An integrated approach to investigate nitrate pollution in groundwater in Shimabara Peninsula, Nagasaki, Japan

Groundwater pollution by nitrate is a common problem in many parts of the world. The agriculturally important Shimabara Peninsula in Nagasaki, Japan, is experiencing this problem. The general source of drinking water in the study area is groundwater; consequently, nitrate pollution is a significant problem. In this study, as a first, groundwater and spring water samples were collected from 2011 to 2021 to determine the status of nitrate pollution and groundwater quality in the Shimabara Peninsula. Based on the water quality of the total of 178 samples, principal component analysis and cluster analysis were used to characterise them. Second, a numerical model of groundwater flow and nitrate transport was developed, and a simulation of nitrate pollution in the northwestern part of the peninsula was performed. Third, in the northwest part of the peninsula, where contamination is more severe, we used coprostanol and sterol ratios to assess the source of contamination. Fourth, we also conducted a factor decomposition approach to understand the tendency of nitrogen supply of each sector to affect pollution for the whole peninsula. As a result of this study, nitrate pollution is severe in the northern part of the peninsula, especially in the northwestern region. The results of the multivariate analysis showed that there were three principal components and 5 clusters. The numerical simulation results revealed that the current contamination situation had been reached over the past 40 years. The improvement of the stratigraphic model allowed us to reproduce the observed changes in nitrate concentrations to a certain extent. The possibility of identifying the cause of contamination by using coprostanol concentration and sterol ratios was demonstrated. The results of the factor decomposition approach revealed that the decrease in nitrogen loading potential from 2013 to 2019 was due to structural change factors in the livestock industry and cropland area factors in agriculture. In other words, in the livestock industry, the shift to animals with lower nitrogen supply has progressed, and in agriculture, the reduction in cropland area is a contributing factor.