A review of the major chemical and isotopic characteristics of groundwater in crystalline rocks of the Canadian Shield

Abstract

Canadian Shield groundwater characterization studies have been conducted over the last four decades through operating mines, underground research areas, and wells installed for a variety of purposes. A newly available database containing chemical and isotopic analytical results of groundwaters includes data from all of these sources and spans the Canadian Shield. The Precambrian Canadian Shield Groundwater and Gas Geochemistry (PCSG³) database provides the most comprehensive data of groundwater chemistry in the Canadian Shield region to date. In this initial review of the PCSG³ database, major ion, Br, water type, δ¹⁸O, δ²H and ³H trends with depth and salinity are described. In future reviews, other aspects of the database will be examined, including minor and trace elements, other isotopes, and gases. The PCSG³ database is comprised of 69 % freshwaters, mostly found at depths <1000 m, and dominated by Ca-HCO₃ and Na-HCO₃ water types; 17 % brackish waters, at depths <2000 m, and dominated by NaCl, CaCl, Ca-HCO₃, and Ca-SO₄ water types; 9 % saline waters, mostly at depths >1000 m, and dominated by CaCl, NaCl, Ca-HCO₃, and Na-HCO₃ water types; and 4 % brine samples, identified at depths up to 1800 m, composed entirely of CaCl water types. An increase in all major ion and Br concentrations with depth is observed, except for HCO₃, with groundwaters typically becoming more saline with depth. Variability in salinities at specific depths and at individual sites across the Canadian Shield reflect the effect of (primarily) anthropogenically induced mixing, although heterogeneous geology and hydrogeologic flow paths are also important. When eliminating data that are likely affected by anthropogenically-induced mixing, saline waters are the most affected, with median depths of NaCl and CaCl type waters shifting downward, and median concentrations of NaCl type waters increasing. The isotopic data indicate that groundwaters from across the Canadian Shield reflect a variety of water recharge sources, with many samples plotting along the Global Meteoric Water Line (GMWL). Deviations to the right of the GMWL are indicative of freezing and mixing with drill fluid, while deviations to the left are indicative of isotopic exchange between water and rock and/or silicate hydration over long time scales. Rock/silicate-water interactions over hundreds of million of years resulted in development and isolation of CaCl brines at multiple locations. Cold climate processes, including glacial meltwater recharge and/or ionic concentration during permafrost formation, yielded a third end-member. A conceptual model of the expected groundwater chemistry variation with depth across the Canadian Shield is presented based on an evaluation of the variation of water type and salinity with depth. This model shows that groundwater chemistry changes from shallow groundwater dominated by Ca,Na-HCO₃ to deep groundwater dominated by CaCl, and details intermediate water type and TDS variations with depth.