Delineation of groundwater flow paths using hydrochemical and strontium isotope composition: A case study in high arsenic aquifer systems of the Datong basin, northern China

Abstract

► We delineate the flow paths in high arsenic groundwater aquifer system. ► We model the evolution of chemical and Sr isotopic composition along the flow paths. ► Groundwater flow regime has effect on arsenic enrichment in groundwater. Strontium isotopic compositions and major ion chemistry were used to delineate flow paths in an arsenic affected groundwater flow system in the Datong basin, China. Total of 28 samples including one spring and one river water were collected for major ions and strontium isotopic compositions and concentration analysis. The ion ratios indicate that dissolution of silicates and carbonates is the dominant geochemical process controlling the hydrochemistry of groundwater from two recharge areas at the basin margins, while the water chemistry in the center of the basin is affected by the dissolution of evaporites (including gypsum and halite). Groundwater from eastern and western margin areas have relatively high 87 Sr/ 86 Sr values ranging from 0.72114 to 0.72604 and from 0.71119 to 0.71151, which are consistent with expected values for groundwater dominantly affected by dissolution of silicates. However, in the discharge area, the groundwater samples had lower 87 Sr/ 86 Sr values and varied between 0.71016 and 0.71753. The contour map of δ 87 Sr in groundwater shows the general tendency of decrease from the western and eastern margin areas to the discharge area. The plot of 87 Sr/ 86 Sr vs. Sr/Na indicates that interactions between Quaternary aquifer sediment and groundwater in the recharge areas along the flow paths control the hydrochemistry and strontium isotopic compositions of groundwater. By contrast, groundwater samples from the discharge area are plotted on the mixing line, indicating that mixing of groundwater from recharge area with low 87 Sr/ 86 Sr values groundwater could be the controlling factor on their hydrochemistry and strontium isotopic compositions. Four main flow paths of groundwater were inferred from hydrochemical and isotopic data. The results of PHREEQC inverse modeling matched quite well with the results of strontium isotopic and ion compositions along the flow paths. The distribution of high arsenic groundwater in this area could be attributed to the combined effect of the vertical flow and local groundwater flow on the redox conditions of the groundwater system.