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a Division of Ecosystem Sciences, Dep. of Environmental Science, Policy, and Management, Univ. of California, Berkeley, CA 94720-3110 USA
ghodrati{at}nature.berkeley.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONObjectivesMaterials and methodsNOTESResults and discussionSummaryREFERENCES |
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Abbreviations: BTC, breakthrough curve • SMC, Split Macropore Column • UMCM, uniform matrix–constructed macropore systems
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONObjectivesMaterials and methodsNOTESResults and discussionSummaryREFERENCES |
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Many of the mechanistic studies of macropore flow in soil have been performed in quasi-natural systems such as undisturbed soil columns or columns seeded with earthworms or root systems. These studies have been successful in demonstrating that hydrologically active macropores can be formed by earthworm activity, the degradation of roots, or simply by a continuous network of interpedal zones (Bouma and Anderson, 1977; Booltink and Bouma, 1991; Li and Ghodrati, 1994, 1995). Other studies have focused on specific aspects of macropore flow dynamics such as the effects of tillage, pore continuity, soil surface coverage, precipitation intensity, and antecedent water content (i.e., soil wetting history) (Andreini and Steenhuis, 1990; Shipitalo et al., 1990; Edwards et al., 1993; Granovsky et al., 1994; Shipitalo and Edwards, 1996).
Another group of studies has used artificial systems to evaluate macropore flow processes. These systems include uniformly packed soil matrices containing constructed macropores (Czapar et al., 1992; Ela et al., 1992; Heathman et al., 1995; Li and Ghodrati, 1997), or completely artificial matrix–macropore systems such as glass matrix–macropore systems (Wan et al., 1996), and porous plate–capillary tube systems (Phillips et al., 1989). Artificial systems have been invaluable in testing theories regarding macropore flow in soils. The uniform matrix–constructed macropore systems (UMCM) have shown that bi- and poly-modal breakthrough curves (BTC) can be produced by transport through macropores during miscible displacement experiments. This phenomenon has been seen in several field experiments (Ehlers, 1975; Currie et al., 1979; Jury et al., 1986; Jabro et al., 1991; Flury et al., 1994; Tsuboyama et al., 1994) and undisturbed soil column experiments (Elrick and French, 1966; Cassel et al., 1974; Bouma and Dekker, 1978; Mallants et al., 1994; Ward et al., 1994; Heathman et al., 1995; Li and Ghodrati, 1997).
Several authors have suggested that, in addition to surface connected macropores, subsurface macropores play an important role in the preferential flow of water and solutes (Quisenberry and Phillips, 1976; Li and Ghodrati, 1997). Using their glass micromodel method, Wan et al. (1996) also showed the ability of subsurface macropores to transmit water and solutes. None of these studies were able to quantify mass transport through macropores. However, Li and Ghodrati (1997) noted significant increases in saturated hydraulic conductivity (KS) when macropores were present and that macropore flow initiated below the assumed KS. In contrast, Czapar et al. (1992) did not observe transport through subsurface macropores.
The artificial systems of Phillips et al. (1989) and Wan et al. (1996) have provided several insights to the mechanisms and dynamics of macropore flow. Using their pressure plate/capillary tube system, Dunn and Phillips (1991) showed that the fraction of hydrologically active macropores (i.e., effective macroporosity) is much less than the actual macroporosity of the soil. By the same system, it was also shown that water can move through macropores under a negative matric pressure (Phillips et al., 1989). This work was further strengthened by Wan et al. (1996) who also demonstrated that in their micromodel system, macropore flow occurred under slightly negative matric pressures while the matrix was nearly saturated. These authors also demonstrated the dynamics and continuity of the macropore water film during macropore flow. Their findings support the theory postulated by Quisenberry and Phillips (1976) that a continuous film of water is necessary to initiate macropore flow.
The majority of studies performed in UMCM type systems have noted several limitations. These include macropore configurations which are subject to change, become clogged, or even collapse during the experiment (Czapar et al., 1992; Ela et al., 1992; Heathman et al., 1995; Li and Ghodrati, 1997). This limits the ability to quantify the macropore size, shape, density, etc. Further, because of their "black box" nature, these systems do not permit examination of the interfacial dynamics (such as mass exchange rates) between the matrix and macropore. To investigate the nature of this mass exchange between a macropore and its surrounding matrix, a system must be used which (i) allows the use of a variety of different soil types or porous media, (ii) permits access to the macropore-matrix interface for direct examination of all transport processes which are occurring at that interface, and (iii) assures the integrity of the macropore structure at all water regimes.
To meet these challenges, we have developed a simple system with which to study matrix–macropore interfacial dynamics. The split macropore column (SMC) is a semi-cylindrical column containing a macropore of fixed diameter. The SMC allows one to control a number of important physical parameters such as the macropore size, density, and continuity for displacement studies, and provides an opportunity to visually observe the macropore's impact on flow processes at the critical matrix-macropore interface. Surface runoff can also be included or easily removed—with certainty—from the macropore component. Furthermore, the integrity of the macropore is maintained by a mesh screen and the homogeneity of the matrix flow can be monitored. The boundary conditions of this system are such that water enters the upper boundary of the column at a constant rate; the system is at steady state and no surface runoff into the macropore occurs.
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Objectives |
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TOPABSTRACTINTRODUCTIONObjectivesMaterials and methodsNOTESResults and discussionSummaryREFERENCES |
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Materials and methods |
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TOPABSTRACTINTRODUCTIONObjectivesMaterials and methodsNOTESResults and discussionSummaryREFERENCES |
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To use the SMC for mechanistic studies of macropore flow, the system must satisfy several constraints. Specifically, to be able to build upon the existing database of macropore flow experiments, this semi-cylindrical system must have the same flow characteristics as a typical cylindrical column. As a result, the semi-cylindrical geometry and macropore screen mesh should have no effect on flow through the system or interfere with packing the column homogeneously.
The Split Macropore Column shown in Fig. 1 is based on these principles. It consists of a semi-cylindrical plexiglas tube with a flat plexiglas plate covering its vertical planar surface. The planar surface has a continuous longitudinal slit along its center representing the open planar surface of the semi-cylindrical macropore. To produce the semi-cylindrical boundary of the macropore and to prevent its possible collapse during miscible displacement studies, a 30-mesh stainless steel screen shaped into a semi-cylinder, is attached to the inside of the slit. The relatively large openings of the 30-mesh screen allow easy passage of water and air while preventing collapse of the macropore. The bottom of the Split Macropore Column is formed with the same stainless steel screen mesh producing a matric potential of zero (atmospheric) at the boundary.
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b) was 1.54 ± 0.01 g/cm3 and the volumetric water content (
V) was 0.36 ± 0.02 m3/m3. The Botella clay loam had a bulk density (b) of 1.25 g/cm3 and a volumetric water content (V) of 0.47 m3/m3. Water content was measured gravimetrically before and after each experiment. Percentage saturation for the silica sand and Botella clay loam was calculated to be 86 and 89%, respectively, assuming a particle density of 2.65 g/cm3. Silica sand was chosen for the majority of experiments, specifically the system evaluation, due to its high hydraulic conductivity. In addition, since the silica sand is a light colored soil, movement of the red dye tracer was much easier to observe than in a darker soil, such as the Botella clay loam.
System Evaluation
During the development of this system, several experiments were conducted to test the validity of using this system for studying macropore flow processes. Some of the possible problems we considered in constructing and testing the column were (i) flow instabilities as a result of non uniform column packing; (ii) heterogeneous flow due to sharp edges at the screen mesh/column junction and where the planar and cylindrical surfaces meet (Fig. 1); and (iii) flow along the screen mesh surface rather than as a result of the macropore itself. Four tests were performed on the split macropore column to determine the effects of packing, macropore edge, and macropore screen on preferential flow. Red dye (FD&C red #40) was used as a tracer since its transport is visually observable. In silica sand, the red dye exhibited little retardation and was assumed to be a conservative tracer for the purposes of visual observations. During these experiments, the large/large SMC
was used and the influx rate was set to the saturated hydraulic conductivity of the matrix–macroporous system (JP, see below). The entire macropore screen was sealed from the inside with clear tape. First different packing procedures were tested to determine which procedure produced uniform flow through the column. Development of an appropriate packing method allowed us to test the effects of the macropore edge on the production of preferential flow while the macropore was sealed.
Two experiments were then performed to determine the effect of the macropore screen on macropore flow. The first involved a miscible displacement in a silica sand packed SMC with a detachable screen where the screen was removed after the column had reached a steady state. In the second experiment, the Botella clay loam was used to seal the macropore from outside. The Botella clay loam was chosen because of its relatively fine texture (25% clay) to seal the macropore but to still allow interaction between the sand matrix and macropore screen. First, the clay loam was dry packed into the macropore from the outside of the column, then carefully wetted with a water spray. Finally, transparent tape was applied over the clay loam and surrounding plexiglas to minimize readjustment of the soil during the experiment.
Column Packing
Columns were packed uniformly to a height of 42 cm. Prior to packing, the top 4 cm (from 40–44 cm from the bottom of the column) of macropore screen was sealed from inside (Fig. 1 and 2) . As a result, the top 2 cm of the soil surface did not have an open macropore. This was done to ensure that the surface added water will flow only through the matrix (in this case for 2 cm) before encountering any macropore.
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Tracer Application and Analysis
In addition to the red dye, both nitrate and bromide were used as conservative water tracers. A multi-channel peristaltic pump (ISMATEC, Wallington, England)1
was used to establish a steady flux in the soil columns. The irrigation channels were distributed to approximate a uniform dripping of solution onto the surface of the columns. Four channels were used for the small matrix columns and seven channels were used for the large matrix columns. The columns were leached with a solution of 0.006 M CaCl2 in distilled water (Klute and Dirksen, 1986). The tracer solution was applied as a 1-cm pulse (i.e., tracer pulse volume = column surface area x 1 cm) such that a type III boundary condition was used (van Genuchten and Alves, 1982).
During miscible displacement experiments, the column effluent was continuously sampled and collected with a funnel with a side stem. By tilting the funnel slightly, the side stem acted as a reservoir for the column effluent. Small diameter tygon tubing (0.08 cm) was connected to the stem and a variable speed peristaltic pump continuously removed a fraction of the effluent to be analyzed. For columns containing silica sand, the effluent was then passed through a UV absorbance detector (ISCO, Inc., Lincoln, NE) where the solution was optically analyzed for the tracer. Optical measurements of the effluent were converted to nitrate concentration using a measured linear calibration curve
(Chendorain and Ghodrati, 1999). For the Botella clay loam experiments, bromide was used as a conservative tracer. The effluent was sampled and collected using a fraction collector (ISCO, Inc., Lincoln, NE) and analyzed with a bromide selective probe (Fisher Scientific Company, Pittsburgh, PA). Bromide was used as a tracer in the Botella clay loam because the low background concentration of bromide in the clay loam enhanced the sensitivity for measuring macropore effluent diluted into the total effluent. The initial concentrations of nitrate and bromide were both 10 g/L.
Calculation of Macropore Hydraulic Conductivity (JP) of the Soil–Macropore System
After steady state was reached (when influx equaled outflux), the initial flux rate was increased to the highest flux rate possible before water visually ponded on the surface (JP). Similar to Li and Ghodrati (1997), we have defined this flux rate where ponding began as the approximate saturated hydraulic conductivity of the soil–macropore system.
Macropore Flow Experiments
Experiments were conducted with the SMCs and silica sand to determine under what conditions preferential flow occurs in a single macropore system. Once a steady state had been reached in each column, the JP was determined (Table 1). Then a series of miscible displacement studies were performed with red dye in the large matrix, small macropore column and the large matrix, large macropore column at fractions of JP: approximately 33, 66, and 100% JP. This series of experiments was repeated with nitrate as a conservative water tracer in newly packed columns. A series of experiments was also performed in each of the four columns at JP and at 1-cm ponded water with nitrate as the tracer. A number of miscible displacement experiments were performed with Botella clay loam soil to ensure that the occurrence and/or nature of macropore induced flow observed were not unique to silica sand.
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Results and discussion |
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TOPABSTRACTINTRODUCTIONObjectivesMaterials and methodsNOTESResults and discussionSummaryREFERENCES |
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The effects of the screen mesh were tested in two ways. Using the removable screen SMC, we observed that macropore flow patterns were similar to those seen in columns where the screen was left intact (data not shown). However, removal of the screen always resulted in the eventual collapse of the macropore. When the clay loam soil was packed into the macropore, dye movement through the column was uniform (Fig. 2c). Hence, the screen mesh itself did not produce any significant effect on the initiation and propagation of flow through the macropore. After performing these evaluations and approximately 75 recorded dye transport experiments to test the system, we concluded that the SMC system does not artificially produce preferential flow or interfere with flow produced by the macropore. Further, we concluded that the SMC is an adequate system to study the interactions between a macropore and a homogenous matrix.
Macropore Flow Through a Clay Loam Soil
Miscible displacement experiments performed with the Botella clay loam soil illustrate that the observed macropore induced flow processes are not unique to silica sand (Fig. 3)
. It is clear from this BTC that for a soil with a fine texture, a low hydraulic conductivity, and a complex pore network in its matrix (compared with silica sand), the relative amount of water preferentially transported through the macropore (under steady flux conditions and at JP) represents a larger fraction than transported through the matrix. These findings are in agreement with those of Li and Ghodrati (1994, 1995, 1997) who demonstrated that macropore flow occurred in soils with different textures, and that the relative macropore flow was always largest for the finest textured medium. The mean residence time for the second peak is greater than what would be expected had there been no macropore flow based upon the input flux. Since the total flux is a composite of the matrix and macropore flow, it is reasonable to see a later breakthrough time for the matrix zone which has a smaller relative flux.
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Visual Observations of Macropore Flow
The colored dye miscible displacement experiments illustrate the dependence of macropore flow on flux rate (Fig. 4)
. At the flux rate of JP (24.1 cm/h, Fig. 4a), flow at the macropore/matrix interface was extensive and began as soon as the dye front reached the top of the macropore. By 0.06 pore volumes (V/V0), the dye had traveled the entire length of the macropore and was observed exiting the column. As the dye front moved through the column, dye continued to appear in the macropore effluent. Furthermore, as the dye in the macropore moved downward from its point of entry, dye also entered the matrix directly adjacent to the macropore. This "meandering" of the macropore dye developed into a second zone where its transport was still accelerated relative to the rest of the matrix, but retarded relative to flow at the macropore. Finally, the remainder of the dye plume moved uniformly through the unaffected portion of the matrix.
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From our visual observations, it was clear that macropore flow occurred predominantly at the matrix–macropore interface and in the matrix zone directly adjacent to the macropore. Along this interface, only film flow was observed. Only at higher fluxes, where the system was ponded, was excess flow into the macropore (i.e., running water) observed. This was likely a result of the matrix–macropore interface film increasing in thickness to the point where excess flow became visually apparent. This supports the work of Wan et al. (1996) who showed an increase in fracture film thickness as the magnitude of the matric potential increased. As a result of these observations, we will define macropore flow in our system as flow through the macropore, flow at the macropore/matrix interface, and accelerated matrix flow in the matrix directly adjacent to the macropore.
In the zone of accelerated matrix flow, both diffusion and advective flow contribute to the entrance and movement of the tracer through this zone. As dye moved along the matrix–macropore interface, the presence of a concentration gradient likely drove the dye into the matrix zone. It is interesting that dye diffuses into the matrix only at certain positions along the interface. This may be a result of differences in entrapped air and matric potential along this interface. Also, dye does not appear to move into this transition zone from the surrounding matrix. This is supported by Fig. 4 since, at later times, dye is washed out of the matrix zone adjacent to the macropore. Wan et al. (1996) also observed this transitional zone between the matrix and macropore. They explained that the transport patterns in their glass micromodels depicted a decrease in advective transport and increase in brownian diffusion with increasing distance away from the fracture into the matrix zone.
The BTCs produced in a separately packed column with a nitrate tracer indicated that macropore flow disappeared near 70% JP (18.3 cm/h) (Fig. 5) . Other miscible displacement experiments using nitrate in the same column showed the disappearance of macropore flow to range from 70 to 55% JP (data not shown). One explanation for this difference is that the exact amount of macropore flow will vary between replicates (on the basis of differences in entrapped air, particle arrangements, etc.). However, another possibility is that macropore flow occurred in the BTC experiments at flux rates less than 70% JP but was undetected. Since the nitrate transported by the macropore is significantly diluted by the effluent, its concentration may have produced a signal less than the background light absorbance. Li and Ghodrati (1997) show similar breakthrough experiments where the macropore contribution was a small fraction of the total effluent. This is also one possible explanation of why Czapar et al. (1992) did not measure any bimodal BTCs in their subsurface macropore experiments. If possible, separate collection of macropore effluent and matrix effluent should alleviate this problem.
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A number of miscible displacement experiments were performed in the four columns at fluxes below JP, at JP, and at 1-cm head. We chose to present the 1-cm head experiments since the contribution of macropore flow to the breakthrough curve is enhanced. All the miscible displacement experiments we performed had similar breakthrough curve patterns. However, the relative contributions of each macropore associated breakthrough peak decreased as the amount of macropore flow decreased. Figure 6 suggests that, although there is certainly an intrinsic variability between columns, there were some interesting similarities. The transport patterns in the breakthrough curves at 1-cm head all exhibited at least three peaks. The earliest peak, which had an arrival time ranging from 0.03 to 0.14 pore volumes (average 0.06), corresponded to transport directly through the macropore zone. The final peaks were likely produced by transport through the matrix. The peaks between the initial and final peaks (Fig. 6a) were likely the accelerated matrix flow zones directly adjacent to the macropore. Figure 6 shows that these "middle" peaks had a small but definite contribution to the breakthrough curve. Moreover, mass recoveries for all miscible displacement experiments were complete with an average of 103% recovery and 6% coefficient of variability.
The breakthrough curve patterns in Fig. 6 also exhibit a great deal of variability. It is difficult to determine the source of variability in these flow patterns; however, differences in matric potential, surface roughness, and entrapped air near the matrix–macropore interface may be responsible. These factors have been shown to be influential to macropore flow (Phillips et al., 1989; Edwards et al., 1992; Shipitalo and Edwards, 1996; Wan et al., 1996; Tokunaga and Wan, 1997). However, we were unable to determine significantly their relative influences. Further studies aimed at quantifying these mechanisms would be valuable.
Through qualitative evaluation of these graphs, the relative amount of mass transported as a result of "macropore flow" (i.e., interface and transitional flow) was determined. The amount of mass transported as a result of macropore flow at 1-cm head was 38% of the total mass in the large matrix columns and 50% in the small matrix columns. Macropore transport was 40% in the large macropore columns and 48% in the small macropore columns. Along with the JP data, these comparisons suggest that, in these column sizes, the ratio of the matrix to macropore size may be more influential in determining the quantity of macropore flow than the actual size of the macropore.
During these experiments, only a film of water was visible on the macropore surface. Subsequently, the macropore never became completely saturated. This suggests that for different macropore sizes under similar flux conditions, the absolute quantity of mass transported by the macropore should remain unchanged. Only the relative proportions of mass transport between the matrix and macropore should change. A more direct examination of the influence of macropore size is needed to validate this hypothesis.
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Summary |
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TOPABSTRACTINTRODUCTIONObjectivesMaterials and methodsNOTESResults and discussionSummaryREFERENCES |
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Quantification of macropore flow was made difficult because the macropore effluent was diluted by the matrix effluent. Therefore, future quantitative studies using this system should separate the matrix and macropore effluents. This modification would also allow researchers to separate information regarding transport through the macropore zone and transport through the matrix. Then data could be compared by means of variables such as mass balance, transport time, and breakthrough variance. This experimental system will permit examination of many issues raised by our initial data.
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NOTES |
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TOPABSTRACTINTRODUCTIONObjectivesMaterials and methodsNOTESResults and discussionSummaryREFERENCES |
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Received for publication August 21, 1998.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONObjectivesMaterials and methodsNOTESResults and discussionSummaryREFERENCES |
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