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Published online 27 August 2007
Published in Soil Sci Soc Am J 71:1600-1606 (2007)
DOI: 10.2136/sssaj2006.0359
© 2007 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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SOIL PHYSICS

Experimental Investigation of Direct Connectivity between Macropores and Subsurface Drains during Infiltration

Onur Akaya and Garey A. Foxb,*

a Dep. of Biosystems and Agricultural Engineering, Oklahoma State Univ., 115 Agricultural Hall, Stillwater, OK 74078 and Dep. of Civil Engineering, Univ. of Mississippi
b Dep. of Biosystems and Agricultural Engineering, Oklahoma State Univ., 115 Agricultural Hall, Stillwater, OK 74078

* Corresponding author (garey.fox{at}okstate.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Recent research indicates immediate breakthrough of surface-applied contaminants in subsurface drainage by transport through macropores directly connected to the surface. This "direct connectivity" phenomenon was verified and investigated by conducting infiltration experiments (1-cm ponded water at the soil surface) in a laboratory soil column (sandy loam soil with bulk density of 1.6 g cm–3) with a vertical artificial macropore placed directly above or shifted away from a lateral subsurface drain. The experimental setup allowed surface-connected and buried macropore lengths to be varied from the surface to the subsurface drain depth without unpacking or disturbing the soil column between experiments. It was observed that the longer the buried macropore length (i.e., as the macropore approached the soil surface), the more rapid the response at the drain outlet in addition to an increased percentage of total drain flow through the macropore (35–40%). Breakthrough with surface-connected macropores was significantly faster than with buried macropores, suggesting that breaking surface connectivity of macropores by tillage may be an important management strategy. For surface-connected macropore experiments, the average ratio of steady-state total (macropore and matrix) to matrix flow rates decreased as the distance from the drain increased: 2.4, 2.1, and 1.6 for distances of 0, 6.25, and 12.5 cm, respectively. Extrapolating this data to distances beyond 12.5 cm suggested that macropores located within 20 to 25 cm of the drain act as though directly connected in this sandy loam soil. This research verifies the "contributing area" concept hypothesized in previous field and numerical modeling studies.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Artificial subsurface drainage is an important component of successful agricultural water management in areas with shallow groundwater. Concerns exist, however, about the rapid transport of pesticides (Fox et al., 2004), pathogens (Jamieson et al., 2002; Shipitalo and Gibbs, 2005), and nutrients (Kladivko et al., 1999) from the soil surface to groundwater through macropores. Preferential flow in macropores can lead to rapid transport of surface-applied contaminants to the subsurface (Magesan et al., 1995; Kladivko et al., 1999; Shipitalo and Gibbs, 2000). Wetting fronts propagate to significant depths by bypassing matrix pore space (Brusseau et al., 1992; Kladivko et al., 1999; Castiglione et al., 2003). The influence of macropores increases as soil saturation increases. Therefore, the ability to model the interrelationship between macropore-facilitated contaminant transport and subsurface drainage systems, where soil is consistently near saturation, is important for evaluating potential environmental contamination.

Contaminant fate and transport models can predict chemical fate under numerous hydrologic and soil conditions and agricultural management practices. The cost of performing field research under all of the expected conditions (soils, climate, and chemical applications) is prohibitive. Generally, the bimodal pore structure of the soil can be approximated by a two-domain flow regime model. The two domains include a highly permeable macropore zone and a low-permeability or impermeable soil matrix zone (Ahuja et al., 1995; Villholth and Jensen, 1998; Nieber, 2001). Solutes and contaminants are transported within the two flow domains based on convective and dispersive transport. Mass transfer between the regimes is assumed to be due to water flow and solute diffusion as described by a first-order mass transfer equation. Transfer is characterized by a single parameter called the transfer rate coefficient, which is a function of the geometry and dimensions of the macropore structure (Villholth and Jensen, 1998; Villholth et al., 1998).

The difficulty in developing mathematical formulations of flow and transport processes in such systems is the inherent heterogeneous soil and macropore properties. Questions exist as to whether two-domain, mass-transfer-based models appropriately capture the physical processes within these heterogeneous systems. Exchange processes between macropore and soil matrix domains may be rate limited, constraining total and instantaneous mixing within the soil matrix. Rate-limited solute transfer can lead to nonideal transport (Hodges and Johnson, 1987). More sophisticated diffusion, mobile–immobile representations may be required. The drawback is that models that are more elaborate require a greater number of parameters that may or may not be readily measurable (Villholth and Jensen, 1998).

Additional difficulties arise when applying models to systems with subsurface drainage (Haria et al., 1994; Kladivko et al., 1999; Lennartz et al., 1999; Paasonen-Kivekas et al., 1999). Recent research indicates immediate breakthrough of surface-applied solutes and contaminants in subsurface drainage by extraordinarily efficient transport through directly connected macropores (Villholth et al., 1998; Fox et al., 2004). Shipitalo and Gibbs (2000) observed macropores created by a deep-burrowing (anecic) species of earthworm in a silt loam soil that allow water to transfer directly to subsurface drains. Their study included the use of smoke injected into drain lines to observe transmission to the soil surface; they then grouped the macropores as smoke-emitting and non-smoke-emitting macropores. Smoke-emitting macropores were located within 50 cm of the drain line and the distance from the subsurface drains correlated with infiltration rate. The rate at which water entered earthworm burrows declined with the log of distance from the drain tile. The field observations of Shipitalo and Gibbs (2000) documented numerous macropores reaching the subsurface drain depth with fairly uniform shape (average diameter, 7.5 mm) in the vicinity of the drain. The plastic replicas of the burrows revealed that these macropores were as close as 2 cm from the tile.

Fox et al. (2004, 2007) modified a pesticide transport model, the Root Zone Water Quality Model (RZWQM), to include direct connectivity by routing a user-specified express proportion of water and chemicals in macropore flow directly from the soil surface to the drains. They used an estimated 2% express fraction based on the assumption that directly connected macropores are located within 25 cm of the drain (10 m total distance between subsurface drain lines) to model a field site with silty clay to loam soils. The modified model more appropriately captured the immediate breakthrough of pesticides during rainfall events shortly after pesticide application (Fox et al., 2004, 2007).

This laboratory study had two main objectives: (i) to quantify the flow components in matrix and macropore domains for different macropore lengths changing from zero (no macropore) to surface (full macropore open) without unpacking or repacking of the soil column, thus maintaining the same soil structure, and (ii) to investigate the importance of direct drain connectivity of open-surface and buried macropores on subsurface drainage in terms of breakthrough times and total flow. This research also aimed to verify field and modeling observations regarding the potential of macropores to be directly connected to subsurface drain lines. A relationship was derived between the ratio of steady-state discharge for experiments with macropores vs. matrix flow experiments as a function of distance from the subsurface drain.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
A box-shaped plexiglas soil column (Fig. 1 ) with dimensions of 50 by 28 by 95 cm was used in the flow experiments. A 6-cm-diameter perforated polyvinyl chloride subsurface drain tube was installed over the impervious bottom on the center axis of the column for the primary purpose of simulating a 0-cm pressure head subsurface drainage boundary condition. Pencil-size tensiometers (bubbling pressure = 100 cm H2O, Soil Measurement Systems, Tucson, AZ) were installed at 10, 40, and 70 cm above the drain to measure soil pore-water pressure heads. All tensiometers were equipped with pressure transducers (ASDX001, Invensys, Milpitas, CA). A datalogger (CR10X, Campbell Scientific, Logan, UT) continuously received and transmitted information to a computer for automated pressure monitoring. Digital scales (EK-12Ki, A&D, Milpitas, CA) recorded the outflow from the soil matrix for shifted and centered surface-connected macropore experiments and from both the macropore (discharge from the tube connected to the bottom of the macropore) and soil matrix (collected from the subsurface drain tube) in centered buried macropore experiments.


Figure 1
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  		Fig. 1. (a) Experimental setup and (b) closeups showing the direct connection of the macropore and the subsurface drain.

 
All of the experiments were conducted using a sandy loam soil (Table 1) collected from the upper soil profile (10–40 cm) of an agricultural field near Little Topashaw Creek, Chickasaw County, Mississippi. To allow a ponded surface boundary condition, the distance between the subsurface drain and the packed soil surface was set to 75 cm. Before packing the column, a solid wooden rod wrapped first with a layer of Al mesh (2 mm) and then a nylon mesh (0.1 mm) was placed vertically in the center of the column and inserted into a hole in the side of the subsurface drain (Fig. 1). Retracting the rod to various levels within the mesh that remained in place enabled the macropore to be intentionally clogged to various degrees to quantify drainage under variable macropore conditions. The Al mesh provided stability of the artificial macropore after removing the rod, while the nylon mesh prevented soil particles from backfilling and clogging the macropore during experiments (Fig. 2 ). This configuration resulted in a macropore diameter of 1 cm. The macropore construction allowed us to perform numerous experiments without disturbing the soil matrix structure; hence the soil properties, e.g., hydraulic conductivity and porosity, remained the same from experiment to experiment. Maintaining soil property consistency is difficult when repacking soil columns. A plastic tube was connected to the macropore at the subsurface drain. This tube allowed differential monitoring of the preferential flow in the macropore vs. matrix flow into the subsurface drain.


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  		Table 1. Field-measured soil properties of the sandy loam soil used in the macropore–subsurface drain column experiments (data from Fox et al., 2006).

 

Figure 2
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  		Fig. 2. The artificial macropore: (a) different buried macropore lengths obtained by pulling the rod out incrementally and (b) the structure of the macropore.

 
Two types of flow experiments were performed to investigate macropore–matrix interactions: centered and shifted macropores (Fig. 3 ). The centered macropore was placed directly above the subsurface drain and with direct connection as described above, whereas the shifted macropore was placed 6.25 or 12.5 cm away from the subsurface drain but ending at the level of the drain. For both sets of experiments, the column was packed in 2.5-cm increments to obtain a dry bulk density of 1.6 g cm–3. The surface boundary condition was 1-cm ponded water for 3 h for centered and 2 h for shifted macropore experiments. Water input was terminated after these times but data collection (soil pore-water pressures, macropore outflow, and drain outflow) continued for 24 h. A ponded boundary condition was obtained with a mariotte-type infiltrometer supplying distilled water containing 0.55 g L–1 CaCl2 and 0.2 g L–1 Thymol to prevent dispersion of soil particles and microbial growth within pore spaces.


Figure 3
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  		Fig. 3. Laboratory soil column with (a) centered and (b) shifted macropores. Dots represent locations of tensiometers.

 
For both centered and shifted macropore experiments, two types of macropores were simulated: buried and surface connected. Buried macropores had no connection to the soil surface; however, they could have direct connection to the subsurface drains depending on their location with reference to the drain. For example, the centered buried macropore had direct connection to the drain, whereas the shifted buried macropore had no direct connection to either the soil surface or the drain. Various buried macropore lengths were achieved without unpacking the soil by gently extracting the rod from the column (Fig. 4a ). For buried macropore experiments, ponded water at the soil surface had to flow through the soil matrix before entering the macropore. The first infiltration test of centered and shifted buried macropore experiments was a zero-length buried macropore, i.e., no macropore effect. This infiltration experiment quantified matrix flow to the subsurface drain. Then, a set of four infiltration experiments (buried macropore lengths of 20, 40, 60, and 72 cm) was performed for centered and shifted macropores.


Figure 4
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  		Fig. 4. The sequence of laboratory soil column experiments for (a) buried macropores and (b) surface-connected macropores. Dots represent locations of tensiometers.

 
The second set of experiments consisted of infiltration experiments with open-surface, or surface-connected, macropores; hence, ponded water at the soil surface was allowed to enter the macropore. In this second set of experiments, the length of the macropore was adjusted by packing the macropore to a specified height with the sandy loam soil used for the soil matrix (Fig. 4b). The first run of this experimental set for centered and shifted macropores consisted of a fully open macropore. For the centered macropore, this resulted in a direct connection to both the soil surface and the subsurface drain. Since ponded water was allowed to directly enter into the macropore at the soil surface, an immediate outflow occurred from the macropore domain, as expected. To prevent immediate outflow for the centered macropore, the macropore tube was closed at the bottom, forcing the flow to diffuse back into the soil matrix before exiting from the subsurface drain. This allowed us to evaluate the maximum diffusion that could take place from the macropore to the soil matrix by comparing the results with the matrix flow experiment. Additional infiltration experiments for centered and shifted macropores consisted of open surface macropores ranging in length from 15 to 75 cm formed by packing sandy loam soil at the bottom of the open macropores.

To compare the results from both sets of infiltration experiments, near-hydrostatic soil water content in the soil profile was ensured before beginning each experiment. Therefore, the soil column was allowed to drain until the readings of soil pore-water pressure heads at the top (T1–4, 70 cm above the drain), middle (T5–8, 40 cm above the drain), and bottom (T9–12, 10 cm above the drain) tensiometers reached values of approximately –70, –40, and –20 cm of H2O, which corresponded to near-hydrostatic initial water content conditions.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Based on steady-state infiltration measurements, the packing resulted in a soil column with a saturated hydraulic conductivity, Ks, of approximately 1.6 cm h–1 (CV = 12.8% based on seven experiments with various packings). Figure 5 presents measurements of cumulative outflow for matrix flow (no macropore present and wooden rod inserted into the drain) and for various centered buried macropore lengths along with their associated matrix and macropore flow components. Matrix and macropore outflow were expressed as a percentage of the total drain flow after 24 h. Total outflow (summation of macropore and matrix components) for centered buried macropore experiments was the same as in the no-buried-macropore (matrix flow) experiment (Fig. 5). Breakthrough times were also only slightly faster for buried macropores than matrix flow experiments and occurred approximately 120 min after initiation of ponding.


Figure 5
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  		Fig. 5. Breakthrough curves for matrix flow and centered buried macropore experiments.

 
The portion of the total flow diverted into the macropore increased as the length of buried macropore increased (approximately 35% for 20- and 40-cm buried macropore lengths and approximately 40% for 60- and 72-cm buried macropore lengths); however, the increase was not linearly proportional to the increase in the buried macropore length. Even with a 300% increase in buried macropore length when passing from 20 to 60 cm, the portion of the macropore outflow increased only about 5%. This suggested that the interaction between the macropore and matrix domains was mostly restricted to the soil profile near the subsurface drain. The pore-water pressure data supports this conclusion in that only the bottom tensiometers (T9–12, 10 cm above the drain) reached positive soil pore-water pressures when the soil matrix reached steady state, promoting a mass transfer from matrix to macropore in that region (Fig. 6 ). Uniformity of the wave front arrivals among different buried macropore lengths suggests that the sensitivity of the matrix domain to these changes were minimal.


Figure 6
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  		Fig. 6. Soil pore-water pressure data from matrix flow and centered buried macropore flow experiments (T1–4, T5–8, and T9–12 are tensiometers located 70, 40, and 10 cm above the drain, respectively).

 
Best management practices aimed at removing the direct connection between macropores and the soil surface, such as surface tillage to disrupt the continuity of macropores, will delay breakthrough times and discharge by converting surface-connected macropores to buried macropores. It should still be noted, however, that as much as 35 to 40% of the total flow may be diverted through the remaining buried macropores after soil pore-water pressure buildup in the soil matrix, as observed by the bottom tensiometers (T9–12 in Fig. 6).

Before conducting the centered open-surface macropores, the rod was inserted back into the subsurface drain to verify the original matrix flow experiment. This experiment also verified that no preferential flow occurred along the side wall of the column or macropore as a result of repeated experiments. The total outflow as well as breakthrough times for matrix flow remained unchanged. Minimal differences were hypothesized to be the result of reduced conductivity by surface crusting with time.

The centered surface-connected macropore experiments consisted of two different lengths: 75 and 50 cm from the ground surface. The initial and boundary conditions remained the same except that ponded water was allowed to enter directly into the macropore, since it was surface connected. Figure 7 presents the outflow from the subsurface drain for these two experiments, along with the outflow from the original matrix flow experiment. As a result of the surface-connected macropore, the steady-state infiltration rate for the soil column with a 75-cm macropore increased approximately five times compared with the infiltration rate of the matrix flow experiment without a macropore. The sensitivity of the matrix flow infiltration rate and breakthrough time to changes in macropore length was greater with open-surface macropores than with buried macropores. Due to lower breakthrough times and the magnified diffusion from macropore to matrix domain, the surface-connected macropore experiments reached steady state earlier (15–30 min) than the buried macropore experiments (120 min). While the bottom tensiometers (T9–12) reached positive soil pore-water pressures 240 min after the start of the infiltration test for buried macropores, tensiometers reached positive pore-water pressure heads in 15 min for surface-connected macropores (Fig. 8 ).


Figure 7
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  		Fig. 7. Breakthrough curves expressed as the ratio of total drain flow to drain flow from the matrix for centered surface-connected macropore experiments.

 

Figure 8
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  		Fig. 8. Soil pore-water pressure data from centered surface-connected macropore flow experiments (T1–4, T5–8, and T9–12 are tensiometers located 70, 40, and 10 cm above the drain, respectively).

 
The shifted macropore setup followed the same infiltration experiments as the centered macropore. Also, the infiltration rate was controlled by the matrix domain above the buried macropore as in the experiments of centered buried macropores. The breakthrough curves had almost no sensitivity to the changes in shifted buried macropore length. Conversely, surface-connected macropores showed an increase in the infiltration rate and a decrease in breakthrough times. The total flow for shifted surface connected macropores decreased, however, compared with centered macropores of the same length.

For example, for macropores located 12.5 cm away from the subsurface drain, the average ratio of steady-state total discharge (matrix and macropore) to steady-state matrix discharge was 1.1, 1.2, and 1.6 for surface-connected macropores of lengths 15, 35, and 55 cm, respectively. Therefore, the length of surface-connected macropores is important for drain connectivity. Field work by Shipitalo et al. (2004) indicated that macropores can extend the entire distance between the ground surface and a subsurface drain. These results directly impact recent suggestions to decrease the depth of subsurface drains for increased residence time in the saturated zone and therefore contaminant removal (Davis et al., 2000; Skaggs and Chescheir 2003; Burchell et al., 2005; Sands et al., 2006). Moving subsurface drains closer to the surface may promote direct macropore connectivity.

For three replicate 55-cm surface-connected macropore experiments with a single macropore shifted varying distances from the subsurface drain, the ratio of steady-state discharge between total flow (macropore and matrix) and matrix flow decreased from 2.4 (CV = 18.7% based on three experiments) for a centered macropore to 2.1 (CV = 15.5% based on three experiments) for a 6.25-cm shifted macropore to 1.6 (CV = 15.4% based on three experiments) for a 12.5-cm shifted macropore. If one increases the length of surface-connected macropores (i.e., 75 cm), results are fairly equivalent to the results for the 55-cm surface-connected macropores. As the length of surface-connected macropores decreased, the ratio of total flow to matrix steady-state discharge approached unity for all distances from the subsurface drain, indicating no effect of the macropore.

The total outflow ratios for both the centered and shifted 55-cm surface-connected macropores were plotted as a function of distance away from the subsurface drain (Fig. 9a ). A linear relationship (slope = –0.06 cm–1, intercept = 2.4, and R2 = 0.98) was assumed between the ratio and the distance from the subsurface drain, which indicated an effective distance (contributing area) of 20 to 25 cm. This distance is similar to that observed by Shipitalo and Gibbs (2000), especially if one considers the infiltration rates of smoke-emitting macropores (i.e., those in physical connection with the drain) vs. non-smoke-emitting macropores (Fig. 9b). Smoke-emitting macropores within 20 cm of the drain line (average infiltration rate of 164 mL min–1 with a CV = 58%) possessed infiltration rates higher than the average plus one standard deviation of infiltration rates for non-smoke-emitting macropores (average of 37.9 mL min–1 with a CV = 84%). Smoke-emitting macropores between 20 and 50 cm possessed an average infiltration rate of 82.6 mL min–1 (CV = 54%). The laboratory-measured value also matched the proposed express fraction (2% or 20 to 25 cm of the 10-m total distance between subsurface drain lines) used by Fox et al. (2004, 2007) for modeling pesticide transport through directly connected macropores in loam to clay soils.


Figure 9
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  		Fig. 9. (a) Ratio of total (macropore and matrix) to matrix steady-state discharge for 55-cm surface-connected macropores shifted at three distances from the subsurface drain (ratio of unity suggests no effect of the macropore and errors bars represent one standard deviation from triplicate experiments) and (b) field data of Shipitalo and Gibbs (2000) indicating the infiltration rate of smoke-emitting macropores.

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The ability to conduct buried and open-surface macropore flow experiments without the need for disturbing the soil matrix provided valuable insight into macropore–soil matrix interactions with the presence of a subsurface drain. The experiments demonstrated that buried macropores may play a role in flow and contaminant transport by diverting as much as 40% of matrix flow when directly connected to subsurface drains and after buildup of soil pore-water pressure. No direct short-circuiting occurred with buried macropores. Open-surface macropores do not require direct connectivity with subsurface drains for rapid drain flow response. Considering the rapid breakthrough times of the surface-connected macropores, macropore flow is critical for surface-applied contaminants. Solute applications incorporated directly into the soil matrix should be considered as a best management practice along with tillage operations that convert surface-connected macropores to buried macropores, thereby removing macropore continuity. Best management practices that suggest moving subsurface drains closer to the soil surface may promote increased direct connectivity. Experiments with shifted macropores were able to represent field conditions regarding the decrease in the infiltration rate as the lateral distance between the macropore and the drain increases, suggesting an effective distance or contributing area of 20 to 25 cm for directly connected macropores in the sandy loam soil used in the experiments. These results verify assumed express fractions used in pesticide fate and transport modeling of directly connected macropores.


   ACKNOWLEDGMENTS
 
This project has been supported by the USDA CSREES NRI Seed Grant (2004-35102-14890). We thank Glenn Wilson (Soil Scientist/Hydrologist, USDA-ARS National Sedimentation Lab., Oxford, MS) for assistance in experimental column design, setup, and operation, and Martin Shipitalo (Soil Scientist, USDA-ARS North Appalachian Experimental Watershed, Coshocton, OH) for research discussions on the direct connection between macropores and subsurface drains. We also acknowledge Gene Walker (Technician, Univ. of Mississippi, Oxford) for column construction and laboratory support.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication October 17, 2006.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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