Seasonal and spatial changes in groundwater environment in Omama alluvial fan, Gunma Prefecture: focusing on nitrate-nitrogen

Abstract

In this study, we focused on nitrate-nitrogen (NO₃^--N) contamination of groundwater in Omama alluvial fan in the Gunma Prefecture and examined the spatial distribution and seasonal variation of NO₃^--N concentrations during a periodic survey over nine months. The analysis results showed that the groundwater for more than 70% of the 27 surveyed sites was affected by human contamination. Moreover, high concentrations of NO₃^--N were measured in areas with numerous agricultural sites. Strong correlations observed between NO₃^--N and chlorine (Cl^-)·sulfate ion (SO₄^2-) suggested that NO₃^--N contamination of groundwater could be attributed to the use of fertilizers in these areas. Furthermore, the seasonal variation of the NO₃^--N concentration and groundwater level seemed to be correspond to the precipitation, the wettest periods are considered to have higher NO₃^--N concentrations Besides, it was clarified that the groundwater pollution is not transient, but certain extent in the long-term in fact, there was some in which the environmental standard value was exceeded during the research period.