Formation Mechanism Of Strontium-Rich Groundwater In Xintian County,Hunan Province

Karst groundwater is the most important source of drinking water in the karst mountains of southwest China.With the rising living standards of the general public,requirements for groundwater resource quality are becoming more stringent,and mineral water containing trace elements necessary for the human body has been gaining increasingly high popularity.The development of mineral water in karst area has become one of the main means to get rid of poverty and get rich in the impoverished mountainous areas,which is paid more and more attention by the governments at all levels.Based on field survey and analysis of the strontium-rich groundwater occurring in the D3s formation in Xintian,hunan province,under the guidance of scientific theory of the groundwater system,herein we comprehensively analyzed the hydrogeochemistry,and isotope tracing,in conjunction with laboratory experiments investigating water-soil-rock interactions and hydrogeochemical simulations to systematically explore the forming conditions of strontium-rich groundwaterunder the characteristics of calcium-rich alkaline geochemistry and unique karst hydrogeological structure.The main findings are as follows.1.The hydrogeochemical characteristics and spatial–temporal variation laws of Sr-rich underground water are clarified.The Sr-rich descending springs within the study area are of HCO3-Ca hydrochemical type.Sr-rich underground water from pumping wells is dominated by HCO3-Ca and HCO3-Ca·Mg hydrochemical types.Water chemical composition is mainly influenced by karst water-bearing media,environment,lixiviation duration and cation exchange,etc.Spatially,the Sr²⁺concentration of underground water tends to increase from the south,north and west towards the central east.This is basically consistent with the flowing direction of underground water.Sr-rich water is chiefly distributed in the Shetianqiao Formation of Devonian age.Analysis of typical profiles of underground flows indicates that the recharge area of Sr-rich underground water is the Devonian Xikuangshan Formation,with a Sr²⁺content of<0.20mg/L;runoff discharge and discharge areas are lithologically the Devonian Shetianqiao Formation,with an underground water Sr²⁺concentration of>0.20mg/L.Along the groundwater flowing direction,the concentration of Sr²⁺tends to increase;this is related to that long water-rock interaction duration increases with underground water runoff path elongating.Into the discharge area,the concentration of Sr²⁺decreases due to surface water mixing.Temporally,Sr-rich surface karst and descending springs generally tend to decrease in Sr²⁺concentration with rainfall increasing.This is correlated to the dilution effect of rainfall.Due to the short groundwater runoff path and rock lixiviation duration,surface karst springs have Sr²⁺concentrations lower than descending springs.The Sr-rich pumping wells in the recharge runoff and discharge areas have Sr²⁺concentrations generally increased with rainfall.This is mainly because groundwater from pumping wells is usually mixed water of shallow and deep underflow zone.In the wet season,the underground water level is lifted,high-Sr underflow water rises,the content of Sr²⁺in underground water of pumping wells is increased.In the dry season,the water level decreases,groundwater dominates relatively low-Sr,shallow underflow zones and the concentration of Sr²⁺is decreased in groundwater of pumping wells.Due to the long groundwater runoff path and lixiviation duration,the content of Sr²⁺in groundwater of pumping wells within the discharge areas is generally higher than that of groundwater of pumping wells of the recharge runoff area.As rainfall increases from June to October,surface rivers diluting Sr²⁺of groundwater of pumping wells of the discharge areas intensifies,leading to the content of Sr²⁺of groundwater of pumping wells in the discharge areas lower than that of the recharge runoff area.2.Isotopic characteristics and implications of Sr-rich groundwater are analyzed?¹⁸O and?D data of the study area imply that Sr-rich underground water is mainly sourced from atmospheric precipitation.Deuterium surplus“d”indicates that evaporation in Sep.is stronger than in Jun and Sr-rich groundwater of pumping wells is more greatly influenced by Sr-rich descending springs.The correlation between“d”value and TDS indicates that the Sr²⁺content of descending springs is greatly influenced by runoff conditions and residence time.The correlation between“d”value and TDS is weak and deuterium surplus is possibly collaboratively influenced by evaporation and concentration and cation exchange,etc.?¹³C data suggest that groundwater of pumping wells may be in a CO2 closed system and enjoys poor runoff conditions;descending spring groundwater may be in an open CO2 system and enjoys favorable runoff conditions.?³⁴S data indicate that S-rich groundwater-hosting strata have gypsum.⁸⁷Sr/⁸⁶Sr ratios and recharge elevation imply that descending springs show fast recharge and discharge characteristics,and low Sr²⁺and ⁸⁷Sr/⁸⁶Sr ratios.Groundwater of pumping wells has a long runoff path and high Sr²+and ⁸⁷Sr/⁸⁶Sr ratios.The Devonian Shetianqiao Formation argillaceous limestone,limestone and marl are the sources of Sr²⁺of groundwater.3.The dissolved organic matter(DOM)characteristics and the interaction mechanism between Sr²⁺and DOM of Sr-rich ground water are revealed.The 3D fluorescent technique combining with PARAFAC was applied for the first time to investigate the DOM composition,sources and the experiment on the interaction mechanism between Sr²and DOM.The formation environment differences between the two discharge types of Sr-rich groundwater are revealed.From Sr-rich groundwater of the study area,three dominant fluorescent components,i.e.,humus(C1 and C2)and tryptophan(C3),were identified.Of them,ground water of pumping wells is dominated by C3 and descending springs are dominated by C1 and C2.BIX,humification index(HIX)and fluorescence index(FI)indicate that DOC of pumping wells is dominated by endogenous input,suggesting pumping wells are formed in a closed environment.Descending springs are dominated by endogenous input,suggesting descending springs are formed in an open environment.Correlation analysis indicates that under the karst hydrogeologic environment,the existence of DOM and Sr²⁺in ground water are correlated.The combination of tryptophan and tyrosine in DOM with Sr²⁺is more stable.Tryptophan,compared to tyrosine,combines in priority with Sr²⁺,transforming Sr²⁺in a free dissolved state into an ion in a DOM-combined state.The mobility of Sr²⁺in a DOM combined state intensifies,quickening the lixivation in soil and rocks.4.The formation mechanism of ground water is revealed.The chemical components of Sr-rich groundwater are sourced from high-Sr marl,limestone and argillaceous limestone of the Devonian Shetianqiao Formation,and pumping wells have weaker hydrodynamic conditions than descending springs,leading to Sr²⁺concentration higher in pumping wells than in descending springs.Descending springs and pumping wells are dominated by dissolution of Sr CO3.The dissolution volume of Sr CO3 in pumping wells is higher than in descending springs,and it's the reason for the high Sr²⁺in the pumping wells.With low calcium and high magnesium and rich in gypsum was more conducive to the formation of strontium-rich groundwater.The action of water with rocks and soil and the indoor simulating experiment of regional deep circulation indicate that lixiviation time and flowing rates are a significant factor for the content of Sr²⁺,the longer water-rock action and the lower flowing rates,the higher content of Sr²⁺.The calcite in the strontium-rich spring in the study area was first close to saturation and the strontium was gradually dissolved along the direction of groundwater runoff.The main source of strontium in the study area is the dissolution of strontium.The simulation results show that the difference in the dissolution of calcite and dolomite leads to the difference in the correlation between Sr²⁺,Ca²⁺and Mg²⁺in the mechanical well.The synchronous dissolution process of dolomite and strontium can explain the internal reason for the highest correlation between strontium and magnesium in groundwater in the study area.This paper has three main innovations:1,Applying the multi-isotope technique of?¹⁸O,?D,?¹³C,?³⁴S and?⁸⁷Sr/⁸⁶Sr,etc.,a closed environment with low calcium and high magnesium,rich in gypsum,and strontium was more conducive to the formation of strontium-rich groundwater.the groundwater mainly comes from atmospheric precipitation,while the strontium mainly comes from carbonate rock in the the Devonian Shetianqiao Formation marl.2,By establishing the Phreeqc hydrogeochemical reverse modul,the dissolution difference of calcite,dolomite and strontiantite is the main cause for the chemical composition difference of water in descending springs and pumping wells as well as the most important factor for discriminating the formation of Sr-rich groundwater of other areas.