters. This approach allowed us to include additional parameters in the optimization process. A sensitivity analysis was performed to determine the most sensitive hydraulic and solute transport parameters. Soil water contents were found to be most sensitive to the n parameter in van Genuchten’s soil hydraulic model, followed by the saturated water content (.s), while solute concentrations were most affected by .s and DL. For these reasons, we estimated .s and n for the various soil horizons of the sequential optimization process during the first step, and only DL during the second step. Sequential estimation somewhat improved predictions of the cumulative infiltration rates during the first irrigation event. It also significantly improved descriptions of the soil water content, particularly of the upper horizons, as compared to those obtained using simultaneous estimation, whereas deep percolation rates of water did not improve. Solute concentrations in the soil profiles were predicted equally well with both optimization approaches.

STANMOD: Model Use, Calibration, and Validation

This article provides an overview of STANMOD, a Windows-based computer software package for evaluating solute transport in soils and groundwater using analytical solutions of the advection-dispersion equation. The software integrates seven separate codes that have been popularly used over the years for a broad range of one-dimensional and multi-dimensional solute transport applications: the CFITM, CFITIM, CXTFIT, CHAIN, and SCREEN models for one-dimensional transport, and the 3DADE and N3DADE models for multi-dimensional transport. All of the models can be run for direct (forward) problems, and several (CFITM, CFITM, CXTFIT, and 3DADE) can also be run for inverse problems. CXTFIT further includes a stochastic stream tube model assuming local-scale equilibrium or nonequilibrium transport conditions. The 3DADE and N3DADE models apply to two- and three-dimensional transport during steady unidirectional water flow assuming equilibrium and nonequilibrium transport, respectively. Nonequilibrium transport can be simulated using the assumption of either physical nonequilibrium (two-region or mobile-immobile type transport) or chemical nonequilibrium (two-site partial equilibrium, partial kinetic sorption). The STANMOD software comes with a large number of example applications illustrating the utility of the different codes for a variety of laboratory and field-scale solute transport problems.

The effects of preferential flow and soil texture on risk assessments of a NORM waste disposal site

This paper investigates the environmental fate of radionuclide decay chains (specially the (238)U and (232)Th series) being released from a conventional mining installation processing ore containing natural occurring radioactive materials (NORMs). Contaminated waste at the site is being disposed off in an industrial landfill on top of a base of earth material to ensure integrity of the deposit over relatively long geologic times (thousands of years). Brazilian regulations, like those of many other countries, require a performance assessment of the disposal facility using a leaching and off-site transport scenario. We used for this purpose the HYDRUS-1D software package to predict long-term radionuclide transport vertically through both the landfill and the underlying unsaturated zone, and then laterally in groundwater. We assumed that a downgradient well intercepting groundwater was the only source of water for a resident farmer, and that all contaminated water from the well was somehow used in the biosphere. The risk assessment was carried out for both a best-case scenario assuming equilibrium transport in a fine-textured (clay) subsurface, and a worst-case scenario involving preferential flow through a more coarse-textured subsurface. Results show that preferential flow and soil texture both can have a major effect on the results, depending upon the specific radionuclide involved.

Batch soil adsorption and column transport studies of 2,4-dinitroanisole (DNAN) in soils

The explosive 2,4,6-trinitrotoluene (TNT) is currently a main ingredient in munitions; however the compound has failed to meet the new sensitivity requirements. The replacement compound being tested is 2,4-dinitroanisole (DNAN). DNAN is less sensitive to shock, high temperatures, and has good detonation characteristics. However, DNAN is more soluble than TNT, which can influence transport and fate behavior and thus bioavailability and human exposure potential. The objective of this study was to investigate the environmental fate and transport of DNAN in soil, with specific focus on sorption processes. Batch and column experiments were conducted using soils collected from military installations located across the United States. The soils were characterized for pH, electrical conductivity, specific surface area, cation exchange capacity, and organic carbon content. In the batch rate studies, change in DNAN concentration with time was evaluated using the first order equation, while adsorption isotherms were fitted using linear and Freundlich equations. Solution mass-loss rate coefficients ranged between 0.0002h-1 and 0.0068h-1. DNAN was strongly adsorbed by soils with linear adsorption coefficients ranging between 0.6 and 6.3Lg-1, and Freundlich coefficients between 1.3 and 34mg1-nLnkg-1. Both linear and Freundlich adsorption coefficients were positively correlated with the amount of organic carbon and cation exchange capacity of the soil, indicating that similar to TNT, organic matter and clay minerals may influence adsorption of DNAN. The results of the miscible-displacement column experiments confirmed the impact of sorption on retardation of DNAN during transport. It was also shown that under flow conditions DNAN transforms readily with formation of amino transformation products, 2-ANAN and 4-ANAN. The magnitudes of retardation and transformation observed in this study result in significant attenuation potential for DNAN, which would be anticipated to contribute to a reduced risk for contamination of ground water from soil residues.

Transport and retention of surfactant- and polymer-stabilized engineered silver nanoparticles in silicate-dominated aquifer material

Packed column experiments were conducted to investigate the transport and blocking behavior of surfactant- and polymer-stabilized engineered silver nanoparticles (Ag-ENPs) in saturated natural aquifer media with varying content of material < 0.063 mm in diameter (silt and clay fraction), background solution chemistry, and flow velocity. Breakthrough curves for Ag-ENPs exhibited blocking behavior that frequently produced a delay in arrival time in comparison to a conservative tracer that was dependent on the physicochemical conditions, and then a rapid increase in the effluent concentration of Ag-ENPs. This breakthrough behavior was accurately described using one or two irreversible retention sites that accounted for Langmuirian blocking on one site. Simulated values for the total retention rate coefficient and the maximum solid phase concentration of Ag-ENPs increased with increasing solution ionic strength, cation valence, clay and silt content, decreasing flow velocity, and for polymer-instead of surfactant-stabilized Ag-ENPs. Increased Ag-ENP retention with ionic strength occurred because of compression of the double layer and lower magnitudes in the zeta potential, whereas lower velocities increased the residence time and decreased the hydrodynamics forces. Enhanced Ag-ENP interactions with cation valence and clay were attributed to the creation of cation bridging in the presence of Ca2+. The delay in breakthrough was always more pronounced for polymer-than surfactant-stabilized Ag-ENPs, because of differences in the properties of the stabilizing agents and the magnitude of their zeta-potential was lower. Our results clearly indicate that the long-term transport behavior of Ag-ENPs in natural, silicate dominated aquifer material will be strongly dependent on blocking behavior that changes with the physicochemical conditions and enhanced Ag-ENP transport may occur when retention sites are filled.

A hybrid finite volume-finite element model for the numerical analysis of furrow irrigation and fertigation

Abstract This study presents a hybrid Finite Volume – Finite Element (FV-FE) model that describes the coupled surface-subsurface flow processes occurring during furrow irrigation and fertigation. The numerical approach combines a one-dimensional description of water flow and solute transport in an open channel with a two-dimensional description of water flow and solute transport in a subsurface soil domain, thus reducing the dimensionality of the problem and the computational cost. The modeling framework includes the widely used hydrological model, HYDRUS, which can simulate the movement of water and solutes, as well as root water and nutrient uptake in variably-saturated soils. The robustness of the proposed model was examined and confirmed by mesh and time step sensitivity analyses. The model was theoretically validated by comparison with simulations conducted with the well-established model WinSRFR and experimentally validated by comparison with field-measured data from a furrow fertigation experiment conducted in the US.

Invited PaperNumerical Model For Simulating Multiple SoluteTransport In Variably-saturated Soils

In this paper we discuss a two-dimensional model, CHAIN 2D, for simulating the variably-saturated movement of water, heat, and multiple solutes involved in sequential first-order decay reactions. The model assumes that the solutes can reside in all three phases, i.e., liquid, solid, and gaseous. The solute transport equations consider convective-dispersive transport in the liquid phase, diffusion in the gaseous phase, nonlinear equilibrium or nonequilibrium sorption, linear equilibrium volatilization, zero-order production, and first-order degradation reactions which provide the necessary coupling between solutes involved in the sequential first-order decay reactions. Nonlinear sorption is described by a generalized formulation which can be simplified to either a Freundlich or Langmuir equation. Nonequilibrium sorption may be described using both one-site or two-site kinetic reactions. The model considers temperature dependence of all transport and reaction parameters. Typical examples of first-order decay scenarios are the simultaneous movement of interacting nitrogen species, radionuclides, organic phosphates, and pesticides and their metabolites.