Study On Hydrochemical Composition Formation And Evolution Mechanisms Of The Groundwater In Oasis Of Arid Areas

Groundwater resources has become an extremely significant water sources for oasissurvival and development. However, with the growth of population and rapid development ofsocio-economy, irrational utilization of groundwater resources has triggered to a series ofwater resources and environmental issues, such as the decline ceaselessly of groundwaterlevel, deterioration continuously of groundwater quality, and diminution increasingly ofavailable groundwater quantity. These problems seriously threat to the sustainabledevelopment of oasis. Therefore, it has a great significance to carry out study onhydrochemical composition formation and evolution mechanisms of the groundwater in oasisof arid areas, reveal spatial and temporal evolution laws of gourdwater chemical compositionunder the long-term influence of the human farming, and use water chemistry information tounderstand comprehensively and systematically groundwater systems of arid Oasis. All ofthese play an important role to evaluate sustainable exploitation of groundwater resources,formulate rational exploitation model of groundwater, and promote harmonious developmentof nature and mankind.Under the guidance of system theory, the paper takes a typical arid area oasis, namelyYaoba Oasis in Alashan as the research object.. By applying all kinds of integratedinformation, such as groundwater chemical compositions, isotope, hydrogeology etc studiesthe temporal and spatial variations and evolution laws of the groundwater hydrochemistry. Onthe basis of these studies, the interaction mechanisms between groundwater and theenvironment is revealed, and the information of the recharge sources, recharge methods,runoff channels, excretion ways and the main geochemical roles in the groundwater is derivedquantitatively or semi-quantitatively. These study results provides an effective way forunderstanding groundwater system correctly. The main research results are that:(1) The paper also conducts statistical analysis on some major hydrochemical indicatorsof groundwater among different irrigation seasons over Yaoba Oasis. The results indicate thatregardless of the exploitation or non-exploitation period, the variation trend of major ioncompositions of phreatic water are in accordance with that of artesian water basically. Theconlusion could be acquired that seasonal change has little effect on hydrochemical spatialvariability. In addition, the hydrological and chemical characteristics of phreatic water is moresusceptible to spatial variation than those of artesian water, which is mainly affected byaquifer media, topography, hydro-meteorological conditions and human activity and so on.Among those water chemical characteristics, Na⁺, Cl^- and SO₄^2- has more spatial variability,which are the main variables determining salinization. Confined water is less suffuring fromexternal interference, therefore, the salinization is weak.(2) The analysis of groundwater chemical compositions measure display that thedistribution of groundwater chemical types and Total Dissolved Solids (TDS) with spacepresents the following features:â‘ In vertical direction, from up to down, TDS varies fromhigh to low, and water chemical types range from complex to easy, which is a reversegeochemical zonation characteristics.â‘¡In horizontal direction, for the northeastern, TDS islower and water chemical type is HCO₃-SO₄. The hydrochemical characteristics of southwestern are high TDS, poor water quality and Cl-SO₄ type water; furthermore, it isobvious that TDS varies with season and TDS in non-mining period is higher than miningstage at overall production, while the change of water chemical types with season is notobvious.(3) The study on all kinds of ions ratio coefficients of groundwater (e.g.γ_Cl^-/γ_Ca²⁺,γ_Mg²⁺/γ_Ca²⁺,γ_Na⁺/γ_Mg²⁺,γ_Cl^-/γ_HCO2^2-,γ_Cl^-/γ_SO4^2-,γ_Na⁺/γ_Cl^-show the results as below:generally speaking, in research area, water chemical compositions of groundwater flowsystem is affected by hydraulic condition and groundwater accumulative action with runoffflowing, becoming poor from northeast to southwest with hydraulic condition, andassembling high saline ions like Cl^-, Mg²⁺ and Na⁺. The increasing of various salt densityprovides a good environment for salitation.(4) According to the data of groundwater chemistry in research area, the evolutionchateristics of hydrogeochemical compositions with time could be obtained. Sincegroundwater was exploited by human being, the TDS has been keeping on an increasing trendat speed of 40mg/L per year in overall. However, with the difference of years and locations,the raising extents are not even. Southern has higher increasing rate than middle part andnorthern. The hydrochemical type has changed from the original type of HCO₃-Ca·Mg toCl·SO₄-Na·Mg. The amount of Ca²⁺ and Mg²⁺ in groundwater has reached saturation statebasically. The amount of HCO₃^- is increasing, but the ratio of HCO₃^-to total ions is reducing.Groundwater contains more Na⁺, SO₄^-, and Cl^- than before. All those reveal that waterchemical field changes toward salination and alkalization, water chemical type developstoward complex, and water quality is becoming poor gradually.(5) Theδ¹⁸O,δD values of groundwater concentrating along the rainfall distribution line,marks that groundwater is recharged by precipitation in Yaoba Oasis. Furthermore, thedistribution feature that theδ¹⁸0 andδD values of northeastern are bigger than those ofsouthwestern, indicates the two regions have different groundwater cycles. Namely, thegroundwater of southwest area is almost kept in a colder climate environment; while in theNortheast area, due to water cycle is more positive for the environment, the groundwaterrecharge is in a warmer climate environment. Vertically, the hydrogen and oxygen isotopecontent are very similar with each other, indicating that all layers of the groundwater keepclose hydraulic contact in vertical direction.(6) The measured value of tritium in its surrounding areas groundwater approve that: inYao Oasis area, the Quaternary pore groundwater originated from precipitation, whichbasically is recharged vertically by precipitation Unit preserved by storage accumulationrather than modern atmospheric precipitation and surface water. but recharge. There is littlegroundwater recharge of the Quaternary groundwater in the Oasis's east mountain valley tothe study area. The pore phreatic water of north part and surface floods are the main rechargesource of Oasis's groundwater. The diving of Western desert is another source of supply.(7) The results of shallow groundwater hydrogeochemical simulation in study area showthat: the mixing action plays a crutial role in the formation of current water chemistry condition of shallow groundwater. However, the process is not simple mechanical mixing; inthe north, along with the mixture of three supply sources (shallow groundwater, floods, desertdiving), some reaction occurred involving calcite dissolution, dolomite and gypsumprecipitation, carbon dioxide escape, calcium ion parsing and sodium ion adsorption; in TaoSuLakes region of the southwest, along with two types of recharges (shallow groundwater andflood) mixing, the precipitation of dolomite, gypsum and rock salt, the dissolution of calcite,fluorite and dissolved carbon dioxide, as well as Absorption of calcium and sodium ions alsotake place. Analysis of deep groundwater hydrogeochemical simulation results interpret that:in the process of deep groundwater flowing, water-rock interaction occurred refering to manyreactions. Dolomite, gypsum, rock salt, potassium salt dissolved into groundwater, and theirtransfer capacity changed from small and large, which caused calcite precipitation. At thesame time, cations undertook exchange between Na+, Ca2+ on the flowing path. In theprocess, Ca2+ entried into the water, and Na+ was adsorbed to the surface of the aquiferparticles. This reaction caused a significant change of water chemical compositions for theflow in the northern part, showing the water chemistry characteristics from recharge to runoffzone.(8) The factors affecting and determining the groundwater chemical characteristics of theOasis and their evolution laws include physical chemistry action, soluble salt content insurface soil as well as man-made acitivity.