Mountain System Recharge (MRS) processes are the natural recharge pathways in arid and semi-arid mountainous regions. However, MSR processes are often poorly understood and characterized in hydrologic models. Mountains are the primary source of water supply to valley aquifers via multiple pathways including lateral groundwater flow from the mountain block (Mountain-block Recharge, MBR) and focused recharge from mountain streams contributing to mountain front recharge (MFR) at the piedmont zone. Here, we present a multi-tool isogeochemical approach to characterize mountain flow paths and MSR processes 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. Isotope tracers helped to validate MSR processes. Novel application of End-Member Mixing Analysis (EMMA) using conservative chemical components revealed three MSR end-members: (1) evaporated Ca-HCO3 water type associated with MFR, (2) non-evaporated Ca-HCO3 and Na-HCO3 water types with short residence times associated with shallow MBR, and (3) Na-HCO3 groundwater type with long residence time associated with deep MBR. We quantified the contribution of each MSR process to the valley aquifer using mixing ratio calculation (MIX). Our results show that deep MBR is a significant component of recharge representing more than 50% of the valley groundwater. Greater hydraulic connectivity between the Sierra Nevada and Central Valley has significant implications for parameterizing Central Valley groundwater flow models and improving groundwater management. Our framework is useful for understanding MSR processes in other snow-dominated mountain watersheds.

Numerous socio-economic activities depend on the seasonal rainfall and groundwater recharge cycle across the Central American Isthmus. Population growth and unregulated land use changes resulted in extensive surface water pollution and a large dependency on groundwater resources. This work combines stable isotope variations in rainfall, surface water, and groundwater of Costa Rica, Nicaragua, El Salvador, and Honduras to develop a regionalized rainfall isoscape, isotopic lapse rates, spatial-temporal isotopic variations, and air mass back trajectories determining potential mean recharge elevations, moisture circulation patterns, and surface water-groundwater interactions. Intra-seasonal rainfall modes resulted in two isotopically depleted incursions (W-shaped isotopic pattern) during the wet season and two enriched pulses during the Mid-Summer Drought and the months of the strongest trade winds. Notable isotopic sub-cloud fractionation and near-surface secondary evaporation were identified as common denominators within the Central American Dry Corridor. Groundwater and surface water isotope ratios depicted the strong orographic separation into the Caribbean and Pacific domains, mainly induced by the governing moisture transport from the Caribbean Sea, complex rainfall producing systems across the N-S mountain range, and the subsequent mixing with local evapotranspiration, and, to a lesser degree, the eastern Pacific Ocean fluxes. Groundwater recharge was characterized by a) depleted recharge in highland areas (72.3%), b) rapid recharge via preferential flow paths (13.1%), and enriched recharge due to near-surface secondary fractionation (14.6%). Median recharge elevation ranged from 1,104 to 1,979 m asl. These results are intended to enhance forest conservation practices, inform water protection regulations, and facilitate water security and sustainability planning in the Central American Isthmus.