Effects of the Japanese Nitrate Directive Plan (NDP) to curb groundwater nitrate-nitrogen content in the Miyakonojo River basin

There are urgent needs to evaluate different management techniques to improve groundwater quality. To maintain a good quality of groundwater in the iyakonojo River Basin (MRB), Japan, the Nitrate Directive Plan (NDP) was implemented in 2004. To assess the efficiency of this plan, we performed multivariate times-series analyses on 26 years (1996–2021) of groundwater nitrate-nitrogen (NO3−-N) data from up to 584 groundwater wells. Results showed that wells with an increasing NO3−-N trend (p ≤ 0.05) increased from 5.8% in 1996 to 13.8% in 2009 and thereafter leveled off with time. The number of groundwater wells with a decreasing NO3−-N trend (p ≤ 0.05) increased from 18.7% in 1996 to 52.5% in 2021. These groundwater wells are primarily distributed in urban and agricultural production areas near rivers. However, the annual change rate of NO3−-N content displayed both increasing and decreasing NO3−-N trend (p ≤ 0.05) over time. But most importantly, the number of groundwater wells exceeding the environmental maximum permissive value reduced by > 60% from 2004 to 2021. Thus, there is statistical evidence that NDP has improved the groundwater quality regarding NO3−-N at the regional basin scale and that nitrogen levels are gradually being reduced