Chemical and Isotopic Constraints on Groundwater Surface Water Interaction in a Wetland Terrain with Implications on Sustainable Agriculture: A Case Study of the Sanjiang Plain, North East China

The Sanjiang Plain in Northeast China is one of the main grain production areas in the country and is supporting a rich biological diversity. However, the wetlands and forest lands have shrunk to one fifth of their original size in the last five decades because of increasing population and land reclamation for agriculture. Identification of the extent of coupling between groundwater and surface water connections is essential to improving agricultural water management in the area. Using a multi-tracer approach involving water chemistry and stable and radio isotopes (²H, ¹⁸O, ³H, ¹³C, ¹⁴C), integrated with data on groundwater regime, demonstrated that it is possible to delineate the mechanism of hydraulic and chemical interactions of groundwater with various surface water sources including rivers, ponds and paddy rice fields of a wetland terrain. Regional variations in hydrogeology are main factors controlling groundwater recharge and regime. Results showed that the evaporative isotopic signature in surface water can be used as a good indicator to study mixing between groundwater and surface water. Groundwater in the confined quaternary aquifer with ages over 50 years that are depleted in heavy isotopes is recharged by lateral flow from nearby mountains. This groundwater is in general not affected by changes in the wetlands and/or rice fields and therefore is less vulnerable to the pollution from fertilizer application, however, with limited yield. On the contrary, the unconfined quaternary aquifer is recharged by rainfall or riverbank infiltration at localities near the rivers. It is more likely for the aquifer to be affected by nutrients released from intensive fertilizer applications, though its yield is rather abundant. It is suggested that surface water should be utilized together with groundwater in order to ensure sustainable water supply for irrigation in the region. (author)