GEOCHEMICAL AND GEOPHYSICAL MONITORING OF SALINEWATER INTRUSION IN KOREAN PADDY FIELDS SANG-HO LEE 1 , KYOUN ¸ G-WOONG KIM 1,∗ , ILWON KO 1 , SANG-GYU LEE 2 and HAK-SOO HWANG 2 1 Advanced Environmental Monitoring Research Center, Kwangju Institute of Science and Technology, Kwangju 500-712, Korea 2 Center for Natural Hazard Prevention Research, Korea Institute of Geology, Mining and Materials, Taejon 305-350, Korea ( ∗ author for correspondence: e-mail: kwkim@kjist.ac.kr) Received 6 July 2000; accepted in revised form 6 August 2001 Abstract. The saline water intruded zone in paddy fields near the seashore can be diagnosed ac- curately by joint exploration with geophysical and geochemical methods. Using the electromagnetic (EM) sounding technique, the weakly consolidated zone which introduces saline water into such an area of near seashore paddy fields in Korea was detected from the variation of electrical conductivity distribution following field irrigation. Vertical electrical sounding (VES) with Schlumberger array and chemical analysis of top soils, and groundwater in the study area verified the intruded zone near the surface. The VES results showed that the intrusion of seawater occurred in the form of a channel down to 30 m below sea level. Geochemical analysis of the top soil samples for the six major elements found in seawater indicated that the region showing high concentrations is concordant with the weakly consolidated zone near the surface. The degree of contamination in the study area was investigated by comparing the soil data with those from a nearby old reclamation field. If remediation work is not done for this intrusion zone, the sodicity degree in the paddy soil is expected to increase compared with the reference site data. Key words: electromagnetic sounding, geochemical analysis, paddy soil, saline water intruded zone, sodicity, vertical electrical sounding 1. Introduction Soil salinity plays an important role in paddy field yield, and currently 30% of worldwide paddy fields are affected by excess salinity (Rowell, 1994). Most prob- lems originated from bad irrigation water quality, but fields located near the sea- shore can be influenced by polluted groundwater through the field basement. The main factor affecting crop yield is known to be sodicity (the relative Na concentra- tion in soil paste water) which prevents the plants from uptaking essential elements such as Ca and K (Baize, 1993). There are numerous paddy fields located near the seaside in Korea, and the KIGAM (Korea Institute of Geology, Mining and Materials) has reported that groundwater contamination by seawater can reach as far as 1 km inland from the sea front (Lee et al., 1993). Geochemical analysis is an affirmative method to verify the saline water intrusion in surface soil and ground- Environmental Geochemistry and Health 24: 277–291, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands.