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Published in Vadose Zone Journal 2:627-632 (2003)
© 2003 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

SPECIAL SECTION - ADVANCES IN MEASUREMENT AND MONITORING METHODS

A Modified Vadose Zone Fluxmeter with Solution Collection Capability

Glendon W. Gee*, Z. Fred Zhang and Andy L. Ward

Pacific Northwest National Laboratory, 3200 Q Ave., Richland, WA 99352
* Corresponding author (glendon.gee{at}pnl.gov).

Received 5 March 2003.



   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
To assess contaminant fluxes in the vadose zone water flux and solute concentrations must be known but they are seldom measured simultaneously at the same location. A water fluxmeter (WFM) with divergence control was modified to measure solute concentrations by adding a funnel and collection vial to the bottom of the meter. Laboratory experiments using coarse and fine sands showed that measured solute concentrations and known water fluxes can be combined to provide estimates of solute flux. Water containing a NO-3 tracer was applied at a rate of 1.97 x 10-8 m s-1 (621 mm yr-1), and water flux was simultaneously measured along with NO-3 concentrations in the outflow water. The general agreement in fitted and measured pore-water velocities suggests that the breakthrough curves of NO-3 measured using the drainage through the WFM can be used to estimate the pore-water velocity of the soil. Solute travel-time through the 60-cm-long wick was <10% of the travel time through the sands and could be neglected. Flow divergence was examined by measuring the soil water content and pressure head at different positions and by measuring the water flux passing through and around the WFM. Divergence was controlled by a 15-cm-high barrier such that more than 80% of the flow passed through the fluxmeter in both soils. Results show that the modified SFM can provide a convenient method for long-term monitoring of contaminant flux.

Abbreviations: TDR, time domain reflectometry • WFM, water fluxmeter


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
CHEMICALS FROM agriculture and industry are being discharged to the subsurface in increasing quantities, potentially reducing the quality of a shrinking groundwater resource (Glennon, 2002; Spalding et al., 2003a, 2003b; Field et al., 2003; Wayland et al., 2003; Bohlke, 2003). It is of interest to farmers, hydrologists, geochemists, and regulators, among others, to quantify these discharges and to find ways to assess the nature and extent of the resultant plumes so that threats to underlying water resources can be properly managed. Water flux and solute concentrations must be known to assess solute fluxes in the vadose zone but are seldom measured simultaneously at the same location.

Water flux within the vadose zone can be measured using indirect or direct methods. Indirect methods include estimation from water-balance evaluation using micrometeorological methods (Hillel, 1980; Gee and Hillel, 1988), derivation from water potential (Richards, 1950), or water storage change (e.g., Gardner and Kirkham, 1952; Topp et al., 1980), heat pulse probes (e.g., Byrne et al., 1967, 1968; Byrne, 1971; Kawanishi, 1983; Ren et al., 2000; Kluitenberg and Warrick, 2001; Hopmans et al., 2002), and tracer techniques (e.g., Si et al., 1999; Zhang et al., 2000a, 2000b). The direct methods are based on intercepting soil water flow with an in situ fluxmeter (e.g., Ivie and Richards, 1937; Cary, 1968, 1970, 1971; Dirksen, 1974; van Grinsven et al., 1988), pan lysimeter (Jemison and Fox, 1992; Chiu and Shackelford, 2000), equilibrium tension lysimeters (Brye et al., 1999), or passive capillary samplers (e.g., Holder et al., 1991; Boll et al., 1992; Knutson et al. 1993, Knutson and Selker, 1994; Rimmer et al., 1995; Louie et al., 2000). The installation of these devices for direct flux measurement breaks the capillary connectivity of soil and hence enhances lateral flow. The enhanced lateral flow is also called flow divergence. Flow divergence can be reduced by precisely matching wick length and diameter to soil type (Boll et al., 1992) when using wick samplers.

Recently, Gee et al. (2002) developed a vadose zone WFM with divergence control to measure drainage fluxes in field soils. It concentrates flow into a narrow sensing region filled with a fiberglass wick. The wick applies suction, proportional to its length, and passively drains the meter. Water flux through the meter is measured with a self-calibrating tipping bucket. A divergence barrier was used to minimize or eliminate the divergence of flow. Their results show that control of the divergence barrier height and knowledge of soil hydraulic properties are required for reducing flow divergence. The WFM has the capability of providing continuous and reliable monitoring of unsaturated water fluxes ranging from 1 to more than 1000 mm yr-1.

A variety of vadose zone sampling methods are available to measure contaminant concentration. Soil cores yield water-quality samples representing the solute concentration in micro- and macropores (Steenhuis and Muck, 1988). However, multiple samples cannot be taken from the same location in the field. Soil suction-cup samplers extract water from the soil by manually applying a suction within a porous cup (e.g., England, 1974; Shaffer et al., 1979). This method tends to preferentially sample the concentration of water in the smaller pores and often misses the fluid moving in the larger (macro) pores. Another way to sample water and solutes moving in the vadose zone is to use wicks to apply capillary suction (Brown et al., 1986; Holder et al., 1991; Boll et al., 1992). Brown et al. (1986) found that the fiberglass wicking did not adsorb inorganic ions (Br-, Cd2+, and NO-3) or organic chemicals (toluene, trichloroethylene, ethyl benzene, and naphthalene). Wick samplers used by Brown et al. (1986) and Holder et al. (1991) consisted of a 30 by 30 cm pan with one tube in the center. The drawbacks of their wick design are that divergence was not accounted for and that chemicals entering the sampler at the sides need to travel a considerable distance to the center while solutes near the middle flow without delay, giving rise to a larger instrument dispersion coefficient.

We modified the vadose zone WFM of Gee et al. (2002) to collect soil water samples in addition to measuring water flux. Solute flux and total mass of solute were determined from measured values of water flux and solute concentration. Laboratory experiments were conducted to evaluate the modified WFM.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Water Fluxmeter with a Solution Collector
Modification of the WFM of Gee et al. (2002) was made to accommodate solution collection (Fig. 1). Specifically, we added a third funnel (Funnel C of Fig. 2a) beneath the tipping bucket. Beneath this lower funnel we also added a small (20 mL) vial as a solution collector. Water runs directly from the tipping bucket into the solution collector. A small diameter (0.8 x 10-3 m i.d.) flexible tube was placed in the bottom of the vial and extended to the soil surface so that a water sample could be withdrawn by applying suction (Fig. 2b). Holes were drilled in the top of the vial so that when the solution collector is full, the older, resident solution overflows. The small volume of the collector vial helps minimize mixing of the collected solution. Frequent sampling gave adequate temporal resolution of solute concentrations. Solute flux, Js(t), which is the solute mass that passed through the WFM per unit area per unit time, was calculated using

[1]
where C is solute concentration, Jw is water flux, and t is time. The total mass of solute passed through per unit area (MPA) from time t1 to t2 is determined by

[2]



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  		Fig. 1. Schematic of a modified vadose zone water fluxmeter with a solution collector.

 


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  		Fig. 2. Schematic of the modified water fluxmeter and experimental setup (not to scale).

 
The transport of a conservative tracer within the soil, isolated by the divergence controller of the WFM, may be described by the one-dimensional convection–dispersion equation:

[3]
where D is the dispersion coefficient, z is distance, and v is pore-water velocity, defined as

[4]
where {theta} is soil water content. In the experiments described in the following section, effluent samples were taken and hence flux-averaged concentration is used. For a step-function solute input of constant concentration, C0, at t = 0 into a semi-infinite soil column at constant water velocity, the flux-averaged concentration is expressed by the analytical solution (Leij and van Genuchten, 2002):

[5]
where erfc = 1 - erf, with erf being the error function, and the subscript "s" denotes a step-function input of solute. If a step input occurs at t = {tau} instead of t = 0, then Eq. [5] becomes

[6]

Using Eq. [6], the analytical solution of a pulse input of C0 for a duration of {Delta}t can be obtained by subtracting two step functions, of which one occurs at t = -0.5{Delta}t and the other at t = 0.5{Delta}t:

[7]
where Cp is the flux-averaged concentration at z from a pulse input.

Laboratory Tests
The schematic of the experiment setup is shown in Fig. 2. Coarse-sand and fine-sand materials were packed into two 0.305-m-diameter columns about 1.5 m in length. The coarse sand was obtained from the Department of Energy's Hanford Site near Richland, WA at a location called the North Caisson, part of the Buried Waste Test Facility (Rockhold et al., 1988). The fine sand was also from the Hanford Site and is similar to what Fayer et al. (1999) described as a dune sand, a material used as a surface cover treatment in the Field Lysimeter Test Facility located near the Hanford Meteorological Station (Fayer et al., 1992). The coarse sand has a saturated hydraulic conductivity (Ks) of 1.51 x 10-4 m s-1 and the fine sand a Ks of 2.00 x 10-5 m s-1. A WFM was buried inside each of the columns with the top of the fluxmeter at a soil depth (z, positive downward) of 0.15 m (Fig. 2a). The two columns and the two WFMs were the same as those described in Gee et al. (2002), except the two WFMs were modified by adding each a solution collector. For experimental convenience, the collectors were placed outside of the columns and were attached by a tube to the bottom of the WMFs (Fig. 2a and 2b). A shower head–type water applicator with a diameter of 0.30 m was placed on each of the soil columns. Thirty-two 27G1/2 needles were attached to each applicator. The water applicators were connected to a programmable syringe pump (Kloehn Co. Ltd, Las Vegas, NV). Water was applied to each column at a pulse rate of 0.086 mL s-1 for 30 s once every 1800 s, equivalent to an average rate of 1.97 x 10-8 m s-1 (621 mm yr-1), a flow rate that is a small fraction of what is typically tested in column breakthrough curve studies. The pulse rate was about 60 times the average application rate, and this allowed water to go through all the needles attached to the applicator and be delivered uniformly over the soil surface. There were two drainage ports at the bottom of each of the soil columns, one for drainage through the WFM and the other for drainage around the WFM. Compared with the total depth of the soil in and above the WFMs (i.e., 0.3 m), the length of the wick and the soil included in the topmost funnel (A in Fig. 2) is nearly twice the length of the soil above it (0.66 m). However, the travel times for soil in the topmost funnel and in the wick were much smaller than for an equivalent depth of soil because the funnel and the wick have smaller cross-sectional areas. We used an equivalent travel distance to approximate the tracer transport in the soil in Funnel A and through the wick. The equivalent travel distance of the tracer through Funnel A was estimated as the height of a cylinder that would have a diameter of the WFM (i.e., 0.2 m) and a volume of the funnel. The equivalent travel distance of the tracer through the wick was estimated as the height of a cylinder that would have the volume of the wick and the same water flux as that in the soil within the WFM. The water content of the wick (0.59 m3 m-3) was determined by weighing the wick dry and then weighing after wetting and draining. Thus, the equivalent travel distance through the 0.06-m-high Funnel A was about 0.02 m and that through a 0.60-m-long 0.025-m-diameter wick was about 0.05 m for the WFM with the coarse sand and 0.03 m for the WFM in the fine sand. Therefore, the total tracer travel distances of 0.37 m (for transport in the coarse sand) and 0.35 m (for transport in the fine sand) were used in Eq. [4] through [6] when the parameters v and D were optimized. Since the sample collected represented the temporally average concentration, an average time was used as sampling time.

Soil water content ({theta}) and pressure head (h) were measured using time domain reflectometry (TDR) and tensiometry methods (Fig. 2a). Two-prong TDR probes, 0.15 m long and 2 x 10-3 m in diameter, were installed vertically at two soil depths, 0 to 0.15 and 0.15 to 0.30 m. There were six TDR probes at each soil depth, three of which were evenly distributed at radial distance R = 0.07 m from the center of the cylinders, and the other three were at R = 0.125 m. The porous ceramic cups of tensiometers were 1.5 x 10-2 m in length and 5 x 10-3 m in diameter. Each column had six tensiometers installed at locations (R,z) of (0,0.10), (0,0.20), (0,0.30), (0.125,0.10), (0.125,0.20), and (0.125,0.30) m. Water contents were measured using a 1502C Tektronix Cable Tester (Tektronix, Beaverton, OR). Pressure heads were measured using a Tensimeter pressure transducer (Soil Measurement Systems, Tucson, AZ). For the soils tested, we determined from computer modeling that effective steady-state conditions were attained within 0.1 m of the surface so that intermittent pulsing produced an effective steady flow condition at a depth of no more than 0.1 m below the surface. Further, there was no indication from the TDR or tensiometers that the pulsed input was affecting the water contents or tensions at the monitoring depths.

Deionized water was applied uniformly to the top of each column. When the flow in the two columns reached steady state, the source water was replaced by a 22 mg L-1 NO-3 (in KNO3) solution. After 4.15 x 105 s (4.8 d), the NO-3 solution was replaced by deionized water. During the 3-mo experiment, the solution at the drainage collectors (about 20 mL total) was sampled as often as twice a day and with sufficient frequency to capture shape and magnitude of the breakthrough curves. Nitrate concentrations of the samples were analyzed using the cadmium reduction method (American Public Health Association, 1989, p. 4.137–4.139). Drainage, through the WFMs, as well as that bypassing the WFMs, was collected, and the respective volumes were reported on a daily basis. The drainage recovery efficiency (DRE) of a WFM was calculated using

[8]
where Jwm is the measured soil water flux and Jwa is the applied flux.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Breakthrough Curve and Solute Flux
The NO-3 breakthrough curves (Fig. 3) of the coarse and fine sands were determined by analyzing the solute concentration of the samples obtained at the bottom of the WFM. Note that the time was zeroed at the center of the input NO-3 pulse. The background NO-3 concentration of the effluent from the coarse sand was about 1 mg L-1 (Fig. 3a), and this number was subtracted in the following analysis. Although NO-3 was applied to both soils at the same time and the same rate, it was transported faster in the coarse sand than in the fine sand. The peak arrival time of the center of mass was 1.76 x 106 s (20.4 d) in the coarse sand and about 3.22 x 106 s (37.3 d) in the fine sand. The difference in arrival times in the two soils was due to the difference in steady-state water contents. The coarse sand had lower average water content than the fine sand (Table 1); hence, according to Eq. [4], the coarse sand had a higher pore-water velocity than the fine sand.



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  		Fig. 3. Nitrate breakthrough curves measured by the modified water fluxmeter in (a) coarse sand and (b) fine sand.

 

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  		Table 1. Steady-state soil water content at the infiltration rate of 1.97 x 10-8 m s-1 (621 mm yr-1). Reported water contents are the mean of three observations. The tops of the WMFs were at the soil depth of 0.15 m. The radius of the WFM was Rwfm = 0.102 m.

 
The transport of nitrate may be described by Eq. [5] through [7]. Parameters v and D were optimized to the NO-3 concentration data using Mathcad 2001i (MathSoft, Inc., 2001). The optimized pore-water velocities in the coarse and fine sands were 2.08 x 10-7 and 1.02 x 10-7 m s-1, respectively (Table 2). The pore water velocities were also calculated by substituting the applied water flux (i.e., 1.97 x 10-8 m s-1) and the average water content at R = 0.07 m into Eq. [4] (Table 2). The optimized pore-water velocity differs from the observed values (Table 2) by -7.3% to +28.4% for the coarse sand and the fine sand, respectively. While not perfect, the general agreement in fitted and measured pore-water velocities suggests that the breakthrough curves of NO-3 measured using the drainage through the WFM can be used to estimate the pore-water velocity of the soil. The dispersion coefficient, D, measured in the coarse sand was 40% lower than that in the fine sand. The dispersion coefficient is expected to be inversely related to pore water velocity so the higher D in the fine sand is consistent with the observed lower pore water velocity for the fine sand.


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  		Table 2. Measured and optimized pore-water velocities and the dispersion coefficients for coarse and fine sands.

 
After the water flux and NO-3 concentration was known, the NO-3 flux was calculated using Eq. [1] and is shown in Fig. 4. Using Eq. [2] and multiplying by the WFM cross section, the total NO-3 applied to each WFM was 6.59 x 10-3 g, while the nitrate moved out of the bottom of the WFMs during the experiment were 6.95 x 10-3 and 6.23 x 10-3 g for the coarse and fine soils, respectively. Thus, using the modified WFM and chemical-analysis results of the concentration of solutes, the solute flux and the total mass transferred through the vadose zone during a specific period of time can be determined. The modified WFM provides a convenient way for long-term monitoring of contaminant transport.



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  		Fig. 4. Nitrate flux in the coarse and fine sands.

 
One of the concerns in using the modified WFM to measure solute breakthrough curves is the solute travel time in the wick within the WFM. Although the length of the wick is about 0.6 m, the solute travel time is still small due to its much smaller cross-sectional area than that of the WFM. For example, the cross-sectional area of a 0.025-m (1 in.)-diameter wick is only 1.6% of that of a 0.2-m (8-in.)-diameter WFM. For the measured volumetric water content of the wick of 0.59 m3 m-3, the solute travel time through a 0.6-m-long wick is approximately equal to the travel time through approximately 0.04 m of soil. Thus, the solute travel time through the fiberglass wick is small enough to be neglected.

Another concern is the mixing of solution in the solution collector. Due to the diffusion of solutes in the solution collector, the solute concentration can be considered to be the temporally averaged value. If the time between two samplings is small relative to the total break though time, the effects of the mixing in the solution collector should be also small. For example, in our experiment, from the time the NO-3 was injected it took about 20 d for it to break through in the coarse sand and about 37 d in the fine sand. The time between samplings was about 1 d, during which the concentration change was small. Hence, the concentration in the solution collector is a reasonable representation of the average concentration during the sample collecting period. The time corresponding to this concentration should be the time approximately at the middle of the sample collecting period rather than the time the sample was taken.

Divergence and Drainage Recovery Efficiency
Divergence of flow was examined by measuring the values of the pressure head (h) and water content ({theta}) within and directly above the WFM and by measuring h and {theta} of the bulk soil at the same depth. The presence of a horizontal hydraulic gradient would indicate that divergence or convergence occurred. Table 1 lists the steady-state water contents and Table 3 the pressure heads at different positions. The differences in pressure head ({Delta}h) between the soil positions R > Rwfm, where Rwfm is the radius of the WFM, and those at R < Rwfm ranged from -0.05 to +0.09 m, with an average of 0.007 m. This suggested that there was no detectable difference of pressure head for both of the soils, considering that the measurement error of a tensiometer can be ±0.02 m or more. The differences in water content ({Delta}{theta}) between the soil positions R > Rwfm and those at R < Rwfm were from 0.0 to 0.035 m3 m-3, with an average of 0.014 m3 m-3. These indicate that the soil at R < Rwfm was slightly wetter than that at R > Rwfm. The results of h and {theta} indicate that there was only minor divergence away from the WFMs.


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  		Table 3. Steady-state soil water pressure head at the infiltration rate of 1.97 x 10-8 m s-1 (621 mm yr-1). The tops of the WMFs were at the soil depth of 0.15 m. The radius of the WFM Rwfm = 0.102 m.

 
The calculated drainage recovery efficiencies (DRE) for the WFMs in the coarse and fine sands are plotted in Fig. 5 using the measured drainage volumes and Eq. [8]. At an early stage, the DRE values were not stable due to the unsteady soil water condition. When the flow was at steady state, the DREs for both the coarse and fine sands also became stable. The average DRE values were 80% for the coarse sand and 81% for the fine sand. The measured DRE value for the coarse sand was lower than the reported 100% by Gee et al. (2002), but the DRE value for the fine sand was higher than the reported 40%. The reason that the DRE values differed from those previously observed may be that the soils were repacked for the current test, so pore geometries could have been different. In addition, the flow rates in these tests, while similar, were not identical to those reported by Gee et al. (2002). Issues with divergence (e.g., low DRE values) can be solved by extending the height of the diversion barrier. Modeling similar to that reported by Gee et al. (2002) has indicated that extending the diversion barrier to a height of 0.6 m ensures that DRE approaches 100% for most soils.



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  		Fig. 5. Drainage recovery efficiency (DRE) of the water fluxmeters in the coarse and fine sands.

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The WFM of Gee et al. (2002) was modified by including a drainage collecting system. In addition to measuring water flux, the modified WFM allows coincident sampling of drainage at desired times to determine solute concentration. Laboratory experiments, using a NO-3 tracer, flowing through columns of coarse sand and fine sand, demonstrated that solute travel times estimated with optimized parameters were in reasonable agreement with observed travel times. Solute travel time through the wick is small and considered negligible. Using water-flux data and solute-concentration data, solute flux and total loads of contaminants can then be determined simultaneously at the same location in the field. Therefore, the modified WFM provides a convenient approach to long-term monitoring of contaminant transport.


   ACKNOWLEDGMENTS
 
This work was funded by the U.S. Department of Energy as part of the Hanford Ground Water/Vadose Zone Integrated Project Science and Technology Initiative. We thank Karen Waters-Husted, who provided valuable help in recording and collecting drainage data. We also acknowledge the late John Cary, who brilliantly pioneered vadose zone fluxmeter work and provided much of the inspiration for our efforts. Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle under Contract DE-AC06-76RL01830.


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




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