Water quality monitoring and its biogeochemical processes of river water and groundwater in Saijo, Japan

Abstract

In this study, to clarify the current status and causes of nitrate-nitrogen increase, we analyzed groundwater at 7 sites and river water at 3 sites (Kamo, Nakayama, and Takamatsu Rivers) in Saijo City for major cations and anions, nutrients, and stable isotope ratios of NO₃^-. The average NO₃^- concentrations (umol/L) in the Kamo River, Nakayama River, and Takamatsu River were 28.8, 47.0, and 134, respectively, and were higher in the Takamatsu River, which has a catchment area on the western hillside of the city where fruit trees are planted. On the other hand, the groundwater showed a large variation from 17.8 to 536, with lower values in groundwater with low water temperature and higher values in groundwater near the hills on the west side of the city. The average Si concentrations (umol/L) in the three rivers were 98.9, 151, and 258, respectively, and those in groundwater were 116-354, showing a similar trend to that of NO₃^-. The Kamo River system and low-temperature groundwater are thought to have their sources at higher elevations, resulting in lower concentrations of various ions and Si. The high NO₃^- concentrations in the river water and groundwater on the west side of the city are considered to be largely influenced by fertilizer application, especially ammonium sulfate, in fruit tree cultivation.