Groundwaters with total dissolved solids up to 9.5 g/l occur at depths of <50 m in the Paleozoic strata of the Niagara Peninsula. Major ions and trace elements are 1.5–40 times higher than in the same strata elsewhere in Ontario. The δ18O and δ2H values (δ18OH2O = −6.9‰ to −15‰; δ2HH2O = −49.4‰ to −105‰) of groundwater suggest that meteoric precipitation is the primary source with a small portion recharged during Pleistocene glaciation. Tritium (up to 17.42 TU) and coliform bacteria (up to 600 counts/100 ml) data indicate groundwater is generally recently recharged and undergoes a series of modifications that progressively increase salinity. Elemental concentrations and δ34S of dissolved sulphate (δ34SSO4 = + 5‰ to + 45‰) indicate that the salinization is due to multiple processes: (1) road salt inputs into shallow groundwater, (2) upward flow of basinal brines that mix with meteoric groundwater, (3) gypsum and anhydrite dissolution, and (4) sulphide oxidation of minor metallic sulphide minerals. Comparison with water from flowing abandoned gas wells suggests some are acting as conduits allowing upward flow of deeper (Appalachian Basin) brines. The distribution of brine-impacted wells suggests natural vertical pathways also contribute to upward transport and mixing of brine.

Supplementary material: The full set of δ34SSO4 in sulphate data are available at: https://doi.org/10.6084/m9.figshare.c.4012885

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