A Chlorine-bromine-iodine Ternary Diagram And Its Implications For The Origin Of Iodine In Shallow Groundwater Of The Pearl River Delta

As an indispensable trace element for the synthesis of thyroid hormones in the human body,iodine plays a crucial role in the metabolism,growth,and development of human beings.Insufficient or excessive iodine intake for a long time will cause thyroid diseases and even thyroid cancer.Since about 70%of natural iodine exists in seawater in nature,marine iodine is characterized by a concentrated distribution and a high degree of enrichment.In recent years,thyroid disorders induced by excess iodine and the hydrogeochemical behavior of iodine have been frequently reported worldwide.Most of them occur in coastal regions of various countries,such as China,Denmark,Japan,and Chile.The Pearl River Delta（PRD）is a typical coastal regions in China.It has undergone two transgression-regression events since the Quaternary due to global climate changes and eustatic movement,forming sedimentary strata consisting of marine-continental interbeds.Meanwhile,transgressions and regressions provide favorable conditions for the interchange of iodine in oceans and continents.On the other hand,the PRD is a typical area undergoing rapid urbanization in China.With the rapid development of urbanization and industrialization,large amounts of domestic sewage and industrial wastewater have been discharged into the PRD at will,severely polluting surface water and groundwater in the PRD.It has been reported that intensified human activities have a certain impact on the enrichment of groundwater with a high iodine concentration（also referred to as HIC groundwater）in the PRD.However,different water environments have multiple types of iodine speciation,which are interchangeable.Therefore,it is uncertain about the presence of other types of iodine speciation in the groundwater of the PRD and their proportions.Moreover,the infiltration of reducing sewage during rapid urbanization leads to more complex enrichment and origin mechanisms of iodine in the groundwater of the PRD.At present,there is still a lack of convincing evidence and identification means for the source and origin mechanisms of HIC groundwater in the PRD.Besides,it is necessary to further ascertain the effects of different aquifer types and different degrees of urbanization on the distribution of HIC groundwater in the PRD.This study investigated the shallow groundwater system in the PRD.Based on previous experience in the identification of groundwater pollution sources using the chlorine/bromine ratio and taking the distribution of chlorine,bromine,and iodine in different water bodies as a starting point,this study proposed a new method for identifying the sources of iodine in groundwater,i.e.,the chlorine-bromine-iodine ternary diagram.Using this diagram,this study effectively identified the response mechanisms of iodine in shallow groundwater of the PRD to rapid urbanization expansion and other human factors and geological background conditions.Moreover,this study generalized a conceptual model for the origin of shallow HIC groundwater in the PRD under the background of rapid urbanization.The results of this study will provide a basic scientific basis for the management,development,and utilization of groundwater resources in the PRD.The major progress and achievements of this study are as follows:1.Hydrochemical characteristics and occurrence patterns of HIC groundwater in the PRDBased on the hydrogeological structure and groundwater flow characteristics,this study divided the PRD into areas of alluvial-proluvial,marine-alluvial,marine depositional,lacustrine-swamp,fissured,and karst aquifers.Moreover,it expounded on the hydrochemical characteristics and the spatial distribution patterns of iodine in different aquifers through a contrastive analysis.As shown by the results,the groundwater system in the PRD has an iodine concentration of up to 1080μg/L,which is far greater than the criterion for the iodine concentration of Class III groundwater,with an over-limit ratio of 5.48%.HIC groundwater in the PRD is distributed primarily in marine-alluvial aquifers,followed by alluvial-proluvial and fissured aquifers.From the angle of different land use categories,HIC groundwater is distributed mainly in urban and suburban areas and sporadically in agricultural areas.Compared with the data in 2006,the average iodine concentration and the proportions of high-concentration and ultra-high-concentration iodine in different hydrogeological units and different types of land generally show a downward trend.The groundwater in the PRD has 92 hydrochemical types,which are dominated by Ca-HCO₃,Ca·Na-HCO₃,Ca·Na-HCO₃·Cl,Ca·Na-NO₃·Cl,and Na-Cl.The HIC groundwater is mainly of Ca-HCO₃,Na-Cl,and Ca·Na-HCO₃·Cl types.The HIC groundwater of the nitric acid type（Na-Cl·NO₃）is present in alluvial-proluvial and fissured units,accounting for up to 50%of the groundwater of the fissured units.The fissured units suffer far more severe anthropogenic pollution of HIC groundwater than other hydrogeological units.2.HPLC-ICP-MS-based characterization of the distribution of iodine speciation in different water sources in the PRDThis study analyzed the iodine speciation characteristics of samples collected from seawater,rainwater,domestic sewage,and shallow and deep groundwater in the PRD.The results show that the five water sources except for rainwater,in which no iodine was detected,vary significantly in the distribution of iodine speciation.Specifically,seawater has similar proportions of iodide,iodate,and inorganic iodine,which are38.53%,27.44%,and 34.04%,respectively;domestic sewage and deep groundwater exhibit almost the same proportions of various types of iodine speciation,with an iodide concentration of more than 80%,an organic iodine concentration of about 14%,and only a small amount of iodate;the iodide,organic iodine,and iodate in the shallow groundwater have proportions of 50.74%,42.20%,and 7.06%,respectively.Therefore,iodides（I-）dominate the iodine speciation in different water sources in the PRD.Organic iodine in the PRD mainly occurs in shallow groundwater and seawater.3.Establishing a chlorine-bromine-iodine ternary diagram for identifying the origin of iodine in groundwater based on the distributions of chlorine,bromine,and iodine in different water sourcesThis study analyzed the chlorine,bromine,and iodine concentrations of eight types of typical end-member water,namely seawater,rainwater,potable surface water,domestic sewage,contaminated surface water,landfill leachate,and deep groundwater,as well as septic tank effluent from typical areas.The comparative analysis results show that these water sources have significantly different chlorine,bromine,and iodine concentrations.The chlorine/bromine ratio of these water sources is in the order of landfill leachate>septic tank effluent>domestic sewage>contaminated surface water>rainwater>deep groundwater>seawater>potable surface water.In other words,the uncontaminated water has the lowest chlorine/bromine ratio,and a higher chlorine/bromine ratio is associated with more severely contaminated water.However,this is not the case for the chlorine/iodine and bromine/iodine ratios,which are the highest in the seawater and deep groundwater and are the lowest in the contaminated water.The chlorine,bromine,and iodine concentrations exhibit significant zoning in the Cl/Br-Cl and Cl/I-Cl diagrams.The average chlorine/bromine and chlorine/iodine molar ratios are 1002 and 21184,respectively.In other words,the concentration of chloride ions is about 1000 times that of bromine ions and 10000 times that of iodine ions.Based on their characteristics,the chlorine,bromine,and iodine concentrations in different types of end-member water were normalized（chlorine/bromine/iodine ratios:1:1000:10000）.Then,the ternary diagram was plotted using the Python and Pycharm software.The results show that:（1）The zone of the bromine end member,where bromine accounts for greater than 80%and chlorine and iodine account for less than 20%each,is the zone of uncontaminated water.This zone is labeled as ZoneⅠ;（2）The zone around the middle of the chlorine-bromine edge,where iodine,chlorine,and bromine account for less than 20%,about 20%?60%,and about 40%?80%,respectively,is the zone of pristine marine depositional water or the zone affected by seawater intrusion.This zone is labeled as ZoneⅡ;（3）The zone of the chlorine end member,where chlorine,bromine,and iodine account for greater than 60%,less than 40%,and less than 22%,respectively,is the zone of contaminated water in a slightly oxidizing environment.This zone is labeled as ZoneⅢ;（4）The zone of the iodine end member,where chlorine,bromine,and iodine account for greater than 60%,less than 20%,and less than 40%,respectively is the zone of csontaminated iodine-rich water in a slightly reducing environment.This zone is labeled as Zone V.85%of the groundwater with an iodine concentration greater than the criterion for the iodine concentration of Class III water falls into this zone;（5）The zone between zonesⅢandⅤ,where bromine,chlorine,and iodine account for less than 20%,60%?80%,and 20%?60%,respectively,is the oxidizing-reducing environment transition zone subjected to anthropogenic pollution.This zone is labeled as ZoneⅣ;（6）The zone in the lower-middle part where chlorine,bromine,and iodine have similar proportions is the zone of mixed water or the zone affected by multiple factors.This zone is labeled as ZoneⅥ.Therefore,water with different sources and circulation pathways shows significant zoning in the ternary diagram,which,thus,can be used to identify the sources and origins of iodine in groundwater.4.Revealing the influencing mechanisms of intensified human activities on iodine enrichment in the groundwater system under the background of rapid urbanization based on the ternary diagram and hydrogeochemical characteristicsBased on the ternary diagram as well as the multivariate information,such as hydrogeochemical characteristics,hydrogeological conditions,land use,and socioeconomic parameters,this study analyzed the influencing factors of HIC groundwater in the PRD using multivariate statistical techniques including cluster analysis and principal component analysis.As shown by the analytical results,the iodine in the shallow groundwater in the PRD originates primarily from the pristine marine sedimentary strata and the infiltration of iodine-bearing sewage caused by human activities（e.g.,domestic sewage,industrial wastewater,landfill leachate,wastewater irrigation,and leaching of iodine-bearing fertilizers）.The enrichment of iodine ions in HIC groundwater is primarily caused by:（1）The degradation of iodine-rich organic matter in marine sedimentary strata increased the HCO₃ concentration,and the competitive adsorption of HCO₃ and I^-promoted the desorption and release of I^-into groundwater;（2）The infiltration of iodine-rich reducing sewage increased the iodine concentration in groundwater;（3）The reducing environment formed by organic matter degradation and the infiltration of reducing sewage led to the reducing dissolution of iodine-bearing minerals,thus promoting the release of iodide ions;（4）The paleo-seawater rich in Cl^-,Na⁺,Mg²⁺,and I^-from marine sedimentary strata entered groundwater with water flow,making the groundwater rich in iodine,as well as Cl^-,Na⁺,and Mg²⁺;（5）Seawater intrusion.Different hydrogeological units vary in the enrichment mechanisms of iodine in groundwater.Marine sedimentary strata are distributed primarily in marine-alluvial and marine depositional areas.Therefore,the major geological sources and driving mechanisms for the formation of HIC groundwater in the marine depositional and marine-alluvial units include the degradation of organic matter and the slow release of retained paleo-seawater in marine strata,and major anthropogenic sources include the leakage and infiltration of HIC sewage in urban and suburban areas.Moreover,a slightly reducing environment is favorable to the enrichment of iodides in the groundwater of the marine depositional and marine-alluvial units.The HIC groundwater in the alluvial-proluvial units originates from the leakage of iodine-bearing sewage in urban and suburban areas and the leaching and infiltration of iodine-bearing fertilizers in agricultural areas.Meanwhile,the HIC groundwater in the alluvial-proluvial units exhibits some reducing characteristics,such as the enrichment of NO₂^-,NH₄⁺,and COD.When a reducing environment transitions into an oxidizing environment under the action of microorganisms and oxygen,the oxidation of iodides will occur,decreasing the quantity of iodides in groundwater.HIC groundwater in the fissured units is distributed in areas with intensified human activities,such as urban and suburban areas,and is rich in nitrate and lead.Owing to the infiltration of nitrate-bearing sewage,lead leaches from the aeration zones of the fissured units and enters groundwater along with water flow.As a result,iodine,nitrate,and lead coexist in the fissured units.It can be inferred that the high-concentration iodine in the fissured units mainly originates from the leakage and infiltration of iodine-bearing domestic sewage.5.Influencing mechanisms of regional hydrogeological conditions on the enrichment of iodine in the groundwater systemThe origin of iodine in the groundwater in the PRD was traced using four different types of diagrams,namely,the Cl/Br-Cl,Br/I-I,Br/I-Br,and ternary diagrams.The results are as follows:（1）The regional distribution of iodine in different hydrochemical types of groundwater does not show horizontal zoning from piedmont areas to plains or basin hinterlands along with the water flow,which is common in large plains and basins.Therefore,the groundwater flow field does not play a dominant role in the migration and enrichment of iodine in the groundwater;（2）For fissured,karst,and alluvial-proluvial aquifers with high hydraulic gradients of groundwater,high permeability,and rapid groundwater circulation in the upper-middle reaches,the groundwater flow field plays a certain role in the migration and enrichment of iodine in local groundwater.With regard to the iodine of a natural geological origin,its migration and enrichment under the influence of groundwater flow field can only make iodine in groundwater enriched from low to medium concentrations;（3）For marine-alluvial and marine depositional aquifers with small hydraulic gradients of groundwater,low aquifer permeability,and slow groundwater circulation in the middle-lower reaches,the groundwater flow field has no significant influence on the enrichment of iodine due to the groundwater runoff conditions;（4）For the study area consisting primarily of hills and mountains in the southern coastal areas,besides regional groundwater flow systems,there may exist many nested multilevel groundwater flow systems with poor hydraulic connections.As a result,the groundwater samples under the current sampling density did not show a significant law of change in the iodine in groundwater.This result also reflects that the iodine in the groundwater does not completely have a natural geological origin but is also induced by anthropogenic pollution.