The Genesis Of High Arsenic Groundwater In The Ecotone Between Polymetallic Sulfide Mining Area And Irrigated Agricultural Area

Polymetallic sulfide deposits are mainly composed by polymetallic sulfides and the important source of arsenic in environment.Polymetallic sulfide mining area refers to the area where polymetallic sulfide deposits are distributed and impacted by polymetallic sulfide mining activities.Irrigated agricultural area refers to the area where irrigation is practiced in agricultural area and influenced by irrigated agricultural activities.Irrigated agricultural activities provide not only recharge,but also salinity,nitrogen,and oxygen for groundwater.Therefore,it has been a key factor in genesis of high arsenic groundwater.Based on the combination of polymetallic sulfide mining and irrigated agricultural activities together,to indicate the migration,transformation,and enrichment of arsenic in groundwater is a challenging study.But because the lack of a typical research area,the knowledge related to this topic is still very limited.Hetao Plain refers to the alluvial plain along the Yellow River between the Inner Mongolia Autonomous Region and Ningxia Hui Autonomous Region.The northern border of Hetao Plain is Yin Mountains,where polymetallic sulfide deposits are distributed in the proterozoic sedimentary rock,with proven reserves of pyrite up to 303,203×106 kg.The mining activities have lasted for over 50 years.Hetao Plain is an important base for marketable grain production in China and famous for the long-historical developed irrigated agriculture.Irrigation using Yellow River water has been started as early as Han Dynasty(4th century BC).Irrigation history has been over 2,000 years before present.In the modern time,after the completion of Sanshenggong Hydro-junction Project with a main irrigation channel of 180-kilometers long in 1959,the total irrigation area has been increased from 190 × 107 m2 to 500 ×107 m2 and Hetao Plain became one of the three largest irrigated agricultural areas.At Hetao Plain,individual geological conditions and anthropogenic activities contributes to the particular pore groundwater system.The groundwater system is bounded by the main drainage channel.The water-bearing media is the alluvial-pluvial sediments mainly consisting of polymetallic sulfide deposits from Yin Mountains in the north of the groundwater system,while the media is the alluvial-lacustrine sediments in the south.The main drainage channel distributed area is the ecotone influenced by polymetallic sulfide deposits in the Yin Mountians area and irrigated agricultural area in the plain,the interlaced zone between alluvial-pluvial sediments in the mountain area and the alluvial-lacustrine sediments from the Yellow River,and the mixing zone of groundwater from the northern mountains area and infiltrated irrigation water in the southern plain.Since 1990 s when the first arsenic poisoning patient was discovered at Hetao Plain,high-arsenic groundwater has been the hotspot attracting international attention.The high-arsenic groundwater is mainly distributed in 19 towns and about 100 villages of Linhe District,Wuyuan County,Hangjinhouqi County,Tumd Rigth Banner of Baotou,Alxa Left Banner,et.al,with a threatened population of over 300,000 and more than 2,000 arsenism patients.Arsenic in groundwater has a concentration ranged from 0.53 to 764.78 ?g/L with a high proportion of As(III).Many fruitful studies in this area have been conducted to understand the geochemical and biogeochemical processes of high arsenic groundwater system at Hetao Plain and to indicate the releasing,migration,and enrichment mechanisms.However,the agreement on the genesis of high arsenic groundwater is not reached yet.And the previous studies didn't relate the influence of polymetallic sulfide deposits and irrigated agriculture.A case of concern is that high arsenic groundwater is exactly distributed in the ecotone between polymetallic sulfide mining area and irrigated agricultural area.Is itcoincidental or consequent? In order to answer this question,this study takes Hetao Plain as the research area,to indicate the sources,migration,and enrichment of arsenic in the ecotone between polymetallic sulfide mining area and irrigated agricultural area,providing a new insight to genesis of high arsenic groundwater.Hangjinhouqi County,the central area of Hetao Irrigation Area and the typical area where high-arsenic groundwater is distributed in China,is selected as the research area of this study.On the basis of geochemical exploration of groundwater,surface water,sediment,and soil,this study explains the dominant hydrogeochemical processes in high arsenic groundwater system,reveals the sources,releasing,and enrichment of arsenic in the groundwater of ecotone,discusses the influence of natural weathering,anthropogenic mining activities,and irrigated agricultural practices on high arsenic groundwater,and indicates the genesis of high arsenic groundwater in the ecotone between polymetallic sulfide mining area and irrigated agricultural area.The study shows that there is a closely inner association between the geological evolution of Hetao Plain,the formation of the irrigation system and the formation of high arsenic groundwater system.The natural weathering of polymetallic sulfide deposits provides original sources for arsenic in groundwater.Human mining activities and irrigated agricultural practices do not only cause secondary arsenic pollution in groundwater system,but also influence the hydrogeochemical and hydrodynamic processes in groundwater system.The study will improve the theories of genesis of high arsenic groundwater,and be helpful to spatially manage water resources and to develop and protect the ecological system in high arsenic area.The following results are gained from this study:1.Ecotone between polymetallic sulfide mining area and irrigated agricultural area(1)Polymetallic sulfide mining area: the area between Yin Mountains and the main drainage channel.Two big-scaled and a number of small polymetallic sulfide deposits are distributed within the area.Tanyaokou polymetallic sulfide deposit is one of the three largest pyrite of China and the important sulfuric acid production base of northern China.Dongshengmiao polymetallic sulfide deposit has the largest and the fourth proven reserves of pyrite and zinc over China,respectively.The proven reserves of pyrite are 86,288,000,000 and 216,915,000,000 kg in Tanyaokou and Dongshengmiao polymetallic sulfide deposits,respectively.Analysis of 50 ores from these two deposits shows that ores have arsenic contents between <5.0 and 78.0 ?g/g with an average of 24.1?g/g.Major arsenic-containing minerals are pyrite,chalcopyrite,siderite,and magnetite.Arsenic contents in mining waste water ranged from 17.7 to 26.6 ?g/L.Arsenic contents in the tailing sand ranged between 6.4-14.3 mg/kg,higher than local geochemical background value 5.2 mg/kg.Soils within 1 km away from the tailings have arsenic contents ranging from 6.6 to 142.6 mg/kg,Pband Zn contents up to 11743 and 1937 mg/kg,respectively.In mining area,groundwater is mainly recharged by surface runoff,lateral flow from bedrock of Yin Mountains,and atmospheric precipitation,while discharges in ways of extraction and evaporation.Groundwater generally flows in N-S direction towards the main drainage channel.The dynamic changes of groundwater levels are mainly impacted by mining activities and extraction for the purpose of irrigation.Groundwater is mostly of HCO3·SO4 and SO4·HCO3 types.Concentrations of SO42-in groundwater reach up to 662.4 mg/L.Groundwaters have pH of 6.6-8.6,EC between 765 and 5090 ?s/cm,Eh between-141.3 and 253.0 mV.Arsenic concentrations in groundwater reach up to 406.3 ?g/L.(2)Irrigated agricultural area: the area between the north of the Yellow River and the main drainage channel including small irrigation channels and small drainage channels nearby the main drainage channel.Borehole sediments change from clay,silt to medium-fine sand with the depth changing from the ground within 30-meter depth.Clay and silt is commonly composed by clay minerals,such as montmorillonite,chlorite,illite,and kaoline.Fine sand generally consists of quartz(up to 45%).Arsenic contents in sediments are between 3 and 21.8 mg/kg.In irrigated agricultural area,groundwater is recharged by irrigation water(Yellow River water)and atmospheric precipitation,while discharges by evaporation.Groundwater generally flows from southwest to northeast.The dynamic changes of groundwater level are influenced strongly by irrigation practices.In summer irrigation(mid June)and autumn irrigation(late September to late October)periods,groundwater level is as much as 2 m lower than un-irrigation period.Groundwaters have pH of 6.5 – 9.2,EC of 763-106300?s/cm,ORP between-169.8 and 253.0 mV.Concentrations of Ca2+,Na+ and Mg2+ are 2.4-457.2 mg/L,33.2-2462.9 mg/L,and 3.8-506.9 mg/L,respectively.Concentrations of HCO3-,Cl-and SO42-are 141.0-1276.2 mg/L,46.3-4495.3 mg/L,and BDL-1475.7 mg/L,respectively.Total dissolved arsenic concentrations are between 0.5 and 764.8 ?g/L.Total phosphate concentrations reach up to 0.56 mg/L,while total nitrate concentration reach up to 190.8 mg/L.DOC concentrations in groundwater ranges from 0.4 to 77.1 mg/L.(3)ecotone: the transitional zone between Polymetallic sulfide mining area and irrigated agricultural area,as well as the mixing area of groundwater from mining area and irrigation area.The ecotone,a the zonal area along the main drainage channel,is the area where various levels of channels intensively distributed.The highest contents of arsenic in borehole sediments occurs in depth of 25 m from SH borehole(34.6 mg/kg)and in depth of 15 m from HF borehole(58.5 mg/kg).Arsenic is well positively correlated with Sb,Fe,Mn,B,V,TC,and TS(R>0.8).Borehole sediments from low-arsenic area are commonly composed by clay and medium sand within depth of 10 m,medium sand in depths of 10-18 m,and fine sand in depths of 18-26 m.The lowest arsenic contents are discovered in depth of 29.5 m.Fe and Mn contents are 9.28-24.10 g/kg,241.81-904.62 mg/kg,respectively.TOC contents are below the detection limit(2.00%).Arsenic contents are negatively correlated with TOC in high-arsenic area,while contrarily in low-arsenic area.In both high-arsenic and low-arsenic areas,arsenic is positively correlated well with Fe and Mn.In the ecotone,groundwater is recharged by groundwater from mining area and irrigation area,as well as atmospheric precipitation.Groundwater discharges in ways of extraction for the purpose of irrigation and evaporation.Groundwater flows along the main drainage channel into Wuliangsu Lake.Groundwaters have an average EC of 3,595 ?s/cm,and an average ORP of-30.23 mV.The average concentrations of Ca2+,Na+ and Mg2+ are 90.6,467.6,and 116.4mg/L,while the concentrations of Cl-and SO42-are 638.1 mg/L and 363.8 mg/L.The average content of the total dissolved As in groundwater is 161.6 ?g/L,in which As(III)has an mean content of 134.5 ?g/L.2.Sources identification of arsenic in the ecotone(1)Polymetallic sulfide depositsDongshengmiao polymetallic sulfide deposit occurs in the ore-bearing formation of Middle Proterozoic Langshan Group comprising a suite of fine clastic rocks-carbonate rocks.The ore-bearing rocks are formed in dry and weakly alkaline strong reductive environment.Ores collected from Dongshengmiao deposit have ?34Spy between 28.0‰-30.9‰ and Co/Ni mass ratios >1,indicating the hydrothermal origin.Combined with contents of other trace elements,it can be inferred that it underwent multi-stage mineralization.Ores collected from Tanyaokou deposit have ?34Spy between 27.0‰ and 39.2‰ with Co/Ni mass ratio between<1 and 15.8,indicating that pyrite mineralization underwent sedimentary diagenetic processes and metamorphic hydrothermal processes.According to previous studies,evidence from Sr isotopes and geological evolution can both demonstrate polymetallic sulfide deposits provide original source for arsenic in groundwater.From results of leaching experiments using ores from the two deposits,leachates of ores from Dongshengmiao deposit have Eh<0 and dissolved sulfide concentration decreased from 572 to 42 ?g/L,while leachates of ores from Tanyaokou deposit have Eh>0 and dissolved sulfide concentration <10 ?g/L.Two leachates have arsenic concentrations of 4.15-6.98 ?g/L and 0.80-1.67 ?g/L,respectively.Ores from Dongshengmiao deposit occur pyrite oxidation and dissolution under natural leaching condition and release As(III),while ores from Tanyaokou deposit have a higher contents of carbonates and release As(V).Before and after leaching,arsenic dominantly exists as sulfide-adsorbed form,such as arsenopyrite,eolite,and pyrite,et.al.Both XPS analysis and Eh-pH plot of Fe–H2O demonstrate the existence of a secondary mineral ferric oxide hydrate after leaching.(2)TailingsArsenic releasing rate from tailing sand is 4.23%,and the dominant specie is As(III).Tailing sand may become the potential source of groundwater arsenic pollution.(3)Sediments in the plainIn geological processes,weathered rocks were moved to the plain and deposited.Sediments in the plain are the direct source of arsenic in groundwater.The highest arsenic content is found in clay or silty clay at depth of 15-25 m.Fe oxides/hydroxides are major carriers of arsenic.Previous studies have indicated that reductive dissolution of Fe oxides/hydroxides in alkaline condition is the major releasing process of arsenic in groundwater.(4)Other potential pollution source: fertilizer,arsenic-containing pesticide,and arsenic-polluted soilThree main kinds of crops in the irrigated agricultural area are wheat,corn,and sunflower,accounting for over 90% of the whole planted crops.Common fertilizers used in the irrigation area are ammonium monohydric phosphate,urea,nitric phosphate,and compound fertilizer.There is no arsenic detected in urea or nitric phosphate.Arsenic content in compound fertilizer is around10.4 ?g/g.Annual average application amount of pesticides is 925×103 kg with an application rate of 1.98 kg/hm2.Arsenic contents in 49 kinds common pesticides range between 0.002 and 22.48 ?g/g,among which arsenic is lower than 1 ?g/g in 37 kinds.Arsenic contents in soils and river sediments reach up to 27.5 ?g/g,much higher than local geochemical background value(11.2 ?g/g).Arsenic-containing pesticides,compound fertilizer,and polluted soil maybe the potential pollution source of arsenic in groundwater.3.Releasing of arsenic in the ecotoneHigher contents of arsenic in sediments occur in clay and silty clay layers.Fine sand is mainly composed by quartz,while clay and silt mainly consist of clay mineral.Fe/Mn oxides or hydroxides are major carriers.Strongly adsorbed arsenic and arsenic coprecipitated with amorphous Fe oxides are dominant species of arsenic in high arsenic sediments.Sediment,from the low arsenic area,with age of 19130±146 BP at depth of 11 m is older than sediment from the high arsenic area in the same depth.Perhaps because the organic matter degrades over time,and the older sediment is more degraded than the younger.Higher arsenic and higher As(?)/AsT ratio(>50%)are always found in environment with Eh<0(-200-0 mV),low dissolved oxygen(1-5 mg/L),low concentrations of U(<5 ?g/L)and Mo(<10 ?g/L).The releasing of arsenic from sediment commonly occurs in reductive condition rich in organic matter.4.Enrichment of arsenic in the ecotone(1)Hydrodynamic processesBoth of vertical leaching and dissolution of halite in vadose zone and evaporation control the Cl/Br ratio in groundwater.Major processes controlling arsenic enrichment in groundwater can be identified by Cl concentration and ?18O composition:(1)lateral recharge and mixing,(2)evaporation,and(3)leaching and vertical mixing.Irrigation water is mainly from atmospheric precipitation,Yellow River,daily waste,and irrigation return flow.Most low-arsenic groundwater is recharged by lateral recharge of Yellow River.In condition that Cl concentration is completely liner correlated with ?18O composition,evaporation possibly is the only factor of Cl enrichment in groundwater.There is no obvious similar correlation found in this study.Therefore,evaporation may be not the major factor controlling arsenic enrichment in groundwater.For most high-arsenic groundwater,?18O composition doesn't change with the increasing of Cl concentration,indicating that leaching and vertical mixing are likely the important processes influencing arsenic enrichment in the ecotone.The ?18O/?2H ratio is paralleled to LMWL,also reflecting groundwater is mostly recharged by Yellow River,irrigation water,and irrigation return flow.(2)Hydrochemical processesCation exchange and wreathing of silicate minerals are the dominant processes controlling groundwater hydrochemistry.In high-arsenic groundwater,there is a weak correlation between PO43-concentration and total arsenic concentration(r = 0.53),possibly because of the competitive adsorption between PO43-and arsenic.It can be deduced that microbiological degradation may lead to reductive dissolution of arsenic adsorbed on the surface of Fe oxides or hydroxides and accelerate the formation of reducing environment due to consumption of oxygen from high contents of DOC(5.0-77.13 mg/L,with an exception of several samples)detected in groundwater.The positive correlation between ?34SSO4 composition and SO42-concentration demonstrates that major processes of sulfur geochemical recycles are reduction of SO42-to dissolved sulfide,even gas H2 S.High-arsenic groundwaters have lower ?34SSO4 composition,indicating that reduction of SO42-is accompanying with reduction and mobilization of arsenic.Some groundwaters have higher NO3-concentrations,implying NO3-has been introduced into reductive groundwater condition by irrigated agriculture,and probably oxidizes Fe(II)to ferric oxide hydrate which is able to adsorb arsenic and resulting in more oxidized As(V)which is easily adsorbed to the particle of aquifer.In high-nitrate groundwater,arsenic concentration is decreasing.5.Conceptual model(1)Dynamic model of groundwater in the ecotone: groundwater is generally recharged by atmospheric precipitation,runoff from Yin Mountains front,lateral recharge from Yin Mountains,and irrigation water,and discharges by anthropogenic extraction and evaporation.Groundwater from mining area and irrigation area is mixed in the ecotone.Groundwater level changes vertically due to anthropogenic extraction for the purpose of irrigtion and evaporation.Groundwater flows along the main drainage channel in direction from southwest to northeast,finally into Wuliangsu Lake.(2)Enrichment model of high-arsenic groundwater influenced by irrigated agriculture: after irrigation,groundwater level rises and thickness of vadose zone becomes shallower.Groundwater is recharged by irrigation water and atmospheric precipitation.Nitrate which may oxidize As(III)to As(V)and phosphate which may competes against arsenic for the adsorption sites possibly are introduced into groundwater by irrigation.Therefore,irrigation will speed up the vertical movement of groundwater and introduce nutriment nitrogen and phosphate reacting with arsenic.The purposes of the study are(1)to provide a new insight to explain the genesis of high-arsenic groundwater,particularly to further analyze the releasing and migration process influenced by irrigated agricultural and mining activities,and(2)to define the area where high-arsenic groundwater is distributed using the ecological concept ?ecotone? to indicate the distribution and genesis of high-arsenic groundwater,beneficial for managing water resource in high-arsenic groundwater area and developing and protecting ecological system.The innovation of the study is to indicate the genesis of high-arsenic groundwater in the ecotone between polymetallic sulfide mining area and irrigated agricultural area and to build a conceptual model,providing a new insight into source and enrichment mechanism of arsenic in groundwater.