Published in Soil Sci. Soc. Am. J. 68:744-749 (2004).
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
DIVISION S-1—SOIL PHYSICS
Passive Pan Sampler for Vadose Zone Leachate Collection
A. Rahman Barzegar*,a,b,
Stephen J. Herbertb,
A. Masoud Hashemib and
C. S. Hub
a Dep. of Soil Science, College of Agriculture, Shahid Chamran University, Ahwaz, Iran
b Ins. of Genetics and Development Biology, Chinese Academy of Sciences, Shijiazhuang, China
* Corresponding author (barzegar{at}pssci.umass.edu).
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ABSTRACT
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Sampling agrochemicals in the vadose zone provides an early warning system for ground water pollution. However, accurate in situ determination of agrochemicals in the vadose zone is often not feasible because of lack of appropriate instrumentation. This paper presents a new passive soil water sampler to effectively collect vadose zone leachate. The passive pan sampler (PPS), and a suction cup lysimeter (SCL) were positioned in soil columns. Two soil types were used. Vacuums applied to both PPS and SCL were adjusted to tensiometer readings in each soil column. Measurements included volumetric soil water content by time domain reflectometry (TDR), soil water potentials by tensiometers at three positions in each soil column, and the cumulative leachate collection (CLC) for the PPS, SCL, and free drainage (FD). Measurements were made at two initial soil water potentials of less than –10 kPa and –20 to –25 kPa, referred to high (HSWP) and low (LSWP) soil water potentials. Results indicated that the PPS performed well in both soil types irrespective of the initial soil water potential. Leachate collected by SCLs in the soils was lower than that collected by PPSs. The maximum CLC for silt loam soil ranged from 9.6 to 10.0 mL cm–2 at both water potentials for PPS, 1.6 to 2.1 mL cm–2 at both water potentials for SCL, and 0 to 2.0 mL cm–2 at HSWP for FD and 0 to 0.7 mL cm–2 at LSWP for FD, respectively. The corresponding values for the sandy loam soil were 8.0, 2.7, and 5.2 mL cm–2, for PPS, SCL, and FD, respectively. The leachate collection efficiencies of PPS at HSWP (98%) and LSWP (99%) were significantly (P < 0.01) higher than for SCL at HSWP (59%) and LSWP (31%), indicating greater performance of the PPS.
Abbreviations: CLC, cumulative leachate collection • FD, free drainage • HSWP, high initial soil water potential • LSWP, low initial soil water potential • PPS, passive pan sampler • RLC, relative leachate collection • SCL, suction cup lysimeter • TDR, time domain reflectometry
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INTRODUCTION
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TO BETTER UNDERSTAND leaching mechanisms of pesticides and nutrients, especially nitrate, we need reliable monitoring techniques. Sampling soil solution for dissolved pollutants in the vadose zone is a difficult task, especially in the presence of preferential flow paths. Although many soil solution-sampling methods are available, no single sampling technique has been proven to be suitable under all conditions. Flury (1996) recommended the need for improved optimal water sampling strategies after reviewing research conducted on transport of pesticides at the field scale.
Various methods are being used to collect leachate to evaluate the fluxes of pollutants to ground water. Sampling tile water is convenient in areas that have tile drainage, but it may underestimate the flow rate (Bergstrom, 1987). Tile effluent is considered to be equivalent to ground water recharge. However, leachate can bypass the drainage lines, and in some cases the drain-tile effluent includes ground water (Kladivko et al., 1991). Soil coring is another method for the leachate collection (Rhoades, 1982). However, soil coring is destructive and labor intensive, limiting its practical application.
Suction-cup lysimeters, first described as artificial roots by Briggs and McCall (1904), are typically used and generally accepted for sampling soil solutions despite some disadvantages (Barbee and Brown, 1986; Litaor, 1988). The porous ceramic SCL has a small contact area with the soil and needs a source of vacuum for sample collection (Grossmann and Udluft, 1991). Comparing SCL with soil extraction methods, Wiesler and Horst (1993) suggested that SCL represents mobile water/nitrate, whereas soil extracts additionally represent less mobile water/nitrate. While the SCL can collect soil solution at a wider range of soil tensions (Barbee and Brown, 1986; Litaor, 1988), it may often miss the critical solute pulse and cannot measure macropore flow and the sampled and flux data are unknown. Further, the vacuum extraction methods, SCL (Daliparthy et al., 1994) and microcup sampler (Hagedorn et al., 1999) have additional difficulties such as unknown volume of soil being sampled when acquiring leachate as soil water potential decreases, and sometimes nonrepresentative sampling of pore sizes (Lord and Shepherd, 1993).
Zero-tension (or pan) samplers collect gravitational water and require that the soil above the samplers be saturated during collection (Barbee and Brown, 1986). They generally have low collection efficiencies ranging from 10 to 58%, as defined by the ratio of observed to expected percolation (Jemison and Fox, 1992; Zhu et al., 2002). To obtain samples, zero-tension samplers require that the soil-pore water above the device attain a water potential greater than or equal to zero. Mostly soil water potentials are less than zero (unsaturated) resulting in flow away from the sampler due to local gradients in matrix potential (Jemison and Fox, 1992). Monolith lysimeters (Saffigna and Keeney, 1977) allow determination of water fluxes. The lysimeter wall, however, can create preferential flow paths (Cameron et al., 1979).
Wick-pan lysimeters have more recently been introduced and use fiberglass wicks to apply capillary suction (Brown et al., 1986; Knutson and Selker, 1994; Steenhuis et al., 1998; Zhu et al., 2002). Boll et al. (1991) evaluated the performance of porous cup samplers, gravity driven pan (zero-tension) lysimeters, and multi-wick-pan lysimeters. Both pan and wick-pan samplers performed well in structured clay soils with dominant preferential flow, and in some sandy soils. However, wick-pan samplers have not performed well in nonstructured loamy soils in which water moves mainly through the soil matrix (Steenhuis et al., 1998). Much of the initial evaluation of wick-pan lysimeters was with intense simulated rainfall events (Boll et al., 1991). When Steenhuis et al. (1998) installed 40 wick-pan samplers (lysimeters) in sandy loam soils they did not work efficiently. Only in early spring, after heavy rainfall, 5 out of 40 samplers collected water. Since the suction created by wick is lower than –5 kPa (Boll et al., 1992) and field capacity is generally close to –30 kPa, part of the leachate cannot be collected by wick-pan sampler.
Because of the difficulties with the aforementioned samplers we contend there is a pressing need to develop and test a soil water collection system that mimics the movement of soil water through the vadose zone, collecting macro-, meso-, and micropore soil water. In this paper, we present a design of a new passive soil water sampler for vadose zone leachate collection. This uniquely designed sampler was tested in two different soil textures at two initial soil water potentials, and compared with the SCL.
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MATERIALS AND METHODS
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Soil Samples
Two soil samples were collected from Agronomy Research Farm in Deerfield, MA, where earlier soil water sampling studies with SCL and wick-pan samplers had been conducted (Daliparthy et al., 1994; Steenhuis et al., 1998). The soil was an Occum fine sandy loam variant (coarse-loamy, mixed, superactive, mesic Fluventic Dystrudepts) low in organic matter content (Daliparthy et al., 1994). Soil samples were taken from 400- to 1000-mm depth of one plot (Soil 1) and from 0 to 400 mm of the other plot (Soil 2). Particle-size distribution of the soils was determined by pipette method (Gee and Bauder, 1986). The sand, silt, and clay contents of Soil 1 were 18, 69, and 14, and for Soil 2 were 45, 53, and 2%, respectively.
Experimental Procedure
Subsequently six soil columns (three replicates per soil type) with a diameter of 340 mm and height of 450 mm were used. The PPS was installed in one side of each soil column, 150 mm from the bottom to allow collection of leachate at the base of the sampler (Fig. 1)
. Three tensiometers were positioned at three depths of 25 and 50 mm above the PPS, and 25 mm below the PPS in soil around the PPS (T1, T2, and T3, respectively). Volumetric soil water content was measured using TDR (Mesa Systems, Medfield, MA). Time domain reflectometry probes were installed horizontally 35 mm above the PPS (similar in design to that of Heimovaara and Bouten [1990] and Vogeler et al. [2000]). Three holes were made on the wall of each cylinder for TDR probes. The TDR probes consisted of three 175-mm long stainless steel rods, 8 mm in diameter, and were spaced 35 mm apart within each three-rod set. The TDR probes were used to monitor water content above the PPSs. A SCL (Soil Moisture Equipment, Sanata Barbara, CA, Model 1990) was positioned vertically in each soil column 50 mm away from the PPS and at the same depth of the PPS position and 200 mm from the TDR probe center. The SCL consisted of a ceramic cup having an outside diameter of 48 mm, connected to a 500-mm long PVC tube with a rubber stopper. Plastic tubing through the stopper were used for applying the vacuum to the sampler, and for retrieval of the soil solution.
Each soil column was placed on a 0.5-mm sieve to allow FD and regular sampling of effluent (Fig. 1). The air-dried soils were packed into the columns to the field bulk density of 1.2 Mg m–3 and depth of soil above the PPS was 300 mm. The soils were put through four wetting and drying cycles. The wetting and drying cycles were used to settle the soils before commencement of the experiment. All soil columns were allowed to drain. Two initial soil water potentials of less than –10 kPa (HSWP) and –20 to –25 kPa (LSWP) were established. At each initial soil water potential, 65 mm of water was added three times to the top of each soil column. The PPS uses tensiometers to measure the soil water potential at the depth of the sampling device. The soil water potential is then used to determine the level of suction from the vacuum reservoir, so that the tension at the surface of the plate is equated to the measured soil water potential. In this way, soil water is not drawn to the sampler by the force of a vacuum as is usual in SCL but rather enters the sampler passively.
Vacuum on both samplers was adjusted to the average soil matric potential determined from the T1 and T2 tensiometers. Leachate was collected every 15 min in the first hour and then after every 30 min. The amounts of leachate collected by PPS, SCL, and FD water were measured. Soil water potentials and volumetric water contents from TDR were also determined simultaneously with leachate collection.
Cumulative leachate collection of the PPS and SCL was determined. The collection surface area of the PPS and SCL were measured and CLC per square centimeter for each device was then calculated (Eq. [1]). This was based on the assumption that the PPSs collected water from the volume of the soil above them, and SCLs collected water from the soil surrounding the surface area of the cup.
 | [1] |
where, CLC equals cumulative leachate collection (mL cm–2), Wi equals leachate collected at the time ti, and S equals surface area of each device. Leachate collection efficiency for each device was defined as the Relative Leachate Collection (RLC), which was calculated by dividing the cumulative leachate by the maximum expected volume of water (V). The V was calculated based on the initial soil water content measured by TDR, the amount of added water, and the TDR reading at time ti.
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Statistical Analysis
Analysis of variance was performed using the SAS statistical package (SAS Institute, 1988). The maximum leachate collection, water potentials and leachate collection efficiencies were compared using the MIXED procedure. The MIXED procedure in SAS (Littell et al., 1996) was also used to determine the nature of any differences between sampling measurement methods. To account for the repeated measures nature of the data, the REPEATED statement was included using a compound symmetric covariance structure (Littell et al., 1998). When a significant (P < 0.05) overall F value was found, the SLICE option was used to test the simple main effects.
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RESULTS AND DISCUSSION
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Preliminary results of the CLC indicated that the PPS collected leachate effectively. The vacuum applied was very close to soil matric potentials measured by tensiometers positioned close to the PPS (Fig. 2)
. Changes in the tensiometer readings and the applied vacuums indicated a consistent trend of changes throughout the experiment (Fig. 2). Statistical analysis indicated that the effects of soil type and time of reading were significant (Table 1). Addition of water to columns of LSWP treatment initially, increased the water potential to similar level to that of HSWP within the first 100 min. Subsequently, tensiometer readings increased as the soils dried out. The results of statistical analysis with the repeated statement indicated that the differences between the applied vacuums and the tensiometers reading were not significant; indicating a similar trend of change in the soil water potentials and the applied vacuums was maintained.
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Fig. 2. Changes in matric potential measured by tensiometers and applied vacuum at low (LSWP) and high (HSWP) soil water potentials. T = tensiometer, and denotes stand for the tensiometer number.
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Table 1. Statistical analysis of the difference between applied vacuum and the matric potential (kPa) measured by tensiometers.
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In the sandy loam soil, the applied water infiltrated rapidly during the LSWP experiment. The soil surface was free of water after 3 to 6 min. For silt loam soil the applied water infiltrated after 6 to 9 min. The SCL started to collect leachate in the soils after 50 min, and ceased after 120 min in sandy loam soil and 180 min in silt loam soil. The PPS started to collect leachate in two soil types after 15 min, and leachate collection continued until the end of the experiment (Fig. 3)
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Fig. 3. The cumulative leachate collection (CLC) of the passive pan sampler, suction cup lysimeter, and the free drainage measured at low initial soil water potential (LSWP). Bars indicate the standard deviation of means.
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Cumulative water volumes collected by different devices in both soil types followed a similar trend. The CLC was highest for the PPS (Fig. 3). The CLC for PPS in silt loam soil was greater than the sandy loam soil. This reflects higher water content in silt loam soil than sandy loam soil at a given matric potential as indicated by TDR readings. The statistical analysis indicated that both samplers had highly significant differences in the CLC values (Table 2). The soil water samplers, soil types, and time of leachate collection were significantly different. Cumulative leachate collection was significantly (P < 0.01) higher under PPS than SCL, indicating the greater performance of the PPS for leachate collections in the two soils. However, the results of statistical analysis with repeated statement showed that the CLC of both PPS and SCL was not significantly different for silt loam at <15 min and for sandy loam at <25 min in LSWP treatment. The corresponding values for HSWP treatment were <8 and <15 min. The calculated RLC (outflow volume to added water volume) for PPS in silt loam and sandy loam soil were 94 and 78%, respectively. However, the SCL had the lowest RLC ranging from 31 up to 59% in the soils. Results of statistical analysis indicated that the effects of sampler types and soil types on RLC were significant, whereas the effects of soil water potential treatments and the interactions between soil water potential, sampler type, soil type and time on RLC were not significant (Table 2).
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Table 2. Statistical analysis of the cumulative leachate collection (CLC) and relative leachate collection (RLC).
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The CLC measured at HSWP is shown in Fig. 4
. The CLC at HSWP for PPS was similar to that of LSWP. Suction cup lysimeter collected leachate more effectively in silt loam soil than in the sandy loam soil at LSWP, which indicates that SCL may have missed some preferential flows. The results are consistent with that of Litaor (1988) and Steenhuis et al. (1995) who reported a loss of attracting leachate by SCL possibly due to inability to collect preferential flow. Among disadvantages of SCL are that they may distort streamlines, and may not collect bypass flow.
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Fig. 4. The cumulative leachate collection (CLC) of the passive pan sampler, suction cup lysimeter, and the free drainage measured at high initial soil water potential (HSWP). Bars indicate the standard deviation of means.
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The leachate collection efficiency for PPS was greater than that of SCL in both soil types measured at either HSWP or LSWP (Table 2). The maximum leachate collection efficiencies for PPS in soils used in this study were 98 and 99% with standard deviation of 7 and 10% for HSWP and LSWP, respectively. The corresponding values for SCL were only 59 and 31% with standard deviations of 26 and 15% for HSWP and LSWP, respectively.
A wide range of leachate collection efficiencies has been reported in the literature. For example, the leachate collection efficiency of the wick-pan lysimeters ranged from 98 to 108% (Boll et al., 1991), 66 to 80% (Brandi-Dohrn et al., 1996), 125% with a coefficient of variation of 36% (Louie et al., 2000), 47 to 206% (Zhu et al., 2002), and 0 to negligible (Steenhuis et al., 1998). Leachate collection efficiency of the zero-tension pans ranged from 10 to 58% (Jemison and Fox, 1992; Zhu et al., 2002). Zero-tension pan samplers mostly collect gravitational water, and consequently, much of matrix water bypassed the sampler. This is particularly true in stuctureless soils where dominant water flow is matrix water flow (Jemison and Fox, 1992; Steenhuis et al., 1995). Our results indicated that PPS had higher leachate collection efficiency and CLCs than SCL and those reported in the literature for wick-pan lysimeters. Although wick-pan lysimeter worked well in structured soils, they did not perform well in sandy soils (Steenhuis et al., 1998). Results presented in this paper indicated a good performance of PPS in both sandy loam and silt loam soils. Results obtained from SCL differed from those reported by Daliparthy et al. (1994) and Hagedorn et al. (1999). These researchers used a fixed vacuum of 33 kPa to collect leachate, whereas in our study we adjusted the applied vacuum to the soil water potentials, which more realistically simulates the field water flow behavior.
The results of this experiment showed that the PPS is a valid and reliable method for collecting the vadose zone leachate. The apparatus proved to be easy to use, and provides accurate results. The greatest challenge in field conditions will be to maintain soil water potential in real time and adjust the level of the vacuum reservoir accordingly. A level of computer-assisted automation will be required. Further work should aim at evaluating the PPSs in the field and at different landscape and soil and crop management systems to determine the efficiency of this method, so that solutes and contaminants movement in the vadose zone can be determined with a greater accuracy than can be obtained with other existing methods. The benefit of the PPS is the equalization of the vacuum applied to the PPS with the water potential of soil matrix.
The higher collection of soil water by PPS and the lower variation compared with SCL suggested that other methods such as SCL might misrepresent the true extent of leaching. Overall, using PPS technology along with the continuous monitoring of soil moisture tension may improve our ability to more accurately monitor and measure nitrate and pesticide leaching.
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ACKNOWLEDGMENTS
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The authors thank three anonymous referees for their invaluable comments, and also Mr. D. Peter from Department of Mathematics and Statistics of University of Massachusetts for helping to perform the statistical analysis.
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NOTES
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The mention of trade or manufacturer names is made for information only and does not imply a recommendation or exclusion.
Received for publication May 7, 2003.
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TOP
ABSTRACT
INTRODUCTION
