ERCZO -- Rainfall Chemistry, Stream Water Chemistry, Throughfall Chemistry, Groundwater Chemistry -- Solute chemistry -- Rivendell -- (2007-2015)

Significant solute flux from the weathered bedrock zone - which underlies soils and saprolite - has been suggested by many studies. However, controlling processes for the hydrochemistry dynamics in this zone are poorly understood. This work reports the first results from a four-year (2009-2012) high-frequency (1-3 day) monitoring of major solutes (Ca, Mg, Na, K and Si) in the perched, dynamic groundwater in a 4000 m2 zero-order basin located at the Angelo Coast Range Reserve, Northern California. Groundwater samples were autonomously collected at three wells (downslope, mid-slope, and upslope) aligned with the axis of the drainage. Rain and throughfall samples, profiles of well headspace pCO2, vertical profiles and time series of groundwater temperature, and contemporaneous data from an extensive hydrologic and climate sensor network provided the framework for data analysis.

All runoff at this soil-mantled site occurs by vertical unsaturated flow through a 5-25 m thick weathered argillite and then by lateral flows to the adjacent channel as groundwater perched over fresher bedrock. Driven by strongly seasonal rainfall, over each of the four years of observations, the hydrochemistry of the groundwater at each well repeats an annual cycle, which can be explained by two end-member processes. The first end-member process, which dominates during the winter high-flow season in mid- and upslope areas, is CO2 enhanced cation exchange reaction in the vadose zone in the more shallow conductive weathered bedrock. This process rapidly increases the cation concentrations of the infiltrated rainwater, which is responsible for the lowest cation concentration of groundwater. The second-end member process occurs in the deeper perched groundwater and either dominates year-round (at the downslope well) or becomes progressively dominant during low flow season at the two upper slope wells. This process is the equilibrium reaction with minerals such as calcite and clay minerals, but not with primary minerals, suggesting the critical role of the residence time of the water. Collectively, our measurements reveal that the hydrochemistry dynamics of the groundwater in the weathered bedrock zone is governed by two end-member processes whose dominance varies with critical zone structure, the relative importance of vadose versus groundwater zone processes, and thus with the seasonal variation of the chemistry of recharge and runoff.

众多研究指出，从风化基岩区（该区位于土壤和风化壳之下）显著的溶质通量。然而，该区域水化学动态的控制过程尚未被充分理解。本研究报告了首次从位于加利福尼亚北部安乔海岸山脉保护区一个4000平方米的一级流域中，对停滞、动态地下水中主要溶质（Ca、Mg、Na、K和Si）进行为期四年（2009-2012年）的高频（1-3天）监测的结果。地下水样品在三个井（下坡、中坡和上坡）中自动收集，这些井与排水轴线对齐。雨水和穿透雨样品、井口空间pCO2的剖面、地下水温度的垂直剖面和时间序列，以及广泛的 hydrologic 和 climate 传感器网络的数据提供了数据分析的框架。在该土壤覆盖的地点，所有径流均通过5-25米厚的风化板岩的垂直非饱和流动产生，然后通过横向流动到达相邻的河道，作为停滞在较新鲜基岩之上的地下水。受强烈季节性降雨驱动，在观察的四年中，每个井的地下水水化学特征重复出现年度循环，这可以解释为两种极端过程。第一种极端过程，在冬季高流量季节的中坡和上坡区域占主导地位，是 vadose 区中较浅的导水风化基岩中的 CO2 增强阳离子交换反应。这个过程迅速增加渗入雨水的阳离子浓度，这是地下水最低阳离子浓度的原因。第二种极端过程发生在较深的停滞地下水中，或者在全年（下坡井）占主导地位，或者在低流量季节逐渐成为两个上坡井的主导过程。这个过程是与方解石和粘土矿物等矿物的平衡反应，而不是与原生矿物的反应，这表明了水停留时间的关键作用。总体而言，我们的测量结果表明，风化基岩区地下水的水化学动态受两种极端过程所控制，其主导地位随着关键区结构、 vadose 区与地下水区过程的相对重要性以及因此化学补给和径流季节变化的化学特性而变化。