As extreme precipitation events intensify hydrologic connectivity and increase in frequency across monsoon regions, deciphering its transient water quality responses becomes crucial for predicting ecological and water security impacts. This study integrates stable isotopes (delta D, delta N-15-NO3-, delta O-18-NO3-) with hydrochemistry in a North China mountain catchment to unravel the dynamics of hydrologic connectivity and its control on solute transport mechanisms during storm episodes. The results showed that extreme rainfall contributed 29% to groundwater recharge, yet elevated total dissolved solids by 18%, primarily through enhanced HCO3- and Ca2+. Streamflow delta D shifted rapidly toward precipitation values during storms, with precipitation-derived water constituting >62.8% of streamflow at peak discharge, highlighting a transient yet intensified connection from groundwater to precipitation with elevated discharge. While most solutes underwent dilution during high flow, Na+, Mg2+, and Ca2+ exhibited near-chemostatic behavior with distinct concentration-discharge slopes on the rising and falling limbs, indicating different transport pathways. Groundwater NO3- concentration rose by 16.5 mg L-1 after the extreme event, driven by an increase in the chemical fertilizer contribution from 16.6% to 25%, thereby increasing ecosystem contamination risks. These findings establish precipitation-mediated connectivity as a dual regulator: sustaining geogenic solutes while accelerating anthropogenic nutrient mobilization.