Integrated approach to investigate groundwater nitrate nitrogen pollution and remediation simulation in Shimabara Peninsula, Nagasaki, Japan

Abstract

Groundwater is the general source of drinking water in the Shimabara Peninsula, Nagasaki, Japan, and consequently, occurring nitrate nitrogen (NO₃-N) pollution in the groundwater is a significant problem. Although various countermeasures have been implemented, nitrate nitrogen concentrations remain serious. Therefore, it is necessary to evaluate effects of different potentially effective countermeasures by simulating various remediation processes using numerical calculations. First, to determine the status of nitrate nitrogen pollution and groundwater quality, we sampled and analyzed 179 groundwater and spring water samples from 2011 to 2021. Principal component analysis (PCA) and hierarchical cluster analysis (HCA) were used to characterize the water quality. A trilinear diagram classified most groundwater samples into Ca-HCO₃ and Ca–(SO₄ + NO₃) types. A small number of samples were classified as Na-HCO₃ type. PCA extracted three principal components, accounting for 82% of the total variance. The extracted principal components indicated that mineral dissolution with water–rock interaction, nitrate nitrogen pollution, denitrification, and seawater pollution control the water chemistry in the study area. HCA classified 179 samples into five clusters. The combination of PCA and HCA results revealed that each cluster had markedly different ion concentrations depending on the degree of influence of each principal component. The nitrate nitrogen concentration ranged from 0.1 to 42.8 mg/L, and the average was 4.5 mg/L. Compared with the Japanese drinking water standard of 10 mg/L, 23 sites (13%) exceeded the standard. The spatial distribution of nitrate nitrogen concentration showed that nitrate nitrogen pollution is particularly severe in the northeastern region. Therefore, a numerical model of groundwater flow and nitrate nitrogen transport was developed to simulate nitrate nitrogen behavior in the northeastern region. To simulate the remediation process from nitrate nitrogen pollution, the reduction in nitrate nitrogen supply from agricultural land (fertilizer) and livestock facilities was assumed to be between 0 and 80% in 20 cases. The simulation results showed that the current pollution situation is a result achieved over the past 44 years. To reduce pollution in the most effective way, a 40% reduction in fertilizer applied to agricultural land is necessary. This is likely to reduce the nitrate nitrogen level in groundwater to permissible levels after approximately 50 years. These simulations of the remediation process are important for the determination of reduction target of the pollutants and necessary administrative decision-making.