National scale evaluation of groundwater chemistry in Korea coastal aquifers: evidences of seawater intrusion.

Pollution of groundwater by seawater intrusion poses a threat to sustainable agriculture in the coastal areas of Korea. Therefore, seawater intrusion monitoring stations were installed in eastern, western, and southern coastal areas and have been operated since 1998. In this study, groundwater chemistry data obtained from the seawater intrusion monitoring stations during the period from 2007 to 2009 were analyzed and evaluated. Groundwater was classified into fresh (<1,500 μS/cm), brackish (1,500-3,000 μS/cm), and saline (>3,000 μS/cm) according to EC levels. Among groundwater samples ( n = 233), 56, 7, and 37% were classified as the fresh, brackish, and saline, respectively. The major dissolved components of the brackish and saline groundwaters were enriched compared with those of the fresh groundwater. The enrichment of Na and Cl was especially noticeable due to seawater intrusion. Thus, the brackish and saline groundwaters were classified as Ca-Cl and Na-Cl types, while the fresh groundwater was classified as Na-HCO and Ca-HCO types. The groundwater included in the Na-Cl types indicated the effects of seawater mixing. Ca, Mg, Na, K, SO, and Br showed good correlations with Cl of over r = 0.624. Of these components, the strong correlations of Mg, SO, and Br with Cl ( r ≥ 0.823) indicated a distinct mixing between fresh groundwater and seawater. The Ca/Cl and HCO/Cl ratios of the groundwaters gradually decreased and approached those of seawater. The Mg/Cl, Na/Cl, K/Cl, SO/Cl, and Br/Cl ratios of the groundwaters gradually decreased, and were similar to or lower than those of seawater, indicating that Mg, Na, K, SO, and Br, as well as Cl in the saline groundwater can be enriched by seawater mixing, while Ca and HCO are mainly released by weathering processes. The influence of seawater intrusion was evaluated using threshold values of Cl and Br, which were estimated as 80.5 and 0.54 mg/L, respectively. According to these criteria, 41-50% of the groundwaters were affected by seawater mixing. [ABSTRACT FROM AUTHOR]