Regional hydrochemical study on salinization of coastal aquifers, western coastal area of South Korea

In order to evaluate the salinization in the western coastal area of South Korea, we performed a regional hydrochemical study on a total of 356 shallow groundwaters sampled within 10 km from the coastline. About 13, 5, and 37% of the samples exceed the drinking water standards for total dissolved solids, chloride, and nitrate, respectively, indicating significant deterioration and salinization of groundwaters. The concentrations of analyzed solutes are very wide in range, suggesting that the hydrochemistry is controlled by several intermixed processes such as seawater mixing, anthropogenic contamination, and water-rock interaction. Based on the interpretation of cumulative probability curves for Cl^- and NO₃^- concentrations, groundwaters are grouped into four types, as follows: (1) type 1 waters (37.6%) that are relatively poor in Cl^- and NO₃^-, which indicates little contamination due to seawater mixing and/or anthropogenic pollution; (2) type 2 waters (2.8%) enriched in Cl^-, indicating considerable effects of seawater mixing; (3) NO₃^--rich, type 3 waters (58.1%) which show significant anthropogenic pollution; and (4) type 4 waters (1.4%) enriched in both Cl^- and NO₃ ^-, demonstrating the effects of both seawater mixing and anthropogenic contamination. The results of water-type classification are well confirmed by statistical analyses of hydrochemical data. As an effective tool for evaluating the relative degree of seawater mixing, we propose the 'Seawater Mixing Index' (SMI) based on the concentrations of Na, Mg, Cl, and SO ₄. Most type 2 and type 4 waters have SMI values greater than 1. At least about 5 and 60% of well groundwaters are considered to be affected by salinization due to seawater mixing and anthropogenic contamination, respectively.