A New Method for Characterization of Porewater Chemistry in Low-Permeability Sedimentary Rocks

Celejewski, Magda

A New Method for Characterization of Porewater Chemistry in Low-Permeability Sedimentary Rocks

Presented on Wednesday, May 7, 2014

Magda Celejewski¹ and Tom A. Al², (1)Department of Earth Sciences, University of New Brunswick, Fredericton, NB, Canada, (2)Deptartment of Geology, University of New Brunswick, Fredericton, NB, Canada

Characterization of porewater chemistry in low-permeability rocks can provide insight into the origin and residence time of porewater, the history of fluid movement, and the nature of transport and reaction processes. However, the measurement of porewater chemistry in low‑permeability rocks is challenging because of the small fluid volume and the difficulty of extracting representative samples. Several techniques for porewater characterization are available, but the results they provide can be affected by ion exchange and mineral dissolution, and they may require independent porosity measurements. The objectives of this work are to develop and test a method of extracting representative samples of in situ porewater from low‑permeability rocks and to accurately quantify solute concentrations in the extracted porewater. The method involves extraction of porewater by absorption into hydrophilic cellulosic membranes. The masses of solutes extracted with the porewater are measured by inductively coupled mass spectrometry (ICP‑MS), and the mass of extracted water is measured by near infrared (NIR) spectrometry. In-situ porewater solute concentrations are obtained by normalizing solute mass to water mass. Laboratory experiments using controlled additions of brine to cellulosic membranes confirm that the method is capable of providing porewater solute concentrations with precision and accuracy that are within the limits suggested by USEPA Method 6020A for analysis of saline water samples by ICP-MS. This presentation will provide a detailed description of the absorption method and show the results of a field trial in which concentrations of major porewater ions (Na⁺, K⁺, Mg²⁺, Ca²⁺, Sr²⁺, Cl^-, Br^-) in a low-permeability shale (K < 10^-12 m/s, porosity < 10 %) were quantified and compared to results from the crush-and-leach method obtained from paired samples. The results of the comparison demonstrate that the absorption method provides precise porewater chemistry data that are free from ion exchange artefacts.

MP3 Presentation

Magda Celejewski

Department of Earth Sciences, University of New Brunswick, Fredericton, NB, Canada

Magda Celejewski completed a B.Sc.Hons. at the University of Toronto with a double major in Environmental Biology and Chemistry and an M.Env.Sci. at the University of Toronto with a direct focus on earthworm metabolomics. She is currently a Ph.D. candidate at the University of New Brunswick investigating environmental hydrogeochemistry.

Tom A. Al

Deptartment of Geology, University of New Brunswick, Fredericton, NB, Canada

Tom Al is an Associate Professor in the Department of Geology at the University of New Brunswick. His research interests include mineral-water reaction processes that affect transport of contaminants in groundwater and surface water, geochemical and hydrologic processes controlling the release and transport of metals from sulfide-bearing mine waste, and geochemical reaction processes associated with in-situ oxidation of organic contaminants in groundwater.

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