High-resolution pore-water sampling with a gel sampler

Sediment pore-water profiles were sampled at high resolution (millimeter scale) with a polyacrylamide gel probe. This simple procedure involves inserting a l-mm-thick gel held in a plastic probe into sediment. The gel reaches diffusive equilibrium in < l-2 h. For anions, the gel was sectioned, back-equilibrated into distilled-deionized water, and anions determined by high-performance liquid chromatography. Laboratory trials showed recovery of 104*4% Cl, 102&2% NOs, lOl& 1% SO4, and 102-t2% NH,,. For Fe and Mn, the gel was fixed in 0.0 1 M NaOH for 3 h, subsectioned, extracted with 1 M HNO,, and analyzed by atomic absorption spectrometry. Field trials were undertaken in Esthwaite Water, a seasonally anoxic lake in the English Lake District. Gel probe data compared well with conventional pore-water extractions. The chemistry of interstitial waters, both freshwater and marine, has been used to study biogeochemical processes in Recent sediments (e.g. Santschi et al. 1990), and gradients at the sediment-water interface have been used to calculate the fluxes of geochemically important solutes into or out of the sediment (e.g. Kelly et al. 1987; Kuivila et al. 1989). Pore waters have been sampled by a variety of techniques, including centrifugation (Hamilton-Taylor and Morris 1985), in situ sampling (Kuivila et al. 1989), and peepers (Hesslein 1976). These conventional methods have a common limitation in that sampling at a resolution of finer than several millimeters to centimeters is problematic because of construction and practical handling difficulties. In order to study biogeochemical processes in organic-rich sediments or fluxes through the benthic boundary, it is desirable to sample at higher resolutions. Over the past decade, oxygen microelectrodes have been used to resolve concentration