Hydrogeochemistry in the part of Ariyalur region, Perambalur district, Tamil Nadu, India**

Hydrogeochemistry of a region is a reflection of the hydrodynamic process, lithological composition and physical constraints. Case study has been carried out in a varied lithological terrain with Archaean, Gondwana, and Cretaceous rocks to unravel the hydrogeochemical process. The study area is part of Ariyalur region lying between North latitude 10°42’00” - 11°12’00” and East longitude 78°42’00” - 79°0’00”. The nature of groundwater is explained by the Johnson plot, which indicates high Ca+Mg, SO₄+Cl and HCO₃+CO₃ facies during pre and post monsoon seasons. Geochemical processes of the area are explained by Gibbs plot where evaporation process dominates the groundwater chemistry. The quality of water for irrigation was estimated by USSL classification indicating high salinity and low sodium hazard, satisfactory for plants having moderate salt tolerance on soils. Doneen plot reveals that groundwater samples from both the seasons fall in class I indicating water is good for irrigation purpose. Silicate minerals are less saturated, and are comparatively under saturated with respect to carbonate minerals. Dilution of water chemistry during the post-monsoon season in the Archaean formation and mixing in the Cretaceous formation are noted.