The Mechanism of Groundwater Salinization and Its Control in the Yaoba Oasis, Inner Mongolia

The arid area is one of the most concerned areas among the water resources researchers and economists. Northwest China will be an important developing region of China in the 21st century. Yaoba is a well-irrigation oasis within this arid area, which is located in the Alxa area west of the Helan Mountains and next to the Tengger desert in the east. It has contributed greatly to the local stock raising and agriculture since its development in 1970. However, the groundwater which the oasis depends on to survive has been getting salinized gradually and more serious in recent years.A comprehensive study was carried out using the methods of groundwater environment isotope analysis, lithofacies and palaeogeography, calculations of water-rock interaction and the existing form of chemical components in groundwater etc. It has been found that the salinization of groundwater is mainly caused by reinfiltration water solving the salt in soil which is deposited simultaneously with the sediments and accumulated in the vadose zone due to strong evaporation in the arid climate for the running of an irrigation scheme.The saline groundwater in the Taosu Lake area southwest of the oasis is preliminarily regarded as the main source of groundwater salinization. However, it has been found that the groundwater has great differences in the D, 18O, H3 and NO3- concentrations with that in the oasis, that is, the former plays only a limited role in the salinization of groundwater in the oasis.The HST3D model (a computer code for the simulation of heat and solute transport in a three-dimensional groundwater system) was used to simulate the TDS movement in the oasis. A good simulation result has been obtained with the mean simulation error of 4.7% and the simulation trend is reasonable. It is proved that there is 3.0- 4.0′ 105 m3/a of reinfiltrated irrigation water in 3 main spots in the oasis. The oasis irrigation manner, winter irrigation to wash salt, the increase of wheat planting area, and no amount limit to water users (the farmers) are the main factors for causing the increase of water consumption as well as the groundwater salinization. The control measures on the groundwater salinization should include the establishement of reasonable irrigation regulations, water use limitation measures, flood utilization project, saltwater discharge, sealing of the upper layers to prevent the saline water to enter the pumping wells.