Journal of Environmental Quality 30:573-583 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORT
VADOSE ZONE PROCESSES AND CHEMICAL TRANSPORT
Detailed Characterization of Solute Transport in a Heterogeneous Field Soil
Fernando Garridoa,
Masoud Ghodratib,
Chris G. Campbellb and
Michael Chendorainb
a Centro de Ciencias Medioambientales, Consejo Superior de Investigaciones Científicas, Serrano 115-dup. E-28006-Madrid, Spain
b Ecosystem Sciences Division, Dep. of Environmental Science Policy and Management, Univ. of California, 151 Hilgard Hall, Berkeley, CA 94720-3110
Corresponding author (ghodrati{at}nature.berkeley.edu)
Received for publication December 16, 1999.
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ABSTRACT
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There is a necessity for improved physical understanding of solute transport processes in heterogeneous soil systems. In situ nondestructive techniques like time domain reflectometry (TDR) and fiber optic miniprobes (FOMPs) permit the collection of unique measurements of solute transport processes in soils for the purposes of model development and validation. This study examined the application of TDR and FOMPs to measure solute transport at various points laterally and at two depths in a heterogeneous clay-loam soil. A miscible displacement experiment was performed at a constant irrigation flux to examine the applicability of these probes to field soils. In their first application to a field soil, the FOMPs were successfully calibrated and performed well in measuring solute breakthrough curves. Two flow regimes were identified in the soil profile, the first where lateral spreading of the solute occurred in the surface horizon, followed by convergence into preferential flow pathways in the second transport zone. The measured transport response was heterogeneous with at least two identifiable vertical flow phases. It was demonstrated using transfer function modeling and data from a corresponding laboratory study that the FOMPs were measuring the slower phase, while the TDR probes captured a composite of the fast and slow phases. The combination of these two techniques may be a means to separate solute transport phases in heterogeneous media and relate laboratory column results to field studies.
Abbreviations: BTC, breakthrough curve • CLT, convective lognormal transfer function • EC, electrical conductivity • FOMP, fiber optic miniprobe • TDR, time domain reflectometry
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INTRODUCTION
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PART of the difficulty in applying deterministic modeling approaches to soil solute transport has been the significant effect of soil heterogeneity, including macropores, irregular vertical anisotrophy, and other structural factors, on transport processes (White, 1985; Beven and Germann, 1982). The importance of preferential flow to solute transport has been demonstrated in numerous field studies (Flury et al., 1994; Edwards et al., 1993; Ghodrati and Jury, 1990, 1992; Andreini and Steenhuis, 1990). In order to account for preferential flow, it has been suggested that transport in one dimension may be divided into two flow phases associated with matrix and preferential flow pathways (Grochulska and Kladivko, 1994; Kladivko et al., 1991; Utermann et al., 1990). This approach most often requires measurements of transport processes or parameters to characterize and separate the flow phases.
Nondestructive in situ techniques, like time domain reflectometry (TDR) and fiber optic probes, provide data at high resolution, which may be used to validate solute transport models such as two-phase models, the convection dispersion equation (CDE), or the stochastic convective lognormal transfer function (CLT) (Jury and Sposito, 1985). For example, TDR has become an accepted method for estimating in situ transport parameters for models like the CDE (Campbell et al., 1999; Jacques et al., 1998; Persson and Berndtsson, 1998; Vanderborght et al., 1997), CLT (Jacques et al., 1998; Persson and Berndtsson, 1998; Vanderborght et al., 1997), and mobile–immobile models (Mallants et al., 1994). However, most of these studies have been performed under controlled laboratory conditions that may not extrapolate well to field conditions (Beven et al., 1993).
One of the first studies using TDR to measure solute transport in the field was performed by Kachanoski et al. (1992). These authors also suggested the first simple calibration of the TDR for relative solute concentration measurements. Other studies followed, with a majority of them occurring in sand and loam soils (Jacques et al., 1998; Radcliffe et al., 1998; Rudolph et al., 1996). For example, Rudolph et al. (1996) were able to analyze the spatial and temporal variability in solute transport at various depths in a 10-m-long sandy plot at the USGS Cape Cod Toxic-Substances Hydrology Research Site with spatial and temporal resolutions of about 50 cm and 7.5 min, respectively. Another field scale study has been performed recently by Jacques et al. (1998) in silty loam and loamy soils. As the authors point out, the TDR probes in this study did not detect preferential flow as a result of the low temporal resolution necessary to multiplex 120 TDR probes (2-h sampling interval).
In addition to TDR, measurement devices are needed that are capable of examining solute transport in soils at various scales (Binley et al., 1996). Recently, Nissen et al. (1998) recognized the need for smaller-scale measurements of water content and developed a coiled TDR probe only 15 mm long. Another relatively new technology is fiber optic probes that can measure solute transport at a point scale (10 mm3) (Ghodrati, 1999), which is far less than the volume-averaged measurements of the existing TDR probes on the order of 102 to 104 cm3 (Garrido et al., 1999a).
The fiber optic miniprobe (FOMP) system developed by Ghodrati (1999) measures the intensity of light reflected back into the probe when a fluorescent tracer passes its measurement volume. The light intensity is calibrated to the concentration of a fluorescent tracer so that solute breakthrough curves (BTCs) can be constructed. The smaller size of the probe allows for detailed characterization of the spatial variability in transport at very small scales (Garrido et al., 1999b). Recently, a 20-channel multiplexed fiber optic system was employed by Ghodrati et al. (2000) for replicated measurements of tracer transport in a large silica sand column. The authors demonstrated that the FOMPs appear to measure resident solute concentration.
These FOMPs have also been used to measure solute transport in clay loam soil (Campbell et al., 1999; Garrido et al., 1999b). Measurements from the FOMPs have been compared with those from TDR probes and continuous soil solution samplers in a repacked column (Campbell et al., 1999). The authors concluded that measurements of solute transport were probe-specific, resulting from both measurement scale and probe bias.
This paper discusses a detailed quantitative solute transport study performed in the field using two in situ nondestructive measurement devices, the FOMPS and TDR probes. The study was performed to (i) test the application of the relatively new FOMPs, as well as the TDR probes, to measure transport in a heterogeneous clay-loam field soil; (ii) collect evidence of preferential flow processes; (iii) examine similarities between the measurements taken at the different sample volumes of the FOMPs and TDR probes; (iv) compare the lateral and vertical variability in probe measurements; and (v) determine if BTCs measured in the field may be fitted to transfer functions for further analysis.
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MATERIALS AND METHODS
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Study Site
Our study was performed in a natural field soil under a pine tree plantation at the University of California Russell Reservation. The site was selected in a level valley bottom away from areas receiving significant foot or machinery traffic. The soil of the area is classified as a Botella clay loam (fine-loamy, mixed, superactive, thermic Pachic Argixeroll). However, particle size analysis from samples collected at the site showed the actual texture classification to be closer to a clay (Table 1). The surface soil horizon was not disturbed, some pine needles were brushed away, and a few annual grasses were cut, but careful attention was given to maintaining the integrity of the soil surface.
Plot Design and Installation
A small (0.82 x 0.82 m) plot was selected and covered with a tent to control any influence from natural rainfall during the solute transport studies. The plot consisted of eight horizontally inserted 0.20-m-long TDR probes, four at each depth of 0.10 m and 0.20 m. Twenty FOMPs were installed vertically into the soil with 10 at each depth (Fig. 1)
. Each FOMP was placed so that the surrounding drip irrigation points were in a consistent triangular pattern. The distance between each FOMP and the nearest drip point was 5 cm. Eight additional 0.05-m-long TDR probes were inserted vertically into the soil surface. The plot was divided into quarters, each containing a TDR and three FOMPs at both depths (Fig. 1). The nested plot design allows examination at a series of spatial scales from the 0.36-m2 plot, to the four 0.09-m2 subplots, individual TDR probes, and individual FOMPs, respectively, at the two depths.
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Fig. 1. Schematic plot design and instrumentation. Fiber optic miniprobes (FOMPs) and time domain reflectometry (TDR) are identified by their respective numbers at each depth level
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The irrigation system consisted of 41 drip points applied to an area approximately 0.68 m2, which included a 0.225-m-wide boundary area surrounding the plot to promote vertical flow of the solute. Thus, the active measurement area of the plot included the inner 0.36 m2. The water application rate was controlled by two peristaltic pumps connected to large (378 and 756 L) water storage tanks. Drip points were evenly spaced with each point corresponding to about 0.0156 m2 surface area on the plot (Fig. 1).
Multiplexed Fiber Optic Miniprobe System
The fiber optic miniprobe system is based on transmitting a constant beam of light through the input leg of a bifurcated fiber optic miniprobe (3 mm diameter, and desired length) to a location within the soil. At the probe's tip, incoming light interacts with the medium, where it is partially absorbed and partially reflected back into the probe. The reflected signal is transmitted through the output leg to a photodetector and quantified. The intensity of the constant output signal changes when a plume of a fluorescent tracer passes through the area in front of the probe.
For this study we used a multiplexed (20 channel) FOMP system as described by Ghodrati et al. (2000). The FOMP system measures light intensity as an electrical signal that is collected by a datalogger (Fluke Corp., Santa Clara, CA). Light intensity is calibrated to the concentration of a fluorescent tracer by injecting a series of tracer concentrations into the zone directly in front of the probe using the in situ calibration technique of Garrido et al. (2000). Pyranine (8-hydroxy-1,3,6-pyrenetrisulfonic acid trisodium; CAS #6358-69-6) was selected as the tracer, with the added advantage that it may also be measured as a salt by the TDR probes (Campbell et al., 1999). A calibration curve relating the output light intensity to the corresponding standard solutions of the pyranine was then constructed and approximated with a second-order polynomial equation. Using this calibration technique, in situ measurements of solute BTC at different points of observation are possible (Ghodrati, 1999). Light intensity measured with the FOMPs was found to fluctuate in a daily pattern corresponding to temperature. Temperature inside the detector housing was monitored during all experiments so that a correction factor of light intensity could be empirically extracted by performing a linear regression between light intensity and temperature measured at thermocouples.
Time Domain Reflectometry System
The TDR probes used in this study consisted of three pronged probes attached to coaxial cable and a 16-channel multiplexer that connected the coaxial cable to a Tektronix 1502C pulse generator (Dynamax Corp., Houston, TX). The probes were used to carry an electromagnetic pulse from the pulse generator to the soil. The electrical signal that returns to the pulse generator is a function of the material it moves through, in contact with the probes (Topp and Davis, 1984). The apparent permitivity of the material (also known as the dielectric constant) determines travel time of the electrical pulse. The change in this signal may be calibrated to soil water content for a given soil (Topp and Davis, 1984). In addition to water content, bulk soil electrical conductivity can be measured using the change in magnitude of the reflected signal or impedance loading (Topp et al., 1988; Dalton and van Genuchten, 1986). Electrical conductivity is, for a large part, determined by solute concentration, and this has lead to the employment of TDR probes to monitor solute transport using any salt as a tracer.
There have been numerous methods proposed for the calibration of change in the TDR measured relative impedance loading (ZL) to bulk soil electrical conductivity (EC) and then to solute concentration. These methods, and their assumptions, have been reviewed by Mallants et al. (1996). This study employed the methodology discussed for horizontally inserted probes by Campbell et al. (1999) for both the 0.20- and 0.05-cm-long TDR probes. In short, concentration was estimated by calibrating the ZL measurements from the TDR to the electrical conductivity of the solution in the soil pore space. Bulk soil EC was estimated by fitting a geometric constant, Kc, from ZL data obtained while immersing TDR probes into solutions of known concentration using the equation proposed by Nadler et al. (1991):
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Using this equation, the Kc values were empirically determined for each probe of both the 0.20- and 0.05-cm lengths.
Bulk soil EC was then related to solution EC by saturating the soil with two solutions of CaCl2 with known EC. Then, a third relationship was developed for concentration of pyranine and its EC. Once it was established that the relationships between EC of the bulk soil to EC of the soil solution and EC of the soil solution to tracer concentration were linear, then relative EC could be used to obtain relative resident concentration (Cr/Co) for all TDR probes of both 0.05 and 0.20 m length:
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where ECz,t is the EC measured over time at a given depth, ECo is the EC of the initial concentration of the tracer pulse, and ECi is the baseline before the tracer application (Kachanoski et al., 1992). As the ZL measurements are temperature dependent, a correction technique proposed by Heimovaara et al. (1995) was applied, which is a simple linear regression similar to our FOMP temperature correction.
Miscible Displacement Studies
The plot was irrigated for more than a week at 1.5 cm/h until a steady-state water content (measured with the TDR probes) was reached. At steady state, a 20-mm pulse of the fluorescent tracer, pyranine, was applied at a concentration of 4 g/L. One pulse of pyranine was approximately 7 L and was applied in less than 1 h. Once the pulse was applied to the plot, irrigation of fresh water resumed at the same rate. The plot was leached for 4 to 5 d following the pulse application.
The FOMP and TDR systems collected data at 5- and 10-min intervals, respectively. Data measured by the probes was converted to concentration using the calibration curves and BTCs were constructed. Mass recovery was calculated using plot average values for the water content measured by the 0.20-cm TDR probes and the irrigation flux. In order to analyze flow phases measured by the TDR probes the potential to fit the BTCs to transfer function models was examined. The CLT equation used was:
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where µl and 
l are the mean and variance of the natural log transformed solute travel times normalized to depth l (Vanderborght et al., 1996) and Crt* (z,t), as described by Vanderborght et al. (1996), is the time integral resident concentration. The concentration is dependent on both depth and time.
Pyranine was used as a tracer in these experiments and was subject to retardation during movement through the soil. A retardation factor of 1.43 was estimated in previous laboratory studies and applied to Eq. [3] (Garrido et al., 1999b). Adding R results in this equation:
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Equation [4] was then linked in series and fit to field data in an attempt to compare a conceptual two-phase system with the actual field data and to identify different flow phases. The resulting equation was:
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where 
1 and 2 are the respective CLT equations (Eq. [4]) corresponding to the two different flow phases, 
1 is the pore space fraction attributed to the water content of first flow phase, and 2 is the pore space fraction attributed to water content of the second phase. Crt*total (z,t) refers to the total concentration from both flow phases.
Data from a similar miscible displacement study in a repacked column by Campbell et al. (1999) were used to fit the first set of parameters (1, µ1, and 1). Breakthrough curves from this laboratory study were measured with the TDR probes in the same Botella clay loam soil at the same tracer concentration. The laboratory study was assumed to represent matrix flow of the field data, and the curve-fitting exercise used to describe this phase produced excellent fits, with r2 values greater than 0.97.
For the purpose of analyzing the data obtained in the field, the laboratory-fitted parameters were termed as constants representing matrix flow. This assumption is reasonable for the purpose of comparing laboratory and field data given the similarity in soil type, particle size distribution, average bulk density, organic matter, and measurement and pulse application techniques. Table 2 lists the fitted parameters from both the laboratory experiments and the field BTCs. Parameters from the second flow zone (2, µ2, and 2) were then estimated using the laboratory-fitted values as inputs. Sensitivity analysis was performed on both fits of the CLT to the two flow phases, demonstrating the suitability of the parameter estimation (data not shown).
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Table 2. Fitted parameters from both the laboratory experiment (Campbell et al., 1999) and the field breakthrough curves (BTCs) measured by the 0.20-m-long time domain reflectometry (TDR) probes. All parameters are unitless
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In addition to the transfer function analysis, an independent statistical moment analysis was performed to compare the normalized travel time of the BTCs constructed from FOMP data with those from the TDR (Jury and Roth, 1990). The moments also allow a comparison between the modeled and actual travel times of the tracer (Valocchi, 1990).
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RESULTS AND DISCUSSION
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Fiber Optic Miniprobe System Measurements
The FOMPS performed well in the field with a successful calibration for 19 of the 20 probes using the calibration method presented in Garrido et al. (2000). A total of 13 out of the 20 probes residing at the two depths intercepted the transport of the tracer, which is not unexpected due to the small measurement volume of the FOMPs and the heterogeneous nature of the soil. The distribution of the FOMP response may be seen in Fig. 2
, which shows the mass recovery at both depths. Notice the tracer mass measured at the same horizontal point in each depth. Some probe pairs measured high mass at both depths, such as Probes 3 and 8 at 0.10 m depth, and Probes 13 and 18 at 0.20 m. Other points, however, measured high mass at one depth and low mass at the other, such as Probes 9 and 19, and 7 and 17. Finally, not all of the probes that measured tracer at one depth measured it at the other, such as at Probes 6 and 16. All these results indicate that for the sampling volume of the FOMPs there is not a uniform transport response horizontally or vertically, even under the extremely controlled conditions in this study.
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Fig. 2. Plot of the mass recoveries of the fiber optic miniprobes (FOMPs) at different spatial locations in the plot
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Figures 3 and 4
are the BTCs from the FOMPs at the 0.10- and 0.20-m depths, respectively. At the 0.10-m depth some of the BTCs (e.g., Probes 3, 6, 7, and 10) appear to have lognormal distributions with minor irregularities. The BTCs for Probes 1, 8, and 9, however, have two or more clear peaks (Fig. 3). At the 0.20-m depth, the BTCs have more irregularities; however, Probe 20 does still appear to resemble a lognormal distribution (Fig. 4).
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Fig. 3. Breakthrough curves (BTCs) measured by the fiber optic miniprobes (FOMPs) at the 0.10-m depth for a 20-mm pulse of 4 g/L pyranine. MR = mass recovery
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Fig. 4. Breakthrough curves (BTCs) measured by the fiber optic miniprobes (FOMPs) at the 0.20-m depth for a 20-mm pulse of 4 g/L pyranine. MR = mass recovery
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The concentration signal and time response are different at two depths even with the normalized travel time axis (x axis). The peaks in the BTCs range from 2 to 10 pore volumes at the 0.10-m depth (Fig. 3) and 1 to 5 at the 0.20-m depth (Fig. 4). The average normalized travel time calculated for the BTCs is presented in Table 3. The average travel time to the 0.10-m depth ranges from 15.7 to 1.8 pore volumes, and at 0.20 m this range is 9.2 to 2.4, which is not as great a spread as at 0.10 m. There does appear to be a tendency for those sampling points that record a faster response at 0.10 m to also record tracer sooner at the 0.20-m depth.
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Table 3. The first moment and second centralized moment for the breakthrough curves (BTCs) in unitless time (pore volumes)
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Time Domain Reflectometry System Measurements
The TDR probes performed reasonably well in this clay soil. All probes recorded a cumulative BTC of CaCl2 during calibration (Fig. 5)
and seven of the eight probes successfully measured the transport of the pyranine tracer. One probe at the 0.20-m depth had some mechanical problems and reasonable data from the pyranine pulse were not obtained. It should be noted that the temperature effect can be significant and is not always completely removed by the correction of Heimovaara et al. (1995). This should be considered in studies where diurnal temperature fluctuations are significant, so that soil and air temperature may be measured and an empirical correction performed.
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Fig. 5. Cumulative breakthrough curves (BTCs) of conservative CaCl2 measured during the calibration of the time domain reflectometry (TDR) probes
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Mass recovery at the TDR probes ranged from 54 to 22% at the 0.10-m depth and 73 to 44% at the 0.20-m depth. The BTCs for the four probes at 0.10 m look quite similar (Fig. 6)
, with similar first moments and second centralized moments ranging from 2.1 to 2.8 and 1.3 to 1.5, respectively (Table 3). This demonstrates that although the mass moving over each probe varied, the characteristics of the transport were consistent at the sampling volume of the TDR probes. This result is demonstrated again at 0.20 m (Fig. 7)
, where the first moment and second centralized moment ranged from 1.4 to 1.5 and 0.5 to 0.7.
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Fig. 6. Breakthrough curves (BTCs) measured by the time domain reflectometry (TDR) probes at the 0.10-m depth for a 20-mm pulse of 4 g/L pyranine. MR = mass recovery
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Fig. 7. Breakthrough curves (BTCs) measured by the time domain reflectometry (TDR) probes at the 0.20-m depth for a 20-mm pulse of 4 g/L pyranine. MR = mass recovery
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The mean travel time at 0.20 m was much shorter than at 0.10 m. In fact, the probes at these two depths registered a response at similar times on an absolute scale. These results suggest that the tracer moved slower in the first 0.10 m of the soil and then faster in the next. It is possible that the increase in transport velocity at the 0.20-m depth is the result of tracer funneling into fewer flow pathways. This would lead us to believe that there is a two-phase flow system at least at the greater depth. The shape of the cumulative BTCs in Fig. 5 have a shape that has been suggested by Kutilek and Nielsen (1994)(p. 292) to indicate more than one flow phase, particularly in Probes 7 and 8. While this is only an observational interpretation, we will examine more evidence for the existence of multiple flow phases and the differences in flow with soil depth.
It should be noted that it is possible that the temporal resolution of 10-min sampling intervals is not enough to capture rapid preferential flow. This may be an explanation for the larger mass recovery with depth and an observed average mass recovery for the plot lower than 100%.
Flow Regimes in the Soil Profile
Fluehler et al. (1996) propose three flow regimes, which they call distribution flow in the attractor zone (Zone 1), preferential flow in the transmission zone (Zone 2), and dispersive flow in the dispersion zone (Zone 3). The first flow zone is a relatively uniform spreading of solute, which then converges into preferential flow pathways in the transmission zone (Zone 2). In the transmission zone, a small portion of the total pore space conducts most of the leachate vertically into the third zone, where the soil is less permeable and lateral mixing occurs again.
There is evidence that the soil in this study is responding in a similar manner. A 0.22-m boundary area was included around the active area of the plot in an attempt to produce one-dimensional flow, but it was clearly not completely effective. The tracer was measured with 0.05-m-long TDR probes inserted vertically into the soil surface. We suggest that because the tracer was measured at the 0.05-m depth while there was a 0.10-m distance to the nearest tracer drip source, this is evidence of significant lateral flow. At times, the tracer moved more than 0.10 m laterally before moving 0.10 m vertically in the soil. For example, the timing of both the arrival and median of the tracer BTCs measured at the 0.05-m probes (Fig. 8)
occurs slightly before the arrival and median of the tracer measured at the 0.10-m depth by the 0.20-m-long TDR probes. Thus, the tracer entered the soil and moved laterally 0.10 m to the small TDR probes in a shorter time than it took to move vertically the same distance. It is difficult to interpret the BTCs in Fig. 8 because they include two dimensions of flow; however, they illustrate the significant lateral displacement of solutes near the soil surface. This lateral bypass of the TDR probes and the potentially inadequate sampling resolution (e.g., 10 min) may explain the low mass recoveries and slow travel time measured by the 0.20-m-long TDR probes.
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Fig. 8. Response measured by the 0.05-m-long time domain reflectometry (TDR) probes inserted into the first 50 mm of the soil horizon
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The different flow patterns between Zones 1 and 2 occurring with soil depth are illustrated by the 0.20-m-long TDR probes. Using the data from the pyranine BTCs and the cumulative BTCs, the fraction of the soil water actively participating in transport (st) was estimated as:
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where Jw is the input water flux, tmedian is the median travel time, R is tracer retardation (if any occurs), and L the travel length (White et al., 1986). The st values from the conservative CaCl2 tracer (R = 1) cumulative BTCs at the 0.10-m depth for the TDR were 0.36, 0.33, 0.43, and 0.40 for Probes 1 to 4, while for the 0.20-m depth these values were 0.12, 0.12, 13, and 0.16 for Probes 5 to 8. These values for the pyranine BTCs (R = 1.43) were 1.4, 0.89, 1.2, and 0.98 at the 0.10-m depth and 0.23, 0.22, 0.24 for those probes that measured BTCs at the 0.20-m depth. The fact that two of the pyranine BTC st values are more than 1.0, which is theoretically impossible, may be the result of an R value for the surface 0.10 m of the soil greater than 1.43. It is possible that there is some disparity between the field R value and that measured in the laboratory (Kookana et al., 1992).
Regardless of the R values used in the calculation, there is a consistent relationship between the st values estimated for each soil depth. The first 0.10 m always has a much greater value and at the 0.20-m depth these values were observed to be between 13 and 24% of the pore space available for transport of the tracers. Thus, it appears that the st values identify the transition from Fluehler's Zones 1 and 2 in our plot, occurring somewhere between 0.10 and 0.20 m in the soil. We also suspect that only those FOMPs that happened to be close to the pore space actively participating in transport in Zone 2 intercepted the tracer, while the other FOMPs did not.
The description of the transport zones would explain the lateral flow measured by the 0.05-m TDR, the faster arrival of tracer at the 0.20-m depth, the nonuniform response measured by the FOMPs, and that fewer FOMPs measured the tracer at the 0.20-m depth. There also exists evidence for preferential flow, giving additional proof for the first two described solute flow zones. Consequently, this plot design allows for the examination of the conceptual delineation of the flow zones of Fluehler et al. (1996) to this plot by measuring the lateral flow using the 0.05-cm TDR probes, demonstrating the existence of the preferential flow zone (Zone 2), and with the st values from the 0.20-m TDR probes at the two depths.
Comparison of Probe Type and Sampling Volume
The difference in measurement volume between the FOMP and TDR resulted in different values for travel times and solute spreading. The FOMPs monitored solute transport at a small scale while the TDR collected volume-averaged data. Differences in effective solute dispersion measured at the two scales of the FOMP and TDR probes in a repacked soil column were reported by Campbell et al. (1999). However, in the laboratory a relationship between measurement scale and solute travel time was not observed. The authors suggested that in a natural structured soil where macrodispersion occurs, it would be possible that the average velocity of the solute could also differ at different scales. Scale dependence in solute travel time has been demonstrated recently by Radcliffe et al. (1998). These authors suggest that the scale dependency was the result of hydrodynamic dispersion (macrodispersion).
Our results also demonstrated the scale dependency in solute travel time. The average travel time of all the FOMPs that saw the tracer was 5.3 pore volumes, while the average travel time measured by the TDR probes was 2.0 pore volumes. The second centralized moment also varied between the probes with average values for the FOMP and TDR of 11.1 and 1.1, respectively. In this study, the difference in probe measurements appears to result from the scale dependence of microdispersion and macrodispersion.
The variability was also much greater at the small scale of the FOMP probes. Coefficients of variation (CVs) of µ for the FOMP BTCs was 87% at the 0.10-m depth and 51% at 0.20 m. These values for the second centralized moment were even greater at 143 and 75%, respectively. The CVs of µ for the TDR were 9 and 3% at each of the two soil depths, and for the second centralized moment these values were 5 and 17%. Therefore, this paper demonstrates that in heterogeneous field soils, transport parameters and the spatial variability resulting from soil structure in those parameters from different probe types may be systematically different, probably resulting from the volume of the soil measured.
Flow Phases and Transfer Functions
The BTCs from the TDR probes, which collected a more integrated sample, were fit to a bimodal resident concentration transfer function model (TFM) based on the CLT (Eq. [5]). Utermann et al. (1990) used a bimodal distribution to represent depth distribution of a tracer, while in this case the bimodal distribution will represent the BTC of the vertical flux of solute over time.
Table 2 presents the fitted parameters from use of Eq. [5]. The corresponding fits are provided in Fig. 9
. As described, the first flow phase represents matrix flow and is denoted as subscript one in Table 2. The other more rapid flow phase (subscript two, Table 2), the bimodal distribution measured in the field, was found by fitting model parameters to the TDR data using a sum of squares technique.
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Fig. 9. The bimodal convective lognormal transfer function (CLT) model fitted to the four time domain reflectometry (TDR) probes at the 0.10-m depth in the soil
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Figure 9 demonstrates the fit of the bimodal CLT model to the TDR data for the four TDR probes at the 0.10-m depth. The first phase in this case corresponds to the laboratory column, which is probably matrix flow with a constant µ1 value of 1.59. The second phase is more rapid flow with µ2 values ranging from 0.83 to 0.98 pore volumes. The matrix flow phase appears to correspond with movement of the tracers seen in the FOMP BTCs. Thus, the FOMP measurements correspond to the slower (matrix) flow phase, while TDR measures a combination of that phase and a more rapidly occurring phase. We should note that it is still possible that there is a faster preferential flow phase that was not captured by the 10-min sampling interval of the TDR. However, it is clear that the combined use of TDR and the FOMPs measuring transport at two different sampling scales may be a method to separate flow phases observed to explain transport at larger scales.
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CONCLUSIONS
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This study was the first application of FOMPs to a field soil and they performed well, with a successful calibration for 19 probes and BTCs measured at 13 probes. The FOMPs are a useful tool for observing transport at a small scale in heterogeneous soils. It is suspected that only the FOMPs close to the active transport volume measured any tracer. It was also demonstrated that, at the sampling volume of the FOMPs, there is not a uniform transport response horizontally or vertically, even under the controlled conditions in this study.
The TDR probes worked reasonably well in this clay soil. The existence of preferential flow would explain the low mass recoveries of some TDR probes. Moreover, it is possible that the temporal resolution of 10-min sampling intervals may not be enough to capture rapid preferential flow. The tracer measured by the 0.05-m TDR probes illustrates that significant lateral displacement of solutes may occur in the first 0.05 m of the soil. In combination with estimates of the apparent fraction the soil water participating in transport, two flow zones were identified including (i) a distribution flow zone at the surface that converged solute into (ii) a preferential flow zone beginning somewhere between 0.10 and 0.20 m and continuing deeper into the soil profile.
In this study, both the first and second centralized moment were different between the FOMP and TDR probes, probably resulting from sampling volume. As a result, the combined use of the TDR and FOMPs measuring transport at two different sampling scales may be a method to separate flow phases. Analysis of these phases with transfer functions suggested a relationship between laboratory measurements and transport in the field soil. The combined application of the FOMPs and TDR to observe different flow phases deserves further investigation.
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ACKNOWLEDGMENTS
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Dr. Garrido has been supported by a postdoctoral grant from the Ministerio de Educacion y Cultura (Spain). The authors also wish to thank the Kearney Foundation of Soil Science and the Horton Research Grant administered by the American Geophysical Union for their support.
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INTRODUCTION
