Assessment of Groundwater Quality in the Jajrood River Basin, Tehran, Iran: A Coupled Physicochemical and Hydrogeochemical Study

Background: The physicochemical composition of groundwater is affected by the quantity and quality of surrounding aquifers which are in turn recharging from adjacent river waters. Methods: In the present study, 20 surface and 16 groundwater samples were collected in pre- and post-monsoon season from the Jajrood River basin, Tehran, Iran. The samples were analyzed for 18 physicochemical water quality characteristics to assess the river and groundwater qualities. Hydrogeochemical analyses of groundwater samples were also performed to determine the Water Quality Index (WQI) for drinking and evaluate factors governing the water quality characteristic in the study area. Accordingly, the Piper diagram and Gibbs and Chadha plots were drawn to assess seasonal variations in hydrochemical facies and processes in the basin. Subsurface soil samples were also examined with respect to the structure, elemental composition, and multi- elemental trace analysis. Results: Results showed the abundance of major ions in the order of Ca⁺² Na⁺ Mg⁺² K⁺ for cations and HCO ⁻ SO ²⁻ Cl⁻ NO ⁻ F⁻ for anions. In general, all drinking groundwater samples met WHO permissible limits except for Chemical Oxygen Demand (COD) and HCO ⁻. Moreover, the water is categorized as Ca-Mg-HCO₃ type. Subsurface soil analyses demonstrated quartz and calcium carbonate as the main phases of soil structure, suggesting the enrichment of groundwater with temporary hardness.