Arsenic Enrichment And Mobilization In The Holocene Alluvial Aquifers Of Southwestern Bangladesh

High arsenic?As?in groundwater has become a major global concern due to its widespread occurrence.Groundwater with an excessive level of As concentration is a great threat to human health.As contamination of groundwater in Bangladesh is a serious natural calamity and public health hazard.It is considered that high As in groundwater in Bangladesh is the largest mass poisoning of population in history and the arsenicosis is the largest chemical threat to the public health.High As concentrations in groundwater?>10?g/L?generally occur in Holocene alluvial aquifers.The aquifers of Southwestern Bangladesh are developed from Holocene floodplain and Pleistocene deposits.The major aims of this study was to identify the causes of As enrichment in the Holocene alluvial aquifers,evaluate the groundwater quality and hydrogeochemical characteristics and determine the controlling factors of its mobilization processes in the Holocene alluvial aquifers of Southwestern Bangladesh.In this study,hydrochemical characteristics and factors influencing As distribution and release were investigated in Southwestern Bangladesh.For that purpose,a total of 78groundwater samples?50 in 2015 and 28 in 2017?were collected from the tube-wells those the depth varied from 21 m to 113 m.These collected groundwater samples were analyzed to measure the major hydrogeochemical components and trace elements in groundwater.In addition,the isotopic analysis also was calibrated for the groundwater samples?30 samples collected in 2015?of Southwestern Bangladesh.However,a hydro-chemical approach was also employed to examine the occurrence of geogenic As as well as the controlling factors of As release in the aquifers.Multivariate statistical analyses such as Hierarchical Cluster Analysis?HCA?and Factor Analysis?FA?were applied on the hydrochemical data to identify the major hydrogeochemical processes in sedimentary aquifers.Moreover,the spatial and temporal analyses were done as well to observe the variation of As and other hydrogeochemical contents in the groundwater of this study area.The analytical results showed that As concentration in groundwater varied from 6.05 to590.7?g/L?average 58.31?g/L?,and about 82%samples contained As exceeding 10?g/L in groundwater samples collected in 2015.Elevated As concentration mostly occurred in shallow aquifers,which was associated with reducing conditions.In contrast,78.57%?22 out of 28 samples?and 60.71%?17 out of 28 samples?groundwater samples?gathered in 2017?had As more than the WHO recommended value?10?g/L?and Bangladesh standard?50?g/L?.The significant correlation of As and NO₃^-indicates that NO₃^-might be attributed from the use of phosphate fertilizers and a responsible factor to enhance As in aquifers.Nevertheless,the concentrations of Si,B,Mn,Sr,Se,Ba,Fe,Cd,Pb,F,U,Ni,Li,and Cr were within the safe limit.The study also reported that the occurrence of high As and presence of trace metals?TMs?in groundwater may be resulted from local anthropogenic activities,like irrigation,intensive land use and application of agrochemicals.High As groundwater was distinguished with weak alkaline pH value,moderate HCO₃^-,medium-high EC and total dissolve solid?TDS?,and low NO₃^-,K⁺,Cl^-and SO₄^2-concentrations.The groundwater was dominated by Ca·Mg-HCO₃ type water that was different from other high As aquifers distributed in the arid area.The positive correlation between TDS and EC values of groundwater samples demonstrated the salinity in groundwater.The study also found the co-existence of elevated As and salinity in the aquifer system.This salinity maybe resulted from the hydrolysis of silicate and alumino-silicate minerals.Heavy isotopic and low concentrated Cl^-groundwater indicated that groundwater is recharged from the infiltration of rainwater and probably by river water.In addition,the heavier?¹⁸O in some deep groundwater samples demonstrated the leakage of top shallow aquifers through the aquitards of the Southwestern Bangladesh.The analytical isotopic result showed that the deep groundwater samples were isotopically heavier than those of shallow groundwater samples suggesting that closely hydraulic connection and frequent conversion relationship exist between river shallow groundwater and deep groundwater.The relationship of Cl^-and?¹⁸O confirmed that evaporation is one of the main ways for discharging of groundwater.Moderate alkalinity and the reductive condition,resulted from biochemical reactions,were the key controlling factors of As mobilization in aquifers.Elevated pH value along with de-coupling of As and HCO₃^-plays a vital role in mobilizing As to aquifer system.Moreover,the positive relationship of As and Si denoted that the As is transported under the biogeochemical environment.The reductive suspension of MnOOH increases the dissolved As loads and redox responsive elements such as SO₄^2-and pyrite oxidation those act as the main mechanisms for As release in groundwater.The reductive suspension of Mn?IV?-oxyhydroxides also accelerated the As mobilization process in the groundwater of Southwestern Bangladesh.As is mobilized by anaerobic leakage from the brown-clay and gray-sand into the groundwater.Infiltration from irrigation return and surface wash water were the potential factors to remobilize As.The weak loading of Fe suggested that the release of Fe and As was decoupled in sedimentary aquifers in this area.Over exploitation of groundwater and the competitive ion exchange process also responsible to release As in aquifers.Environmental isotopes(?¹⁸O and?²H)confirmed that evaporation did not play a vital role in the process of As enrichment in the groundwater.Moreover,a study was performed on the occurrence of high As groundwater of two As affected regions?Datong Basin,China and Southwestern Bangladesh?.To conduct this comparative hydrogeochemical study some of 83 groundwater samples were collected from Datong Basin,China to analyze the major hydrogeochemical components and trace elements in groundwater.High As groundwater was observed in the low-lying central parts of Datong Basin,which are composed of the Holocene alluvial and lacustrine aquifers.The elevated As concentrations ranged from 0.31 to 452?g/L,and distributed in depths between 20-45 m.As enriched groundwater was mainly Na-HCO₃ type water,and characterized by higher pH value,high Na⁺,low Ca²⁺,SO₄^2-and NO₃^-along with moderate TDS.The alkaline and reducing subsurface environment facilitate the leaching of As in sedimentary aquifers.The release and distribution of As in aquifers resulted from the reduction of As-carrying crystalline iron?Fe?oxide/hydroxides and oxidation of organic matter.