An Integrated Study of Geoelectric Vertical Sounding and Hydrogeochemistry in the Riverside Alluvium around Buyeo Area, Korea

The water quality of alluvial aquifers in agricultural areas is sensitive to the behaviour of agricultural chemicals (e.g., fertilizers, pesticides, and lime) and, also, to the geologic conditions. It is important to know the characteristics of the aquifers (e.g., depth, spatial distribution, and soil types) and the relationship between subsurface geology and the groundwater contamination for its effective use and management in the future. In order to provide the subsurface information of the lithology and the groundwater zone for hydrogeologic interpretations, an integrated study using twenty vertical electric soundings (VES), direct observation of lithology from two boreholes and hydrochemical data from irrigation well has been conducted in the riverside alluvium near Buyeo area, Korea. Main results of this study are as follows. The depth of main groundwater table is getting slightly deeper toward the river. The boundary between surface sandy and silty soils extends to the subsurface at depth of groundwater table. The vestige of an ancient river channel, such as an oxbow lake, can be identified by a lateral continuation of perched aquifer parallel to the river on the resistivity profiles. This perched aquifer materials are composed of clay-rich silt soils, which prohibit the infiltration of oxygen and nitrate from the land surface. Therefore, the groundwater of the main aquifer below the oxbow lake shows the very low NO3 level and Eh values under the strong anoxic condition. Surface resistivity contour map indicates that the resistivity varies with a NW-SE trending zonal distribution and increases toward the river. This result shows an agreement with the spatial distribution of surface soils, implying that the variation of surface resistivity is mainly controlled by surface lithology or soil type. On the other hand, the distribution of water resistivities is correlated with that of total dissolved solids (TDS) concentration, while the earth resistivity of aquifer shows a different spatial distribution from those of water resistivity and TDS. It is interpreted that the earth resistivity of aquifer might represent the variations of soil type rather than water chemistry in the study area. The present study shows that the geoelectric sounding survey with the complement of borehole lithology and hydrochemical data can provide an inexpensive and useful method for delineating the subsurface hydrogeology in the riverside alluvial aquifer.