Effects of Yindaruqin irrigation project on groundwater chemical compositions in Qinwangchuan basin in Gansu Province

Abstract: Hydraulic project alerts hydrological cycle and changes biochemistry of water as a result. In this paper, we investigated the change in groundwater chemistry in Qinwangchuan basin as affected by the Yindaruqin irrigation project. Groundwater samples were taken from the basin and were chemically analyzed before and after the irrigation project was in operation. The change in groundwater chemistry was analyzed using methods including statistics, Gibbs graphs and ions ratios. The results showed that the coefficient of variation (CV) of all main ions was larger than 10%, with their CV in the plain area being higher than that in the loess hill area. The total dissolved solids in the groundwater were currently high, with Na+ being the dominant cation and Cl- and SO42- the dominant anions. Salty and brackish water was found in most parts of the basin, and fresh water with total dissolved solids less than 1.0 g/L existed only in the middle of the basin. The total dissolved solids in the groundwater decreased from the loess hills at the edge of the basin towards the central plain area, and the associated hydro-chemical type of the groundwater changed gradually from Cl·SO4- Na type to HCO3·SO4- Na type. Before the irrigation project was constructed, the main geochemical type of the groundwater was Cl·SO4- Na, Cl·SO4- Na·Mg and SO4·Cl - Na. At earlier operating stage of the project, leaching of soluble salts from the topsoil led to an increase in total dissolved solids and hardness in the groundwater. With the decrease in soluble salts in the topsoil due to leaching, however, the total dissolved solids in the groundwater have been in decline since 2009, especially in the center of the plain where the total dissolved solids in the groundwater in some areas had reduced to < 1.0 g/L from > 3.0 g/L and the groundwater type changed from Cl·SO4-Na to HCO3·SO4-Na, HCO3·SO4-Na·Ca·Mg, and Cl·SO4·HCO3- Na, respectively. In contrast, the groundwater chemistry in the loess hills at the edge of the basin did not show significant change due to limited irrigation, and its chemical type is still Cl·SO4-Na·Mg despite a slight increase in total dissolved solids and total hardness. The groundwater type at Dangpu area in the southern basin changed from Cl·SO4-Na to Cl·SO4-Na·Mg, with a light decrease in total dissolved solids and total hardness. The change in groundwater chemistry in the Qinwangchuan basin is affected by both natural and anthropogenic factors. The infiltration and salt leaching due to irrigation increased the total dissolved solids in the groundwater initially followed by a dilution. Rock weathering and dissolution of minerals increased the total dissolved solids, and groundwater evaporation also increased the total dissolved solids in areas where the groundwater table was shallow. The ions in the groundwater originated from silicate weathering and dissolution of halite, along with the evaporation-induced condensation. Industry did not affect groundwater chemistry but the application of chemical fertilizer led to an increase in NO3- in the groundwater. These results provide a useful dataset for sustainable management of the groundwater resources in proximal areas of the Qinwangchuan irrigation project.