Soil Science Society of America Journal 64:809-811 (2000)
© 2000 Soil Science Society of America
DIVISION S-10-WETLAND SOILS
Constructing Simple Wetland Sampling Devices
Matthew J. LaForcea,b,
Colleen M. Hanselb and
Scott Fendorfb
a Div. of Soil Sci., Univ. of Idaho, Moscow, ID 83844-2339 USA
b Dep. of Geological and Environ. Sci., Stanford Univ., Stanford, CA 94305-2115 USA
laforce{at}stanford.edu
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ABSTRACT
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Wetland soils and sediments are commonly monitored for nutrient or contaminant cycling. Oftentimes, sample collection and subsequent transport to the laboratory proves to be difficult. We detail the construction and use of three inexpensive devices for sampling and transporting sediment and solution in wetlands: dialysis cells (peepers) for solution measurements, a piston coring device for sediment sampling, and a portable vacuum-chamber sample transporter. Peepers were fabricated using either 5- or 20-mL filter tubes placed at 2- or 5-cm intervals into slotted polyvinyl chloride (PVC) tubes. On the basis of a laboratory investigation, trace elements reached a steady state in our peepers within 10 d. The piston coring device was constructed from 1.9-cm diam. PVC tubing and was used to remove intact sediment (
20-cm) cores. The transporter was constructed from 25-cm diam. PVC pipe and was designed to carry field-collected materials back to the laboratory while minimizing sample alteration caused by exposure to O2. Our results indicate that four purges at 0.05 MPa, followed by filling with N2, resulted in negligible O2 levels in this transporter.
Abbreviations: ICP, inductively coupled plasma • PVC, polyvinyl chloride
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INTRODUCTION
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DIALYSIS CELLS (peepers) have been used for in situ monitoring of dissolved constituents in saturated sediments (Hesslein, 1976; Brandl and Hanselmann, 1991; Carignan, 1984; Carignan et al., 1985; Harper et al., 1997; Webster et al., 1998). Unfortunately, previous peeper designs are expensive and do not allow for sufficient sample volume to monitor pH, Eh, and aqueous-phase contaminants (Hesslein, 1976). The efficiency of peepers depends on equilibration time of the analyte, the analyte's diffusion coefficient, its adsorption–desorption properties, the surrounding ambient-solution temperatures, and sediment porosity (Carignan, 1984). Carignan et al. (1985) determined that dialysis cells and pore water centrifugation yielded similar results for metal analysis. However, peepers have several advantages over centrifugation including in situ monitoring of trace elements, quick and efficient sampling times, and minimal temperature and O2 (g) diffusion effects.
A proper sediment-sampling device needs to minimize frictional resistance and sample deformation while keeping the sediment stratigraphy intact. Therefore, we use a simple, cost-effective piston coring device. Piston coring devices are commonly used in limnological work to efficiently collect sediment samples (Blomqvist, 1991; Fischer et al., 1992). In addition, piston coring devices allow for a maximum amount of sample with minimal sample mixing. Although piston coring can alter the stratigraphy of a sample because of compaction (Baudo, 1990; Quinn and Clyde, 1998), it is an efficient technique for extracting sediment cores (Baudo, 1990; Blomqvist, 1991).
Proper sample transport to the laboratory is essential for minimizing sample alteration. Samples removed from wetlands or lacustrine environments, through piston or other coring devices, may be anoxic (Faulkner et al., 1989). Consequently, samples must be kept anaerobic during transport to the laboratory. There is a lack of information regarding the construction and use of a device designed to transport materials under oxygen-limited conditions. To achieve this task, we designed a simple transporter to limit sample alteration during transport to the laboratory. The purpose of this manuscript is to describe three devices we constructed to sample wetlands. We detail the construction, use, and efficiency of a dialysis cell (peeper) for monitoring soil pore water and a portable vacuum chamber to transport field-collected materials back to the laboratory. Additionally, we briefly describe the construction of a simple piston coring device.
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Materials and methods
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We used two different peeper designs in our investigations: one for monitoring pH and Eh (mV) and a second, higher-resolution peeper for monitoring spatial elemental distributions. Our first peeper was created by placing 2.5-cm-diam. by 8-cm-long (20-mL-volume) filter tubes (prefitted with sterile 0.20-µm nylon filters [Lida Maxispin centrifugal filter tubes, Lida Manufacturing, Kenosha, WI]) at 5-cm intervals into a slotted 70-cm tall PVC supporting rod (Fig. 1)
. The filter tubes were then filled with 20 mL of double-deionized water and placed in the wetland for 2 wk. The higher resolution peeper was created by placing 1-cm diam. by 5-cm long (5-mL volume) filter tubes (containing sterile 0.20-µm nylon filters [Centrex MF-5 sterile, disposable centrifugal microfilters, Schleicher & Schuell, Keene, NH]) at 2-cm intervals into a 40-cm tall PVC supporting rod (Fig. 1). Upon retrieval from the wetland, the solution collected from the first peeper was monitored for pH and Eh (mV), while trace element concentrations were sampled from the second higher resolution peeper. In the field, aqueous samples were removed from the filter tubes, acidified with concentrated HCl, and kept at 4°C until analysis.
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Fig. 1 A side view of dialysis cells (peepers) used for monitoring pH and Eh and obtaining in situ aqueous elemental distributions
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We conducted a laboratory investigation to determine the time required for the peepers to reach steady state with the surrounding solution. Filter tubes were filled with 5 mL of double-deionized water and then placed in contact with equal amounts of 5-mg L-1 solutions of As, Fe, Mn, and Zn in a double-deionized water matrix. Solutions were separated by a 0.20-µm nylon filter and the peepers were stored upright. Triplicate samples were collected every 2 d, filtered, and acidified before analysis. The concentration of these contaminants were measured by inductively coupled plasma (ICP) optical emission spectrophotometry (Thermo Jarrell Ash IRIS ICP-OES, Franklin, MA) with a 10% accuracy range; quality control was checked every 15 samples. Detection limits were defined by 3
(where is the standard deviation) of five blanks (Vander Voet and Riddle, 1993). Detection limits were as follows: As was 0.04 mg L-1; Fe was 0.01 mg L-1; Mn was 0.01 mg L-1; and Zn was 0.02 mg L-1.
The piston coring device was constructed of 5-cm diam. PVC tubing. Each tube was 40 cm tall and the top was covered by a washer to prevent water from filling the core during sediment collection. Inside the core, a rubber stopper (#9) was used as a piston. A hole was drilled through the center of the stopper and fitted with a threaded plug. The threaded plug could be directly screwed into a threaded rod equipped with a handle. The device functions by pushing the PVC tubing into the sediment while pulling the rubber stopper up at an equal rate inside of the tubing, creating a vacuum. The vacuum allows for 20-cm cores of sediment to be collected while maintaining sediment stratigraphy. Cores collected by this method were immediately capped and placed upright in a transporter.
All samples collected in the field need to be transported to the laboratory in a manner that minimizes sediment and solution alteration by O2. To achieve this task, a portable vacuum-chamber transporter was constructed. The dimensions of the transporter were designed so that it can fit directly into the transfer chamber of a N2-purged glovebox. The transporter's body (45-cm height) was made from 25-cm diam. PVC pipe with a 0.2-cm thick Plexiglas bottom and a removable O-ring–sealed 0.2-cm thick Plexiglas top. The top was secured by threaded bolts embedded in the 25-cm PVC pipe and capped with wing nuts. Two ports were on top: one for a vacuum gauge and the other for a three-way, female, threaded ball valve. One port of the three-way ball valve was used for vacuum and the other port for purging the transporter with N2 gas (Fig. 2)
. A 12-V corrosion-resistant vacuum pump (Fisher Scientific, Santa Clara, CA) was used for evacuation. Both the vacuum and a 12-V battery can be easily placed inside the transporter and taken into the field. A small, portable N2 tank was used to purge the transporter. The tank was equipped with an N2–gas regulator and attached to the sampler's ball valve via Gates MegaFlex 0.15-cm SAE, 4000 Max PSI, flame-resistant, high-pressure tubing (Gates Rubber, Denver, CO). The tubing was fit at the end with a 0.1-cm male body connector (for easy insertion into the ball valve) and a 0.1-cm Swagelok stem quick connector (Milton Quick-Change A-style coupler; Swagelok, Solon, OH) for easy attachment to the N2 regulator. The transporter works as follows: Samples collected in the field were (i) immediately placed in the chamber, (ii) placed under a 0.05-MPa vacuum, and (iii) purged with N2 gas. The latter two steps were repeated four times to minimize O2 levels. The chamber was left in an N2–gas environment for transport back to the lab.
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Fig. 2 Sample transporter used for placing materials under an N2–purged vacuum and minimizing O2 diffusion during transport to the laboratory
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A Crowcon Triple Plus Portable O2 gas sensor (Crowcon Detection Instruments, Abingdon, UK) was used to determine the efficiency of the transporter. The sensor was calibrated outside the transport chamber at ambient O2 levels of 21.2%. The sensor was then placed inside the device to determine O2 levels during evacuation and purging.
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Results and discussion
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The effects of size and dimension greatly influence the time required for peepers to reach a steady state with the surrounding aqueous environment (Harper et al., 1997; Webster et al., 1998). The peeper equilibrium experiment indicates that after 10 d, As, Fe, Mn, and Zn concentrations reached a steady state in the dialysis cells (Fig. 3)
. Thus, 2 wk was determined to be a sufficient time for steady state to be reached in aqueous environments. In porous media, solute diffusion rates will be slower; consequently, these peepers should be tested in situ to determine the time needed for an analyte to reach steady state for a given site. Our laboratory findings are consistent with Carignan et al. (1985), who determined that only 5 to 10% metal concentration differences occurred when peepers were removed from the field after 1 wk compared with 4 wk. Thus, 2 wk is a conservative estimate for the time necessary to reach steady state in porous media under field conditions.
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Fig. 3 Changes in As, Fe, Mn, and Zn concentrations as a function of time within membrane-separated filter tubes. Error bars indicate the standard deviation for the mean of the three replicates
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The results of the transporter purging test indicate that after four consecutive evacuations (0.05 MPa) and purges with N2 gas, the O2 gas concentrations were negligible and remained so for >3 h (an assumed time required for sample transport to the laboratory). Thus, four purges of N2 were sufficient to limit O2 concentrations within the transporter.
Using the aforementioned instruments, sampling wetland systems can be inexpensive, easy, and effective. We have described three devices (peepers, piston corer, and transporter) that can be used for sampling the aqueous and solid phase of wetlands while minimizing sample alteration from the time of sample collection in the field to the time of analysis in the laboratory.
Received for publication August 17, 1998.
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REFERENCES
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66(4):
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[Abstract]
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ABSTRACT
INTRODUCTION
