Formation Mechanism Of Fresh Groundwater In Yanchi Area

“Ning Dong Energy and Chemical Base” is one of the13national hundred million tonsmines which are the key zones for development. As an important part of the base, Yanchi areais located in the eastern part of Ningxia Hui Autonomous Region, and southern margin of theMaowusu desert. Moreover, Yanchi is one of the water-shortage regions in the northern China,where groundwater is major water-supply source. However, in this area, the index of TDS ispretty high and the fluorine ion concentration is out of limits seriously, hence, the waterquality is overall poor. Therefore, water resource has become the key factors which restrictthe local development. Fortunately, fresh water was found in northern Yanchi area by fieldsurvey. The formation mechanism of fresh groundwater in Yanchi area is studied in this paper,which can provide scientific basis for reasonable groundwater exploration, utilization andprotection. Furthermore, it is of great significance to ensure the base’s construction, developlocal economy and improve people’s living condition.In this paper, the vadose zone and saturation zone is considered as a whole, and there is aclose relationship between them. Based on this theory, this paper uses data analysis, fieldreconnaissance, laboratory simulation, hydrogeochemical simulation and calculation ofwater-salt balance to conduct research. Then, the factors affecting groundwater hydrochemicalcharacteristics are analyzed, the formation mechanism of fresh groundwater in Yanchi area isrevealed. The results are summarized as following:（1）By studying spatial distribution characteristic of groundwater quality, the followingconclusions can be reached: Under natural conditions, groundwater chemical characteristicshave obvious zonality and regional in the study area. In western and southern study area, thesalinity is greater than1g/L, and fluorine ion consentration is greater than1mg/L, hence,water quality is worse. In the northern part, the salinity is lower than1g/L, fluorine ionconsentration is lower than1mg/L, so water quality is rather preferably.（2）With the typical soil samples of vadose zone for soluble salt testing, it can beconcluded as follows. In the southern area, the lithology of vadose zone is mainly thequaternary aeolian alluvial, whose soluble salt content is high; soluble salt content and ionscontent in the upper part of vadose zone are higher than those in the lower part; obvioussalinization is found in the earth surface of Xijingtan and Beiwangquan areas. In the northernpart, the lithology of vadose zone is mainly aeolian sand, whose soluble salt content is low;the soluble salt content rises slightly with the increase of depth, and salt shows enrichmentwhere the soil particles become finer; the vadose zone and groundwater is in close contact, so that the salinity uplink and downlink frequently.（3）Indoor leaching experiment is conducted with aeolian sand and fine sandstone, thentheir source supply abilities are got and water-rock reaction. are divided into three stages.Thefirst is mass transfer controlled stage, the second is mass transfer and interfacial reactioncombinative controlling phase, the last is interfacial reaction controlled stage.（4）From the analysis of water quality formulation process, it can be indicated that thesoil and the rock component are the basic provenances of groundwater chemical component.Hydrodynamic conditions can affect recharge, flow, discharge and renewal speed. Theintensive evaporation results in the salt enrichment.（5）By calculation of the regional water-salt balance, the following conclusions areobtained: in the northern part, the recharge and discharge difference of salinity is-2.81×103kg/a (calculate as Cl-). Groundwater salinity can be reduced by0.12mg/L per year.（6）Aeolian sand has special properties, which leads to the large rainfall infiltrationcoefficient, weak water-rock interaction, leaching with less salt and small capillary rise height.Therefore, aeolian sand plays a decisive role in the formation of fresh groundwater.Furthermore, the groundwater from Maowusu Desert mixes with the groundwater of theresearch area, which has dilution effect and is important for the formation of freshgroundwater. Affected by all factors, the northern part salt input is in equilibrium with theoutput. In the north of Yanchi area, the groundwater system doesn’t accumulate salt, salt islow in the vadose zone soil and aquifers, most recharge water has low salinity. Consequently,fresh water is formed after a long period of evolution.