Isotope Geochemical Studies On Arsenic Mobilization In Groundwater In Datong Basin

Elevated levels of arsenic (As) in groundwater has caused worldwide attention due to the risk of As toxicity from drinking water sources. In Datong Basin for this study, naturally occurring arsenic (As) in Holocene aquifers has undermined the success of supplying the residents with safe drinking water. Diverse mechanisms governing arsenic mobilization in natural environments have been scrutinized by worldwide scholars to contribute profound perspectives and detailed reacting models of arsenic geochemical behavior. As concentrations in groundwater is predominantly managed by the redox conditions, presence of metal oxides/hydroxides and related biogeochemical processes, which are governed by the geological attributes of the aquifers. More recently, anthropogenic influences, such as agricultural irrigation, has caught increasingly greater attention to the As enrichment in groundwater systems.The research work contained within this dissertation serves to shed light onto the occurrence of high levels of As and the hydrogeological. geochemical and biogeochemical processes which affect As mobility within groundwater system in Datong Basin. An integrated approach by combining multiple isotopes with hydrogeochemistry of groundwater was tested to improve the understanding of governing redox reactions and adsorption/desorption as well as biogeochemical process. The explanatory variables included oxygen isotope (δ18O-H2O) and hdrogen isotopes (δD and3H), nitrogen isotope of nitrate (δ15N-NO3), sulfur isotopes of sulfate and sulfide (δ34S-SO4and δ34S-Sulfide), oxygen isotope of sulfate (δ18O-SO4) and molybdenum isotope (δ98Mo). Integrated interpretation of multiple isotopes proves to be useful to provide effective evidences about As release and sequestration resulting from shifts in redox conditions and subsequent redox reactions.Detailed investigation on the hydrogeochemistry of groundwater in Datong basin indicates that the groundwater can be divided into four main groundwater types:Na-HCO3water, Ca+Mg-HCO3water. Na-Cl water and Na-Cl+SO4water. The study reveals that the local and regional scale variations in groundwater composition, and the levels of As concentrations are generally low in the groundwater abstracted from the basin’s marginal areas where Ca+Mg-HCC3water prevails. High As groundwater is commonly Na-HCO3water. Another remarkable characteristic of the groundwater geochemistry is that the Mg2-concentration is commonly high, especially that in the central basin. The Mg enrichment was considered as a result of coupled processes of minerals weathering (such as dolomite and magnesites). anthropogenic (such as fertilizer application) and microbial activities (such as BSR). In the organic rich sediment in Datong Basin, major chemicals including Mg2+,Ca2+,SO42-and HCO3-contents relied on vigorous bacterial metabolisms. As enrichment was revealed fatally related to the redox conditions in groundwater other than the salinization process since no clear correlation was observed between As and CI’. To enlarge this sequitur, evapotranspiration may not be a significant controlling factor of As mobilization. Redox proxies determinations from the arsenic enriched groundwaters indicated that high arsenic groundwater mainly occurs in the reducing environment. Detectable S2-and NH4+in groundwater were further considered as markers for the contribution of bacteria such as sulfate reducing bacteria and denitrifying bacteria. Both As mobilization and immobilization caused by BSR were present in the aquifer in Datong Basin, while the priority of these processes was circumstantially controlled by the Fe2+and S2-concentrations which were related to the Fe-sulfides solubilities.Water isotope (δ18O-H2O and δD) data indicate a local precipitation origin of groundwater. Shifting of δ18O and δD apart from the LMWL was proposed as a result of the mixing of direct vertical recharge and evaporation of shallow groundwater which can lead to a parallel shift away from the LMWL. The relatively homogenized δ18O values in deep groundwater reflect the effect of mixing of groundwater along the flow path. The primary factors affecting groundwater isotopic compositions are:(1) precipitation;(2) evapotranspiration;(3) mixing of groundwaters with various isotopes signatures with irrigation return water. Non-linear correlation between tritium concentration (3H) and depth might result from pumping of aged deep groundwaters from deep aquifers. Generally, arsenic concentrations in groundwaters with different ages ranged widely. Groundwater residence time showed neglectful control on arsenic enrichment. Relatively higher As contents occurred in the young groundwaters. As interpreted by the3H concentrations of groundwater from different depths, mixing of irrigation groundwaters with various ages and As concentrations played important role in arsenic distribution in the aquifer.Microorganisms’ metabolisms involve many important geochemical processes including reduction/oxidation, precipitation/dissolution, and adsorption/desorption which significantly modify the geochemical fate of As. To gain insights into nitrogen cycling influencing arsenic mobility, nitrogen isotope of nitrate (δ15N-NO3) was detected. Nitrogen isotope investigation suggested that the highest δ15N-NO3values in groundwater (up to+28.0%o) might be contributed from point input of animal manure since cow and sheep husbandry are important industries in Datong basin. Negatively correlated nitrate concentration and δ15N-NO3ratio suggested that N isotope of nitrate has experienced significant isotopic fractionation from denitrification. Pyrite oxidation driving denitrification might be present in the aquifer. Co-precipitated As onto the Fe-oxyhydroxides might desorbed into groundwater due to Fe transformations during denitrification process. The weak positive correlation between As and δ15N-NO3ratio within the groundwaters carrying moderate δ15N-NO3values was consistent with this assumption.In an effort to better understand sulfur biogeochemical cycling affecting As mobilization, isotope signatures of dissolved sulfate and sulfide were analyzed. High δ34S-SO4, along with elevated δ18O-SO4of dissolved sulfate in groundwater in Datong Basin was considered to result from biological sulfur cycling of bacteria sulfate reduction (BSR) and re-oxidation of sulfide and the intermediates produced by BSR. Relatively high δ34S-Sulfide values excluded the sulfate contribution from sulfide oxidation. On the other hand, CAS evaporites were proposed to be a potential sulfate source. High δ18O-SO4values, which deviated from both CAS evaporites and fertilizers, may derive from nitrogen cycling involving nitrification, denitrification and sulfide oxidation, as atmospheric oxygen with comparatively high δ18O was incorporated into SO42-Mixing of irrigation water with different δ34S-SO4and δ18O-SO4values may also play important roles in governing isotopic compositions of sulfate in groundwater. Larger sulfur isotope fractionations△34SSulfide-SO4are associated with high As concentrations. Mechanisms of As release from the aquifer sediments into groundwater could be explained as:oxidation of sulfide produced by BSR could fuel denitrification; with the consumption of NO3-, reductive dissolution of Fe-oxyhydroxides (e.g., goethite) was facilitated; consequently, arsenic was desorbed into groundwater.An effort was made for the first time to validate the correlation between As concentration and Mo isotope of groundwater. Isotopic composition of dissolved Mo in groundwater (δ98/95Mo) was first reported in this study, ranging from-0.12‰to+2.17‰. with an average ratio of+1.1‰, displaying a relatively heavier ratios than those in freshwaters. Sanggan River was detected with a comparable δ98Mo ratio of+0.72‰to the documented mean riverine898Mo value of+0.7‰. Compared to the euxinic environment, sulfide concentrations in groundwater were significantly lower. However, S2-measured data demonstrated that a slowed formation of Mo-Fe-S might take place in Datong Basin, which might cause notable Mo fractionation. The partially positively correlated S2-and δ98Mo in some groundwaters might result from the preferential precipitation of light Mo by the formation of thiomolybdates.δ98Mo in groundwater was more related to Fe than Mn. The elevated δ98Mo in groundwater accompanied with progressive Mo decrease was considered as a consequence of reductive ferrihydrite dissolution and re-adsorption of Mo. This process is depicted as re-adsorption of isotopically light Mo from the groundwater. Arsenic concentration was found weakly positively correlated with δ98Mo ratio in Datong Basin, indicating that large Mo fractionation resulted from reductive dissolution of Fe oxyhydroxides could be used as a signal of As release into groundwater. In addition, a new mechanism controlling arsenic mobility was proposed in this study, based on the results of Mo isotopic data interpretation:Mo played as a competitor with As in co-precipitation with Fe-sulfides. The similarity of formation conditions for As-Fe-S and Mo-Fe-S could be delineated as the dynamo of this mechanism.