POLYACRYLAMIDE GELS, MICROELECTRODES, AND CENTRIFUGATION-COLORIMETRIC MEASURES: A COMPARISON OF POREWATER ANALYSIS METHODOLOGIES

Sediment diagenesis and associated redox environments are import for understanding elemental cycles, chemical fate, transport, storage, and estuarine behavior and response to external stressors (e.g., nutrient loads, hypoxia, and other contaminants). 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. Ideally, ex-situ methods will mimic in-situ conditions, will not introduce error, disturbance, or contamination to the analyte of interest, and extraction times will be expedient. However, 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 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 was conducted in Bay St Louis, Mississippi at multiple locations to compare recovery and accuracy of these analytes as measured via traditional methods (e.g., the use of solid state microelectrodes and porewater extraction via centrifugation) and the DGT method. Results indicate that the DGT method recovery is 78% in the laboratory experiments and field results align with traditional methods. As such, we recommend the DGT method as a reasonable alternative to traditional geochemical methods for porewater measurement of dissolved H2S, Mn2+ and Fe2+.