Reactive Transport Of Arsenic Groundwater Quality Of Datong Basin Aquifer, China

Various factors define high arsenic groundwater at Datong, northern China similar to various studies done elsewhere around the world. The aridity, reliance solely on groundwater, slow groundwater flow (high residence time) and almost closed system maintains the levels of high arsenic (As) in groundwater through evaporative concentration, draw-down effect, water rock interaction and insignificant flushing out of arsenic waters from the system. Such combination of factors and the accompanying mechanisms leading to arsenic mobilization and consequences on human health demands high end investigations that could potentially assist in abating the problem of rampant arsenic. Using reactive transport studies in Datong seems to be an upcoming field as most studies based their studies on inverse modeling and flow models. Especially more important is the application of reactive transport models in high arsenic groundwater characterized by poor or very low dissolved oxygen (anoxic) aquifer requiring carefully crafted reaction networks (database).The specific objectives of the study were to (1) interpret and reveal possible factors promoting the observed spatial distribution of hydrochemistry in the field, (2) assess the impact of sediment geochemistry and groundwater recharge on arsenic mobilization, (3) model the impact of cation exchange, mineral dissolution/ precipitation and local conditions on the reactive transport and distribution of arsenic along a discredited space and time, and (4) extricate the significant mechanisms involved in arsenic mobilization.This research used field data to elucidate the causes of the variation across a flow path (1-Dimension) and the reasons for the observed distribution in 2-dimension space in a subset domain located in the center of the Datong basin. The study concentrated on the shallow aquifer typically around 20 meters from the ground surface with an effective groundwater depth of one meter. Field data was collected on-site, hydrochemical data and sediment geochemical characteristics analyzed in the laboratory to evaluate the general hydrochemistry and nonreactive distribution of arsenic to elucidate the potential mechanisms leading to arsenic mobilization. From these observations, two reactive transport models were developed using TOUGHREACT and PHT3D to predict and identify geochemical (or combination of) factors governing high concentration of arsenic in groundwater at Datong basin.This research had three chapters (3 to 5) dedicated to results and conclusions drawn from them:(1) The general hydrochemistry and sediment geochemistry of the study site were analyzed in the laboratory and results evaluated. The principal groundwater type in the study site was Na-HC03 with arsenic as high as 2627.1 μg L-1 in groundwater and 26.21 mg kg-1 in sediments. The evaluation showed that presence of high HCO3 could be instrumental in arsenic mobilization besides high pH and very reducing conditions. Furthermore, reduction of Fe(II) to Fe(III) and SO4/HS cycle may explain the occurrence of As(III) fueled by shuttled electrons from organic matter degradation in sediments. Conversely, the results explained that sequestration of arsenic by sulfide formation from SO4 reduction is a probability though insignificant. The species distribution using nonreactive transport (MODFLOW/ MT3DMS) show that in mildly reduced conditions As(V) is predominant relative to more reduced conditions, and co-occurring with Fe(III) and Al.(2) The 1-dimension flow path simulation using TOUGHREACT identified that the evolution of major chemical constituent, changes in pH, and cation exchange along a flow path had an apparent impact on arsenic mobilization. From the simulation, it was reasonable to conclude that reduction of Fe(III), As(V) and SO4 affects localized distribution of arsenic. Furthermore, the model deduced that arsenic desorption from Fe/Mn-hydroxide/oxide is a probable prime mechanism for arsenic mobilization in reducing conditions observed at Datong Basin. The observations further suggested that silicate weathering produces high Na content in this aquifer that promotes arsenic mobilization by affecting sediments stability.(3) The distribution of arsenic in the study site and its co-occurrence with other elements was modeled/predicted using PHT3D. It was shown from the model that despite groundwater allowed to spatially evolve, distribution of redox sensitive elements remained in conformance with evolution of mildly to strongly reducing conditions. The model supported the observations made in Chapter 3 that arsenic distribution is influenced by coexistence of Mn with Fe-hydroxides/oxide which are responsible carriers of arsenic in the study area. Furthermore, the modeled distribution highlights the significance of pH/pe zonation and cation exchange in the system as factors responsible for the display of varied As(III)/As(V) concentrations besides surface complexation reactions.The novelty in this study is the understanding of arsenic mobilization using reactive transport in sodic groundwater characterized by anoxic conditions using a customized database suitably developed for Datong basin. The study contributes to research by providing a customizable database for TOUGHREACT whose database is very scarce and especially difficult for anoxic or DO deficient aquifers. Furthermore, the two models and results will act as a basis in future reactive transport modeling as this is the first time this has been done. Lastly, although the plot is smaller in size compared to the greater Datong Basin, this work contributes significantly to the regional study by offering valid information that could prove useful in management of groundwater quality in the area as well as anticipate the regional spatial and temporal changes.