Hydrochemical Relationship between Water in Drainage Ditches and Shallow Groundwater in Qingtongxia Irrigation Area

【Objective】 Change in groundwater in Qingtongxia Irrigation areas is mainly due to the leaching of drainage ditches and infiltration of field irrigation. The exchange between surface water and groundwater is frequent but complex. This paper analyzes the hydrochemical relationship between water in drainage ditches and their adjacent shallow groundwater. 【Method】 The study was based on hydrochemical data measured from water samples taken from both drainage ditches and shallow groundwater. Their correlations and transformation were analyzed using statistics, Gibbs model, Piper chart, Schoeller chart, cation alternating adsorption chart and other methods. 【Result】 The water in the drainage ditches was more alkaline in the off-irrigation period, and slightly alkaline or alkaline in the irrigation period. In contrast, the shallow groundwater was neutral or weakly alkaline in the off-irrigation period and weakly alkaline or alkaline in the irrigation period. The TDS of the water in the drainage ditches was higher in irrigation period than in off-irrigation period, while the TDS of the shallow groundwater remained approximately same, though it was significantly higher than that of the drainage ditches. The content of cations in the water in both the drainage ditches and shallow groundwater were ranked in the order of Na+>Ca2+>Mg2+>K+, while the anions were in the order of Cl->SO42->HCO3-. The difference in ion concentration in the shallow groundwater near the drainage ditches was greater in the vertical direction than in the horizontal direction. The δ18O and δD in the ditch water and shallow groundwater were approximately the same during both irrigation and off-irrigation periods, and their spatial variation was small. The isotope dilution by shallow groundwater was noticeable at the depth of 30 m. 【Conclusion】 The chemical compositions of ditch water and shallow groundwater are controlled by evaporation or rock weathering, in addition to hydrodynamic mixing. The hydrochemistry in the shallow groundwater in the south of the area depends on rock weathering, while that in the middle and north of the area is largely modulated by evaporation. Depending on the depth of groundwater table, the drainage ditch can ether discharge or drain the groundwater due to the rise of groundwater table following irrigation. The average ratio of irrigation water withdrawal was 81.54%. For regions where groundwater table is deep, the drainage ditch is the main source of groundwater, accounting for 84.62% of water supply to the groundwater.