Hydrochemistry of Groundwater in Quaternary Aquifers of the Phrae Basin, Northern Thailand

This study presents the baseline hydrochemical characteristics of groundwater in the Phrae Basin based on 120 samples. Groundwater is classified into five groups, with hydrochemical characteristics across three aquifers showing no significant seasonal differences, although varying degrees of mineralization were observed. PhreeqC modeling suggests that most samples are undersaturated, nearing equilibrium. The correlation matrix and PCA indicate that alkalinity, HCO3–, and CO32– influence pH and EC, while Ca2+ and Mg2+ correlate with carbonate rock dissolution. The SO42–/Cl– ratio points to pyrite oxidation and anthropogenic impacts. Rock–water interactions and the dissolution of mineral phases mainly control ion concentrations in groundwater. Ion exchange, silicate weathering, and clay mineral absorption are potential sources of Na+ ions, rather than halite dissolution. PCA analysis showed anthropogenic effects in shallow and intermediate wells. Groundwater quality is primarily controlled by CO2(g), calcite, aragonite, and dolomite dissolution in the east, and hematite and goethite dissolution in the west, near igneous rocks, which are dominant comprising pyroclastics, basalt, andesite, granodiorite, and some diorite. Gibbs diagrams confirm rock weathering as the dominant geochemical process in the Quaternary aquifer. Our results provide clear evidence that cation exchange and recharge mechanisms are the dominant processes controlling groundwater hydrochemistry in the study area. This study establishes the baseline hydrochemistry of groundwater in the Phrae Basin, Thailand, highlighting natural and anthropogenic influences on groundwater quality.