The Isotopic and Hydrochemical Studies of Groundwater and Modified Mn Sequential Analysis of Sediments at the Two Riverbank areas

This research is to investigate the hydrogeological and hydrogeochemical characteristics and to know the factors affecting the groundwater chemistry. The isotopic study was executed by using boron and strontium isotopes which are used to infer the effect and the source of high NO3 in the groundwater at Mankyung riverbank area, with an intensive agricultural cultivation such as rice paddy and greenhouses. The riverbank area is also important as the pathway of the nonpoint source contamination of nitrate from an agricultural land. According to the research results, the vertical hydrogeological structure of the Mankyung river area was classified into three zones, silty sand layer, gravel sand layer, and weathered layer of granite, and the gravel sand layer has the highest hydraulic conductivity. The results of hydrochemical logging and boron and strontium isotopic analyses showed that the groundwater of the site has been influenced by seawater and chemical fertilizer. The hydrochemistry of shallow groundwaters showed the influence of land use, paddy and greenhouse which was caused by the different redox environment of groundwater. Another problem frequently observed in bank filtered groundwater is the high Mn that exceeds the level of water quality standards. To understand the source and occurrence, we executed the hydrogeologic structure, groundwater quality analysis and modified sequential analysis of Mn for sediments. The high Mn is mainly caused by the microbial reduction reaction, which is proved by the sequential analysis of Mn, exists as an 3/4 fraction.