Geochemical Processes Of High Arsenic Groundwater System At Western Hetao Basin

Hetao Basin(the Great Bend of Yellow River)of Inner Mongolia is located in the arid-semiarid region of northwestern China,which is one of the representative arseniasis-affected areas in China,where more than 300,000 victims of arseniasis.Hangjinhouqi county at the western part of Hetao Basin is the most serious and representive endemic arseniasis area in Inner Mongolia.About 76,000 people exposured in arseniasis area in 35 villages from 9 towns,among them there have been 1169 arseniasis patients.Most groundwater from arseniasis area contained high As with the concentration from 0.35 to 1.74 mg/1,which exceeds maximum contaminant level(MCL)based on the health risk associated with arsenic in drinking water(10μg/l)more than 100 times.Up to date,about 50%residents are still using groundwater with high contents of arsenic,TDS or high fluoride as the only source of potable water.Geographically, arseniasis-affected areas stretch southwest-northeast along Yin Mountains front.Residents have taken high arsenic groundwater as their potable water from generation to generation,most of them suffer from the arseniasis to different extent,which affect their daily life and normal production activity.Long-term intake of the high arsenic groundwaters has caused waterborne arsenic poisoning,which seriously endanger the redients and impede the sustainable development of local economy and society.National and local government attach great importance to this problem,20 billion RMB will be spent to resolve the difficulty on safe potable water in rural area, especially the endemic arseniasis and fluorosis area.Therefore,better understanding the genesis of high As groundwater is of pratically significance in correctly guiding current water source transforming and removal of arsenic from groundwater.Arsenic contamination in groundwater is a serious environmental issue all over the world, which is also the hot topic in environmental study.Hetao basin is a fault basin formed in Mesozoic and Cenzoic,which experienced geological environment changes since Cenozoic, including tectonic movement,climate change,flowage and watercourse change of Yellow River and son on.Furthermore,irrigation using delivered Yellow River water has a long history for thousands of years at Hetao Plain.Groundwater system in Hetao was influenced by both natural conditions and anthropogenic activities,which makes the arsenic enrichment and mobilization more complicated.Thus,the study on major factors and geochemical processes controlling arsenic mobilization in shallow aquifers at Hetao Basin has important scientific meaning,based on the investigation of the regional hydrogeological background and distribution of high As groundwater in the arseniasis area,the speciation analysis of As in both groundwater and sediments,discussion on natural and anthropogenic factors affecting As enrichment and release.The framework of evolution of sedimentary environment in Hetao basin is discussed based on analysis of changes of tectonic movement,paleo-climate,paleo-geology environment.The form and evolution of groundwater in Hetao basin are internally associated with the evolution of sedimentary environment.The changes of sedimentary environment and corresponding climate process directly affect regional groundwater flow and hydrogeochemical evolution.Groundwater system evolution is discussed at long-time scale in view of paleo-climate,paleo-tectonic movement,paleo-geology environment and hydrogeology.The general pattern of groundwater evolution is the most important background for scientifically understanding the geochemical process occurred in groundwater system and for reasonably comprehending the origin and enrichment for some minor elements such as As,B in groundwater system.Hangjinhouqi is selected for our case study.According the analysis of regional hydrogeotogical setting and large amounts of groundwater chemical data,the basic hydrochemical characteristics is summarized as follows:Most water samples have high TDS, with Cl and HCO₃ as the dominant anions and Na as the dominant cation.The chemical composition of groundwaters varied from the margins towards the low-lying areas in the centre of the study area.Along the Yin Mountains front in the northern part of this area,as well as the southern part receiving recharge from the Yellow River,shallow groundwaters are generally of Na-HCO₃ or Na-HCO₃-Cl type,while in the central parts of Hangjinhouqi,mostly Na-Cl-HCO₃ or Na-Cl type.The study area is located in the Hetao fault depression basin and yellow river diverting irrigation plain with arid and semi-arid climate.Due to paleogeography,hydrology,geological Structure and human activity,with dissolution,mixture,evaporating concentration,ion exchange, oxidation and reduction,the components in groundwater transport and enrich,representing different chemical characteristics.Because of the yellow river's rechannel and irrigation,the fresh water,salt water,high-arsenic water and high-fluorine water distribute interlacedly, hydrochemical enviroment is very complicated.Due to differences of groundwater flow condition and hydrogeological units, hydrochemical classification represents zonation in spatial variation.From fan margin to plain center,the flow condition weaken,water quality changes from fresh to salty,salinity increases gradually,and the groundwater types from fresh bicarbonate water turn to saline chloride-sulfate water.The study area affected by yellow river,salinity is relatively low(TDS＜1.5 g/l),mainly belong to carbonate water with little bicarbonate-chloride water.According to hydrogeological condition,sedimentary environment and human activities,the shallow groundwater in Hang jinhouqi is classified into six zones:piedmont soluable-filter zone (â… ),yellow river lateral seepage zone(â…¡),mining affecting zone(â…¢),reductive environment zone(â…£),mixing zone(â…¤),depression concentrated zone(â…¥).Among them,piedmont soluable-filter zone locates in groundwater recharge region,with HCO₃ as main anion and Na⁺ as main cation in groundwater.Water quality in this zone is good;yellow river lateral seepage zone (â…¡)affected southern yellow river,flow condition is better than central part with slow flow and strong evaporation,with HCO₃^- still as main anion and lowest As and Fe contents;groundwater in mining affecting zone(â…¢)has abnormal SO₄^2-content,due to oxidation of pyrite,the highest amount could be 1550 mg/l,and the average TDS reaches 2313 mg/l.The hydrochemical type belongs to sulfate-chloride groundwater,reductive environment zone(â…£)is close to piedmont plain,representing strong reductive environment from west to northeast,with the highest contents of DOC,ammonia-nitrogen,sulfide,HPO₄^- and Ba;mixing zone(â…¤)locates in transition region from front of proluvial fan to low-slope plain.With hydraulic gradient decreasing,sedimentary in aquifers becomes fine,permeability turns weak,yellow river infiltrates into ground,hydrochemical characteristics become complicated,with chloride-bicarbonate water and bicarbonate-chloride water as main types;depression concentrated zone(â…¥)is in depression zone of the plain centre and the front of proluvial fan, with lacustrine deposited fine sand and clay layers between aquifers,which made groundwater flow slow.In arid climate,strong evaporation results into highest contents of TDS and main components,the water type are mainly chloride type or chloride-sulfate type.The main hydrogeochemical process defining the hydrochemical characteristics of the study area includes:hydrolysis process,dissolution-precipitation process,evaporating concentration,ion exchange,mixture,oxidation and reduction.The primary mineral,such as aluminosilicate,hydrolysis process forms secondary mineral kaolinite,and increases the contents of K,Ca,Na,Mg,HCO₃^-;However,with the TDS and Cl^- increasing,the enrichment of Ca and Mg is limited by calcite and montmorillonite precipitation;drought climate with slow flow makes the evaporating concentration as the main process increase TDS,Na⁺ and Cl^-.In the middle of the plain,the sediment contains silt and clay interlayer,rich in organic components,forming strong reduction environment,which is helpful for arsenic enrichment.The mixture of yellow river and groundwater lead to complex hydrochemical environment.Hydrogeochemical behavior and accumulative mechanism of several important trace elements,such as B,Br,F was discussed,especially the genesis of groundwater salinization and the relationship between soda-water environment and high concentration of boron,arsenic and fluoride.With the stable isotope analysis(hydrogen oxygen isotopes and strontium isotope),the dissertation identified the main factors controlling the hydrochemical characteristics of the study area,the ratio of strontium isotope revealed the flow direction and sources direction of regional groundwater.The main understandings are as follows:1).The study on hydrochemisty and hydrogen oxygen isotopes of the study area suggestes that evaporating concentration played an important role in accumulation of Na,Cl and some trace elements,such as B,Br,As,F.2).The genesis of groundwater salinization is associated with geological structure,evolution of paleoclimate,variation of quaternary sedimentary environment,groundwater flow condition and human activities.3).Sodawater environment is benefitfle for simutaneous accumulation of As,F, B.With pH increasing,arsenic will come into groundwater desorbing from minerals;alkaline condition with low concentration of Ca is helpful for hydroxy displacement in groundwater;high concentrtion of HCO₃ makes boron easier to desorbe from organic matters,clay minerals and metal oxide.4).Long-term irragation from yellow river leads to groundwater table increasing, which speeds up salinity accumulation in surface soil and shallow groundwater;besides, geochemical data of trace elements and isotopes verify that irrigation from yellow river playes a significant role in releasing some chemical element(such as boron)and mixing effect.5).The study on strontium isotope suggest that piedmont groundwater affected by bedrock lioxiviation with high ⁸⁷Sr/⁸⁶Sr background value,⁸⁷Sr/⁸⁶Sr value is higher that other samples,which may affected mainly by mixing process and evaporation effect.In addition,the components of strontium isotope reveal the regional flow direction and verify that the original source of arsenic in groundwater is bedrock weathering which is close to polymetallic sulfide deposit.The formation of high arsenic groundwater in Hang jinhouqi is relative to strong reductive environment,representing high level DOC,HCO₃^-,NH₄⁺,sulfide,methane gas in some region and low content of nitrates and sulfate.Forming reductive environment is inseparable with basin tectonic activities and change of sedimentary environment.Quaternary in Hetao basin is composited by river-lake sedimentary system.In middle pleistocene,giant thick lake sediments was formed in basin.Mudstone rich in organic matter and humus developed well in north of the basin,besides,a lot of buried herbaceous plants provied material sources for methane bacteria growth.Hetao basin experienced fault-developing stage and depression sedimentation stage, while Hang jinhouqi located in the centre of sedimentation,with 5000m-thick pliocene formation and 1.85 mm/a sedimentary speed.The fast sedimentation leaded to the geochemical environment which was helpful for accumulation and biochemical reaction of organic matter.High-arsenic groundwater mainly represent As(â…¢),accounting to 41～98%of total soulable As,average value 85%.This also verified that the groundwater is in strong reduction environment.It should be mentioned that the particle arsenic in groundwater of the study area is up to 172.5μg/l,more than 1/3 water samples the particle arsenic over 50μg/l.The proportion of particle arsenic to total arsenic is about 10%,with the highest exceeding 30%.Based on quantitive hydrochemical data,using factor analysis method,the disstertation identified four factors controlling hydrochemical characteristics in high-arsenic groundwater: salinization factor,reducing environment factor,carbon effecting factor and absorption factor. Among them,the the first three factors affect obviously.Salinization factor and carbon effecting factor is theprominent characteristics different from other high arsenic aquifers.The hydrogeochemical characteristics are quite different from those from other high-As groundwater regions under reducing conditions,such as Bangladesh,West Bangal and Hungary,as well as Datong basin in Shanxi province and Huhhot Basin in Inner Mongonia of China,due to extremely strong evapotranspiration resulting from the arid climate,also affected by regional geology,sedimentology,palaeoclimate evolution,paleohydrogeology,water-rock interaction and human activities.The evolution of high-As shallow groundwater in Hangjinhouqi with high content of DOC,HCO₃^-,NU₄⁺,and sulfide,low content of nitrates and sulfates and highly concentrated methane in some area,which reaches 5107.7μg/l is closely correlated to the intense reducing environment caused by quaternary lacustrine sediments and rapid subsidence of the basin.According to the characterization of the reducing environment in groundwater,different districts have been through different oxidation-reduction processes.The driving force for reducing environment is varying and different indicators can concentrate in the same region at the same time.Two cored boreholes(Shahai and Tuanjie village)were drilled in hydrogeologically distinct areas with the depth of 50 m.The observed range of total As concentrations in sediments are 7.7-34.6 mg/kg and 6.8-58.5 mg/kg,respectively.The concentrations correlate positively with Fe oxides.High sulfur content is detected in the sediments of Shahai,up to 0.194%assocaiated with high As concentration(34.6 mg/kg)with the depth of 25 m.Another borehole was drilled for comparison in Erdaoqiao in the south of our study area,where the concentration of As in groundwater is quite low(2μg/l).The range of As concentration in the sediments is 3-21.8mg/kg, with the average value of 9.9 mg/kg.Up to one third of arsenic is ammonium oxalate-extractable and taken to be associated largely with Fe oxides.Only 2%of arsenic is hydroxylamine hydrochloride-extractable and taken to be associated with Mn oxides.Selectively sequentical extraction procedure has been used to investigate sediment As associations.Results show that approximately 35%of total As extracted by PO₄(strongly absorbed As)in sediments,also higher percentage of Oxalate-extractable As(As incorporated in amorphous iron oxides),concentrated HNO₃ and H₂O₂ extractable As(As associated with sulfides and organic matter),HCl-extractable As (targeting acid volatile sulfides,carbonates and amorphous metal oxides).The release of As into solution is believed to be by desorption coupled with reductive dissolution of the Fe oxide minerals.The release of As in groundwater system is an integration of macro-micro multifactor;its cause of formation is affected macrocosmically by regional geological structure,quaternary environment revolution and hydraulic dynamics,and also microcosmically by mineralogical properties of sediments and chemical constituents in groundwater.The former factor is the precondition for the formation of high-As groundwater and the latter is the controlling factor. Besides,irrigation using deliverd Yellow River water impose some influence on the formation of high-As groundwater to some extent.The major advances achieved in this dissertation are as follows:1)trace elements geochemical behavior in Hetao basin such as As,F,B was systematicly studied,the influence of irrigation using deliverd Yellow River exerting on the hydrogeochemistry was also discussed;2) the formation mechanism of high-arsenic groundwater was analyzed from macro to micro scope on the basis of hydrogeology,regional geological structure,paleosedimentary evolution, hydrogeochemical and sedimentary characteristics study and arsenic speciation analysis.