A high - resolution pore water sampler for sandy sediments

In this paper, we present a new technique for collecting pore-water samples in coarseto fine-grained sandy sediments. We have used the technique both for in situ sampling and for pore-water extraction from sediment cores brought back to the laboratory, in both cases with a depth resolution as fine as 1 cm. The key device is a long, thin (;2 mm) stainless steel tube with a specially designed tip that functions as a filter. Pore-water samples are collected by inserting the tube into the sediment at the desired depth and applying a light suction from a syringe connected to the tube. The sample is injected immediately into a vial through a syringe filter and stored for later analysis. The technique has some clear advantages over other methods that are used commonly in sandy sediments. In comparison with lysimeters or sippers, it gives pore-water samples with a much higher depth resolution, which often is needed to accurately estimate fluxes across the sediment-water interface or transformation rates of solutes within the sediments. In comparison with dialysis cells or peepers, the new technique gives measurements that truly represent the time of collection. This can be crucial when non– steady-state systems are studied. Furthermore, disturbance of the sediment is minimal because of the small dimension of the tube. This is especially important in vegetated sediments, in which the deployment of larger conventional samplers is likely to cause significant disturbance. Finally, sample extraction is less laborious than other techniques, requiring only one site visit and typically ,1 min of sampling time per sample. In laboratory tests using a dye tracer, we showed that pore water was drawn evenly toward the tube tip from all directions in the surrounding sediment. On the basis of these results, we provide guidelines for choosing the appropriate sampling depth intervals as a function of the sample size. Data from representative pore-water profiles are presented for PO4 from vegetated coarse-grained carbonate sediments in the Bahamas and NH4 from fine-grained unvegetated sands in a temperate coastal lagoon in Virginia. Concentration profiles of pore water in sediments are used extensively in studies of biogeochemical processes. Vertical gradients of pore-water solutes have been used to calculate fluxes across the sediment-water interface (Glud et al. 1995; Charpy-Roubaud et al. 1996; Urban et al. 1997). Furthermore, estimates of carbon, nitrogen, and phosphorus transformations within the sediments, such as mineralization, denitrification, and benthic photosynthesis, can be obtained from the profile curvature (Revsbech et al. 1981; Dalsgaard and Revsbech 1992; Berg et al. 1998; McGlathery et al. pers. comm.). Microelectrodes (Revsbech et al. 1980) provide an