Contributions of C3/C4 organic materials and carbonate rock to dissolved inorganic carbon in a karst groundwater system on Miyakojima Island, southwestern Japan

► It is evaluated the controlling processes of DIC in a karst groundwater system. ► The DIC are originated from calcite dissolution and C3 and C4 organic materials. ► The average mass fractions of these were calculated as 46.4%, 18.5%, and 35.1%. ► The results regarding the origins of DIC agreed well with land use distributions. ► In particular, with the spatial distribution of forest and sugarcane fields. Groundwater δ 13C values and chemical compositions were employed to quantitatively evaluate the controlling processes and sources of dissolved inorganic carbon in a karst aquifer system on Miyakojima Island (MI), southwestern Japan. Most MI groundwater is Ca–HCO 3 type water, but some Na–Cl, Ca–Cl, or Na–HCO 3 type groundwaters occur, formed mainly by seawater intrusion and partly by cation exchange reactions. Calculations using the WATEQ4F and PHREEQC programs revealed that all MI groundwater, not just the Ca–HCO 3 type, was strongly influenced by calcite dissolution in a karst aquifer system open with respect to soil CO 2, initially derived from organic materials with mainly around 10–50 matm as pCO 2, and that most groundwater was saturated with respect to calcite. Moreover, oxidation of ammonium sulfate (applied as fertilizer) added H + to the MI groundwater, resulting in surplus calcite dissolution. These processes sufficiently explained contents of alkalinity, DIC, and Ca 2+ in the groundwater, and were evaluated by model calculation using the PHREEQC program with measured groundwater δ 13C and pH values. The results showed that the average mass fractions of dissolved inorganic carbon (DIC) initially from calcite dissolution and C3 and C4 organic materials in the groundwater were 46.4%, 18.5%, and 35.1%, respectively. The model results regarding the origins of DIC in the groundwater agreed fairly well with land use distributions on MI, in particular, with the spatial distribution of forest and sugarcane fields, and gave us some constrains of groundwater recharge areas and flowpaths.