| Karst groundwater constitutes the major water supply source for industry, agriculture and urban domestic water use in the Liulin spring area. With rapid economic and social development, the contradiction between water demand and water supply become an increasingly serious problem. The impact of human activities(such as artesian of unclosing exploratory hole in stagnant area, over exploitation of karst groundwater, construction of reservoir, etc.) on karst groundwater system become more and more obvious, and result in changes in boundaries, structures, cyclic conditions, hydrodynamics and hydrogeochemical characteristics of the system. These changes have threatened local water supply security and sustainable development of local economy and society. Thus, according to storage conditions and hydrodynamics characteristics of karst groundwater, the research on kasrt groundwater for hydrogeochemical processes controlling hydrochemical characteristics, exploring isotopic signature and its water cyclic significance, deducing the source of main anions and cations, have certain theoretical significance for understanding karst groundwater system and its spatial and temporal evolution rules, and have practical significance for reasonable exploitation and conservation of karst groundwater.Based on analysis of regional geographic and geomorphic, geologic and hydrogeologic conditions, the rules of karst groundwater formation, cycle and hydrogeochemical evolution are studied using groundwater system methods in combination with hydrogeochemical analysis and isotopic tracing technology. The main results are listed as follows:(1)Liulin spring karst groundwater system is typically an arid and semiarid karst groundwater system, with predominant water-bearing media composed by dissolved pore and corrosion fissure and main flow mode being diffuse flow. According to circulation conditions, karst groundwater system could be divided into recharge area, flow-through area, discharge area and stagnant area in plane, and into shallow circulating system and deep circulating system in vertical direction. The karst groundwater in shallow circulating system mainly discharge from Liulin spring groups, while in deep circulating system is stored in stagnant area by increasing its water pressure under natural conditions. The spring water is caputured by deep circulating system after excavation of Henggou artesian well and the cyclic depth of this system is deepened.(2) Temperature and concentrations of Ca2+, Mg2+, Na+, SO42-, Cl- in karst groundwater increase along flow path, while p H values and p E values show a decreasing trend. The redox states of groundwater varied from oxidation state to reduction state. The gradients of physical and chemical components in groundwater are lower in east and central part and increase in west part. Compared with 1995, karst groundwater is characterized by increasing ion concentrations in the east part, especially in Lishi synclinal basin with intensive groundwater withdrawals, and decreasing ion concentrations in the west part in 2011.(3) The δ2H and δ18O values of groundwater samples generally fell into the two flanks of the Local Meteoric Water Line, indicating that karst groundwater basically originate from precipitation infiltration. The d-excess values are higher in recharge area and stagnant area, lower in flow-through area and discharge area, indacting water vapour forming karst groundwater in recharge and stagnant area come from a relatively dry marine enviroments. The 3H contents of karst groundwater is larger in recharge area, then discharge area, and the lowest in stagnant area, suggesting an order for runoff conditions from best to worst by recharge area, discharge area, flow-through area and stagnant area. In the upgradient of Liulin spring, the HCO3- mainly stem from soil CO2, while primarily come from dissolution of carbonate rocks in downgradient. The samples located in upgradient of Liulin spring represent the modern groundwater features, while showed remarkable increase in downgradient of Liulin spring with 14 C age of ~13000yr in Henggou 1# and an increasing gradient of 1990 yr/km.(4) The saturation index of calcite, dolomite and aragonite are greater than 0 in karst groundwater system, indicating a oversaturated state, yet the values of gypsum, anhydrite, halite and CO2(g) are less than 0, indicating a undersaturated state. The results of inverse hydrogeochemical modeling show that dissolution of calcite mainly occur during infiltrating of rainfall in soils and unsaturated zones, while the mineral precipitate in subsequent runoff and discharge processes. The dissolution of gypsum and halite and Ca-Na or Mg-Na exchange are found in all flow paths. Though saturation index of dolomite a greater than 0 in most samples, dolomite still dissolves along flow path due to the dedolomitization. The mixture between surface water and groundwater occur in these flow paths from recharge area to runoff area and from runoff area to discharge area. The results of hydrochemical compositions prediction show that TDS of springs will increase if the mount of surface water leakage is significantly decreased, however TDS in Henggou will decrease under current discharge conditions.(5) The dominant hydrogeochemical processes controlling chemical composition of kasrt groundwater are different for various areas in this spring area, and are predominant by leaching, mixture and ion exchange in recharge and discharge area, and by leaching, ion exchange and redox reaction in flow-through and stagnant area. The mixture between stagnant water and capturing water occur after the excavation of the Henggou artesian well. The Ca2+ in groundwater mainly derive from dissolution of calcite, dolomite and gypsum; Mg2+ mainly stem from dissolution of dolomite; HCO3- come from dissolution of calcite, dolomite and soil CO2. SO42- primarily stem from dissolution of gypsum and oxidation of sulfide mineral, and the percentage of the two sources are 81% and 19%, respectively. About 9%-15% of SO42- is reduced to H2 S and this percentage is increasing along flow path. In the recharge, flow-through and discharge area(including Shangqinglong spring group, Yangjiagang spring group, Longmenhui spring group and Zhaidong spring group), Na++K+ is mainly from dissolution of halite and ion exchange; Cl- mostly originate from dissolution of halite. While in the discharge area( Liujiageda spring area) and stagnant area, Na++K+ and Clmainly derive from dissolution of halite. The heat sources of karst groundwater in stagnant area consist of local geothermal gradient, local normal geochermal, radiogenic heat and exothermic of gypsum dissolution, and the contribution of these sources are 45%, 30%, 19% and 6%, respectively. |
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