Groundwater chemistry in the nitrate contaminated area in Shimabara, Nagasaki Prefecture, Japan

Abstract:

Groundwater contamination by nitrate from agricultural fields is a problem shared by many parts of the world. Shimabara, Nagasaki prefecture, Japan is an important agricultural district experiencing this problem. In Shimabara, drinking water relies on the groundwater. In this study, groundwater samples were collected at 40 locations such as residents and municipal waterworks wells, springs and rivers from August 2011 to November 2013. NO₃-N concentration of 15 samples exceeded 10 mg L^-1 (drinking water standard in Japan). Maximum NO₃-N concentration was 26.6 mg L^-1. Correlation coefficients were calculated between ion components of collected samples (n=277). NO₃^- had the highest positive correlation with Cl^-(r =0.956) and had positive correlation with K⁺(r=0.679), SO₄^2-(r=0.654) and Ca²⁺(r=0.593), respectively. The results revealed that Cl^- and K⁺ related to livestock wastes, SO₄^2- related to chemical fertilizers and Ca²⁺ related to calcareous materials. Main source of NO₃^- is from livestock wastes. To understand groundwater chemistry in detail, principal component analysis (PCA) and cluster analysis were carried out. Result from the PCA, chemical characteristics of groundwater was summarized by the first principal component and the second principal component. Both of two principal components reflected nitrate contamination and ion dissolution from aquifer matrix during groundwater flows. Result from the cluster analysis, chemical characteristics of groundwater was classified into four clusters. Nitrate polluted samples into specific cluster and the rest samples were classified into other clusters depending on the original water quality.