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.

Methods
We used hydrochemical and isotope data from the US Geological Survey Groundwater Ambient Monitoring and Assessment (GAMA) program (Bennett et al., 2017) and processed these data for End-Member Mixing Analysis (EMMA) and mixing ratio calculations using MIX program.