The Study Of Groundwater Renewability In The Northern Area Of Ordos Cretaceous Basin

Ordos basin is the important energy sources and chemical industry base of China,which plays an important role in the realization of Western Developing Strategy. Butdue to the arid and semiarid climate, the contradiction between the water resourcessupplyand demandis veryobvious. Thenorthern areaof Ordos Cretaceous Basinis themain water supply of this area in the future, where the water resources are abundantrelatively. As we all known, the groundwater renewablity is the important basis ofevaluatingthegroundwaterresourcesandestablishingthereasonabledevelopingpatternof groundwater resources. So that it is needful to studythe groundwater renewabilityinthenorthernarea.This thesis is on the basis of Chinese geological investigating project"Groundwater chemical evolution and water circulation research in Ordos CretaceousGroundwaterBasin("12120103302-ZT2).Basedonthegeologicalandhydrogeologicalconditions of northern area, this paper first analyzes the characteristics of geochemicaldistribution and the isotopic composition of natural waters, what is used to illustrate thelaw of groundwater circulation. Then it evaluates the groundwater renewablity ofnorthern area by renewal rate as the index. This work doesn't only find out thegroundwater renewablityof northern area, which is the basis of groundwater reasonabledevelopment and scientific management, but also explores some problems in thecalculation of renewal rate. Therefore this study is significant in practice and theory.Andthemainachievementsofthispaperareasfollows:1,The distribution characteristics of the groundwater chemistry. Because ofthe impacts of landform, rock characters, rock facies and groundwater circulation, thevaluesofTDS,Cl-,SO42--,K++Na+,Ca2+andMg2+aregenerallyhighinthewestofstudyareawhilelowintheeast.In horizontal dimension, divided by Yanchi-Sishililiang surface watershed, the concentrations ofTDS andmainions are generallylowintheeast,andthe groundwateris mostlyHCO3type water. So there is no classical horizontal distributionlaw here. Butin the west, the concentrations of TDS and main ions rise slowly from the surfacewatershed to west and northwest, with the changes of chemical types which aregenerally from HCO3type water to SO4·Cl one. There is an obvious distribution law inhorizontaldimensionalongtheflowlineofgroundwater.In vertical dimension, the concentrations of TDS and main ions rise with theincreasing depth of groundwater especially in Molinhe-Yanhaizi groundwater systemand Yanchi-Dousituhe groundwater system, while the area of HCO3type water reducesobviously. The TDS distributes with the characteristics of shallowmedial>deep nearthedownstream of MolinheRiverandDosituheRiver.Forvertical distribution,thegroundwaterchemistryis homogenousnear the surface watershed and in Wulanmulunhe-Wudinghe groundwater system. ButinYanchi-DousituhegroundwatersystemandMolinhe-Yanhaizigroundwatersystem,itshowsthedelaminatingcharacteristicstoacertainextent.2,Theisotopiccompositionofnaturalwaters.Theisotopicresearchfindsouttherelationships of"3-types water"transformation in the study area. Both surface waterand groundwater are originated from precipitation. It is proved that groundwater isrechargedbyprecipitation,butsurfacewaterisgenerallyrechargedfromgroundwater.And the composition of ?D and ?18O in groundwater in the east ofYanchi-Sishililiang surface watershed is more enriched than the west. In verticaldimension, ?D and ?18O in shallow groundwater is more enriched than medial and deepgroundwater. The isotopic composition of groundwater in vertical dimension ishomogenous near the surface watershed and in the east, but it shows a littledelaminatingcharacteristicinthewest.3,The characteristics of groundwater circulation. The tracing ofhydrochemistry and isotopes shows the characteristics of circulation that thegroundwater is recharged by precipitation near the surface watershed, and flow to theeastandwestofthestudyarea.TheareasneartheYanchi-SishililiangandXinzhaosumusurface watersheds are recharge areas. The areas near the eastern, western and northernboundaries are discharge areas. The groundwater flows from recharge area to dischargearea. In the east of Yanchi-Sishililiang surface watershed, the vertical circulation playsan important role in groundwater circulation, but in the west, the horizontal flow isinstead of vertical circulation. By constructing the circulation models of classical cross section, the groundwater circulation is concluded into three classes: local circulationsystem,mediumcirculationsystemandregionalcirculationsystem.4,The calculation of groundwater renewal rate. First, the EPM, LM and LPMfor calculating groundwater renewal rate are educed. Then the corrections of 14C inputconcentration are made. At last, according to the circulation model and flowcharacteristics of groundwater, the renewal rates of shallow groundwater are calculatedby3Hand14C,usingthemodelofwellmixedreservoir.Andtherenewalratesofmedialand deep groundwater are calculated by 14C, using the model of mixing in equalproportions.5,The evaluation of groundwater renewability. In terms of the groundwaterrenewalrate,theevaluationofrenewabilityinthethreelayersgroundwaterisperformed.Most renewal rates oftheshallowgroundwaterarelagerthan1.0%/a,whiletherenewalrates of medial groundwater are between 0.1 and 1.0%/a, and the renewal rates of deepgroundwater are less than 0.1%/a. The renewability of groundwater near the surfacewatershed and in the east is better than the west. In verticaldimension, the renewabilityin shallow and medial groundwater is better, which is suitable for developing on thebasis of reasonable evaluating. But the deep groundwater can't be used ordinarilybecauseofits'weakrenewability.