NMT Rio Hondo Geochemistry, General Chemistry (July 16th-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担忧与日俱增，明确地表水体的补给来源变得愈发关键。在美国西部诸州（如新墨西哥州），多数地表水资源均源自山地流域。然而，此类山地系统内地下水与地表水的相互作用机制仍鲜有研究探明。研究人员在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表明，可能存在大量地下水补给地表水的情况，而这一点在既往研究中基本未被关注。对地表水、地下水及泉水的δ18O和δ2H稳定同位素分析，以及皮珀三角图（Piper diagrams）分析结果显示，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流域汇水面积增大而升高，而稳定同位素特征则保持恒定，这表明该流域水体的补给来源相似，但在不同径流路径上经历了不同程度的地球化学演化。SiO₂与Ca²+、Na+、Mg²+及K+的相关性分析结果显示，从流域源头到出口，地下水溶质浓度呈现出空间演化特征。我们提出假说：地表水溶质浓度升高受控于深层、地球化学演化程度更高的地下水补给。这一结论与Frisbee等人（2011年）在萨瓜奇流域（Saguache Watershed）的研究发现类似，尽管本研究的流域规模更小，且地质背景存在差异。鉴于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，才能达到观测到的化学组成。若大量古老地下水补给地表水，将使地表水系统具备更强的气候变化缓冲能力。此类流域中的深层地下水组分此前基本未被探索。本研究为上述假说提供了支撑，并表明地表水的深层地下水补给可能发生在比既往研究报道更小的流域尺度上。