THE POTENTIAL OF THE UNSATURATED ZONE IN GROUNDWATER RECHARGE IN ARID AND SEMIARID AREAS

The solute and isotope profile of the unsaturated zone(UZ)in arid and semiarid areas, can not only provide groundwater recharge information, but also record the past climate and environment changes. Based on the last 10-year's study in the Ordos Basin(including the Loess Plateau in the south part and the desert plateau in the north part), we have combined evaluations of the thick UZ with those of the saturated zone and used multiple tracers to promote the potential of UZ in groundwater study in arid and semiarid areas, including reconstruction of groundwater recharge history, determination of groundwater recharge mechanism, assessment of impact of vegetation changes on groundwater recharge and evaluation of whole processes of groundwater pollution. the results show that shallow groundwater in arid and semiarid areas is often not in equilibrium with near-surface boundary conditions. This is especially the case where the unsaturated zone is thick and recharge rate is limited. A case study from west part of the desert plateau in the Ordos Basin shows that the present diffuse recharge beneath the grassland is 0.11-0.32 mm/year, based on the chloride mass balance of seven soil profiles. the chloride accumulation age is approximately 2, 500 years at a depth of 13 m in the unsaturated zone. the average Cl content in soil moisture in the upper 13 m of the unsaturated zone ranges from 2, 842 to 7, 856 mg·L^-1, whereas the shallow groundwater Cl content ranges from 95 to 351 mg·L^-1. the corrected ¹⁴C age of shallow groundwater ranges from 4, 327 to 29, 708 years. in typical tablelands in the Loess Plateau, the length of time required for annual precipitation to reach the water table ranges from decades of years to hundreds of years with soil moisture velocity of 0.1 to 0.3 m·a^-1 and shallow groundwaters are all pre-modern water and paleowater, the Cl and NO₃ contents in the upper UZ are considerably higher than those in the deep UZ and shallow groundwater because of recent human activities. the homogeneous material of the UZ and relatively old groundwater age imply that piston flow is the dominant recharge mechanism for the shallow groundwater in the tableland. The"Returning Farmland to Forest and Grassland" project in the Loess Plateau and plantations in desert areas have decreased groundwater recharge and increased solute content in UZ moisture, which can be used to quantitatively determine the changes in recharge rates. the paper has strengthened the role of the UZ in groundwater recharge study in arid and semiarid area, to which should be paid more attention in groundwater resources assessment and groundwater pollution investigation.