Abstract Changing precipitation patterns are projected to reduce snowpack storage and late summer stream flows in mountain headwaters of the western United States. Ecosystems, agriculture, and municipalities depend on late summer flow in streams supplying water to the mountain front. Therefore, improved understanding of groundwater storage contributions to mountain streams is increasingly important. In this work, we use 87Sr/86Sr ratios and 234U/238U activity ratios ([234U/238U]) as indicators of water-rock interaction contributing to runoff, groundwater, and surface-subsurface hydrologic exchanges along Hyalite Creek, a mountain headwater tributary within the upper Missouri River basin. Main stem and tributary flow was sampled longitudinally from 2016 to 2018, focusing on presumed baseflow conditions in February and August. Changing stream chemistry of the main stem is associated with groundwater inflows from local springs discharging from Madison Group limestones, and inputs of water from the Archean gneiss exposed along the mountain front. We use mixing models to estimate that locally in the stream reach traversing a spring outlet, distinct inflows from the Madison aquifer contribute ~4% of streamflow during baseflow conditions. Contributions to streamflow by local inflows from Archean gneiss varied seasonally, with increases in 87Sr/86Sr values in Hyalite Creek that suggest contributions of ~2% in August to ~8% in February; at the same time, decreasing [234U/238U] values indicate an otherwise undetected additional endmember, and 222Rn assays show that total inflows are likely more extensive. Our results reveal distinct groundwater contributions to streamflow from specific rock units within this mountain headwater system, and elucidate pathways of mountain stream flow generation in lithologically diverse watersheds.

中文翻译：

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Sr 和 U 同位素揭示了岩性结构对沿山地源头集水区（Hyalite Canyon，MT）地下水贡献的影响

摘要 不断变化的降水模式预计会减少美国西部山区源头的积雪储存和夏末溪流。生态系统、农业和城市依赖于夏末向山前供水的溪流。因此，更好地了解地下水对山区溪流的贡献变得越来越重要。在这项工作中，我们使用 87Sr/86Sr 比率和 234U/238U 活动比率 ([234U/238U]) 作为水岩相互作用的指标，这些相互作用有助于沿山源支流 Hyalite Creek 的径流、地下水和地表-地下水文交换在密苏里河流域上游。从 2016 年到 2018 年，对干流和支流进行了纵向采样，重点是 2 月和 8 月的假定基流条件。主干河流化学变化与从麦迪逊群石灰岩排放的当地泉水流入的地下水以及从沿山前暴露的太古代片麻岩流入的水有关。我们使用混合模型来估计在穿过弹簧出口的局部河流中，来自麦迪逊含水层的明显流入在基流条件下贡献了约 4% 的流量。太古宙片麻岩的局部流入对水流的贡献随季节变化，Hyalite Creek 的 87Sr/86Sr 值增加，表明 8 月的贡献约为 2%，2 月的约为 8% 同时，降低 [234U/238U] 值表明存在未检测到的额外端元，222Rn 分析表明总流入量可能更广泛。