湘西北地区岩溶地下水水化学与氘氧同位素特征分析

Analysis on D/18O and hydrochemical characteristics of karst groundwater in northwestern Hunan Province

湘西北地区岩溶地下水水化学研究是地下水资源合理开发与利用的保证,文章在全面采集区内地下水水样进行水化学和同位素分析的基础上,利用氘氧同位素和综合水文地球化学研究方法对该区地下水的来源与组分成因进行探讨。结果表明:(1)研究区内岩溶水化学特征整体上三个区之间差异不大,但各区之间地下水组分的来源与成因仍有较大的不同,其主要来源于碳酸盐岩矿物的溶滤,并伴有不同程度的石膏等其他矿物溶滤,在龙山地区(I区)以方解石/白云石、石膏溶滤为主;永顺—凤凰地区(Ⅱ区)内,酉水流域地下水主要以白云岩溶滤为主;武水流域地下水中钙镁离子浓度受灰岩、白云岩溶滤作用共同影响。石门地区(Ⅲ区)主要离子来源于灰岩溶滤,地下水中钠钾离子、氯离子有多种来源;(2)研究区内氘氧同位素体现出明显的大陆效应和高程效应,泉相对井和暗河地下水系统具有相对封闭性,氘盈余则反映泉相对于暗河是一种快循环、短停留时间的系统。

Understanding hydrochemistry of karst water in northwestern Hunan is the foundation for reasonable utilization of water resources.The study on the causes of water sources and the evolution process of solute components is of great scientific value and practical significance.However,most of the research on this area has focused on the optimal allocation of water quantity and the evaluation of local water quality in the past few years.There is lack of research on hydrological geochemical characteristics and control factors based on groundwater systems,and hence a need of further research on the systematic understanding and utilization of local karst groundwater.This study takes the karst water system in northwestern Hunan Province as a case,aiming to reveal the characteristics,the evolution law and causes of hydrochemistry in this area.The research findings are hoped to play a positive guiding role in the sustainable development,and utilization and integrated management of regional groundwater resources.On the basis of hydrochemical and isotopic analyses of water samples collected in the study area,the source and composition origin of groundwater in this area have been explored with isotope D and 18O and with the comprehensive hydrogeochemical research method of multivariate statistics and hydrochemistry(piper trilinear diagram and ion scale coefficient).The results show that although there is little difference among three study zone on the whole,the sources and genesis of groundwater components are still quite different.According to the pipper three-line chart and the Gibbs chart,it can be seen that the main component of groundwater mainly derives from the dissolution and leaching of carbonate,accompanied by different degrees of the leaching of gypsum and other minerals.Concentrations of potassium,sodium ions and chloride ions are generally low,which is related to the low content of salt rocks in the carbonate formation and weak effect of evaporative concentration.Because the alternating adsorption of cations is weak,it is not the main influencing factor of the groundwater chemistry in the study area.Longshan area(Zone I),mainly presents the leaching of calcite/dolomite and gypsum.The water abundance in karst aquifers is high and the interaction between water and rock is rapid.In Yongshun-Fenghuang area(Zone II),groundwater in the Youshui basin mainly presents the leaching of dolomite and the concentrations of potassium and sodium ions are affected by the leaching of rock salt.In the Wushui basin,the joint leaching effect of limestone and dolomite that influences the concentrations of calcium and magnesium ions in groundwater,together with the leaching of silicate rock,augments the concentrations of potassium and sodium ions.In Shimen county(Zone III),the main source of ions comes from the leaching of limestone,and there are multiple sources of sodium,potassium,and chloride ions in groundwater.Besides,the recharge source of groundwater in the study area is mainly atmospheric precipitation,groundwater generally replenished directly through sinkholes, karst pipelines, karst funnels, etc. The isotope D and 18O of groudwater in thestudy area presents obvious continental and elevation effects. It is believed that springs show more closure propertiescompared with wells and underground river systems. Since the study area is a karst landform in an arc-shapedmountainous area, the terrain is severely cut, and the elevation of the sampling point is not very representative of theaverage supply elevation of springs and underground river systems. The deuterium surplus reflects that the springsystem is the one with a faster cycle and shorter retention compared with the system of underground river. However,the underground river has a longer supply source, a wider range of replenishment, a longer movement time and alonger flow of water underground, and stronger water-rock interaction.