Spatial trends of nitrate pollution and groundwater chemistry in Shimabara, Nagasaki, Japan

Groundwater contamination by nitrate is a common problem in many parts of the world. The agriculturally important Shimabara district in Nagasaki, Japan, is experiencing this problem. The general source of drinking water of the study area is groundwater and consequently the nitrate contamination is a significant problem. For this reason, a groundwater investigation was performed and water samples were collected at 40 locations including residential areas, public water supply wells, springs, and rivers from August 2011 to November 2013. Results showed that nitrate nitrogen (NO₃-N) concentration is exceeding the Japanese drinking water quality standards (10 mg L⁻¹) at 15 locations. Maximum NO₃-N concentration was 26.6 mg L⁻¹. Nitrate ($${\text{NO}}_{3}^{ - }$$NO3-) was strongly correlated with Cl⁻ (r = 0.96), K⁺ (r = 0.68), $${\text{SO}}_{4}^{2 - }$$SO42- (r = 0.66), and Ca²⁺ (r = 0.59), respectively. The high correlations with Cl⁻ and K⁺ are related to livestock waste. Corresponding correlation with $${\text{SO}}_{4}^{2 - }$$SO42- is related to chemical fertilizers and Ca²⁺ to calcareous material to neutralize acidic soil. Both the first and second components in principal component analysis reflect ion dissolution from aquifer matrix during groundwater flow along the mountain side towards the lower reaches of the alluvial fan. Using hierarchical cluster analysis, chemical characteristics of groundwater were classified into four clusters. One cluster is strongly related to the nitrate contaminated groundwater and the other clusters reflect the origin of the major ions in the groundwater.