Isogeochemical characterization of mountain system recharge processes in the Sierra Nevada, California

Mountain System Recharge processes are significant natural recharge
pathways in many arid and semi-arid mountainous regions. However, Mountain
System Recharge processes are often poorly understood and characterized in
hydrologic models. Mountains are the primary water supply source to valley
aquifers via lateral groundwater flow from the mountain block (Mountain
Block Recharge) and focused recharge from mountain streams contributing to
focused Mountain Front Recharge at the piedmont zone. Here, we present a
multi-tool isogeochemical approach to characterize mountain flow paths and
Mountain System Recharge in the northern Tulare Basin, California. We used
groundwater chemistry data to delineate hydrochemical facies and explain
the chemical evolution of groundwater from the Sierra Nevada to the
Central Valley aquifer. Stable isotopes and radiogenic groundwater tracers
validated Mountain System Recharge processes by differentiating focused
from diffuse recharge, and estimating apparent groundwater age,
respectively. Novel application of End-Member Mixing Analysis (EMMA) using
conservative chemical components revealed three Mountain System Recharge
end-members: (1) evaporated Ca-HCO3 water type associated with focused
Mountain Front Recharge, (2) non-evaporated Ca-HCO3 and Na-HCO3 water
types with short residence times associated with shallow Mountain Block
Recharge, and (3) Na-HCO3 groundwater type with long residence time
associated with deep Mountain Block Recharge. We quantified the
contribution of each Mountain System Recharge process to the valley
aquifer by calculating mixing ratios. Our results show that deep Mountain
Block Recharge is a significant recharge component, representing 31 to 53
% of the valley groundwater. Greater hydraulic connectivity between the
Sierra Nevada and Central Valley has significant implications for
parameterizing groundwater flow models. Our framework is useful for
understanding Mountain System Recharge processes in other snow-dominated
mountain watersheds.

山地系统补给（Mountain System Recharge）过程是诸多干旱与半干旱山区的重要天然补给路径。然而，当前水文模型对山地系统补给过程的认知与表征往往不足。山地通过山块侧向地下径流（山块补给，Mountain Block Recharge）与山地溪流的集中补给，为河谷含水层提供主要水源，其中山地溪流的集中补给会在山麓带形成山前集中补给（Mountain Front Recharge）。本研究针对美国加利福尼亚州图莱里盆地北部的山地径流路径与山地系统补给问题，提出了一套多工具同位素地球化学分析方法。研究团队利用地下水化学数据划分水文化学相，并阐释了从内华达山脉到中央谷地含水层的地下水化学演化过程。稳定同位素与放射性地下水示踪剂分别通过区分集中补给与弥散补给、估算表观地下水年龄，验证了山地系统补给过程。本研究创新性地将基于保守化学成分的端元混合分析（End-Member Mixing Analysis, EMMA）方法应用于该研究区，识别出三类山地系统补给端元：（1）与山前集中补给相关的蒸发型Ca-HCO3型水；（2）滞留时间较短、对应浅层山块补给的非蒸发型Ca-HCO3与Na-HCO3型水；（3）滞留时间较长、对应深层山块补给的Na-HCO3型地下水。研究通过计算混合比例，量化了各类山地系统补给过程对河谷含水层的贡献占比。结果显示，深层山块补给是重要的补给组分，占河谷地下水总量的31%至53%。内华达山脉与中央谷地之间较强的水力连通性，对地下水流模型的参数化工作具有重要指导意义。本研究提出的分析框架，可为其他以积雪为主导的山地流域的山地系统补给过程研究提供参考。