Abstract Introduction Study Area Coring & Pore Water Extraction Analytical Methods Results & Discussion Summary Acknowledgments References PDF Version

Pore water samples were obtained by coring and subsequent squeezing of the core using a specially designed stationary piston coring device and squeezing apparatus (Fig. 2). A detailed description of the coring and squeezing apparatus is presented elsewhere (Orem and Lerch 1997). The squeezing device is a modified version of that developed by Jahnke (1988). The advantages of this approach for pore water extraction in wetland sediments containing rooted aquatic macrophytes are discussed by Howes et al. (1985). Piston cores were taken using a monopod, an acrylic core tube, a PVC piston with a pair of o-rings, an aluminum cutter with cutting teeth, and a set of iron handles (Fig. 2A). The core tube has a series of holes at 2 cm intervals tapped for a 1/4-28 thread, which act as exit ports for pore water from different depths during squeezing. The holes are sealed during coring with 1/4-28 roundhead nylon screws and small o-rings.

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Figure 2. Diagram of sediment piston coring device and pore water squeezer. The coring device (A) consists of a plexiglas core barrel, aluminum cutter, polyethylene piston, and iron handles. During coring, the piston is attached to a monopod (not shown) with steel cable. Pore water is obtained from a core by attaching the core barrel to a support, adding a second piston, and gradually compressing the core using threaded steel rods and threaded plates attached to the support (B). Pore water from various depth intervals exits the core through holes in the side of the core barrel, passes through a syringe filter, and is collected in appropriate vessels (C). See text and Orem and Lerch (1997) for more details.

In the Everglades we chose coring sites where the peat surface appeared undisturbed, and the vegetation was not so thick as to impede access. Airplane cable attached to the piston was fed through the core tube, the piston was tapped into the core tube with a rubber mallet, and the cutter was screwed onto the end of the core tube. The airplane cable was then attached to the monopod, and the iron handles were clamped around the core tube at a convenient height. A core was obtained by first twisting and pushing the core barrel with the handles to allow the serrated edge of the cutter to slice through any surface mat of rootlets and plant debris. Once through the root mat the core barrel easily penetrated into the peat while the piston remained stationary at the peat surface. We generally used core barrels of 4, 3, and 2 feet in length. Once the core barrel was emplaced in the peat to the desired depth, the cable was removed from the monopod pole and wrapped tightly around the handles to prevent the piston from slipping during recovery of the core. The core was then lifted from the peat with the handles (some effort required), and the bottom was capped and taped. The eye bolt and cable unit was usually removed from the piston at this point by unscrewing it with a wrench. Every attempt was made to keep the core as vertical as possible during return to base for pore water extraction. Cores for geochemical analyses of the peat were obtained in an identical manner.

Squeezing of the core for pore water extraction was begun as soon as possible after return to an appropriate base (boat launch area, motel parking lot, laboratory, etc.). The cutter and cap on the end of the core tube were removed and the core tube was placed into the squeezer support and secured with aluminum clamps (Fig. 2B). A squeezing piston with two o-rings was tapped into the bottom of the core barrel with a rubber mallet, and a push piston (no o-rings) attached to a threaded steel rod was screwed up until it made contact with the squeezing piston. The core was raised with the bottom piston by turning the lower threaded rod arrangement with a socket wrench until the core was in the desired position relative to the sampling ports on the side of the core barrel. A sampling port above the sediment surface was opened by removing the roundhead nylon screw to allow excess overlying water to escape during the positioning of the core, then reclosed. Sufficient overlying water is left above the sediment core so that no air is trapped below the upper squeezing piston. Next, An upper push piston attached to a threaded rod was screwed down until it abutted the top squeezing piston (the latter already in place from the coring operation). The nylon screws were removed from the sampling ports at the depth intervals to be sampled and were replaced with 1/4-28 male to female luer fittings. The luer fittings contained a tightly-rolled quarter piece of 5.5 cm diameter Whatman filter paper as a pre-filter. One end of the fitting was attached to a piece of rigid plastic tubing 2-3 cm in length (Fig. 2C). The rigid tubing extends into the center of the peat matrix to minimize the chance of sampling water that flows along the inner wall of the core tube (Jahnke, 1988). Syringe filters (Gelman 25 mm, 0.45 µm ion chromatography Acrodisc) were locked onto the luer fittings and sampling containers were attached to the syringe filters with a small piece of flexible tubing (Fig. 2C). We used disposable plastic syringes for collection of samples for dissolved gases and pH, and small plastic bottles with plastic fittings pushed through the lids for collection of pore water for other dissolved constituents (nutrients, anions, cations, and dissolved organic carbon). All plastic fittings, tubing, and containers used in the pore water extraction were soaked overnight in 10% HCl and thoroughly rinsed with deionized/distilled water prior to use.

Pore water began to flow from the sampling ports into the collection vessels as the steel rods from the top and bottom were alternately advanced, squeezing the peat and forcing the pore water out the sampling ports. After the initiation of flow, only the bottom threaded rod and piston was turned, avoiding potential problems of excessive compression of the surface sediment and smearing of the pore water profile at the surface (Jahnke, 1988). The pressure on the core was maintained by continuing to screw down the bottom threaded rod and piston about every 10-15 min or when the pore water flow seemed to diminish significantly. Squeezing continued for 2-5 hours, depending on the quantity of pore water desired and the characteristics of the peat. Aliquots of pore water for immediate analysis of volatile dissolved constituents (e.g. dissolved sulfide, pH, alkalinity) were collected as soon as sufficient sample was available. Syringe filters were normally changed once or twice during the course of squeezing to maintain proper flow. This was done by reducing pressure on the core barrel (e.g. slightly retracting the threaded rod and piston), and quickly replacing the filter. Occasionally, some depth intervals would produce "dry holes" or little pore water recovery for no apparent reason, possibly due to blockage of the rigid plastic tubing in the peat. Typical yields of pore water (excluding "dry holes") ranged from 20 to 60 ml for each interval. Peat cores were compressed by 4-11 cm by squeezing. Squeezing was terminated simply by retracting the threaded rod and piston and removing the sample containers, syringe filters, and luer fittings. The core tube was removed from the squeezer, the nylon screws and washers were replaced in the pore water sampling ports, and the core was extruded. After cleaning, the core tube was ready to be used again.