Seasonal Differences and Source Estimation of Groundwater Nitrate Contamination

Abstract

Small-scale field research was conducted in an agriculture-dominated alluvial fan in Kofu, Japan, where nitrate contamination was found in groundwater. The aim of this study was to examine the seasonal differences in nitrate contamination and to quantify the relative contribution of pollutant sources in groundwater. Proportions of δ¹⁸O-NO₃^-/δ¹⁸O-H₂O were evaluated to identify the seasonal differences in N leaching during transport from soil to groundwater. This indicated a significant difference in the ranges of nitrate concentrations between seasons. Three major nitrate source end-members (manure, soil, and upland groundwater) were identified based on isotopic (δ¹⁸O-NO₃^- and δ¹⁵N-NO₃^-) and major ion compositions. The contribution of each nitrate source was quantified using a method of multivariate mixing and mass balance calculation. In this method, the assumption is made that the groundwater chemistry is a result of mixing as well as water-soil interaction. Results showed that soil was the major source of NO₃^- (45%), followed by upland groundwater (30%) and manure (25%).