Monitoring And Coupled Modeling Of The Hydraulic And Hydrogeochemical Processes Of Arsenic Transport In Hyporheic Zone

Natural As groundwater contamination is a serious problem in many areas around the world, especially in Asian countries. The fate and transport of arsenic in groundwater system is influenced by hydrogeologic conditions and geochemical and biogeochemical processes. Due to the extremely complex geochemical reactions in aquifers system and many uncertain factors, numerical modeling has been regarded as the most cost-effective method.Datong Basin is located in an arid and semi-arid region of Shanxi Province, northern China. Groundwater has been the major source of potable water for drinking and irrigation purpose. High arsenic concentration has been detected in Datong groundwater, with the maximum value being up to1820μg/L. Long-term intake of high-As groundwater has caused endemic arsenic poisoning in Datong. Science1990s, a lot of work have been done to understand the genesis of Datong high arsenic groundwater. The results indicated that the arsenic in the Quaternary aquifer systems mainly originated from the Archean metamorphic rocks and Mesozoic coal-bearing strata around the Basin. The major processes of arsenic mobilization are most likely linked to As desorption from Fe oxides/oxyhydroxides and the reductive dissolution of the Fe-rich phase in the aquifer sediments under reducing and alkaline conditions.In fact, the hydrogeolocial conditions play an important role in As release. In recent years, many studies have demonstrated the effect of hydrodynamic conditions on dissolved As distribution in the aquifer. However, the previous studies on high-arsenic groundwater in the Datong Basin have been mostly focused on the geochemical and biogeochemical processes controlling As transport in the groundwater system. No systematical investigations were conducted to discuss the linkage between As concentration and groundwater flow paths in this area. Since studies of groundwater flow are helpful to understanding the enrichment of As in the groundwater affected by natural or anthropogenic changes in the hydrological cycle, clarifying the relationship between hydrodynamic conditions and arsenic behavior in groundwater is becoming essential.For this study, we selected a typical high arsenic groundwater site for detailed monitoring. At first, based on the one-year continuous monitoring work with one month interval for water level and water chemistry, a three-dimensional transient groundwater flow model with realistic assumptions of hydraulic parameters and boundary conditions of the geological structure was conducted with MODFLOW to reveal the relationships between groundwater dynamics and As concentrations in shallow contaminated groundwater systems. Then a short-term artificial flooding experiment was conducted to further understand the effects of groundwater and surface water interactions on arsenic transport in the adjacent aquifer. Finally, a one-dimensional reactive transport model occupied with biogeochemical processes of arsenic was conducted with PHREEQC to recognize the major processes controlling arsenic mobilization in adjacent groundwater system during the period of groundwater and surface water interaction. The main contents of this paper and findings are summarized as below.1. The relationships between groundwater dynamics and As concentrations in shallow contaminated groundwater systems were understood.A one-year continuous monitoring work was conducted under natural condition with one month interval for groundwater level and groundwater chemistry. Based on the three-dimensional transient groundwater flow model, the following main findings were obtained:(1) Groundwater arsenic concentration significantly increases from irrigation season to non-irrigation season, with the fluctuation range of2.8-46.3μg L-1and3.5-181.5μg L-1, respectively. A slight decrease of oxidation reduction potential (ORP) value presents from irrigation season to non-irrigation season, fluctuating between-6.6mV and-141.1mV and between-61.1mV and-134.9mV, respectively. During irrigation season, groundwater HS-concentration has a narrow fluctuation range of1-5μg L-1, but with a much wider fluctuation range of2-12μg L-1during non-irrigation season.(2) Groundwater numerical simulation indicates that irrigation can increase groundwater level and reduce horizontal groundwater velocity and thereby accelerate vertical and horizontal groundwater exchange among sand, silt and clay formations.(3) Results of net groundwater flux estimation suggest that vertical infiltration is likely the primary control of As transport in the vadose zone, while horizontal water exchange is dominant in controlling As migration within the sand aquifers. Recharge water, including irrigation return water and flushed saltwater, travels downward from the ground surface to the aquifer and then nearly horizontally across the sand aquifer.(4) The maximum value of As enriched in the hyporheic zone is roughly estimated to be1706.2mg day-1for a horizontal water exchange of8.98m3day-1close to the river and an As concentration of190μg L-1.(5) A possible mechanisms of As transport in the aquifer can be discussed within the framework of groundwater dynamics. First, in the process of downward movement of irrigation return water and salt flushing water, oxygen and organic matter are carried into the aquifers, which can not only oxidize the dissolved As in the vadose zone but also desorbed As into the groundwater. Second, the horizontal groundwater flux plays a dominant role in the saturated zone since the vertical groundwater flux is too small to be neglected. Horizontal water exchange may also cause As dissolution and release into the groundwater or promote the transport of dissolved As toward a more reductive environment. Consequently, the As concentration increases along the flow paths over the depth from17m to25m below the ground surface.2. The effect of short-term flooding on arsenic transport in groundwater system was investigated.Hydrogeological and geochemical approaches were combined to investigate the impact of a short-term artificial flooding event on water chemistry at this field monitoring site. By monitoring the groundwater physical-chemical parameters including redox potential, major ions and trace elements concentrations, and isotope compositions of18O/16O,2H/1H and87Sr/86Sr, we have the following key findings:(1) The groundwater levels fluctuate to respond to the fluctuation of surface water, as a result of groundwater-surface water interaction.(2) The δ18O and δ2H shift away from local meteoric water line may be related to the mixing with surface water having higher values. The δ18O and δ2H values of shallow groundwater samples increase after the flooding, indicating the effect of mixing with infiltrating surface water, since there is no precipitation during the flooding period.(3) The87Sr/86Sr ratios in groundwater samples reflects the contribution of silicate rock weathering, and the shift of87Sr/86Sr ratios of post-flood groundwater samples away from those of pre-flood ones but towards the surface water should be related to their mixing in the aquifers.(4) The variations of water temperature and TDS provide us some clues about surface water and groundwater interaction:when surface water with lower temperature and higher TDS percolates into the aquifers, there should be a decline in groundwater temperature and a rise in TDS. Correlations between Cl-and Na+, Mg2+, Ca2+and SO42-concentrations reveal surface water infiltration into groundwater trigged by the flooding. Cl-concentration increases in both surface water and groundwater after flooding, although that it is much higher in groundwater than in surface water, indicating there must be some other sources of groundwater Cl-, such as the dissolution of halite in the unsaturated zone.(5) The close positive relationship between As change and Cl change (γ2=0.59,α=0.01for all samples; γ2=0.99, a=0.01for only shallow samples) suggests a possible process of arsenic transport in groundwater:the vertical downward shift of the dissolution by weathering of detrital As-bearing minerals in shallow aquifer.(6) When surface water infiltrates into the aquifer, oxygen and organic matters can be brought into the groundwater system, which will accelerate the oxidation of organic matters and induce more reducing environment, as reflected by the lowering of ORP values and rasing of HS-and Fe(II) concentrations of our groundwater samples before and after flooding. A more reducing groundwater environment will induce the reduction of sulfate minerals and Fe-oxyhydroxides to produce sulfide. NH4-N contents, much like HS-, increase from0.19to0.68mg L-1before and after flooding indicating ammonification under reducing conditions. Therefore, the reductive dissolution of Fe-oxyhydroxides and bacteria-mediated reactions may be other important processes controlling arsenic mobilization in groundwater.3. The major processes controlling arsenic mobilization in groundwater system during the period of groundwater and surface interaction were discerned.Inverse model and forward model were conducted to calculate the mole transfers of phase and figure out the major processes controlling arsenic transport in groundwater system under the impact of surface-groundwater interaction, respectively. The main results and findings are concluded as follows:(1) Inverse modeling results indicate that the increase of Ca2+、Na+、Cl-、SO42-、Sr2+concentrations is mainly related to the dissolution of dolomite and albite and the precipitation of calcite, with the maximum quantity of dolomite dissolution and calcite precipitation being up to9.651mmol/kg H2O and15.73mmol/kg H2O, respectively. Ion exchange is another important process.(2) Fe(III) surface complexation model results demonstrate that a primary process controlling arsenic mobilization in aquifers during the period of groundwater and surface water interaction is the adsorption onto Fe(III) oxides/hydroxide. The quantity of sorbed arsenic onto HFO is1.06mmol L-1and1.56mmol L-1, respectively.(3) Both redox model and adsorption-redox model illustrate the importance roles of redox reactions in arsenic mobilization in groundwater system associating with surface-groundwater mixing.(4) For the monitoring well (Well1-2S) closest to the riverbed, the adsorption of Fe(III) oxides/hydroxide and related redox reactions, e.g. the reduce dissolution of As(V), Fe(III), SO4and NO3, together control the arsenic mobilization; while for the monitoring well (Well2-2S) further away from the riverbed, the effect of arsenic adsorption onto Fe(III) oxides/hydroxide seems to have more significant impact on arsenic mobilization than that of redox reactions.The major innovative advances achieved in this dissertation are as follows:1) new approaches were provided for high arsenic groundwater studies by carrying out monitoring of geogenic arsenic transport in hyporheic zone and modeling the relevant coupled reactive solute transport;2) by short-term artificial flooding experiments, the impact of surface-groundwater interaction on arsenic transport was investigated. A new model of the genesis of high arsenic groundwater was proposed:abundant oxygen and organic matter were introduced into the hyporheic aquifers by the infiltrating surface water to accelerate oxidation of instable oragnics and nutrients and facilitate the occurrence of strongly reducing conditions in the aquifers. Reductive dissolution of ferric oxyhydroxides was consequently intensified to release sorbed arsenic into groundwater.