NMT Rio Hondo Geochemistry, Stable Isotopes (July 16-18th 2012)

With growing concerns about declining snowpack, warmer temperatures, and land use changes, it is becoming increasingly
important to determine the sources that contribute to surface water. In western states, such as New Mexico, most of the surface
water is derived from mountainous watersheds. However, the interaction between the groundwater and the surface water within these
mountain systems is poorly understood. Geochemical data collected from a mesoscale (~200 km2) watershed in northern New Mexico
indicate there may be significant groundwater contributions to the surface water that have largely been ignored in previous studies.
Stable isotopic analysis of δ18O and δ2H and Piper diagrams for surface water, groundwater, and spring water are not geochemically
distinct. Surface water solute concentrations for most constituents increase as a function of the drainage area while the stable
isotopic signature remains constant, suggesting that the water is sourced from similar areas but has undergone differing degrees
of geochemical evolution along different flow paths. Plots of SiO2 vs Ca2+, Na+, Mg2+, and K+ show evidence of spatial evolution
of groundwater with solute concentrations from the headwaters to the watershed outlet. We hypothesize that the increasing solute
concentrations in the surface water are controlled by inputs from deep, more geochemically evolved groundwater. This is similar
to what Frisbee et al. (2011) saw in the Saguache Watershed, though our watershed is significantly smaller and has a different
geological setting. Due to the chemical kinetics involved, this more geochemically evolved groundwater would require longer residence
time along a given flow path to achieve the observed chemical compositions. Significant contributions of old groundwater to surface
water could result in the surface water system having increased buffering capacity against climate change. This deep groundwater
component in watersheds has largely been unexplored. Our research provides support for our hypothesis and indicates that deep
groundwater contributions to surface water may occur at even smaller scales than previously published.

随着人们对积雪减少、气温升高及土地利用变化的关注度持续攀升，明确地表水的补给来源已变得愈发关键。在新墨西哥州等美国西部州域，绝大多数地表水均源自山地流域。然而，当前学界对这类山地系统内地下水与地表水的相互作用机制仍缺乏深入认知。我们对新墨西哥州北部一处中尺度（约200平方千米）流域采集的地球化学数据开展分析后发现，地下水对地表水的贡献可能十分显著，但这一现象在既往研究中长期被忽视。对地表水、地下水和泉水的δ18O、δ2H稳定同位素分析及派珀图（Piper diagrams）结果显示，三者的地球化学特征并无显著差异。多数组分的地表水溶质浓度随汇水面积增大而升高，但其稳定同位素特征保持稳定，这表明该区域地表水的补给来源相似，但沿不同径流路径经历了不同程度的地球化学演化。以二氧化硅（SiO₂）分别与钙离子（Ca²+）、钠离子（Na+）、镁离子（Mg²+）、钾离子（K+）绘制的相关性散点图表明，地下水的溶质浓度从河源区至流域出口呈现出空间演化特征。我们提出假说：地表水溶质浓度的升高，受控于地球化学演化程度更高的深层地下水补给。这一结论与弗里斯比等人（Frisbee et al., 2011）在萨瓜什流域（Saguache Watershed）的研究结果相似，但本次研究的流域面积显著更小，且地质背景存在差异。由于涉及化学动力学过程，这类地球化学演化程度更高的地下水，需要在特定径流路径上拥有更长的停留时间，才能达到观测到的化学组成。老旧地下水对地表水的显著补给，可能会提升地表水系统应对气候变化的缓冲能力。流域内的深层地下水补给组分，长期以来基本未被探索。本研究为我们的假说提供了支撑，并表明地下水对地表水的深层补给，可能在比既往研究认为的更小尺度上即可发生。