Groundwater in the Zhangjiakou area, North China, is being depleted, but little is known about its hydrological processes. By studying water isotopes and hydrochemistry, researchers found that groundwater is mainly recharged by precipitation. Shallow groundwater is modern, while deeper groundwater is a mix of old and new water. Confined aquifers contain older groundwater, over 60 years old. Groundwater flow velocity ranges from 1.10 to 2.26 meters per year, with recharge rates from 0.034 to 0.203 meters per year. This information helps us understand where the groundwater comes from and how to manage it sustainably.