Evolution of Hydrochemical Characteristics of Shallow Groundwater in the Karst Critical Zone of Southwest China and Its Response to Environmental Changes

• Four-year monitoring reveals warming-driven hydrochemical evolution in karst groundwater. • δ 2 H-δ 18 O isotopes and hydrochemistry identify three distinct groundwater flow paths. • δ 15 N-δ 18 O isotopes indicate soil organic nitrogen as primary nitrate source with nitrification. • Temperature rise (+1.67°C) increases nitrate export despite stable overall water quality. • Carbonate weathering dominates cation inputs (79-84%) with minimal anthropogenic influence.