Use of gel probes for the determination of high resolution solute distributions in marine and estuarine pore waters

Pare water profiles were obtained at high resolution (millimeter scale) in marine sediments using a DET (diffusive equilibration in thin films) gel pr obe. A plastic probe which holds a 1-2 mm thick polyacrylamide gel covered by a 0.45 mu m Millipore filter is inserted into the sediment. The gel was prehydrated in water of similar salinity to the in situ sampling conditions . Chloride and sulphate needed 24 and 48 h for complete front and back equi libration, respectively, while 6-8 h (front) and 4-6 h (back) were used for calcium and alkalinity. Ammonia-N and total CO2 were back-equilibrated for 2 h. Probes were sectioned immediately after sampling, stored for up to 1- 2 days (NH4+, Sigma CO2) or for up to 1-2 weeks (Ca, alkalinity, Cl, SO4, N O3), before they were back-equilibrated into Milli-Q water or 0.7 hi NaCl ( calcium and alkalinity). The gel retains between 3-7% of the total sulphate . A simple procedure has been developed to correct for this incomplete reco very. Recovery tests using seawater and 50% seawater spiked to concentratio ns found in nearshore pore waters, showed recoveries of 101.4 +/- 0.5% (chl oride), 101.4 +/- 0.7% (bromide), 100.3 +/- 0.2% (nitrate), 96.6 +/- 0.7% ( sulphate), 99.7 +/- 0.8% (ammonia-N), 99.1 +/- 1.2% (Sigma CO2), 96.9 +/- 0 .8% (calcium) and 96.8 +/- 1.4% (alkalinity) (mean +/- standard error). Por e water profiles obtained simultaneously using gel probes and conventional techniques (box core-anoxic slicing followed by centrifugation) showed exce llent comparability at cm resolution though features which required higher resolution which could only be seen in the gel profiles. (C) 1998 Elsevier Science B.V. All rights reserved.