Porewater characterization in low-permeability geologic materials using a novel extraction method

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University of New Brunswick
Characterization of porewater chemistry in low-permeability sedimentary deposits provides insights into solute transport mechanisms and the origin and residence time of fluids. Recent interest in solute transport through low-permeability rocks defined by low hydraulic conductivity (<10−11 m/s) and diffusion dominated solute transport has been prompted by requirements for long-term geological sequestration of CO2 and isolation of various waste forms, including nuclear waste. Additionally, development of landslide potential in sensitive marine muds with low hydraulic conductivity is dependent on solute distribution through the deposit. However, representative sampling of porewater from materials with low permeability and small porewater volumes remains a challenge. The main objective of this thesis is to develop and establish a novel method for extraction and characterization of in-situ porewater by capillarity and diffusion, targeting only the water in the most interconnected pore spaces. This method uses a cellulose-based material with a low chemical background and exhibiting no preferential sorption of major porewater solutes. The feasibility of porewater extraction by this cellulosic material from sediments and rocks with low hydraulic conductivity is demonstrated on samples from the Opalinus Clay, Switzerland, the Upper Ordovician Lorraine Group shale, Quebec, the Upper Ordovician shales of the Michigan Basin, Ontario, and the Champlain Sea mud, Ontario. Results of Cl− and Br− masses obtained by the paper absorption method from shale and clay-rocks produce systematically lower Cl− : Br− ratios than those from a crush-and-leach method. The magnitude of the difference in Cl− : Br− ratios is inversely related to pore throat diameters, and is attributed to size-specific anion exclusion effects. A benchmarking experiment conducted on Champlain Sea mud demonstrated that porewater solute concentrations measured in paper-absorption and centrifuge extracts compare well. However, alkaline-earth metal concentrations are systematically lower in porewater extracted by the paper absorption method, indicating potential sorption to the absorbing paper or fractionation during centrifugation. The results of this work highlight the fact that different porewater extraction methods yield different proportions of porewater from the mobile and bound fractions. As such, understanding the unique characteristics or biases of each method is important when assessing solute distribution and transport.
NATURAL SCIENCES::Earth sciences::Exogenous earth sciences::Sedimentology