| Geothermal resource has become an important type of new energy resources that are rich inreserve, pollution-free, and cost-effective for exploitation. In China, high or medium temperaturegeothermal fields have been studies and exploited for decades. However, fewer works has beendone on low- medium temperature geothermal resources. This status needs to be changed as thedemand for cleaner energy keeps increasing. To investigate and exploit low or mediumtemperature hydrothermal resources in a reasonable way, the origin and water-rock interactionprocesses of the geothermal water should be understood first.There were only several scattered surveys of karst geothermal resources in Taiyuan before1995. Seventeen hot wells have been successively constructed since 1995. Shanxi GeologicEngineering Survey Institute accomplished Hydrothermal Survey of XiShan Mountain in TaiyuanCity, Shanxi Province, and Survey & Evaluation of Hydrothermal Resources in Taiyuan City, in2004 and 2005 respectively. However, all these surveys are by means of the traditionaltechniques, and no systemical work has been done on the genesis of karst geothermal water inTaiyuan.The overall objective of this study was to understand the genesis of karst geothermal waterin Taiyuan. Specifically the purposes of this paper were: (1) to divide the karst water system intoseveral sub-systems according to geologic, hydrogeologic and hydrodynamic characteristics; (2)to describe the temperature distribution of karst water and then locate hydrogeothermal anomalyzones by means of geophysical prospecting, thermal infrared remote sensing, and boretemperature logging; (3) to reveal the flow patterns of geothermal water using the evidences ofwater temperature, hydrogeochemistry, water-rock interaction, and isotope geochemistry, and (4)to develop hydrogeologic conceptual model of low-medium geothermal groundwater incarbonate bedrock in Taiyuan based on the above research findings. This study is the mostcomprehensive investigation of cold karst water, and the most systematic investigation of hotkarst water up to date in Taiyuan. 1. Temperature Distribution of Karst GroundwaterThe temperature distribution of groundwater in Ordovician karst strata was characterizedusing thermal infrared remote sensing, CSMAT geophysical prospecting, and bore temperaturelogging. It is found that:(1) Groundwater temperature tends to be higher from mountain to the basin for all karstwater systems.(2) The courses of groundwater temperature increasing differ for different karst groundsystems.①For North Mountain karst water system, the water temperature is always less than 15℃from recharge area to runoff area, and to discharge area, except some local discharge areas.②For West Mountain karst water system, the water temperature generally increases from northwestto southeast. However, the rates of increase differ in different tectonic zones. Specifically, thetemperature is less than 15℃north from Fenhe River, high abnormally in southeastZhenchengdi-Qinxu syncline, low abnormally in Wangfeng-Sangei-Ximing primary groundwaterrunoff area, increasing gently along groundwater runoff route in Shixiang-Yuanqianfeng area,and increasing rapidly in mountain-front fracture zone.③For East Mountain karst watersystem, the water temperature increases gradually from southwest to northeast.④In the basin,Qinxian, Xiwenzhuang, and Xiaodian areas are with abnormal high water temperature.(3) The sag in the basin adapts to reserve karst geothermal water. However the hugethickness of cover strata makes it high cost to exploit karst geothermal water there.2. Hydrogeochemistry of Karst GroundwaterCold/hot groundwater and surface water in different sites and depth, and precipitation, weresampled. Sixty-seven groups of water sample were gained and analyzed for hydrochemicalparameters, major and minor elements and gas composition. Then the hydrogeochemistryic datain different areas within each karst water system were compared. It is found that:(1) From recharge area to runoff area, and to local discharge area, the karst water changesfrom HCO3 to HCO3·SO4, and to SO4 for hydrochemical type, and from less than 0.5 g/L toabout 1 g/L, and to more than 2 g/L for TDS (total dissolved solid). The concentration of SO42-,Ca2+ and Mg2+ also increases along groundwater flow path, but that of HCO3- keeps invariablegenerally. The concentration of Sr,Si,Fe,F and some trace elements increases as karst watertemperature increases.(2) In hydrochemical Piper trilinear diagram, cold and hot water locate different zones.Water samples between them indicate the mixture between cold and hot water.(3) Spatial distribution of hydrochemical data shows that cold karst water in discharge areashould come from local groundwater flow system while the hot one in deep part from theregional system.3. Water-Rock Interaction of Karst Water System Proper geochemical thermometers were selected to calculate the temperature of geothermalreservoir. The maximal depths of hydrothermal circulation were calculated using the localgeothermal gradient, and SI (Saturation Index) of each mineral in all water samples usingPHREEQC. The above results were combined with analysis of relationship between eachchemical component, and relationship between temperature and each component, to infer thehydrogeochemical behaviors of Ca2+, Mg2+, SO42-, F, Si, and Sr. It is found that:(1) Geothermal water in Taiyuan is so-called "immature water", which means that ionexchange between water and rock is still under equilibrium. Consequently the quartzthermometric scales are suited to calculate temperature of geothermal reservoir. According to theresults of quartz thermometer, the temperature of geothermal reservoir in Taiyuan is between35℃and 68℃. The hydrothermal circulation depth is between 898 m and 2192 m.(2) The hydrochemistry of geothermal water in Taiyuan is controlled mainly by carbonatedissolution and mixing between cold water and hot water.①Ca2+, Mg2+ and HCO3- in rechargearea of each karst water system mainly originate from the dissolution of carbonate and dolomite,while Ca2+ in the discharge area has one additional source, namely the dissolution of gypsumalong groundwater runoff route.②Evolution of karst water hydrochemistry is caused by bothdolomitization and calcite deposition.③From discharge area to discharge area of each karstwater system, whatever water temperature, SO42- in water is increasing and always in excess,which indicates that SO42- must have additional sources besides dissolution of gypsum. Theymay be oxidation of H2S gas from deep strata or oxidation of pyrite.(3) All geothermal water samples show high concentration of F, Si, Sr and Fe.Concentration of Fe, Si, and Sr ions is mainly controlled by solubility of fluorite, quartz andchalcedony, and celestite, respectively.4. Isotope Geochemistry of Karst GroundwaterAt the same sites/times where/when water samples for hydrochemical analysis were gained,thirty-seven, twenty-two, thirty-five, fifteen, and fifteen groups of water sample were sampledfor analysis ofδD, 3H andδ18O, 14C andδ13C, 87Sr/86Sr,δ34S andδ18O in sulfate, and noble gas,respectively. Then the isotope geochemical data in different areas within each karst water systemwere analyzed. The results are showed as follows:(1) Hydrogen and Oxygen Isotope①The relationship betweenδD andδ18O values in water samples was compared withmodern MWL, which indicates that karst groundwater should originate from old precipitation,and experience concentration effect of evaporation.②Karst water shows D and 18O isotopicstratification, indicating that groundwater in different hierarchies of flow system originates fromdifferent aged precipitation.③Spatial distribution ofδD, 3H andδ18O values in karst waterdemonstrates the mixture between cold water and hot water.④Groundwater rechargeelevations were calculated usingδD andδ18O values in water samples. The results show that recharge elevations of hot water in West Mountain, cold karst water in West Mountain, and hotwater in East Mountain should be 1700～2000 m, 1500～1700 m, and 1700～1900 m, respectively.⑤The relationship between EC andδ18O values shows that hot water in discharge areas shouldexperience the longer water-rock interaction than cold karst water at the same areas. It indicatesthat there are different hierarchies within the same karst groundwater flow system.(2) Carbon Isotope①According to 14C data, hot waters in the karst water systems of West Mountain and EastMountain should be 13000～18000 and 10000～20000 years old respectively, and should originatefrom the colder precipitation in late Pleistocene.②According toδ13C data, CO2 gas in karstwater should originate from biologic effect or carbonate metamorphism.(3) Strontium Isotope①Through the analysis for sources of Sr isotope, it is found that spatial distribution of87Sr/86Sr ratio for karst water is the result of typical water-rock interaction in carbonate strata.Spatial variation of the 87Sr/86Sr ratio indicates there are different hierarchies within the samekarst groundwater flow system.②Relationships between 87Sr/86Sr ratios and 1/Sr values,between 87Sr/86Sr ratios and Sr/Ca ratios, and between 87Sr/86Sr ratios andδ18O values wereanalyzed for all groundwater samples. It is found that karst water in Cambrian strata may bemixed into geothermal water, the karst water systems may have hydraulic connections with eachother in deeper part of discharge areas, and fissure water may mix with pore water. But neither ofthem has hydraulic connection with karst water.③According to the results of Sr isotopemixing model with hot water and cold water as end members, a large proportion of hot water ismixed with cold karst water in the discharge areas.(4) Sulfur and Oxygen Isotope in Sulfate①δ34S values of sulfate in karst water are close to those of evaporite gypsum found incarbonate rock area, which implies that sulfate in karst water comes from gypsum dissolution,not from mantle.δ34S values of sulfate in hot karst water from deep part are a little larger thanthose of evaporite gypsum found in carbonate rock area, which implies that hot water is inreducing environment and may be mixed with water from Cambrian strata.②According torelationships between SO42- concentration andδ34S values of sulfate, and between 14C andδ34Svalues of sulfate, hot water should be in strong reducing environment and few originates frompyrite oxidation.(5) Helium Isotope3He/4He ratios (namely R) and R/Ra ratios in water samples show that helium isotope in hot karst water should be a mixture of that from atmosphere and crust, and the local geothermalwater is not the mixture of karst water and hot water from mantle.5. Flow Model of Geothermal WaterBased on the above studies of hydrogeology, temperature distribution, hydrochemistry andisotope hydrogeochemistry of karst water, the groundwater flow models in three karst watersystem were analyzed comprehensively. The results are summarized as follows:(1) East Mountain Karst Water System and West Mountain Karst Water SystemThe flow models of them are similar. Each is supplied in recharge area, then groundwaterflows through three runoff zones at different depths, and thereby forms three hierarchies of flowsystem, namely local groundwater flow system, intermediate groundwater flow system, andregional groundwater flow system.①For the local flow system, recharge elevation is low, circulation depth is shallow, flowpath is short, flow velocity is large, water temperature is low (mostly less than 15℃) and waterdrainages as cold water, TDS is small, groundwater is young, water-rock interaction duration isshort, and hydrochemical type is HCO3·SO4 in the main.②For the intermediate flow system,recharge elevation is higher, circulation depth is deeper, flow route is longer, flow velocity issmaller, water temperature is higher (mostly from 15℃to 25℃) but water still drainages as coldwater near mountain, TDS is larger, groundwater is older, water-rock interaction duration islonger, and hydrochemical type is HCO3·SO4 or SO4·HCO3 in the main.③For the regionalflow system, karst water was supplied by precipitation in Pleistocene, with the highest rechargeelevation, circulation depth is the deepest, flow route is the longest, TDS is the largest,groundwater is more than 10000 years old, water-rock interaction duration is the longest, andhydrochemical type is SO4·HCO3 or SO4 in the main. In the regional flow system, groundwaterflows very slowly, and drainages as hot karst water with high temperature, even though it ismixed with cold karst water during upward flow.(2) North Mountain Karst Water SystemCompared with East Mountain karst water system and West Mountain karst system, itshows the following characteristics: better permeation because of developed structures;groundwater recharged by recent precipitation; shorter flow path; higherflow velocity; lowerwater temperature (mostly less than 15℃) and TDS; young groundwater; and shorterwater-rock interaction duration. The hydrochemical type is always HCO3—Ca·Mg alonggroundwater flow path.
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