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Characterization of porewater chemistry in low-permeability, clay-rich rocks provides insights into solute transport mechanisms and the origin and residence time of porewater. Extraction of porewater for chemical quantification is challenging, and several methods have been applied including squeezing, advective displacement, crush and leach, and a relatively new technique that extracts porewater by absorption into a cellulosic paper by capillary action. Here we compare porewater Cl− and Br− mass ratios from samples using the paper-absorption and crush-and-leach techniques. Samples were obtained from Upper Ordovician shales in the Michigan Basin in Ontario, the Opalinus Clay at the Mont Terri Rock Laboratory in Switzerland, and the Upper Ordovician Lorraine Group shale in southern Quebec. The data display consistent and reproducible differences among methods for Cl− and Br− mass ratios, with the paper-absorption method producing systematically lower Cl− : Br− ratios. The observed differences in Cl− : Br− ratios are attributed primarily to anion exclusion effects which are stronger for Br− than for Cl−, resulting in higher Br− concentrations in the largest pores that are preferentially sampled by the paper-absorption technique. In addition, calculations suggest that Cl− is more effective than Br− in forming ion pairs and clusters with neutral or positive charge which can enter the diffuse double layer. This causes a further decrease in the Cl− : Br− ratios for the mobile water. One important message from this work is that different extraction methods should not be expected to converge on a unique porewater Cl− : Br− ratio because each method reflects different proportions of the interlayer, diffuse double layer, and mobile fractions of porewater.

Measurement of Cl− : Br− Ratios in the Porewater of Clay-Rich Rocks—A Comparison of the Crush-and-Leach and the Paper-Absorption Methods