The hydrogeochemical assessment of hot springs in Mahallat region, central Iran

The Mahallat hot springs are located in the city of Mahallat, central Iran. This region is famous for its balneotherapy and health tourism attractions. They are located in the transitional zone between the Sanandaj–Sirjan (mainly metamorphosed zone) and the Orumieh–Dokhtar (mainly volcanic zone) structural zones. The host rocks of the region include sedimentary to volcanic rocks (Permian to Quaternary), but Quaternary-aged alluvium and travertine layers are the main outcrops. Additionally, these features reveal that thermal and shallow ground waters had been mixed before hot springs exposure. The chemistry data show that these waters are enriched in Ca²⁺, Mg²⁺, Na⁺, Cl⁻, HCO₃⁻, and SO₄²⁻ as well as U and NO₃⁻. The chemistry of thermal and shallow groundwater samples are graphed in the Cl⁻–SO₄²⁻–HCO₃⁻ ternary and other diagrams. In addition, hot springs having high NO₃⁻-concentrations indicate that thermal waters mix with shallow groundwater, which are attributed to agricultural and other anthropogenic activities. The hot springs have high U concentration that may result in shallow groundwater mixing with andesitic tuff, granodiorite, shale, and schist of the region. Therefore, the local farms and agricultural crops as well as local residents’ health could be at risk of exposure to the U and NO₃⁻ pollution.