Tracing recharge and groundwater evolution in a glaciated, regional-scale carbonate bedrock aquifer system, southern Ontario, Canada

Highlights: • Traditional groundwater tracers tested in carbonate aquifers in a glaciated setting. • High tritium and aerobic chemistry show recharge to bedrock where drift is thin. • Sulphate isotopes are the most informative as a marker of redox and residence time. • Strontium isotopes are ineffective in bedrock from sediment signal overprint. • Regional-scale model of recharge and evolution developed using best tracers. Where carbonate bedrock aquifers are overlain by glacial sediments, these sediments control the locations and geochemical signatures of recharge to the bedrock groundwater system. In these settings, geochemical and isotopic tracer tools may be rendered ineffective due to geochemical fingerprinting that can develop as groundwater migrates through the sediments and carbonates of the recharge pathway. In this investigation, traditional tracer tools are assessed in an 8000 km² study area in southern Ontario, Canada, where the Early Silurian carbonate bedrock aquifers are overlain by glacial sediments. These carbonate aquifers contain significant quantities of high-quality groundwater resources and provide the sole drinking water source to many large cities and private residences. The glacial history of the study area is complex, with the advance and retreat of three ice lobes having deposited sediments that vary widely in permeability, mineralogy, and geochemistry. Results show that spatial trends of higher tritium correlate with aerobic redox chemistry in the carbonate groundwater systems underlying areas of thin or permeable sediment cover. Groundwater chemical evolution beyond recharge areas is assessed with general chemistry, redox characteristics and an investigation of water-rock interactions. A comparison of strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) in bedrock and groundwater shows that long residence times may be required for the isotopic signature of the carbonate bedrock to imprint on the groundwater, though this does not occur consistently. The isotopic composition of sulphate (δ³⁴S_SO4 and δ¹⁸O_SO4) in groundwater was most informative, showing isotopic evidence of sulphide oxidation in recharge areas, and a Silurian sulphur isotopic signature of the host bedrock in areas of thick and low permeability sediment, downgradient of identified recharge areas. The set of tracer tools deemed most useful in this investigation provides the empirical evidence needed to support a conceptual model of recharge and groundwater evolution and is recommended for use in similar settings elsewhere.