Hydrogeochemical and isotopic features of the groundwater flow systems in the central-northern part of Jeju Island (Republic of Korea)

Groundwater from 10 production wells located in lowlands to highlands was investigated from July 2008 to February 2010 for chemical and isotopic compositions (δ18O, δ2H, and δ13CDIC) to understand groundwater recharge processes and to assess geochemical conditions of groundwater in the volcanic aquifers. The groundwater in lowlands had elevated concentrations of Cl and NO3 with greater seasonality when compared with those from higher areas, indicating that groundwater in the lowlands is affected by contamination. Based on δ18O values and altitudinal variations, groundwater was classified into two regional flow systems with different flow paths. The regional flow systems are likely connected from highland to lowland areas, considering consistent δ18O values and major ion contents throughout the seasons. Of the two regional flow systems, the wells clustered in one regional flow system showed low δ13CDIC values (average − 14.3‰) and high PCO2 (average 2.04 in -logPCO2), reflecting relatively high contribution of soil CO2 to groundwater. The other regional flow system with low PCO2 (average 2.39 in -logPCO2), however, consisted of two highland wells with enriched δ13CDIC value, − 9.3‰ and two lowland wells with relatively depleted δ13CDIC value, − 13.7‰. These findings indicate that the two regional flow systems were mixed with local recharge from soil layers during recharge. The δ18O and δ2H values in groundwater from the regional flow systems were not clearly consistent with the local meteoric water line due to mixing of summer and dry season precipitation. The isotopic variations showed a phase lag and cyclic temporal patterns, suggesting that a considerable portion of groundwater was derived from fast recharge with short residence times on monthly time scales. This study suggests that groundwater in lowland areas in Jeju Island can be significantly contributed from highland areas, which was revealed by chemical and isotopic compositions, especially, altitude effect and seasonal variation of δ18O value. We expect that the knowledge on the volcanic aquifer system can be useful for management of groundwater resources including projects (e.g. artificial recharge technology) which has been carried out to mitigate water poverty over the world.