Source,Transformation Of Nitrogens And Significance For Arsenic Enrichment In High Arsenic Groudwater Based On Nitrogen And Oxygen Isotopes

High arsenic groundwater is widely distributed around the world,which has a serious impact on people's health.Microbial-mediated reduction of iron-manganese oxides is the main process of groundwater arsenic enrichment,during which organic matter acts as the electron donor.As one of organic matter decomposition products,concentration of ammonium can indicate the degree of organic matter decomposition.Therefore,high arsenic groundwater often has high concentrations of ammonium and low concentrations of nitrate.However,it is still unclear about source,migration and transformation of ammonium and nitrate,and their influence on mobilization and enrichment of groundwater arsenic.This study,based on characteristics of nitrogen and oxygen isotopes in nitrate and nitrogen isotope in ammonium in the Hetao basin,investigated the source and transformation of nitrogen and the significance on arsenic accumulation.The main conclusions of the study are shown as follows.Groundwater in the Hetao Basin originates from atmospheric precipitation and is strongly affected by evaporation.The TDS gradually increased from the piedmont area to the plain,and the hydrochemical type changed from Ca-SO₄-HCO₃ to Na-Cl.There is a obvious redox zoning in groundwater,with ORP decreasing gradually from the piedmont to the plain.Concentrations of As,Fe,Mn,NH₄⁺,Fe?II?gradually increased,while ORP,DO,NO₃^-,SO₄^2-/Cl^-decreased from the piedmont area to the plain.Sandy sediments had higher pH and lower TOC,EC,NH₄-N,while clay and silty clay sediments often had lower pH value and higher TOC,EC,TN,NH₄-N.NH₄-N in clay sediments may be originated from mineralization of organic matter.NH₄⁺of clay sediments were obviously higher than those of sandy sediments.Distribution of NH₄⁺in groundwater was similar to that of sediments,indicating that NH₄⁺in groundwater originated from desorption from sediments.?¹⁵N-NO₃??¹⁸O-NO₃ were lower in the priedmont area,with?¹⁸O-NO₃ falling in the range of the theoretical nitrification.Thus,nitrification was the main process occurring in the priedmont areawas.In the transition area and the plain area,there was a positive correlation between?¹⁵N-NO₃ and?¹⁵N-NO₃,both of which were negatively correlatied with ln?NO₃^-?.It indicated that denitrification process occurred in this area with organic matter as the electron donor.¹⁵N,¹⁸O enrichment factors indicated that other nitrate reduction processes occurred in the plain region,including the Dissimilatory Nitrate Reduction to Ammonium?DNRA?.?¹⁵N-NH₄ and the relationship between groundwater NH₄⁺and other ions showed that groundwater NH₄⁺was mainly derived from the mineralization of organic matter.In the sub-oxic zone of the transition area,anaerobic ammonium oxidation resulted in the increase in?¹⁵N.In the shallow aquifer of the plain area,?¹⁵N-NH₄ in groundwater was obviously lower than the values of organic matter mineralization,confirming that the DNRA process occurred.When concentrations of NH₄⁺were high,the degree of decomposition of organic matter was high,leading to strong reducing conditions.In those conditions,the reductive dissolution of iron oxides occurred,and therefore mobilized sediment arsenic into groundwater.In addition,As?V?was reduced to As?III?,which is conducive to arsenic mobility in aquifers.