Trends and Present Conditions of Nitrate-Nitrogen Concentration of Groundwater in the Miyakonojo Basin for the Past 16 Years

Abstract

In the sixteen years between 2001 and 2016, we analyzed the nitrate-nitrogen concentration change, as well as the frequency distribution of fluctuation patterns (rising, descending, and leveling) of the nitrate-nitrogen concentration of groundwater in the Miyakonojo Basin. We also analyzed and summarized the trends of the nitrate-nitrogen concentration where the concentration at the measurement points exceeded 10 mg･L^-1. The relative frequency of the high nitrate-nitrogen concentration in the groundwater has decreased and the fluctuation pattern showed that the leveling-off pattern decreased, and the descending pattern has increased. These changes seem to have occurred in the past 16 years. However, in the past 5 years, many measurement points indicated that the concentrations were leveling-off. Also, it was found that in 64% of measurement points where the nitrate-nitrogen concentration of the groundwater exceeded 10 mg･L^-1, the concentration had improved to 10 mg･L^-1 or less by 2016. However, during the 16-year period, it was found that 2.2% of the measurement points were consistently and significantly over 10 mg･L^-1, while 2.8% of measurement points indicated fluctuations of measurements above and below 10 mg･L^-1.