The researchers studied how surface water and groundwater are connected in the second Songhua River basin. They used stable isotopes and hydrochemistry to analyze water samples. They found that water in the upstream area had different isotopes than water in the downstream area. Human activities affected the type of water in the area. Shallow groundwater contributes about 50% to deep groundwater, with 20% of shallow groundwater flowing back into the river in the plain area. These findings help understand how to use surface water and groundwater sustainably.