Variation and Its Driving Mechanisms of Groundwater Arsenic Induced by Intensive Groundwater Extraction

Author:Zhang Zhuo

Supervisor:guo hua ming


Degree Year:2019





High arsenic groundwater is widely distributed throughout the world and poses a serious threat to human health.In order to ensure the safety of groundwater extraction in high arsenic areas,the influence of groundwater extraction on arsenic enrichment in groundwater has gradually attracted attention.Therefore,this study mainly investigated variations in natural groundwater flow,chemical evolutions,and the driving mechanisms of groundwater arsenic variations.The main findings are shown as follows.The iron oxide-coprecipitated arsenic is the most abundant arsenic form.This fraction accounts for 64%of total arsenic in alluvial fans and 53%in flat plain.In the alluvial fans,Fe oxides have weak and strong crystallization,while they are dominated by amorphous and weakly crystallized phases in the flat plain.Adsorbed arsenic is the second abundant arsenic form.Adsorbed arsenic represents 23%of total arsenic in the alluvial fans,and 33%in the flat plain.It can be seen that arsenic in the sediments of the flat plain is more easily released into the groundwater through biogeochemical and desorption processes.The depression cone induced by groundwater extraction changes the natural flow direction of groundwater in the flat plain,and the Shahai Lake is shifted from groundwater discharge area into groundwater recharge area,being supported by stable isotope data.The water from the lake can reach the center of the cone in 20 to 28 years.In addition,groundwater extraction induces a water head difference between shallow and deep groundwater,which drives the recharge of shallow groundwater into the deep aquifer seasonally.This vertical mixing of shallow groundwater and deep groundwater is confirmed by the seasonal variation of inert components in groundwater.It takes only43 to 166 days for shallow groundwater to penetrate the aquitard.From non-irrigation to irrigation season,the mixing proportions of shallow groundwater to the affected deep aquifer increase from 21%to 53%in the alluvial fans,while they increase from 53%to86%in the flat plain.The change of groundwater recharge source in the plain area leads to the increase of TDS and the shift of hydrochemical type from Na-SO4-HCO3 to Na-Cl-SO4,and introduces labile organic matter with high biological index(BIX),which enhances microbial activities in groundwater.Near the alluvial fans,the seasonal recharge of shallow groundwater results in the fact that chemical type in the affected deep groundwater is changed from Ca-Na-HCO3 to Ca-Na-SO4-HCO3.In the flat plain,the seasonal recharge of shallow groundwater leads to the increases in groundwater TDS,DOC concentrations and biological index(BIX)in the the affected deep aquifer.The hydrochemical type is changed from Na-Cl-HCO3 to Na-Cl-SO4.The recharge of the lake water in the plain area introduces labile organic matter into groundwater and promotes the reductive dissolution of iron oxides,which leads to an increase in groundwater arsenic concentration.In the alluvial fans,the seasonal recharge of shallow groundwater with low arsenic and high dissolved oxygen into the deep aquifer results in a decrease in arsenic concentrations of deep groundwater.However,in the plain area,the seasonal recharge of shallow groundwater,with high levels of arsenic,biological index(BIX)and Na/Ca0.5 molar ratio,directly or indirectly leads to an increase in arsenic concentration in the deep groundwater by introducing high arsenic groundwater and promoting reductive dissolution of iron oxides and desorption of arsenic.