제주도 서부지역 지하수의 수질 특성 및 질산성질소 오염원 규명

Abstract

The characteristics of groundwater quality (such as water pollution level of studied items, evaluation of water pollution level through their distribution patterns, correlation between water quality items, and water quality variability according to precipatation, etc), were investigated for all groundwater wells except for those due to the failure of motor pump, unused wells, owner’s disapproval of their use, in the western part of Jeju Island (Hanlim-eup, Hangyeong-myeon, and Daejeong-eup), which are known to be increasing in pollution due to the existence of various contaminants. Moreover, water quality characteristics and identification of contamination source using stable nitrogen and oxygen isotopes for nitrate nitrogen (NO3-N) which is used as an indicator of groundwater quality pollution in Jeju Island, were also examined. The concentrations of HCO3 - and K + mainly produced by the reaction of groundwater and geological media were high in Hanlim-eup, and those of NO3-N, Cl - , and SO4 2- and etc, caused by anthropogenic source or seawater inflow rather than natural source, were high in Hangyeong-myeon, indicating that the groundwater quality in the latter was relatively affected by anthropogenic contaminants, compared with other areas. According to the evaluation of groundwater quality using the skeletal box-and-whisker plot, two groundwater wells were thought to be affected by seawater inflow, and groundwater pollution level was higher than that of other areas due to seawater inflow or surface pollutants in Hangyeong-myeon, and in Daejeong-eup, the average groundwater quality pollution level was increased by a large distribution of groundwater wells with higher abnormal values than in other areas. According to the correlation analysis between groundwater quality items, NO3-N showed a normal or high correlation with SO4 2- , Ca 2+ , and Mg 2+ in all the areas but in the correlation between NO3-N and stable nitrogen isotope ratio (δ 15N-NO3 - ), the correlation coefficient (r) was 0.358, -0.464, and –0.369 in Hanlim-eup, Hangyeong-myeon, and Daejeong-eup, respectively, assuming that the source of nitrogen pollution was different between in Hanlim-eup and in Hangyeong-myeon and Daejeong-eup. Looking at the groundwater quality variability according to precipitation, it was high in the order of Hangyeong-myeon > Hanlim-eup > Daejeong-eup for NO3-N, indicating that the concentration of NO3-N was the most affected in Hangyeong-myeon and the least affected in Daejeong-eup by precipitation, and was high in the order of Hanlim-eup > Hangyeong-myeon > Daejeong-eup for the other items. As a result of examining the changes in water quality type with NO3-N concentration by the Piper diagram, it was similar in Hangyeong-myeon and Daejeong-eup, i.e., in Hangyeong-myeon and Daejeong-eup, compared with in Hanlim-eup, anions were further directed into Cl type at NO3-N concentration of 3 mg/L or less, and at NO3-N concentration of 10 mg/L or above, anions were further evolved into SO4 + NO3 type and cations were more evidently directed into Ca + Mg type, assuming that the source of nitrogen pollution was different between in Hanlim-eup, and in Hangyeong-myeon and Daejeong-eup. As a result of examining the stable oxygen and nitrogen isotope ratio in nitrate (δ 15N-NO3 - and δ 18O-NO3 - ) for all the groundwater wells (711 wells) in this study, the values of δ 18O-NO3 - were in the range of –6.46∼+16.89‰, indicating that nitrate was mainly derived from nitrification process, and those of δ 15N-NO3 - were mostly in the range of +3～+8‰, which was considered to have been affected by two or more sources of nitrogen pollution. Looking at the source of nitrogen pollution with study area, the values of δ 15N-NO3 - - viii - were high and low in Hanlim-eup, and in Hangyeong-myeon and Daejeong-eup, respectively, indicating nitrate nitrogen source was mainly derived from manure and sewage, and derived from soil and chemical fertilizer, respectively.