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A New Device for Collection of Interstitial Water
from Wetland Sediments

Methods


Sampler design
The sampler (Figure 1) we designed is similar to both "in situ" samplers and diffusion-controlled samplers and has desirable qualities of both. The sampler is inserted in wetland sediments and can be used to draw repeated samples of interstitial water from identical locations. Because of a mild pressure gradient, interstitial water diffuses into the sampler over 2-3 days and thereafter it functions as a diffusion-controlled sampler until samples are removed. Disturbance of benthic sediments is restricted to initial installation and bias resulting from sediment disturbance is minimized. The profiles sampled can be much finer than in conventional "in situ" samplers.

Our sampler has modules that collect interstitial water from 2 depth strata; samples are removed via permanently mounted tubes. Each collection module holds approximately 100 ml of interstitial water. A simple modification could produce multiple ports capable of drawing samples from several sections of a sediment profile for detailed work on chemical gradients. An evacuation tube is used to remove the sample and a fill tube is used to convey N2 from a pressurized container to collection modules; pressure from N2 forces interstitial water out of the modules and into evacuation tubes. The fill tube is plugged when not in use to prevent atmospheric gases from contaminating the sample and to exclude insects; the evacuation tube is placed in a O2 trap to vent N2 as each module fills and to prevent contamination. Both tubes are routed to the surface of the wetland where connections to a portable N2 tank are made and samples collected.

Sample collection
The sampler we designed (Fig. 1 and Fig. 2) was used to collect water samples from profiles 2.5 cm and 11.25 cm deep in sediments. Schedule 80 PVC pipe fittings (available through domestic plumbing suppliers) and 0.635 cm thick plexiglass were used to form the sides and ends of collection modules. Porous ceramic (Soilmoisture Equipment Corp., Santa Barbara, CA 93105; use of brand names does not constitute endorsement by the U.S. Government) was epoxied to the inside of 1.5 cm holes drilled in 5 cm PVC nipples to facilitate diffusion of water from sediment into collection modules. Pore size of the ceramic material used for the unit described was 1.2 microns but pore sizes from .16-6 microns are available. Standard pipe taps cut into the plexiglass circles received threaded barbed plastic fittings. Rigid polyethylene tubing was connected to each barbed fitting to form separate and continuous lines for sample evacuation and venting. Schedule 80 PVC couplers were used because other PVC schedules did not provide an internal lip for sealing a gasket against plexiglass circles. Tubing leads to the water surface from collection modules were housed inside 5 cm schedule 40 PVC pipe of an appropriate length. Disturbance to sediments was minimized by coring a 4.8 cm prehole (Swanson 1978) before seating probes to desired sampling depths. We drilled a hole in the PVC pipe that encased the evacuation and venting tubes to permit water to fill the pipe to reduce the buoyancy of the unit. Rubber stoppers were used to plug sample evacuation tubes to exclude insects and block gas exchange with the atmosphere. For the same reason, venting tubes were inserted in a water trap when not extracting samples. Approximate cost of materials for the unit described was US$25. The first set of samples collected after installation were discarded to avoid potential bias from sediment disturbance.

Figure 1: Illustration of sampling device   Figure 2: Expanded internal view of sampling device

Figure 1.  Sampling device for collection of interstitial water from wetland sediments. 1. Sch 40 PVC cap 2. Rubber stopper 3. Sample evacuation tube 4. Sample venting tube 5. Test tube water trap 6. Clamp 7. Sch 40 PVC pipe 8. Hole to reduce buoyancy 9. Sch 40 PVC threaded male adapter 10. Sch 80 PVC coupler 11. Sch 80 PVC nipple (module #1) 12. Holes to expose sediment to ceramic membrane 13. Sch 80 PVC nipple (module #2) 14. Sch 40 PVC threaded cap.

Figure 2.  Expanded internal view of lower and of the sampling device. 1. Sch 40 PVC pipe 2. Hole to reduce buoyancy 3. Sample evacuation tube (module #1) 4. Sample venting tube (module #1) 5. Sample evacuation tube (module #2) 6. Sample venting tube (module #2) 7. Sch 40 PVC male adapter 8. Sch 80 PVC coupler 9. Internal lip in Sch 80 couplers to facilitate a water tight seal with gasket 10. Threaded male nylon barbed fitting 11. Threaded female nylon barbed fitting 12. Sch 80 PVC nipple (module #1) 13. Hole in PVC nipple to expose sediment to ceramic membrane 14. Rubber gasket 15. 0.635 cm thick plexiglass circle 16. Sch 40 PVC nipple (module #2) 17. Cylinder of porous ceramic 18. Sch 40 PVC threaded cap.

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