Process dominance shift in solute chemistry as revealed by long-term high-frequency water chemistry observations of groundwater flowing through weathered argillite underlying a steep forested hillslope
Process dominance shift in solute chemistry as revealed by long-term high-frequency water chemistry observations of groundwater flowing through weathered argillite underlying a steep forested hillslope
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DOI：
10.1016/j.gca.2014.05.011
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发表时间：
2014-09
影响因子：
5
通讯作者：
Hyojin Kim;J. Bishop;W. Dietrich;I. Fung
中科院分区：
文献类型：
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作者：
Hyojin Kim;J. Bishop;W. Dietrich;I. Fung
许多研究表明，来自风化基岩区（土壤和腐泥土之下）的重要溶质通量。然而，在这个区域的水化学动力学控制过程知之甚少。这项工作报告的第一个结果，从四年（2009-2012年）的高频率（1-3天）的主要溶质（钙，镁，钠，钾和硅）的栖息，动态地下水在4000平方米的零阶盆地位于安杰洛海岸山脉保护区，北方加州。地下水样品自动收集在三个威尔斯井（下坡，中坡，上坡）与排水轴对齐。降雨和穿透降水样本、井口顶空pCO 2剖面图、垂直剖面图和地下水温度时间序列，从一个广泛的水文和气候传感器网络的同期数据提供了数据分析的框架。所有径流在这个土壤覆盖的网站发生的垂直非饱和流通过一个5- 25 m厚的风化泥质岩，然后由于地下水停留在较新鲜的基岩上而侧向流到邻近的河道。在强烈的季节性降雨的驱动下，在四年的观测中，每口井的地下水水化学重复一个年度循环，这可以用两个端元过程来解释。第一端元过程，在冬季高流量季节在中，上坡地区占主导地位，是CO2增强的阳离子交换反应，在包气带中更浅的导电风化基岩。这一过程迅速增加了渗透雨水的阳离子浓度，这是地下水的最低阳离子浓度的原因。第二个端元过程发生在更深的栖息地下水和主导全年（在下坡井）或在两个上坡威尔斯井的低流量季节逐渐占主导地位。这个过程是与矿物如方解石和粘土矿物的平衡反应，但不是与原生矿物，这表明水的停留时间的关键作用。总的来说，我们的测量结果表明，在风化基岩区的地下水的水化学动力学是由两个端元过程，其优势随临界区结构，渗流与地下水区域过程的相对重要性，从而与补给和径流的化学季节性变化而变化。
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 m2zero-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 CO2enhanced 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.