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 isotopic signatures than water in the downstream area. Human activities influenced the type of water in the area. Deep groundwater is recharged by both the river and shallow groundwater, with shallow groundwater contributing about 50%. In the plain area, about 20% of shallow groundwater flows back into the river. These findings help us better manage and sustainably use water resources in the region.