Immobile Water Content Research Articles

Sensitivity Analysis of Transient‐MIM HYDRUS‐1D: Case Study Related to Pesticide Fate in Soils

A sensitivity analysis of the HYDRUS‐1D model was performed based on scenarios that represented the fate of a nonvolatile pesticide in three soils. We used the latest dual‐porosity, transient mobile–immobile (MIM) version of HYDRUS‐1D that includes water and solute transfers between mobile and immobile domains. We considered flat bare soils under atmospheric boundary conditions with identical initial pressure head profiles. We targeted instantaneous and mean annual pesticide concentration values at 1‐m depth as relevant output variables. Reference values for most of the parameters were obtained from in situ or laboratory measurements. Automation of parameter set generation and input formatting was achieved using SENSAN. We used a one‐group‐at‐a‐time sensitivity analysis to isolate and identify the effect of single parameter variations whose ranges were potentially different between topsoil layers (Group 1) and deep layers (Group 2). Sensitivity was analyzed by calculating the time‐dependent ratio of variation, i.e., the ratio of relative variation in output concentration to relative variation in parameter input values. As found for other single‐ or dual‐ permeability models, transient‐MIM HYDRUS‐1D was highly sensitive to parameters related to pesticide degradation and sorption, especially the Freundlich exponent associated with nonlinear sorption. Variations in the parameter n of the Mualem–van Genuchten functions had also a very significant impact. Among the MIM parameters, only saturated immobile and mobile water contents were very sensitive parameters, especially those of the topsoil layers. Parameter sensitivities hardly varied among the three soils, but this could be due to their limited contrast.

Predicting the Solute Breakthrough Curve From Soil Hydraulic Properties

Soil hydrodynamic dispersivity and therefore inert solute transport are largely determined by soil hydraulic properties. In short travel times or long distances, diffusion is not a significant process. Thus, in these cases, only the convection of fluid in different pores determines the breakthrough curve (BTC) shape. Because convection of fluid in different pores is described by pore fluid velocity distribution and consequently pore size distribution, it is possible to connect BTC to pore size distribution or pore fluid velocity distribution. The objectives of this study were to develop a model relating soil moisture characteristic (SMC) to BTC and to evaluate model predictions through comparisons with measured values. In this article, an approach that allowed predicting inert solute transport from knowledge of the SMC, parameterized by means of the closed-form van Genuchten-Mualem model, is presented. It was assumed that only part of the total soil porosity contributes to the transport process. The immobile water content and consequently the actual relative pore volume were predicted based on the SMC. A heaviside-type miscible displacement was performed, imposing a saturated unit gradient flow. The derived model was then tested on eight different disturbed soils. Results showed that the modeled and measured BTCs were in good agreement, especially during the initial stages of the breakthrough. It was further shown that only a small part of the soil pores (∼4%-13%) contributes significantly to the convective solute transport. Results indicated that when the relative pore volume is increased, the accuracy of the model decreases. Prediction errors were attributed to estimating immobile water content, the assumption of the tortuosity factor, and neglecting of diffusion, which may become significant as the travel time increases. Abbreviations: BTC: breakthrough curve, SMC: soil moisture characteristics curve, HCC: hydraulic conductivity curve, SHC: soil hydraulic characteristics, VGM: van Genuchten-Mualem model, CDE: convection dispersion equation

Modeling the Spatial Distribution of Nitrogen Leaching from Dairy Farmland

Urine patches on dairy farmland may cause extra loss of nitrogen through leaching and thus lead to potential contamination of groundwater in underlying aquifers. In this study, mathematical models and numerical simulations were used to quantitatively investigate the most important factors affecting the total quantity and spatial distribution of N‐leaching from dairy farmland soils. A comprehensive two‐dimensional model was derived by integrating models on pasture growth, heat transport, water flow and solute transport, and various N transformation processes. Through numerical simulations, the influence of soil texture (hydraulic conductivity, dispersivity, immobile water content, and “diffusion” path length and conductivity of macropores), urine patch density, quantity of irrigation and N fertilizer application on total N‐leaching, and spatial variations in N‐leaching were analyzed. Results showed that for all cases investigated, spatial variations in N‐leaching at the bottom of root zone were very large, which is the major reason urine patches would introduce extra N‐leaching losses. Spatial variations in N‐leaching were strongly affected by the density of urine patch coverage, soil hydraulic conductivity, dispersivity, preferential flow in macropores, and irrigation, but little by N fertilizer application and the presence of soil immobile water. Total N‐leaching was strongly affected by density of urine patch coverage, soil hydraulic conductivity, irrigation, N fertilizer application, and preferential flow, but only slightly by the dispersivity and immobile water.

Effect of immobile water content on contaminant transport in unsaturated zone

The effect of immobile water content on contaminant advection and dispersion in unsaturated porous media has been studied using a formulation which assumes that even in the immobile liquid zone, there occurs minor liquid flow. An approach is proposed for computing this liquid velocity. One dimensional numerical simulation is carried out to study the effect of immobile water content on the time of appearance of contaminant at the bottom of an unsaturated column. The numerical model developed is verified using analytical solutions for simple advective-dispersive cases. From the simulations, it was observed that increase in the immobile water content leads to quicker appearance of contaminant.

Hydraulic Conductivity, Immobile Water Content, and Exchange Coefficient in Three Soil Profiles

We used tension disk infiltrometers to determine the hydraulic conductivity at the matric potential of −0.1 kPa (K−0.1), the immobile water fraction (θim/θ), and the mass exchange coefficient (α) in three soil profiles of an agricultural field cropped with winter wheat (Triticum aestivum L.). A significant effect of soil horizonation was found for K−0.1, θim/θ, and α and could be related to soil structure as determined either by tillage for surface horizons or by pedogenetic factors for subsurface horizons. The seedbed had lower K−0.1 values (31 to 99 mm h−1) than the plowed layer under the seedbed (94–430 mm h−1). The θim/θ was highly variable in the seedbed (0.213 to 0.882), while α had values ranging from 0.0006 to 0.0115 h−1 Values of θim/θ and α in the plow layer under the seedbed were similar but less variable than those found in the seedbed. In the subsoil, clay‐enriched illuvial horizons had K−0.1 values similar to those found in the seedbed, but lower than those of the heavy clay horizon (average value of 260 mm h−1) and the limestone horizons (average value of 450 mm h−1). In the illuvial and heavy clay horizons, both θim/θ and α parameters had a small variability. In the limestone, we found a high variability of both θim/θ values (0.506–0.933) and α values (0.0006–0.0424 h−1) associated with high average values. The presence of subsoil horizons with large θim/θ and low α values may have a significant impact on ground water contamination.

Colloid and Bromide Transport in Undisturbed Soil Columns: Application of Two‐Region Model

Bromide tracer breakthrough and natural soil colloid leaching curves for undisturbed soil columns were used to characterize dissolved and suspended matter transport at the field scale. Data from 33 columns of 20‐cm diameter and 20‐cm height were used. Columns were collected in a grid of 25 by 30 m at an agricultural field. A two‐region (mobile–immobile water phase, MIM) solute transport model was fitted to data. The model was used to estimate bromide and colloid transport parameters including mobile and immobile water contents (θm, θim), bromide and colloid advective velocities (vBr, vColl), and mobile–immobile mass transfer coefficients (αbr, αcoll). Both soil physical properties and transport parameters were highly variable across the sampling field. Comparison of bromide transport parameters with basic soil physical properties revealed that vBr was proportional to soil clay content and bulk density (ρb), but αBr was inversely proportional to these parameters. Colloid transport parameters, vColl and αColl, however, showed only a weak correlation with clay content and ρb. Also, vColl was typically three to four times higher than vBr. The colloid velocity was generally higher than the bromide velocity, implying size exclusion of colloids. The spatial distributions of soil physical properties, bromide and colloid transport parameters, and leached quantities of particles were compared. The results suggested that bromide and colloid mass transfer (diffusion) were not controlled by the same soil physical conditions, and that soil structure and macropore flow are more important than the quantity of dispersible colloids in controlling colloid leaching.

Effect of subcritical hydrophobicity in a sandy soil on water infiltration and mobile water content

AbstractRecent research shows that most soils are more or less water repellent. Already subcritical water repellency may cause incomplete soil wetting and preferential flow. Both processes potentially reduce the residence time of water and solutes in the vadose zone, resulting in an enhanced risk of groundwater contamination. The objective of the present paper is, therefore, to evaluate the impact of reduced soil wettability on the soil water infiltration rate and to investigate the tendency towards preferential flow with the analysis of the immobile water content in the infiltration zone.In november 2002, a field experiment was done in a coniferous forest, 30 km N of Hannover, Germany. Soil hydrophobicity was quantified by measuring the contact angles. The hydraulic conductivity of the podsolic sandy soil was measured depth‐dependent with a double‐ring tension infiltrometer in three soil horizons. To quantify possible preferential‐flow effects, a LiBr‐Tracer was added to the infiltrating water to evaluate the mobile water‐content fraction after infiltration. Additionally, infiltration rates of water were compared with infiltration rates of ethanol which were determined after water infiltration at the same locations.Results show that the actual water repellency of field‐moist soil was mainly subcritical (contact angle <90°). Water infiltration rates were reduced due to subcritical repellency by a factor of 3–170 compared with ethanol infiltration rates (exclusion of wetting effects). This spatially variable infiltration behavior was not clearly reflected neither by the small‐scale contact‐angle measurements nor by the analysis of the average immobile soil water content in the infiltration zone. We conclude that this specific infiltration behavior of water caused by small‐scale wettability effects may temporarily reduce the local connectivity of water‐flow pathways.

Determination of Solute Distributions in the Vadose Zone Using Downhole Electromagnetic Induction

We conducted a field experiment to evaluate the ability of downhole electromagnetic induction (EM) measurements to track the migration of a salt plume in the vadose zone. A 6.9 g L −1 slug of NaCl was applied to a 3 by 3 m area for 85 d at a flux of 2.7 cm d −1 . Electrical conductivity measurements were collected weekly, using 13 12-m boreholes installed in a 15 by 15 m area. The measured bulk soil electrical conductivity (EC a ) was converted to soil water electrical conductivity (EC w ) using water contents from neutron probe measurements and clay contents from soil cores. The calculated mass of salt in the profile agreed well with the known mass of salt infiltrated when the appropriate immobile water content was assumed. The low water content, θ w , ( a , ( −1 ) measured at the test site presented more resistive conditions than previous studies of this type and were a cause of uncertainty in the results. Good agreement between calculated and known EC w resulted from the ability to measure θ w and EC a in the same locations. Sensitivity analyses showed that the calculated EC w was strongly dependent on the assumed upper limit of immobile water content, and less sensitive to soil temperature and clay content. The results of this study demonstrate that downhole EM methods can accurately characterize water and solute distributions in the vadose zone.

Determination of Solute Distributions in the Vadose Zone Using Downhole Electromagnetic Induction

We conducted a field experiment to evaluate the ability of downhole electromagnetic induction (EM) measurements to track the migration of a salt plume in the vadose zone. A 6.9 g L−1 slug of NaCl was applied to a 3 by 3 m area for 85 d at a flux of 2.7 cm d−1 Electrical conductivity measurements were collected weekly, using 13 12‐m boreholes installed in a 15 by 15 m area. The measured bulk soil electrical conductivity (ECa) was converted to soil water electrical conductivity (ECw) using water contents from neutron probe measurements and clay contents from soil cores. The calculated mass of salt in the profile agreed well with the known mass of salt infiltrated when the appropriate immobile water content was assumed. The low water content, θw, (<15% by volume) and low bulk soil electrical conductivity, ECa, (<100 mS m−1) measured at the test site presented more resistive conditions than previous studies of this type and were a cause of uncertainty in the results. Good agreement between calculated and known ECw resulted from the ability to measure θw and ECa in the same locations. Sensitivity analyses showed that the calculated ECw was strongly dependent on the assumed upper limit of immobile water content, and less sensitive to soil temperature and clay content. The results of this study demonstrate that downhole EM methods can accurately characterize water and solute distributions in the vadose zone.

Nonideal transport of solute and colloidal tracers through reactive zeolite/iron pellets

Solute and colloidal tracer tests were conducted in laboratory columns to examine the hydraulic properties of a foamed zeolite/iron pellet material that was developed for in situ remediation of contaminated groundwater. The colloidal tracer (1 μm polystyrene microspheres) moved through the columns much faster than the nonreactive solute tracer tritiated water, reflecting the interpellet preferential flow paths in the packed material. Flow interruption experiments with tritium and bromide showed concentration rebound of both tracers after the interruption (during elution), indicating the existence of nonadvective zones inside the pellets. Inverse modeling of microsphere data using a physical nonequilibrium transport model yielded immobile water content (θim) equivalent to the intrapellet porosity (0.40), suggesting that the microspheres were excluded from the small intrapellet pores and could only move through the large interpellet pore spaces. Inverse modeling of tritium data using physical nonequilibrium and dual‐permeability dual‐porosity models yielded θim values of 0.1–0.2, confirming the existence of nonadvective zones inside the pellets as suggested by the flow interruption experiments. The dual‐permeability dual‐porosity model also indicated that 6–11% of the total porosity was preferential flow porosity, consistent with the observation of enhanced microsphere transport with respect to tritiated water. Forward modeling with the dual‐permeability dual‐porosity model suggested that the immobile water in the pellets would not significantly affect the removal efficiency of contaminants subject to sorption and reduction. In contrast, the preferential flow porosity would drastically lower the contaminant removal efficiency.

Parameterisation of physically based solute transport models in sandy soils

Immobile water fractions of up to 40% had been reported in sands, and it was therefore relevant to determine if the convection-dispersion equation (CDE) or mobile-immobile model (MIM) should be used as the basic physical model for field studies of solute transport in sandy soils. A review of literature data for granular media indicated that for steady state flow, the dispersion coefficient could be estimated from the grain Peclét number and the mass exchange coefficient from the pore water velocity, but the immobile water fraction was poorly predicted from soil properties.In this study, performed on a sandy soil at Moora, Western Australia, the choice of model was determined after analysis of column effluent breakthrough curves (BTC), sequential tracer experiments, and single tracer experiments on the same core. The latter two methods have recently been introduced in an attempt to independently measure some of the MIM parameters (immobile water content and mass transfer exchange coefficient) in situ. Low immobile water content was found in this sand, with very rapid exchange between the mobile and immobile regions. All 3 techniques gave measured immobile water contents around 10%, which was consistent with most literature values for granular media. The single tracer experiment does not give the mass exchange coefficient α (h–1), but α determined by the sequential tracer technique could not be confirmed by the BTC technique due to the wide 95% confidence interval of the fitted parameter. Although the MIM behaviour was minor and inconsistent in the Moora sand, the choice of model may depend on the problem under consideration. For short column experiments, the CDE and MIM produced similar solute transport behaviour. However, for leaching below the root-zone, the MIM is recommended.

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.

FIELD DETERMINATION OF SOIL HYDRAULIC AND CHEMICAL TRANSPORT PROPERTIES 1

Hydraulic and chemical transport properties are the major inputs in predictive models that simulate the movement of water and chemicals through the vadose zone. However, there is a lack of field measurements of such properties to verify models describing water and chemical movement through the soil. One of the objectives of this study was to use a point source method to determine simultaneously the hydraulic and chemical transport properties at multiple field locations. A second objective was to determine the spatial distribution of such properties across a field. A total of 50 field locations within a 7 × 15-m area were rapidly and simultaneously evaluated for such properties. The hydraulic properties were the saturated hydraulic conductivity (K s ) and the macroscopic capillary length (λ c ). The chemical transport properties were the immobile water content, expressed as a fraction of water content (θ im /θ) and the mass exchange coefficient (a). The hydraulic properties were determined by applying three discharge rates from irrigation dripper lines and measuring the resultant steady-state flux densities at the soil surface beneath each emitter. The chemical transport properties were determined by applying a sequence of three conservative tracers at a steady-state infiltration rate and measuring their resident concentration in the soil. The K s values ranged from 7.5 to 79.0 cm h -1 , with a median of 27.4 cm h -1 (± 16.8). The λ c values ranged from 0.03 to 13.1 cm, with a median of 2.6 cm (± 3.6). The θ im /θ values ranged from 0.36 to 0.88, with a median of 0.57 (± 0.098). The a values ranged from 0.002 to 0.12 h -1 , with a median of0.034 h -1 (± 0.027). The values of the hydraulic and chemical transport parameters were found to be comparable with values reported by studies conducted on nearby field locations on similar soil. Based on semi-variogram analysis, the measured properties were not spatially correlated. Because the method required only 2 days to collect data it should prove useful for future studies that require extensive field measurements of hydraulic and chemical transport properties.

The Feasibility of Shallow Time Domain Reflectometry Probes to Describe Solute Transport Through Undisturbed Soil Cores

Rapid and nondestructive methods for determining solute transport properties are useful in many soil science applications. Recently, a series of field methods and a time domain reflectometry (TDR) method that could estimate some of the mobile‐immobile model (MIM) parameters, immobile water content (θim) and mass exchange coefficient (α), have been reported. The first objective of this study was to determine an additional parameter, dispersion coefficient (Dm), using the TDR method. The three MIM parameters were estimated from the TDR‐measured data, and the estimated parameters were compared with the estimated parameters from the effluent data. The second objective was to determine whether the TDR‐determined parameters from the surface 2‐cm soil layer could be used to predict effluent breakthrough curves (BTC) at the 20‐cm depth. The TDR‐determined parameters were used to calculate effluent BTCs using the CXTFIT computer program. Parameters obtained by curve fitting of the three parameters simultaneously using TDR data were not similar to the parameters obtained from the effluent BTCs. The parameter estimations were improved by fixing one or two independently determined parameter(s) before curve fitting for the remaining unknown parameter(s). The calculated BTCs were similar to the observed BTCs with coefficient of determination (r2) being 0.99 and root mean square error (RMSE) being 0.036. The TDR data obtained from shallow soil layers were successfully used to describe solute transport through undisturbed soil cores.

The Feasibility of Shallow Time Domain Reflectometry Probes to Describe Solute Transport Through Undisturbed Soil Cores

Rapid and nondestructive methods for determining solute transport properties are useful in many soil science applications. Recently, a series of field methods and a time domain reflectometry (TDR) method that could estimate some of the mobile-immobile model (MIM) parameters, immobile water content (θim) and mass exchange coefficient (α), have been reported. The first objective of this study was to determine an additional parameter, dispersion coefficient (Dm), using the TDR method. The three MIM parameters were estimated from the TDR-measured data, and the estimated parameters were compared with the estimated parameters from the effluent data. The second objective was to determine whether the TDR-determined parameters from the surface 2-cm soil layer could be used to predict effluent breakthrough curves (BTC) at the 20-cm depth. The TDR-determined parameters were used to calculate effluent BTCs using the CXTFIT computer program. Parameters obtained by curve fitting of the three parameters simultaneously using TDR data were not similar to the parameters obtained from the effluent BTCs. The parameter estimations were improved by fixing one or two independently determined parameter(s) before curve fitting for the remaining unknown parameter(s). The calculated BTCs were similar to the observed BTCs with coefficient of determination (r2) being 0.99 and root mean square error (RMSE) being 0.036. The TDR data obtained from shallow soil layers were successfully used to describe solute transport through undisturbed soil cores.

Estimation De La Teneur En Eau Immobile Et Du Coefficient D’Échange Selon Deux Méthodes

Use of the mobile-immobile transport equation requires the knowledge of two key parameters: the immobile water content, θim, and the exchange coefficient between mobile and immobile phases of groundwater, αm. In this paper, these two parameters were 1) estimated by a tension infiltrometer, and 2) optimized by a numerical model. For the parameter estimation, a tension infiltrometer was used to apply four tracers to three fixed pan lysimeters that served to monitor variations of water content, bromide transport and drainage volume during a growing season. Estimated values of θim varied from 0.144 to 0.232 cm3 cm−3 (average of 0.191), which corresponds to a θim/θ ratio between 0.52 and 0.81 (average of 0.67). Values of αm ranged from 4.08 10−7 to 1.49 10−5 min−1 (average of 4.57 10−6). Values of θim/θ estimated in the present study were greater than published values, while αm values were less. Optimized parameters θim and αm were obtained by minimizing the difference of the sum of squares between weekly simulated and observed values of bromide concentration and mass drained. Optimized values of θim were less than those estimated with the infiltrometer, with an average difference of 38% for the three pan lysimeters. However, optimized values of αm were slightly different from those estimated with the infiltrometer, with a difference of 4.00 10−6 min−1 for the three pan lysimeters.

A Time Domain Reflectometry Method to Measure Immobile Water Content and Mass Exchange Coefficient

Physical nonequilibrium of water and solute transport in soil has been reported. One of the most common mechanistic models used to describe physical nonequilibrium transport phenomena is the mobile–immobile model (MIM). Two significant parameters in the MIM are immobile water content (θim) and mass exchange coefficient (α). Previously, a method for determining θim and α using sequential tracers (ST) has been used to characterize solute transport. In this work, we present and evaluate a method to estimate θim and α using time domain reflectometry (TDR). The TDR method was tested in laboratory experiments using three 20 cm long by 12 cm diameter undisturbed saturated soil columns. The method used TDR with an application of CaCl2 to obtain resident concentrations as a function of time. The data obtained from TDR were analyzed using a log‐linear equation developed based on the ST method to estimate θim and α. The θim and α estimates from the TDR method were compared with the estimates from the ST method and from effluent data. A conventional inverse curve fitting method (CXTFIT) was used to estimate parameters from effluent data. The means of θim/θ from the TDR method, ST method, and effluent data were 0.31, 0.30, and 0.26, respectively. The means of α from the TDR method, ST method, and effluent data were 0.03, 0.03, and 0.04 h−1, respectively. The values of θim/θ and α from the TDR method were very similar to the estimates from the ST method. In all three columns, the θim estimates from the TDR method were within the 95% confidence intervals (CI) of the estimates from the effluent data. In two of three columns, the α estimates from the TDR method were within the 95% CI of the estimates from the effluent data. The TDR method is relatively simple, rapid, and had advantages over the ST method and conventional methods for measuring solute transport properties.

Evaluation of a Simple Method for Estimating Solute Transport Parameters Laboratory Studies

A two‐domain, physical nonequilibrium solute transport model has been used to describe the transport and fate of solute in soil. The model contains the parameters θim (immobile water content) and α (mass transfer coefficient) which must be determined for a soil before applying the transport model. A simple field method that can estimate both θim and α without measuring extensive breakthrough curves (BTCs) has been presented. The purpose of this paper was to test in laboratory soil columns the simple method of estimating parameters by comparing to the conventional BTC analysis method of parameter estimation. The experiments involved 12‐cm‐long and 4‐cm‐diam. columns packed with five different soil materials. The BTCs were performed on each column using a sequential application of four fluorobenzoate tracers. Each tracer was applied for a different length of time. The soil columns were sectioned at the end of the BTC experiments. The simple method gave results of θim and α based upon the sectioned soil samples, and the BTC analysis gave results of θim and α based upon effluent concentrations. The estimates by the two different methods were from the same experiments. Most of the estimated α values using the simple method were within the 95% confidence interval (CI) of the BTC estimates. For 7 of 10 soil columns, the estimates of immobile water fraction, θim/θ, from the simple method were within the 95% CI of the estimates of θim/θ obtained from BTC data. Breakthrough curves calculated using the θim and α values estimated by the simple method were similar to observed BTCs. The simple method provides estimation of θim and α from easy to obtain soil samples in field and can be used as a first approximation to apply the analytical BTC method.

Comparing Field Methods that Estimate Mobile–Immobile Model Parameters

Recent studies have used field techniques that estimate soil hydraulic and solute transport parameters. These methods utilize a tension infiltrometer to infiltrate either a single tracer or a series of tracers in order to estimate immobile water content (θim) and mass exchange coefficient (α) of the mobile–immobile solute transport model. The objective of this study was to compare two single tracer methods (basic and variance) with one multiple tracer method for estimating θim and α from data obtained on the same field soil location. Hydraulic conductivity (K(h0)) was also estimated using these methods. Research was done at five interrow sites in a ridge‐tilled corn (Zea mays L.) field, and the soil was mapped as a Nicollet series (fine‐loamy, mixed, superactive, mesic, Aquic Hapludoll). The values of θim and α estimated by the multiple tracer method compared well with previously measured values using the same technique on the same field. The θim values for the multiple tracer technique were larger than values derived from the basic single tracer technique. The basic single tracer technique did not take into consideration a mass exchange between θim and the mobile water domain (θm). The α values were less variable for the multiple tracer method than for the single tracer‐variance method. Values of immobile water fraction (θim/θ) for the multiple and basic single tracer techniques ranged from 0.30 to 0.52 and from 0.24 to 0.35, respectively. The values of α for the multiple and single tracer‐variance techniques ranged from 0.06 to 0.9 d−1 and from 0.03 to 60 d−1, respectively. The volumetric water content (θ) changed considerably over the course of the experiment for the estimation of α using the single tracer‐variance method; thus, the assumptions of this technique were compromised. The measured values of K(h0) at the five sites ranged from 0.47 to 1.66 μm s−1 There was evidence that the basic single tracer method underestimated θim and overestimated θm, because this method considers α = 0 during the tracer application.

In Situ Measurement of Solute Transport Coefficients: Assumptions and Errors

AbstractSingle‐tracer and sequential‐tracer techniques for directly measuring the immobile water content, θim, and a sequential‐tracer technique for measuring the solute transfer rate coefficient, α, have been proposed. These techniques have not been subjected to a comprehensive analysis of their assumptions, nor of the errors resulting if those assumptions are violated. Here, an analytical solution to the mobile‐immobile convection‐dispersion equation was used to assess the range of soil and sampling conditions that can lead to accurate estimates of θim and α. Both techniques were shown to result in considerable error in soils possessing high dispersivity (λ) and high α when conductivity (q) was low, or if sampling was done when the cumulative infiltration (I) was too small. For a wide range of conditions, the single‐tracer technique resulted in estimates of θim within 15% of the true value if sampling took place when q > 10 mm h‐1, I > λ, and sampling time <1.25 h. The value of θm/θ had only a small effect on appropriate sampling conditions. Sampling conditions were more restrictive for the sequential‐tracer technique than for the single‐tracer technique. When θm/θ ≈ 0.6, if λ ≈ 5 mm and q > 10 mm h‐1, and I > 30 mm, acceptable values of θim and α were calculated. Sampling conditions became more restrictive as λ increased. If λ > 20 mm, there seemed little possibility for obtaining reasonable estimates of either θim or α. Sampling conditions varied strongly with θm/θ, becoming more lenient as θm/θ decreased.