Hydrochemistry and Dissolved Inorganic Carbon (DIC) Cycling in a Tropical Agricultural River, Mun River Basin, Northeast Thailand

Dissolved inorganic carbon isotope composition (δ 13 C DIC ), together with major ion concentrations were measured in the Mun River and its tributaries in March 2018 to constrain the origins and cycling of dissolved inorganic carbon. In the surface water samples, the DIC content ranged from 185 to 5897 μmol/L (average of 1376 μmol/L), and the δ 13 C DIC of surface water ranged from −19.6‰ to −2.7‰. In spite of the high variability in DIC concentrations and partial pressure of carbon dioxide ( p CO 2 ), the δ 13 C DIC values of the groundwater were relatively consistent, with a mean value of −16.9 ± 1.4‰ ( n = 9). Spatial changes occurred in the direction and magnitude of CO 2 flux through water-air interface ( F CO2 ). In the dry season, fluxes varied from −6 to 1826 mmol/(m 2 ·d) with an average of 240 mmol/(m 2 ·d). In addition to the dominant control on hydrochemistry and dissolved inorganic carbon isotope composition by the rock weathering, the impacts from anthropogenic activities were also observed in the Mun River, especially higher DIC concentration of waste water from urban activities. These human disturbances may affect the accurate estimate contributions of carbon dioxide from tropical rivers to the atmospheric carbon budgets.