Nitrate contamination of groundwater and identification of its cause in Kumamoto City and neighbouring area in Japan

Abstract

Kumamoto City that has a population of 0.7 million depends on the only groundwater for the drinking-water demand. Recently, there are growing concerns about a groundwater contamination by nitrate. In this study, we aimed to reveal the situation and cause of nitrate contamination of groundwater in Kumamoto City and its neighbouring area by a combination of water-quality measurement and the analysis of public data. Nitrate concentration in the groundwater varied according to the area-location, however, most groundwater samples showed an increasing trend for the last few decades, and high nitrate concentration (>10 mg-N/L) was detected even in deep-aquifer samples. The results of nitrogen stable isotopic analysis suggested that nitrate contamination was caused by organic nitrogen. At the same time, the analysis based on public statistical data demonstrated that the nitrogen load by waste of domestic animals drastically increased for the last decades. These results strongly suggested that the waste of domestic animals was the most significant cause for the groundwater contamination in the research area.