Since most nutrients and contaminants contained in sediments are predominantly associated with the solid phase, small changes in their sediment concentrations can translate into relatively large changes in their porewater concentrations. As a consequence, measurements of pore-water concentrations are usually the most sensitive indicator of processes occurring in sediments. Similarly, measurements of concentration gradients in porewaters enable calculations of fluxes within sediments and across the sediment-water interface.
Consequently, several methods have been developed for sampling sediment porewaters. (1) Interstitial water can be extracted by squeezing or centrifugation of successive slices of a sediment core, after it has been collected. (2) Interstitial water may also be collected in situ, using peepers. These are typically plastic devices with a vertical series of small chambers initially filled with high-purity water, which are enclosed within semipermeable membranes. The peepers are inserted into the sediments for an extended period (^weeks) to allow the chambers to equilibrate with porewater, after which they are collected and the water in the peepers is extracted. (3) Interstitial water may also be collected in thin gels (DET-DGT) placed between two plates, which are inserted into sediments to allow equilibration of the gels with porewaters before they are collected and sampled. (4) Finally, microelectrodes can be either inserted directly in the sediment or in sediment cores immediately after sampling. Since there are advantages, disadvantages, and limitations for each of these methods, they are often used in combination to obtain a suite of complementary measurements, along with concurrent measurements of particulate sediment concentrations.
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