| The climate of Mu Us Sandy is a kind of continental droughty climate which is dry, lackof rainfall, and has great quantity of evaporation. And there are abundant groundwaterresources with good quality, but salt water exists in the shoaly lands between sand dunes.Yanchi area borders on the south of Mu Us Sandy, and groundwater is the main water supplysource because it is short of surface water resources. But the content of total dissolved solidsin groundwater is high and water quality is poor in a large proportion of the study area. Andhigh quality fresh water only exists in the Mu Us Sandy which is in the northeastern part ofthe area. Taking groundwater dynamics theory as a guide, this paper is aimed to research theformation, evolution and sustainable exploitation and utilization of freshwater based on theproject of the China Geological Survey—“Groundwater Survey in Ningdong Energy Base”.In the study, numerical simulation method is used to study the formation process and causesof fresh groundwater in protogenic environment, and the migration rules of groundwater flowfield and the interface of salt-fresh water under the influence of human activities are revealed,and the threshold of sustainable exploitation and utilization of groundwater is presented,together with groundwater protective measures. Some major opinions and conclusionsobtained from the study are showing as below:(1) Cretaceous Huanhe group is a set of lacustrine facies fine clastic rocks andargillaceous rocks deposition, and riches in gypsum mudstone, siltstone with gypsum layer.And the environment of brackish—salt groundwater in the study area is formed under theinfluences of high salinity of sedimentary water and sedimentary rocks, drought paleoclimateand slow circulation of groundwater.(2) According to the borehole data, topography and geomorphology and hydrogeologicalconditions of the study area, combining with the research objectives, the simulation range isdetermined and the main water-bearing petrofabric is generalized as Quaternary aeolian sandwater-bearing petrofabric, Cretaceous Huanhe group upper and lower sandstone andmudstone interbedding water-bearing petrofabrics. The hydrogeological conceptual model,mathematical model and numerical model of groundwater flow and solute transport areestablished, and the identification and validation of the model are accomplished.(3) The formation and evolution process of fresh groundwater in protogenic environmentis simulated by the numerical model, and the balance calculation of salt and water and thevelocity partition of groundwater flow in shallow aquifer (layer1in the model) are carried out.The velocity range of0.2~0.5m/d,0.1~0.2m/d,0.05~0.1m/d and <0.05m/d are defined as strong runoff zone, general runoff zone, weak runoff zone and supper-weak runoff zonerespectively. The hydrodynamic conditions in fresh water zone are better than that in saltwater zone, and the average velocity of groundwater flow in shallow aquifer is greater thanthat in deep aquifer.(4) The forming and evolution mechanism of fresh groundwater is analyzed from theaspects of provenance analysis, hydrodynamic conditions, rainfall infiltration and evaporationand concentration. The difference of parent rocks between Cretaceous Huanhe group andQuaternary aeolian sand aquifer is thought to be the most basic controlling factor aboutgroundwater quality distribution, and hydrodynamic conditions play an important role indriving in the formation and evolution of water quality, meanwhile, rainfall infiltration,evaporation and concentration, together with the lithology and salt content of vadose zonehave important influences on the formation, distribution and evolution of the groundwaterquality. Moreover, human activities become a new driving force of groundwater water qualityevolution.(5) The threshold of groundwater sustainable exploitation–19200m3/d of exploitationquantity in Yanchi area is determined by the criteria of seizing ineffective evaporation, thepremise of not exceeding ecological warning groundwater level and the guarantee of waterquality. And in the meantime, the threshold of groundwater sustainable exploitation–about25%of the total groundwater supplies in fresh water zone is proposed in arid and semi-aridareas which are dominated by the vertical exchange of water and fresh water–salt watercoexist. And the measures of dual water supply and comprehensive utilization, rationalexploitation and ecological protection, strengthening groundwater resources exploration,improving the groundwater monitoring system and water conservation are put forward toprotect fresh water resources.These research achievements above provide important guidance and scientificsignificance to the rational exploitation, utilization and protection of groundwater resources inYanchi, Mu Us Sandy and arid areas in the northwest. |
PDF Full Text Request