NMT Rio Hondo Geochemistry, Stable Isotopes (precipitation samples May 31st, June 1st, and June 2nd - July 16th and 17th 2012; June 28-29th 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、气温升高以及土地利用变化的担忧与日俱增，明确地表水的补给来源正变得愈发重要。在美国西部的新墨西哥州等州，绝大多数地表水均源自山地流域，但目前学界对这些山地系统内地下水与地表水的交互作用仍知之甚少。研究人员从新墨西哥州北部一处中尺度（约200 km²）流域采集了地球化学数据，结果显示地下水对地表水或存在显著补给，而这一现象在既往研究中基本被忽略。对地表水、地下水与泉水的δ¹⁸O、δ²H稳定同位素分析及派珀图（Piper diagrams）分析结果显示，三者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，但稳定同位素特征保持恒定，这表明该流域内地表水的补给区域大致相似，但沿不同径流路径经历了不同程度的地球化学演化过程。以二氧化硅（SiO₂）与钙离子（Ca²+）、钠离子（Na+）、镁离子（Mg²+）及钾离子（K+）绘制的散点图显示，地下水溶质浓度从源头到流域出口呈现出空间演化特征。本研究提出假说：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。这与弗里斯比等人（Frisbee et al., 2011）在萨瓦奇流域（Saguache Watershed）的研究发现相似，尽管本研究的流域规模显著更小，且地质背景存在差异。考虑到所涉及的化学动力学过程，这类地球化学演化程度更高的深层地下水，需要在特定径流路径上拥有更长的停留时间，才能形成观测到的化学组成。古老地下水对地表水的显著补给，可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迄今基本未被探索。本研究为上述假说提供了支撑，并表明地下水对地表水的深层补给可能发生在比既往研究报道更小的流域尺度上。