Hydrogeochemical characteristics of groundwater influenced by reclamation, seawater intrusion, and land use in the coastal area of Yeonggwang, Korea

Abstract

The coastal line of Yeonggwang, South Korea, has been continuously changed by reclamation of tidal land to expand the residential and agricultural area, which is critical factor to understand the regional hydrogeochemistry in this area. In this study, we investigate the major factor to influence on the hydrogeochemical characteristics of groundwater in the reclaimed coastal area. Groundwaters from this area can be classified into four groups (A, B, C, and D) by a cluster analysis using their physico-chemical properties. Groups C and D have an enriched NO3– concentration (maximum ~210 mg/L), which is the result of anthropogenic inputs (particularly chemical fertilizers), whereas observations (e.g., high Cl–, Na+, and the correlation between Cl– and major anions/cations) have demonstrated that groups A and B are affected by old seawater intrusion. Agricultural activities also influence Group B because Br–/Cl– ratios vary, and values of the major cations/Cl– of this group are higher than those of seawater and Group A. The underlying organic matter and microbials have been influenced by the reclamation processes and the subsurface environment has experienced sudden changes, which have led to sub-/anoxic conditions that are sufficient to cause redox reactions. Because Mn and Fe reduction processes occur after NO3 reduction finishes, groups A and B have low Eh, dissolved oxygen (DO), and NO3– while they have high Fe2+, Mn2+, and HCO3– with depleted δ13CDIC. Furthermore, SO42– concentrations were very low compared to the estimated values from a simple two-component (seawater and freshwater) mixing model, with the enriched δ34SSO4 values (30.7~57.3‰) in these groups clearly demonstrating the occurrence of SO4 reduction. The reclamation process also appears to affect the recharge time of the regional groundwater system in the study area. Overall, the regional groundwater system in the coastal area of Yeonggwang has been influenced by the reclamation, old seawater intrusion, land use, and reduction process resulting in highly complicated hydrogeochemical characteristics.