Tailing Of Breakthrough Curves Research Articles

Solute transport in a filled single fracture under non-Darcian flow

Filled single fractures (FSF) are commonly seen in the subsurface but have received little attention in terms of flow and transport. This study uses well-controlled laboratory experiments to investigate flow and transport in a FSF under non-Darcian flow conditions. Flow in the experiments is well described by the Forchheimer or the Izbash equations. Non-Fickian transport is found to dominate with early arrival and long tails. The breakthrough curves (BTCs) of the solute transport are modeled by the conventional advection–dispersion equation (ADE), the ADE with retardation (ADE-R) and the single rate mobile–immobile (MIM) model. The ADE is used as a benchmark for comparison. The choice of ADE-R is to acknowledge the fact that mechanical filtration and intake of contaminant particles in dead or disconnected pore spaces can retard the transport. The choice of MIM is to acknowledge the early arrival and long tails of the BTCs. The ADE model poorly describes the transport; the ADE-R model is better than the ADE in fitting the peak concentration, but it is less satisfactory in fitting the tails of BTCs; the single rate MIM model shows the potential to model the transport behavior over a wide range of discharge. The large dispersivity values found in the experiments are consistent with the prediction of Taylor, who had shown that dispersion under non-Darcian flow condition was much stronger than that under Darcian flow.

Transport and Deposition of Suspended Soil Colloids in Saturated Sand Columns

Understanding colloid mobilization, transport, and deposition in the subsurface is a prerequisite for predicting colloid-facilitated transport of strongly adsorbing contaminants and further developing remedial activities. This study investigated the transport behavior of soil-colloids extracted from a red-yellow soil from Okinawa, Japan. Different concentrations of suspended-soil colloids (with diameter <1 μm) were applied, at different flow velocities and pH conditions, to 10-cm long water-saturated columns repacked with either Narita (mean diameter D50=0.64 mm) or Toyoura (mean diameter D50=0.21 mm) sands. The transport and retention of colloids were studied by analyzing colloid effluent breakthrough curves (BTCs), particle size distribution in the effluent, and colloid deposition profiles within the column. The results showed a significant influence of flow velocity: Low flow velocity caused tailing of colloid BTCs with higher reversible entrapment and release of colloids than high flow velocity. Th...

Non-Fickian transport in porous media with bimodal structural heterogeneity

Tracer tailing in breakthrough curves in porous media with two distinct porosities is analyzed in terms of the dynamic responses of experimental fixed bed columns filled either with solid or porous beads. The flow is fast in the column interstitial space between beads (for both solid and porous beads) but slow within the porous beads that act as controlled ‘traps’ constituting an immobile zone. The transport is quantified using a Continuous Time Random Walk (CTRW) framework, which accounts for domains with controlled structural and flow heterogeneity associated with two distinct spatial and time spectra. We first demonstrate that breakthrough curves for a column containing solid glass beads exhibit non-Fickian transport, quantifiable both in fitting and validation mode by a CTRW based on a power law transition time distribution. We then examine breakthrough curves in the porous bead case, obtaining fits with a two-scale CTRW model that accounts explicitly for the two time spectra. Because the porous beads are uniform, tracer trapping within them is described by a simple first-order approximation trap model, with relatively weak capture and relatively faster release rates. The extent of tailing apparent in the porous bead breakthrough curves, due to the traps, can be quantitatively distinguished from the contribution to tailing due to mobile zone non-Fickian transport. A parameter study of the two-scale CTRW adds further insight into the dynamics of the process, showing the interaction between the advective non-Fickian transport and the mass exchange to immobile regions.

Comparison of alternative models for simulating anomalous solute transport in a large heterogeneous soil column

This study compared five different models for evaluating solute transport in a 1250-cm long, saturated and highly heterogeneous soil column. The five models were: the convection-dispersion equation (CDE), the mobile-immobile model (MIM), the convective lognormal transfer function model (CLT), the spatial fractional advection-dispersion equation (FADE) and the continuous time random walk model (CTRW). Each of these models was used to fit the breakthrough curve (BTC) at each distance individually and was also used to fit the BTCs at different distances simultaneously. Dependence of estimated parameters on distance was investigated. The estimated parameters at 200 cm were used to make predictions at subsequent distances. Highly anomalous transport behavior was observed in the column as the BTCs demonstrated significantly irregular shape and long tailing. This study indicated that CDE, CLT and FADE were unable to describe the anomalous BTCs adequately and their parameters changed with transport distance significantly. Compared to CDE, CLT and FADE, MIM better captured the evolution of anomalous BTCs. However, MIM did not explain the distinct BTC tailing satisfactorily. In contrast to MIM, CTRW better simulated the long tails of BTCs. The spreading parameter (beta) of CTRW was close to one and remained approximately constant at different travel distances. To make the comparison of these five models more general beyond the specific transport condition in the soil column, a generic evaluation of the advantages and disadvantages of these five models was presented in terms of their theory framework and a priori knowledge of the model behaviors. (C) 2009 Elsevier B.V. All rights reserved.

Simulation of nonlinear sorption of N-heterocyclic organic contaminates in soil columns

The transport of organic contaminants in porous media is frequently influenced by nonequilibrium sorption and/or nonlinear sorption. In this study, sorption of coal tar related contaminants with different sorption properties, i.e., toluene, quinoline, quinaldine, and benzotriazole, was studied in column experiments using a European reference soil and compared with batch sorption results in order to quantify the governing sorption features. The breakthrough curves (BTCs) were simulated with a versatile 1-D reactive transport model using a one-site first-order sorption approach. Some differences in fitted parameters from batch and column experiments were found and discussed in terms of different sorption mechanisms in different aqueous concentration ranges, effects of solution properties (e.g., pH) and differences in solid-to-solution ratio and accessible sorption sites. The modeling results show that the fitting results were not sensitive to mass transfer coefficients and that a local equilibrium assumption provides excellent agreement with BTCs in our designed column when Damkohler numbers were greater than 20. Nonequilibrium sorption resulting from intraparticle diffusion thus was negligible in the column experiments. Tailing of BTCs nevertheless occurred and was primarily attributed to nonlinear sorption due to specific interactions in the sorption processes rather than to sorption nonequilibrium. Our study demonstrates how column experiments with different concentrations and flow velocities can be designed to obtain reliable sorption parameters for polar solutes with nonlinear sorption isotherms from modeling.

Evidence of Physical and Chemical Nonequilibrium in Lead and Cadmium Transport and Sorption in Acid Soils

Environmental issues regarding soil trace metals often center on mobility and bioavailability. Transport of metals through the soil system could be affected by physical or chemical nonequilibrium processes. In this study, the physiochemical factors involved in Pb and Cd transport and sorption in soils were examined to determine the fate of metal pollutants. A series of metal miscible displacement experiments were executed following a full factorial statistical design including five sources of variability: soil (S1 and S2), metal (Pb and Cd), metal initial concentration (C0), water flux rate (Jw), and depth (0–4 and 4–8 cm). Temporal moment analysis of the measured breakthrough curves (BTCs) revealed that Cd transport is characterized by larger mass recovery values and lower estimated values of the transport timing—mean travel time (μ) and vertical solute spreading (σ2)—than Pb. This indicates the high affinity and retention of Pb in the soil and the high mobility of Cd. On the other hand, the sorption parameters for both Pb and Cd were estimated from a sequential chemical extraction of the metal retained in the soil column. The effect of the sources of variability on both transport and sorption parameters were determined. The functional relationship between sorption and transport parameters was also estimated through a linear regression and canonical correlation analysis. Physical and chemical nonequilibrium in transport processes were demonstrated by BTC tailing, changing BTC characteristics (μ and σ2) with Jw and C0, and semi‐irreversible sorption identified by sequential extraction.

Regularized pilot points method for reproducing the effect of small scale variability: Application to simulations of contaminant transport

Small scale variability of hydraulic conductivity controls ground water contaminant transport and some of the subtle aspects of transport through heterogeneous media (e.g., mixing and tailing). Unfortunately, it cannot be identified. This paper addresses the question of whether the presence of high frequency fluctuations, which define small scale variability, impedes the characterization of large scale variability patterns. In parallel, we investigate whether including small scale variability allows us to reproduce tailing in breakthrough curves. To this end, we solve the inverse problem using the regularized pilot points method for simulating fields of hydraulic conductivity conditioned to available information. Heterogeneity of hydraulic conductivity is represented by two nested variograms simulating small scale (short range variogram) and large scale (long range) variability patterns. Calibrated fields are applied to the prediction of a transport problem. Application to four synthetic examples (with varying importance of the small scale variability) shows that, first, the calibrations reproduce the statistics of the “true” fields and, second, that a good characterization of the small scale variability is not critical for groundwater flow modeling. More important, small scale variability leads to increased tailing in solute breakthrough curves and needs to be acknowledged for proper transport prediction, which is rarely the case in aquifer modeling practice.

Continuous time random walk model better describes the tailing of atrazine transport in soil

Contaminant transport in soils is complicated and involves some physical and chemical nonequilibrium processes. In this research, the soil column displacement experiments of Cl− and atrazine under different flow velocities were carried out. The data sets of Cl− transport in sandy loam fitted to the convection dispersion equation (CDE) and the two-region model (TRM) indicated that the effects of physical nonequilibrium process produced by immobile water on the breakthrough curves (BTCs) of Cl− and atrazine transport through the repacking soil columns were negligible. The two-site model (TSM) and the continuous time random walk (CTRW) were also used to fit atrazine transport behavior at the flow rate of 19.86cmh−1. The CTRW convincingly captured the full evolution of atrazine BTC in the soil column, especially for the part of long tailing. However, the TSM failed to characterize the tailing of atrazine BTC in the soil column. The calculated fraction of equilibrium sorption sites, F, ranging from 0.78 to 0.80 for all flow rates suggested the contribution of nonequilibrium sorption sites to the asymmetry of atrazine BTCs. Furthermore, the data sets for the flow rates of 6.68cmh−1 and 32.81cmh−1 were predicted by the TSM and the CTRW. As to the flow rate of 6.68cmh−1, the CTRW predicted the entire BTC of atrazine transport better than the TSM did. For the flow rate of 32.81cmh−1, the CTRW characterized the late part of the tail better, while the TSM failed to predict the tailings of atrazine BTC.

Comparison of Microscopic and Macroscopic Modeling Approaches for Subsurface Contaminant Transport

The practice of contaminant transport and remediation has shown significant progress in recent years. However, despite the significant progress made, remediation efforts are often delayed by extremely long breakthrough curve tails that render efforts to bring the level of contaminants below the regulatory standards inefficient. One hypothesis is that these long tails are due to the reservoir-like slow diffusive processes in soil micropore zones. This study compares the effects of micropores at macroscopic and microscopic levels and establishes a link between these approaches for validation and calibration purposes. The link between macroscopic and microscopic levels is established through comparisons and testing of the two models while incorporating appropriate scale and boundary effects. Despite the differences in conceptual approaches and simulation time, the two approaches rendered meaningful results. The link helps forecast the effects of micropore zone transport processes in the subsurface efficiently and thus allows development of numerical tools that could contribute towards more efficient remediation design.

Breakthrough curve tailing in a dipole flow field

Studying the tailing behavior of breakthrough curves (BTCs) is useful in the characterization of anomalous transport, matrix diffusion, and kinetic sorption. We analyze how BTCs of nonreactive and sorbing tracers behave at late time in well‐to‐well flow fields in homogeneous media. In the absence of regional flow, an asymptotic solution is derived for the traveltime distribution which follows a power law with exponent −4/3 at late times. With regional flow, the traveltime distribution exhibits a power law with exponent −4/3 over a certain period of time, followed by exponential decay. Dispersion influences the early time behavior but has little effect on the late‐time tailing. We also consider the BTC tailing of tracers undergoing kinetic sorption and diffusive mass transfer into immobile regions. If the memory function characterizing sorption kinetics is exponential, the late‐time behavior of the BTC is controlled by the traveltime distribution and thus follows a power law with exponent −4/3. In case of a matrix diffusion model, the memory function exhibits a power law with exponent −1/2, and the BTC in the dipole flow field follows a power law with exponent −7/6 in an intermediate time range, which differs from the exponent of −3/2 observed in uniform flow. Our analysis demonstrates that the flow configuration has to be considered when the tailing behavior of BTCs is used to characterize mass transfer kinetics. In particular, truncating a well‐to‐well tracer test at times where the traveltime distribution still follows the power law behavior may lead to the erroneous interpretation that the power law tailing of the BTC is caused by matrix diffusion.

Predicting the Tails of Breakthrough Curves in Regional‐Scale Alluvial Systems

The late tail of the breakthrough curve (BTC) of a conservative tracer in a regional-scale alluvial system is explored using Monte Carlo simulations. The ensemble numerical BTC, for an instantaneous point source injected into the mobile domain, has a heavy late tail transforming from power law to exponential due to a maximum thickness of clayey material. Haggerty et al.'s (2000) multiple-rate mass transfer (MRMT) method is used to predict the numerical late-time BTCs for solutes in the mobile phase. We use a simple analysis of the thicknesses of fine-grained units noted in boring logs to construct the memory function that describes the slow decline of concentrations at very late time. The good fit between the predictions and the numerical results indicates that the late-time BTC can be approximated by a summation of a small number of exponential functions, and its shape depends primarily on the thicknesses and the associated volume fractions of immobile water in "blocks" of fine-grained material. The prediction of the late-time BTC using the MRMT method relies on an estimate of the average advective residence time, t(ad). The predictions are not sensitive to estimation errors in t(ad), which can be approximated by L/v , where v is the arithmetic mean ground water velocity and L is the transport distance. This is the first example of deriving an analytical MRMT model from measured hydrofacies properties to predict the late-time BTC. The parsimonious model directly and quantitatively relates the observable subsurface heterogeneity to nonlocal transport parameters.

Impact of upscaling on solute transport: Traveltimes, scale dependence of dispersivity, and propagation of uncertainty

Efficiency constraints in applying numerical models to real problems force the use of a coarse discretization of the domain compared with the detailed scale required for the most adequate description of the system. Upscaling encompasses the methods that transfer small‐scale information to the computational scale. The loss of small‐scale information by upscaling aquifer properties to construct a numerical model largely modifies the true heterogeneous structure of the aquifer compromising the final predictions of solute transport. We present extensive Monte Carlo solute transport simulations in heterogeneous porous media to investigate (1) the impact of upscaling on the evolution of solute plumes and (2) the propagation of uncertainty through upscaling. In doing this we analyze the benefits of using enhanced block dispersion tensors in the advection‐dispersion equation to compensate for the loss of information. It is shown that upscaling does not affect the median traveltimes of particles reaching a control location, but the tail of breakthrough curves is largely underestimated. It is also observed that normal upscaled transport models can largely underestimate the spreading of solute plumes even if block dispersivities are calculated as being representative of within‐block heterogeneity. Discrepancies are mainly attributed to the mass transfer interaction between grid blocks of the numerical model and the use of a simplistic representation of the block hydraulic conductivity tensor. Using the small‐perturbation stochastic approach, we found that mass transfer effects are still significant, even for block sizes of the numerical model greater than 30 correlation scales. Moreover, upscaling is shown to have a substantial impact on the uncertainty predicted by solute transport models, which is not compensated by the use of enhanced block dispersion tensors. For very heterogeneous media, simulation results showed the inability of the classical advection‐dispersion equation to simultaneously reproduce the tail of the breakthrough curve and its uncertainty in the upscaled model. In those cases, results indicated that either a mass transfer model allowing solute mass exchange between high‐ and low‐conductivity zones, encountered within a grid block of the numerical model, or the continuous time random walk model expressing the retardation of solute particles passing through low‐conductivity regions inside a grid block by means of a transition time distribution function might suppose a better upscaled transport model.

Transport of Escherichia coli through variably saturated sand columns and modeling approaches

A sand column leaching system with well-controlled suction and flow rate was built to investigate the effects on bacterial transport of air–water interface effects (AWI) correlated to water content, particle size, and column length. Adsorption of Escherichia coli strain D to silica sands was measured in batch tests. The average % adsorption for coarse and fine sands was 45.9 ± 7.8% and 96.9 ±3.2%, respectively. However, results from static batch adsorption experiments have limited applicability to dynamic bacterial transport in columns. The early breakthrough of E. coli relative to bromide was clear for all columns, namely c. 0.15 to 0.3 pore volume earlier. Column length had no significant effects on the E. coli peak concentration or on total recovery in leachate, indicating retention in the top layer of sands. Tailing of breakthrough curves was more prominent for all fine sand columns than their coarse sand counterparts. Bacterial recovery in leachate from coarse and saturated sand columns was significantly higher than from fine and unsaturated columns. Observed data were fitted by the convection–dispersion model, amended for one-site and two-site adsorption to particles, and for air–water interface (AWI) adsorption. Among all models, the two-site + AWI model achieved consistently high model efficiency for all experiments. Thus it is evident from experimental and modeling results that AWI adsorption plays an important role in E. coli transport in sand columns.

Numerical Analysis of a Short-Term Tracer Experiment in Fractured Sandstone

A short-term, pulse injection tracer experiment conducted in fractured quartzitic sandstone at Kukuan, Taiwan was analyzed. Tracer transport at the test site was dominated by advection but a specific attenuation mechanism leading to breakthrough curve (BTC) tailing also seemed to exist. Matrix diffusion was hypothesized as the transport mechanism that results in the tailing. This hypothesis was proved by comparing the field BTC with numerical simulation results obtained by the general-purpose flow/transport simulator, TOUGH2, based on a single-fracture conceptual model. Due to the lack of accuracy of estimating the interporosity flux by the conventional double porosity model (DPM), TOUGH2 was incorporated with the multiple interacting continua (MINC) scheme to simulate the transient characteristics of the interporosity flux. In MINC, rock matrix is discretized as a series of continua according to the perpendicular distance from the fracture that adjoins the matrix. The closer the rock matrix is to the fracture, the finer the rock matrix is discretized. This concept is fundamentally different from DPM in that rock matrix is no longer treated as a single continuum. Simulation results by TOUGH2-MINC have successfully reproduced the observed BTC tailing even under the dominating advection effect. Sensitivity studies showed that TOUGH2-MINC is sensitive to parameters including fracture aperture (2b), matrix porosity (nm) and effective molecular diffusion coefficient in matrix (Dm). If 2b, nm , Dm , are respectively 200 µm, 2%, 10 -11 m 2 s -1 , and if hydrodynamic dispersion coefficient

An analytical solution for slug tracer tests in fractured reservoirs

The transport of chemicals or heat in fractured reservoirs is strongly affected by the fracture‐matrix interfacial area. Under unsaturated conditions, such as in vapor‐dominated geothermal reservoirs, this area can be estimated from inert gas tracer tests that produce a characteristic tail in tracer breakthrough curves (BTCs). For water‐saturated conditions, molecular diffusion is orders of magnitude smaller, and tails in BTCs from diffusive exchange between fractures and matrix are too weak to be practically useful as a characterization tool for fracture spacings of practical interest. However, recent studies suggest that reversibly sorbing solute tracers can generate strong tails in BTCs that may allow a determination of the fracture‐matrix interfacial area. To theoretically explore such a useful phenomenon, this paper develops an analytical solution for BTCs in slug tracer tests in a water‐saturated fractured reservoir, idealized here as a set of uniformly spaced plane, parallel, identical fractures. The solution shows that increased sorption should have the same effect on BTCs as an increase of the diffusion coefficient. The solution is useful for understanding transport mechanisms, verifying numerical codes, and identifying appropriate chemicals as tracers for the characterization of fractured reservoirs.

Assessment of the predictive capabilities of stochastic theories in a three‐dimensional laboratory test aquifer: Effective hydraulic conductivity and temporal moments of breakthrough curves

Conservative and sorptive tracer experiments were conducted in a highly heterogeneous (σlnK2 = 1.79) and anisotropic (λH/λV = 3.52) three‐dimensional test aquifer under well‐controlled laboratory conditions to evaluate the effective conductivity and temporal moments predicted by stochastic theories. The spatial distribution of lnK in the test aquifer conformed to a statistically homogeneous system, thus allowing evaluation of results from stochastic theories for stationary random fields. Effective hydraulic conductivity Keff in the mean flow direction for the test aquifer was compared with different stochastic theoretical expressions. Breakthrough curves (BTCs) at individual deep‐penetrating observation wells and averaged breakthrough curves at control planes were examined. The mean arrival time and the travel time variance estimated using averaged BTCs from many observation wells at the same control plane for bromide and lithium were slightly overestimated by stochastic theories, yet most of the experimental data ranged within the confidence interval rendered by the uncertainty in the statistical properties of the test aquifer. A highlight of our experimental approach is that the continuous sampling procedure also permitted the evaluation of higher‐order temporal moments that were analyzed to study the asymmetry and peakedness of BTCs by means of the coefficients of skewness and kurtosis, respectively. Small‐perturbation analytical solutions for the coefficient of skewness and kurtosis were provided. It is seen that although stochastic theories based upon small perturbations provide reasonably good estimates of the coefficients of skewness and kurtosis associated with averaged BTCs at control planes, in general, they can largely underestimate the peakedness and tailing of BTCs observed at individual deep‐penetrating observation wells.

Non‐Fickian transport in homogeneous unsaturated repacked sand

It is commonly assumed a priori that solute transport experiments conducted in homogeneously repacked laboratory columns can be described by the advective‐dispersive (AD) or single‐rate mobile‐immobile (MIM) transport model. To investigate this, nonreactive transport through macroscopically homogeneous repacked unsaturated sand was studied at two water contents using laboratory columns of diameter of 11 cm and lengths of 10, 20, and 40 cm. Non‐Fickian behavior was found to dominate transport at this scale, with long breakthrough curve (BTC) tailing persistent over the range of column lengths tested. Measured BTC tails were well described by the single‐rate MIM model at each depth; however, over the range of travel distances studied BTC tailing was better explained as a result of stochastic‐convective (SC) transport or continuous time random walk (CTRW). The SC model was applied using the BTC measured at 10 cm as the probability density function to make predictions to subsequent depths. The CTRW model spreading parameter (β) remained approximately constant across the range of both travel distances and water contents considered. It is concluded that the assumption of a single‐rate MIM model cannot be made a priori for macroscopically homogeneous unsaturated sands. In this case, variability between replicates prevented identification of whether a SC transfer function or CTRW provided the best description of transport. These results demonstrate the variability in transport as a result of heterogeneities in column packing, even for macroscopically homogeneous sand, and emphasize the importance of studying transport over a range of travel distances in order to allow prediction of transport.

Simple screening models of NAPL dissolution in the subsurface

Three simple screening models of nonaqueous phase liquid (NAPL) dissolution in the subsurface are proposed based on the NAPL mass conservation and the assumption of proportionality between the residual NAPL source zone concentration and the remaining residual NAPL mass. The purpose of the proposed models is to predict the solute concentration in the zone of the residual NAPL as a result of dissolution. The predicted source zone concentration decrease is used to simulate and account for the decrease of dissolution rate with time. The proposed simple NAPL dissolution models enable the pseudo-equilibrium formulation to be used and therefore the numerical simulations for field application problems can be simplified compared to the non-equilibrium counterpart. With proper choice of empirical parameters, the proposed simple screening models can work as well as more complex dissolution rate correlation models, such as that of Imhoff et al. [Water Resour. Res. 30 (1994) 307–320]. It is found that the proposed models are very good for quantifying non-equilibrium dissolution, which is characterized by tailing of breakthrough curves. The models are especially useful for situations of small residual NAPL saturation, which are typical for many field applications.

Influence of Initial and Boundary Conditions on Solute Transport through Undisturbed Soil Columns

The effect of initial soil water content (θ) and steady and transient water flow boundary conditions (Jw) on the transport of identical solute pulses was examined in four undisturbed soil columns. Solute breakthrough curves (BTCs) and θ were monitored nondestructively using time domain reflectometry (TDR). Pulse concentrations were predicted from TDR impedances (Ω) using either a steady Jw calibration or a transient Jw calibration. The steady Jw calibration, βs, was estimated from the steady Jw BTC. The transient calibration, β(θ), was estimated from the changes in Ω−1(θ) between three different calibrating solutions of Cl− (1.106, 1.503, and 1.900 g L−1) in the pore space. All β(θ) curves had a very similar decreasing curvilinear shape. At one location the β(θ) curve was high and residual water was not completely replaced by the calibrating solutions. The steady Jw calibration values were generally slightly lower than the equivalent β(θ) value, but collectively they followed a similar decreasing curvilinear trend with steady θs Solute applied to initially dry soil tended to lead the transient Jw wetting front and breakthrough began with the start of cumulative drainage. These BTCs also had the greatest amount of BTC tailing. Solute applied to initially wet soil had less BTC tailing. The least amount of BTC spreading occurred during the steady Jw condition. The two transient Jw experiments had very similar distributions of solute with depth, despite the contrasting initial conditions. The pores actively transporting solute appear to be determined by the Jw boundary condition.

Influence of Initial and Boundary Conditions on Solute Transport through Undisturbed Soil Columns

The effect of initial soil water content (θ) and steady and transient water flow boundary conditions (Jw) on the transport of identical solute pulses was examined in four undisturbed soil columns. Solute breakthrough curves (BTCs) and θ were monitored nondestructively using time domain reflectometry (TDR). Pulse concentrations were predicted from TDR impedances (Ω) using either a steady Jw calibration or a transient Jw calibration. The steady Jw calibration, βs, was estimated from the steady Jw BTC. The transient calibration, β(θ), was estimated from the changes in Ω−1(θ) between three different calibrating solutions of Cl− (1.106, 1.503, and 1.900 g L−1) in the pore space. All β(θ) curves had a very similar decreasing curvilinear shape. At one location the β(θ) curve was high and residual water was not completely replaced by the calibrating solutions. The steady Jw calibration values were generally slightly lower than the equivalent β(θ) value, but collectively they followed a similar decreasing curvilinear trend with steady θs Solute applied to initially dry soil tended to lead the transient Jw wetting front and breakthrough began with the start of cumulative drainage. These BTCs also had the greatest amount of BTC tailing. Solute applied to initially wet soil had less BTC tailing. The least amount of BTC spreading occurred during the steady Jw condition. The two transient Jw experiments had very similar distributions of solute with depth, despite the contrasting initial conditions. The pores actively transporting solute appear to be determined by the Jw boundary condition.