Isotope-based identification of karst water system in Xin'an Spring Region

ZHAI Huwei; HU Xiaobing; ZHANG Kai; MA Yongming; GAO Xubo; JIN Jianhong

doi:10.11932/karst2023y004

Abstract：Xin'an Spring Region is located in the southeast of Shanxi Province,on the western side of the middle-southern section of the Taihang Mountains.The main area lies within Changzhi City,covering an area of 10,950 km2.The overall structure of the spring area is monoclonal,with a strike direction of NNE and a dip toward the NW;the strata are generally flat.Karst groundwater generally flows from west to east,which is opposite to the dip direction of the strata.The flow field is a fan-shaped spring group originating from the north,west,and south,characteristic of a"monoclinic inverted"karst water system. Karst groundwater in Xin'an Spring Region serves as the primary source of urban domestic water for Changzhi City.In recent years,the continuous increase in karst groundwater extraction has led to geological and environmental issues,such as the decline of karst water levels and the decrease of spring flow,which have severely constrained the sustainable development of the local economy and society.To provide technical support for the scientific protection and utilization of karst water resources,this project adopts isotope tracing method to conduct a comprehensive analysis and diagnosis of the spring field.This approach utilizes variations in natural isotopes in water to accurately interpret and identify the recharging runoff pathways within the groundwater system.In the relatively active hydrological cycle area in the central-eastern part of the spring domain,a total of 85 groups of isotope water samples were collected,including 14 groups of surface water samples and 71 groups of karst groundwater samples.The analyzed isotopes include stable isotopes D(2H),18O,87Sr/86Sr,34S and 13C.The isotope composition is expressed as the per mil(‰)difference(δ)between the sample's stable isotope ratio and that of a standard reference,reflecting the direction and magnitude of isotopic variation relative to the standard.Based on the natural principles of isotope fractionation and the distribution of isotope test values,this study interprets characteristics such as boundaries of karst water subsystems,surface water leakage,and mine water contamination. Based on a consensus on the basic framework of the spring region,the isotopic data from water samples were divided into 14 system units.These include the Zhuozhang River,reservoir,Changzhi recharge area,Changzhi runoff area,Xiangyuan recharge area,Xiangyuan runoff area,Pingshun-Huguan recharge area,Pingshun-Huguan runoff area,Licheng recharge area,Licheng runoff area,Lucheng runoff area,Anchang-Zhonghua fault runoff zone,discharge area,and slow flow area. Isotopes in the spring region generally follow these rules:D(2H)and 18O isotopes are the inherent components of water,exhibiting similar distribution rules and characteristics,and originate from atmospheric precipitation,resulting in low δD(2H)and δ18O values.Sr in underground karst water mainly derives from celestite(SrSO4)and strontium rhodochrosite(SrCO3)present in soluble rocks.The 87Sr/86Sr in the same water body does not change due to SR fractionation.In its original state,the 87Sr/86Sr ratio in karst water is low.The 34S isotope in karst groundwater mainly originates from the dissolution of sulfate minerals,so the δ34S value is similar to that of sulfate minerals,and is relatively high.Dissolved inorganic carbon from carbonate dissolution typically exhibits a high δ13C value.Surface water,influenced by evaporation,shows elevated δD(2H)and δ18O values.It is also affected by the high 87Sr/86Sr ratio of non-carbonate rocks and the mixing of multiple water sources.The δ34S value in surface water due to microbial degradation,and the δ13C value is also low.Due to recharge from surface water leakage and mine water pollution,the isotopic values generally fall between these two end-members. Based on the isotopic indication rules outlined above and combined with isotopic test data,the hydraulic connections between various system units are analyzed.A collaborative analysis is conducted incorporating with geological water control structures,water levels,water quality,hydrochemistry,and other relevant factors.Finally,the karst water system in the Xin'an Spring Region is further divided into six zones:western stagnant slow-flow area,Changzhi subsystem,Xiangyuan subsystem,Pingshun-Huguan subsystem,Licheng subsystem,and discharge area. In addition,surface water leakage and mine water pollution are clearly reflected in the data.The δD(2H)and δ18O values of spring water in the discharge area are lower than those of karst well water in the discharge area,indicating that as the karst well water level declines,the karst well water is recharged by surface water from the Zhuozhang River.The Lucheng runoff area is a section of the river with abundant water leakage and is strongly affected by leakage;itsδ13C value is-7.8 ‰,which is close to that of surface water.The 87Sr/86Sr value in the Pingshun-Huguan recharge area is higher than in the runoff area,likely because the southern recharge area is contaminated by pit water.Sampling points near the Taoqing River exhibit very low δ34S value,showing that the karst water is heavily affected by pit water pollution.