Assessment of groundwater quantity and quality indexes and WQI determination for drinking and agricultural uses: Alborz Plain, Iran case study

This study evaluates groundwater quantity and quality variations in Alborz Plain and assesses suitability for drinking and agriculture using the Water Quality Index (WQI). Data from 19 aquifers (2002–2023) were analyzed for key parameters: K⁺, Na⁺, Mg²⁺, Ca²⁺, SO₄²⁻, Cl⁻, HCO₃⁻, NO₃⁻, pH, EC, and TDS. Piper, Scholler, and Wilcox diagrams were prepared using Aq.QA and MATLAB to identify dominant facies and evaluate agricultural and drinking suitability. WQI results indicate “very good” quality in northwest and central regions, declining to “good” in east and improving in southeast. Dominant facies include Ca-HCO₃ or Mg-HCO₃ in northwest, center, and southeast, and Na-HCO₃ in east, with three aquifers exhibiting Na-SO₄, Na-Cl, and Ca-SO₄ facies. W1 and W3 aquifers are suitable for drinking despite high EC but show agricultural limitations in salt-sensitive areas. Long-term quality trends were assessed using Mann-Kendall test and Sen’s slope, while PCA and K-means clustering identified seven stable aquifer groups. SVR and Random Forest models accurately predicted TDS trends. Salinity is mainly influenced by regional geology, including evaporitic and dolomitic rocks, rather than proximity to sea. This integrated approach provides a practical framework for understanding groundwater dynamics and supporting sustainable management at regional and national levels.