Abstract

The most common water types are found to be Ca-HCO₃, Ca-Na-HCO₃ and Ca-Na-HCO₃-Cl in Gwangju groundwater. Groundwater near the Gwangju stream are characterized Ca-Cl water type, with over 50 mg/L of Cland 400 μS/cm of EC. The systematic variation of Cl^?, HCO^3?, EC and δ¹⁸O values in groundwater with distance away from drainages is caused by streamwater infiltration. Stable isotope data indicate that δD and δ¹⁸O values of groundwaters near drainages were enriched by evaporation effect, showing a equation of δD=7.1×δ¹⁸O-1. δ¹⁸O values over -6 ‰ are anomalous in the unconfined groundwater zones, which are influenced by the local surface water enriched in ¹⁸O composition. Groundwater in highland shows remarkably light δ¹⁸O values below -8‰. The infiltration
of streamwater is dominant in unconfined alluvium aquifer near drainages. δ¹³CDIC values (-17.6~-15.2‰) of groundwaters near drainages revealed that dissolved inorganic carbon (DIC) is predominantly originated from natural soil-derived CO₂. δ¹⁵N and δ¹⁸O values of nitrate are 0~17.0‰ and 6.6~17.4‰, respectively. Relationship between δ¹⁵N and δ¹⁸O shows a systematic isotopic fractionation caused by denitrification of 40~60%, suggesting that the major source of groundwater nitrate originated from nitrate of soils, and mixing nitrate of soil and sewage or manure.

Keywords Gwangju, groundwater, hydrochemistry, stable isotope, denitrification