Arsenic Mobilization And Transport In Shallow Aquifer Systems Of Jianghan Plain,Central China

Arsenic (As) is a redox-sensitive toxin that severely impacts environmental quality and human health and it becomes most hazardous when conditions are such that it partitions to the aqueous phase. Himalayan-derived As is contaminating groundwater in South and Southeast Asia. It was estimated that there are over100million individuals exposed to unsafe As levels by drinking untreated groundwater in India, China, Myanmar, Pakistan, Vietnam, Nepal, and Cambodia. Systematic and comprehensive research on the fate and transport, as well as the health effects, of As have been taken mostly in areas of the Ganges-Brahmaputra-Meghna River system, Red River system, and Mekong River system. With the first report of groundwater arsenic poisoning in Jianghan plain,2005, Yangtze River system was added to the list.While arsenic is native to the sediments, its mechanism of mobilization into the aqueous phase and its subsequent residence time in groundwater are the focus for science community and sustainable water management. In addition, the human-induced effects on arsenic concentrations are currently a topic of intense debate. The research presented in this thesis elucidates the coupled hydrological and biogeochemical processes controlling arsenic concentrations within aquifers of Jianghan Plain. In addition to guide water management in Jianghan Plain, this work also provides contrast to research in South and Southeast Asia where arsenic contamination of groundwater represents the largest mass poisoning in history.Our research work was carried out in a typical high-arsenic groundwater system in a field monitoring site (about12km2) along Dongjing River which is a tributary of Hanjiang River. In this study area, we constructed a field monitoring nest including39monitoring wells at three depths (10m,25m and50m) in the groundwater and some other sites in adjacent surface water bodies like rivers, channels and ponds. With this monitoring nest, we conducted hydrogeological monitoring of the groundwater and adjacent surface water for a whole hydrological year. And from our field monitoring results, arsenic release is still going on at current time. And most importantly, groundwater As exhibits large spatial and temporal variations. Spatially high arsenic concentrations were located in25m wells at sites adjacent to surface water bodies of ponds and irrigation channels. Temporally, there is obvious arsenic increase during rainy season and decrease during dry season in groundwater.Various theories have been put forth regarding the modes of arsenic release to the aqueous phase, ranging from the oxidative or reductive degradation of arsenic-bearing solids to competitive ligand displacement by phosphate. Reductive dissolution of Fe(III)(hydr)oxides and concomitant arsenic release has become the most widely accepted explanation of high arsenic groundwater concentrations. In order to evaluate the potential mechanisms of arsenic desorption to groundwater and at where sediment is releasing arsenic, spectroscopic and laboratory batch incubation experiments were conducted with aquifer sediments from the study area. Our results indicated that both the upper aquitard clay sediments and the lower aquifer sand sediments have great potential to release arsenic if sufficient fresh organic carbon is supplied.Another significant feature of the high-arsenic groundwater in the study area is the high dissolved sulfide. In our study area, the recharging surface water often contains high sulfate leading to great sulfate reduction in the groundwater system. To elucidate the influence of high dissolved sulfide on arsenic release and retention, we examined arsenic release and adsorption under high sulfide condition in flow system with natural sediments. The results indicate that, although dissolved sulfide is a strong reductant it can only promote arsenic release for a transient time. Over longer time, it will promote arsenic retention to more stable minerals via promoting iron minerals secondary transformation.With the evidence that the above25m sediment has the potential to release arsenic currently, an important question is whether there is sufficient organic carbon to be recharged to fuel microbially-mediated arsenic releasing. Organic carbon in the aquifer sediments is almost depleted according to our analysis. The major source of organic carbon should be the recharging surface water. The possible way for the infiltration of organic carbon into the aquifers is via the leaky aquitards. To advance understanding of hydrological influences on As behavior within groundwater of Jianghan Plain, the flow system of an As-rich aquifer in Jianghan Plain along Yangtze River was examined. From our results, groundwater flow in this system is vertically through the upper clay layers and horizontally through the sandy aquifer material below. Arsenic released via reductive dissolution within the upper clay layers will transport downward into the aquifer and then horizontally toward a point of discharge. However, the particularity of this system is that the vertical flow is very fast and the horizontal flow is pretty slow, leading to the significant spatial and temporal variations of arsenic concentrations in groundwater.The current distribution of arsenic in aquifers of Jianghan Plain is the result of coupled biogeochemical and hydrologic process, which vary depending on the sedimentology of a specific area and local human perturbations. To reveal the impact of conductive the biogeochemical and hydrogeologic conditions on arsenic release in this study area, we used one and two dimensional reactive transport modeling calibrated with biogeochemical and hydrogeologic field data to simulate the subsurface distribution of arsenic. The results of our simulations support the concept model of arsenic release from upper clay layers and below sand aquifer, where horizontal transport of arsenic is extremely weak, leading to the erratic spatial distribution of arsenic. Constructed ponds and channels provide sufficient source of fresh organic matter and pollutants via leaky aquitards to facilitate arsenic release by sulfidization in the aquifers.The research presented in this thesis seeks to discern the dominant factors controlling arsenic concentrations in Jianghan Plain. The distinguish features of this research include:(1) Synchrotron based XANES, batch incubation and sequential extraction are integrated to analyze the sediment;(2) Long-term continued hydrodynamic-hydrochemical monitoring was conducted in the field;(3) Contraposing the sulfide influence of arsenic release and retention, specific column experiment was carried out;(4) Reactive transport modeling coupling solid phase results and field hydrodynamic-hydrochemical monitoring results was conducted to illustrate the special and temporal distribution of groundwater arsenic;(5) The research was carried out in the typical arsenic contaminated aquifer system, probing a range of spatial scales, from the molecular level to the field scale.