Trend analysis for the nitrate-nitrogen and chloride data of the National Groundwater Quality Monitoring Network for systematic groundwater quality management

In this study, trend analysis was performed for the nitrate-nitrogen (NO₃-N) and chloride (Cl^-) data from the National Groundwater Quality Monitoring Network (NGQMN) of Korea during 2007 and 2013, using Sen’s method, a non-parametric statistical analysis method, for systematic groundwater quality management. The results for individual monitoring wells with different depths were plotted on the national distribution map, and were classified based on the relative concentrations against threshold values (TVs) along with the trends. Of the total of 234 points analyzed, the points corresponding to 'no trend' accounted for the majority with 194 points and 163 points, and the points above TVs were 17 points and 13 points for nitrate-nitrogen and chloride, respectively. The results of the trend analysis according to the depth in each well showed that the trends can be different, and this is closely related to the geology and hydrogeological characteristics of the area where the monitoring well was installed. For monitoring wells showing an increasing trend, the time required to reach TVs in the future was calculated based on the trend slope and the difference between the present concentration and TV. This study can help to establish water quality management framework in areas where groundwater contamination is in progress, and to verify the reliability of water quality predictions by statistical analyses.