| Karst mountainous areas are widely distributed in the southwest of China,with high mountains and low water,lack of surface water,and underground water depth,which makes it difficult for local residents to use water.Nandong subterranean river basin is one of the typical super-large subterranean river basins in southern China.Since the 1960s,the national and local governments have organized a large number of technical forces to carry out hydrogeological survey,research and development demonstration work.Based on the hydrogeological survey of Nandong basin,this paper collected water samples and conducted field tests at 8 observation points in Nandong karst water system in each month of 2019 to realize the sustainable utilization of water resources in Nandong Basin.The complete analysis of 85 groups of water samples and 77 groups of isotopic data were obtained,which were analysised by the methods of multivariate statistics,Piper trigram and Gibbs diagram to study the hydrogeochemical characteristics and formation mechanism of Nandong karst water system.The conclusions were as follows:(1)The content sequence of main cations in the study area was Ca2+>Mg2+>Na+>K+,and the content sequence of main anions was HCO-3>SO42->Cl->NO3-.Controlled by carbonate strata,the hydrochemical types of surface water in the study area are HC03·S04--Ca·Mg(47.92%)、HC03-Ca·Mg(27.08%)、HC03·SO4-Ca(25.00%),and the hydrochemical types of groundwater are mainly HCO3-Ca(75.68%)、HCO3-Ca-Mg(19.22%).(2)The controlled mechanism of water ions in the study area is "rock weathering dissolution and filtration type",which mainly comes from weathering hydrolysis of rocks and soil.According to the Gibbs anion ratio diagram,the content of Cl-in surface water is higher than that in groundwater,which mainly comes from the input of atmospheric precipitation,mineral dissolution,domestic sewage and agricultural farming.The ratio analysis of total hardness to total alkalinity showed that surface water had more other acids involved in the water-rock interaction than groundwater during the flow process.(3)The hydrogen and oxygen isotope analysis show that the initial recharge source of surface water and groundwater in the study area are atmospheric precipitation,and due to strong evaporation,the surface water’s hydrogen and oxygen isotopes lay particular stress on.According to the value range and average value of deuterium surplus parameters,the evaporation speed of seawater in the water vapor source area of local precipitation is fast and the unbalanced evaporation is strong,while the air humidity is low.(4)The DIC content and δ13CDIC value in the water body of the study area show obvious seasonal changes:in the rainy season,the groundwater has low DIC content and δ13CDIC value,the surface water has low DIC content and δ13CDIC value;the dry season has the opposite changes.The influence of a large amount of rain on the dilution of groundwater in the rainy season is greater than that of the soil effect,which occupies a dominant position,resulting in higher DIC content in groundwater in the dry season.The groundwater at different observation points has different DIC content and 813CDIC value.The overall trend is as follows:with the decrease of vegetation cover,the thickness of soil layer decreases,the rocky desertification becomes more serious,the soil loss intensification,and in groundwater,the DIC content decreases,δ13CDIC value is more. |
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