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In-Situ Polyacrylamide Gels to Ex-Situ Microelectrodes and Centrifugation Colorimetric Measures: A Comparison of Porewater Analysis Methodologies


Monitoring and measuring redox reactive porewater chemical species is important for understanding modern sediment diagenetic behavior; however such work can be difficult. Ex-situ methods for porewater analysis are often employed, such as the retrieval of core samples and subsequent porewater analysis using microelectrode and/or removal of porewaters using centrifugation and measurement via colorimetric methods. The battle faced when using ex-situ methods is the possibility for error, disturbance, and contamination of the analyte of interest from field to laboratory. Ex-situ methods will ideally mimic in-situ conditions. In-situ methods give direct undisturbed measurement for maximum accurate results. Polyacrylamide gels can be deployed in-situ and used to capture specific ions of interest through diffusive gradients in thin film (DGT) eliminating these potential uncertainties. DGTs must equilibrate with the sediments for 18 to 24 hours before analysis can be completed. Once the specific ions are captured by the DGTs they are locked to the binding layer of the gel, and colorimetric analysis can be conducted to provide an accurate sediment depth profile. This study reports on a laboratory refinement and field recovery study of the DGT method for specific retrieval of H2S, Mn2+, and Fe2+ at 2cm sediment depth intervals. Field deployment of the DGTs was conducted in Choctawhatchee Bay, Florida and Bay St Louis, Mississippi at multiple locations in each bay to compare recovery and accuracy of these analytes to traditional ex-situ methods (e.g. the use of solid state microelectrodes and porewater extraction via centrifugation). Results indicate that the DGT method maintains a 78% recovery in the laboratory experiments and field results align with traditional methods. As such, we recommend the DGT in-situ method as a reasonable alternative to traditional ex-situ geochemical methods for porewater measurement of dissolved H2S, Mn2+ and Fe2+.