NMT Rio Hondo Geochemistry, Low Chloride and Bromide (Jan 8th-9th, 2012 and March 23rd-25th, 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担忧与日俱增，明确地表水的补给来源变得愈发重要。在美国西部各州（如新墨西哥州），绝大多数地表水均源自山地流域。然而，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。从新墨西哥州北部一处中尺度（约200 km²）流域采集的地球化学数据表明，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被大幅忽略。对地表水、地下水及泉水的δ¹⁸O、δ²H稳定同位素分析结果，以及皮珀三角图（Piper diagram）分析均显示，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却保持稳定，这表明该区域地表水的补给来源区域相似，但在不同径流路径上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+）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。我们提出假说：地表水溶质浓度的升高，由地球化学演化程度更高的深层地下水补给所驱动。这一发现与Frisbee等人2011年在萨瓜奇流域（Saguache Watershed）的观测结果类似，但本次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。流域中的这类深层地下水补给效应，在很大程度上尚未得到深入研究。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效应，可能在比既往研究报道更小的流域尺度上发生。