Assessing Seasonal Biogeochemical Variations in the Mun River Watershed Using Water Quality Data and the Geochemical Mass Balance Method

The Mun River watershed, a vital water resource in Northeastern Thailand and a major tributary of the Mekong River, faces significant water quality challenges driven by climate change and human activities. This study examines seasonal biogeochemical variations in the watershed, with a focus on how climate fluctuations affect water quality and geochemical processes. Water samples were collected from 19 surface sites during the dry and wet seasons of 2024 and analyzed for major dissolved ions. Using the geochemical mass balance method, we quantified rates of mineral weathering and biomass degradation. Our findings reveal a notable shift in hydrochemical facies from Na-Cl dominance in the dry season to Ca-HCO3 dominance in the wet season, indicating reduced salinity and changes in geochemical conditions. Wet season mineral weathering rates averaged 300–700 µmol m−2 d−1, approximately 10–20 times higher than those in the dry season. The highest weathering and biomass degradation rates, ranging from 900 to 1200 µmol m−2 d−1, were observed in the northern subwatersheds, likely due to intensified agricultural practices and underlying geological conditions. These results highlight the urgent need for adaptive watershed management strategies to address the growing impact of climate change on regional water quality.