Quantifying Geochemical Processes Of Arsenic Mobility In Groundwater From The Hetao Basin Using A One- Dimensional Reactive Transport Model

Natural occurrence of high-arsenic?As?groundwater has been found throughout the world,threatening the livelihoods of hundreds of millions of people globally.There is a consensus that As is released from sedimentary As into groundwater by multiple water-rock interaction processes.Therefore,identifying and quantifying geochemical processes controlling As mobilization have become research hotspots in recent years.A typical study area was selected in the Hetao Basin,in which relative contributions of different geochemical processes to aqueous As distributions were studied and quantified by means of field investigation,laboratory experiments,and hydrogeochemical simulations.The output is of great significance in both theoretical and practical values.The main conclusions are shown as follows.?1?Along the approximate flow path,groundwater evolved from oxic-suboxic conditions in the alluvial fan to anoxic conditions in the flat plain,with increases in concentrations of NH₄⁺,Fe???,As,and HCO₃^-,and decreases in Eh and SO₄^2-/Cl^-values.Contents of Fe???oxides and the bound As in sediments decreased along the flow path,while contents of desorbable of As increased from 5.5?mol/kg in the transition area to7.4?mol/kg in the flat plain.?2?The characteristics of As???and As???adsorption both exhibited nonlinear isotherms,with adsorption decreasing with increases in pH from around 7 to 9?typical ambient groundwater pH of inland basins?.Generally,As???adsorption was greater than As???adsorption at pH<7.5,whereas the opposite was true at pH>7.5.Phosphate competed more strongly with As???adsorption than As???adsorption,while HCO₃competed more strongly with As???adsorption than As???adsorption.?3?Aqueous As???was oxidized to As???by interaction with the brown sediment,mainly due to its high content of extractable Mn???oxides.The brown sediment showed weaker affinity for As???and As???than the gray sediments,possibly due to the better crystallinity of the Fe???oxides in the brown sediment.In the two gray sediments containing Fe???oxides with similar crystallinities,As adsorption capacity was related to content of Fe???oxides.Geochemical models were developed to quantitatively simulate As???and As???adsorption onto aquifer sediments.Experimental data were approximated well by these models.?4?Based on the results of spatial evolution of geochemical components in aquifers and the characteristics of As adsorption onto sediments,a 1 D multi components and kinetic-rate geochemical process-controlled reactive transport model was constructed.The unique characteristics of As-prone aquifers in inland basin with high pH and high SO₄^2-concentration were fully considered in the model,in which the three primary mechanisms concerning As mobilization?i.e.,dissimilatory reduction of Fe???oxides,chemical reduction of Fe???oxides,and As desorption?were coupled.Generally,an overall good agreement was achieved between observations and the values predicted by the 1 D reactive transport model.?5?Reactive transport model calculations illustrated that As was primarily released from Fe???oxide reduction?>70%?,and secondarily from As desorption?<30%?.The reductive dissolution of Fe???oxides was primarily induced by organic matter degradation?>75%?and partly by H₂S oxidation?<25%?.More As was sequestered by mackinawite precipitation and adsorption than that released via abiotic reduction of Fe???oxides by H₂S.