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 8,000 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 (δ34SSO4 and δ18OSO4) 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.

中文翻译：

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追踪加拿大安大略省南部冰川区域规模碳酸盐基岩含水层系统的补给和地下水演化。

在碳酸盐基岩含水层被冰川沉积物覆盖的地方，这些沉积物控制着基岩地下水系统补给的位置和地球化学特征。在这些情况下，地球化学和同位素示踪工具可能会因地球化学指纹图谱而变得无效，因为地下水通过补给路径的沉积物和碳酸盐迁移时会产生地球化学指纹。在本次调查中，在加拿大安大略省南部 8,000 平方公里的研究区对传统示踪工具进行了评估，那里的早志留世碳酸盐基岩含水层被冰川沉积物覆盖。这些碳酸盐含水层含有大量优质地下水资源，为许多大城市和私人住宅提供唯一的饮用水源。研究区冰川历史复杂，随着三个冰瓣的进退，沉积的沉积物在渗透性、矿物学和地球化学方面差异很大。结果表明，高氚的空间趋势与薄层或可渗透沉积物覆盖区域下的碳酸盐地下水系统中的有氧氧化还原化学相关。补给区以外的地下水化学演化通过一般化学、氧化还原特性和水-岩相互作用的调查进行评估。基岩和地下水中锶同位素比 (87Sr/86Sr) 的比较表明，碳酸盐基岩的同位素特征可能需要较长的停留时间才能印记在地下水上，尽管这种情况并不一致。地下水中硫酸盐（δ34SSO4 和 δ18OSO4）的同位素组成最能提供信息，显示补给区硫化物氧化的同位素证据，以及厚和低渗透性沉积物区域中寄主基岩的志留纪硫同位素特征，已确定补给区的下坡。在本次调查中被认为最有用的一组示踪工具提供了支持补给和地下水演化概念模型所需的经验证据，并建议在其他地方的类似环境中使用。