Vertical mixing with return irrigation water the cause of arsenic enrichment in groundwater of district Larkana Sindh, Pakistan

Stable isotopes ratios (parts per thousand) of Hydrogen (delta H-2) and Oxygen (delta O-18) were used to trace the groundwater recharge mechanism and geochemistry of arsenic (As) contamination in groundwater from four selected sites (Larkana, Naudero, Ghari Khuda Buksh and Dokri) of Larkana district. The stable isotope values of delta H-2 and delta O-18 range from 70.78 parts per thousand. to -56.01%parts per thousand and from -10.92 parts per thousand to -7.35 parts per thousand, relative to Vienna Standard for Mean Ocean Water (VSMOW) respectively, in all groundwater samples, thus indicating the recharge source of groundwater from high-salinity older water. The concentrations of As in all groundwater samples were ranged from 2 mu g/L to 318 mu g/L, with 67% of samples exhibited As levels exceeding than that of World Health Organization (WHO) permissible limit 10 mu g/L and 42% of samples expressed the As level exceeding than that of the National Environmental Quality Standard (NEQS) 50 mu g/L. The leaching and vertical mixing with return irrigation water are probably the main processes controlling the enrichment of As in groundwater of Larkana, Naudero, Ghari Khuda Buksh and Dokri. The weathering of minerals mostly controlled the overall groundwater chemistry; rock-water interactions and silicate weathering generated yielded solutions that were saturated in calcite and dolomite in two areas while halite dissolution is prominent with high As area. (C) 2018 Elsevier Ltd. All rights reserved.