Effects of short-term flooding on arsenic transport in groundwater system: A case study of the Datong Basin

Abstract

Hydrogeological and geochemical approaches were combined to investigate the impact of a short-term artificial flooding event on arsenic and associated hydrogeochemisty at a field monitoring site of high arsenic aquifers in Datong Basin, northern China. The groundwater levels fluctuated in response to the flooding (the surface water level and groundwater level fluctuated with time in a range of over 80cm and 30cm, respectively), indicating groundwater–surface water interaction. Redox indices, major ions, trace element concentrations, O, H and Sr isotope compositions for surface water and groundwater were determined before and after the flooding. δ2H and δ18O values for all water samples plot close to the global meteoric water line (GMWL); a subset of the samples (shallow wells) averagely increased from −11.78 to −11.26 for δ18O and from −86.59 to −83.49 for δ2H, commensurate with mixing with ~10% surface water. The relationship between compositions of strontium isotope 87Sr/86Sr ratios and Mg concentration also clearly indicates the effect of mixing between groundwater and surface water. Before and after the flooding, there were obvious variations in water chemistry, including arsenic concentrations. Increases and positive correlations between Cl− and Na+, Mg2+, Ca2+ and SO42− following flooding revealed surface water infiltration into groundwater, plus additional input from dissolution of evaporate minerals and/or evaporated pore-water. Organic matter and oxygen are carried into groundwater together with surface water; oxidation of organic matter leads to exhaustion of oxygen and consumption of terminal electron acceptors (e.g. ferric iron, sulfate, nitrate) inducing a more reducing groundwater environment (indicated by decreased ORP values). Consequently, reductive dissolution of iron oxides/hydroxides resulted in an observed elevation of HS, NH4-N, Fe(II), with the concentrations increasing from 5μg/L to 10μg/L, from 0.19mg/L to 0.68mg/L and from 0.06mg/L to 1.86mg/L, respectively. This may explain that the As concentrations were enhanced after surface water flooding (from mean value of 22.6μg/L to 31μg/L). However, a positive correlation was observed between enhance values of As and Cl in the groundwater after flooding is interpreted as reflecting re-dissolution of evaporate crusts and/or saline pore water during infiltration, and this may alone explain the increase in As observed in the groundwater. Surface water As concentration increased to a minor degree following flooding also (from 14μg/L to 15.1μg/L), possibly reflecting return flow of the high As groundwater during a temporary switch to gaining conditions.