Identification of Groundwater Contamination Causes Around Subway Tunnels at a Coastal Area in Korea, Using Hydrogeological and Geostatistical Analyses

The cause for the deterioration of groundwater quality was identified by hydrogeological investigations and geostatisitical analyses at a coastal area in Busan, Korea. The city has many tunnels for three subway lines, for communication cables and for electrical cables under the ground. The groundwater levels of 135 wells were measured during the dry and wet seasons. The average groundwater level was 7.81 m above the mean sea level in dry season, and 9.11 m in wet season. Groundwater was sampled at 135 wells developed in shallow and deep aquifers. The average water temperature in wet season was 5.5 degrees C higher than that in dry season. The average of pH was 0.4 higher, and the average of DO was 0.6 mg/L lower in wet season. The average of EC was 136 μS/cm higher in wet season. The total quantity of discharged groundwater from subway tunnels was 2,282,000 m3/year at the study area. It was 2.4 times the sustainable development yield of groundwater. So the groundwater level was seriously decreased at the study area. The maximum groundwater level around the subway was located at 30 ~ 32 m below the mean sea level. The deep drawdown of groundwater level brought about the inflow of sea water and river water. The groundwater around the subway lines was contaminated by sea water and the salinized water of the Suyeong River. The quality of groundwater was not reached to the standard of potable, domestic, agricultural and industrial uses. The distribution maps of groundwater level and quality were produced using a geostatistical method, kriging. The maps were very useful to find out the causes of groundwater contamination at the study area. The maps identified that sea water and river water infiltrated the inland groundwater and contaminated the groundwater around the subway lines, because the groundwater level was seriously drawdowned by the groundwater discharge from the subway tunnels. The groundwater was also contaminated by seawater intrusion at the coastal area.