The Spatiotemporal Change Features of Groundwater Nitrogen Content in Miyakonojo River Basin, South Japan

Abstract

For preserving good status of groundwater in Miyakonojo river basin, the Nitrate-Nitrogen Reduction Plan has been implemented since 2004. However, the effects of Nitrate-Nitrogen Reduction Plan have not yet been quantitively evaluated. For this purpose, we used groundwater nitrogen data for 21 years (1996-2016) from groundwater observation wells (486-584) for spatial times series analyses (e.g., Mann-Kendall and Spearman’s Rho test). Results show that the number of observation wells with decreasing nitrogen trend (statistically significant level, p≤0.05) increased from 100 to 200 from 2004 to 2016. However, the number of observation wells with increasing nitrogen trend (p≤0.05) increased to 60 wells up to 2012, and thereafter, levelled off little by little with time. The nitrogen change rate for annual groundwater samples was calculated (p≤0.05) based on Sens`s slope. The results show that annual change rate of groundwater nitrogen decreased slowly with time. Thus, there are statistical evidence of improvements from the Nitrate-Nitrogen Reduction Plan based on observations at the regional river basin scale.