Changes of water cycle structure and its effect on water resource development in fans

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Most of cities in the world are developed at alluvial fans in piedmont or along the river. With the developing of economy and industry and the increasing of population, quality and quantity of groundwater are aggrieved by over-exploitation in these areas. In order to address the problem of realistic assessment of groundwater resources potential and its sustainability, it is vital to understand the recharge sources, flow system, hydro-geochemical evolution and the age of groundwater in the alluvial fan. As a main economic center, there are two main alluvial fans in west and south of the North China Plain (NCP). One is the Taihang Mts. alluvial fan, the another is the Yellow River alluvial fan, which can be divided into north and south wings by the Yellow River. However, few studies have paid attentions to water cycle in these fans. The objectives in this dissertation are to understand the water cycle concerning recharge sources, and interaction between surface water and groundwater; to … More make clear hydro-geochemical evolution in fans through which groundwater flows, especially, the fate and transport of nitrate; and to date groundwater age using new method-CFCs (Chlorofluorocarbons) combining with tritium. Several case studies also have been carried out to present rainfall-infiltration processes inmountain area, as well as the long-term effects of irrigation diversion water on the local groundwater system. Groundwater and surface water from local rivers, located at these alluvial fans, were collected for analyzing major ions, nitrate concentration, stable isotope (oxygen-18 and deuterium) compositions, nitrogen isotope (^<15>N), tritium and CFC-12 measurements in the study period. The main conclusions are:1) Recharged by precipitation, groundwater in the Taihang Mts.alluvial fan is characterized by Ca-HCO_3 water type. Ion exchange and absorption effect are expected to the groundwater evolution in the Yellow River alluvial fan. From the mountainous area, to Taihang Mts.alluvial fan, and to the north Yellow River alluvial fan, the groundwater undergoes the evolution from Ca-HCO_3, Na-HCO_3, to Na-SO_4.2) The groundwater age in the south Yellow River alluvial fan ranged from 23 to 50 years dating by the CFC-12 and tritium.3) Identified by <15>^N, the main sources of nitrate in the shallow groundwater came from nitrogen chemical fertilizer and manure use. The nitrate concentrations in the complex alluvial fan were over the drinking water standard. In the south Yellow River alluvial, however, denitrification exists in the recharge process of river water to groundwater.4) The diversion water for irrigation from the Yellow River is freshening the local groundwater remarkably. The freshening front in the shallow groundwater can be estimated to advance at a rate of 200m/year. This means that it will take, at least, another 45 years to change the groundwater composition in the entire study area from Na-SO_4 water type to Ca-Na-HCO_3 water type. Less