Hydrogeochemistry of groundwater from karstic limestone aquifer highlighting arsenic contamination: case study from Jordan

Groundwater wells in the Amman-Wadi Es Sir Aquifer (B2/A7) throughout Jordan are investigated for their arsenic (As) and element-by-element geochemical behavior. Groundwater wells are found to have total arsenic concentrations above the recommended levels designated by the Jordanian drinking water standard, the Environmental Protection Agency of the United States, and the World Health Organization. Arsenic distribution in the aquifer is variable, but it is detected with a concentration of ≥10 ppb in 87 samples out of the analyzed 150 groundwater samples, with a maximum concentration of 173 ppb. Elevated As concentrations can be attributed to several mechanisms. One of these mechanisms is accounted for to the interaction between groundwater and the natural phosphorite deposits in the upper part of the aquifer and oil shale deposits overlying it. The high significant correlation between arsenic, phosphorus, and calcium concentrations in the analyzed groundwater samples suggest that these elements are derived from the same source. Moreover, scanning electron microscopy shows the association of As with the P and Ca in the phosphorite; pyrite is present in the oil shale samples, which were collected from the Muwaqqar formation overlying the aquifer. EDAX analysis shows that substantial As concentrations are present in phosphate and pyrite. This study suggests that the major mechanism responsible for releasing As from the aquifer material into the groundwater is a simple dissolution reaction. However, Piper and Durov diagrams, correlation coefficients, and factor analysis all suggest that water interaction with phosphate and oil shale deposits, sorption of heavy metals, and simple dissolution of iron oxyhydroxides are together the primary factors affecting the chemistry of the groundwater and responsible for the elevated As values in some wells.