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Vol. 39 (1997) No. 1 p. 1-15



To investigate nitrate sources in the aquifer consisting of unconsolidated limestone, stable isotope of nitrogen (δ15N) was measured in shallow groundwater from wells and springs under three land-use settings. The typical land-use settings and corresponding average nitrate concentrations and δ15N values were : residental areas,7.1mg/L and 8.0‰; agricultural areas,7.2mg/L and 5.2‰; and natural, undeveloped areas,5.3mg/L and 5.6‰, respectively. Average 15N values of urine sewages and commercial inorganic fertilizers were 18.10‰ and 0.5‰, respectively. Values of δ15N greater than 7‰ in residental areas suggest that nitrogen from sewages exists in groundwater. Values of δ15N less than 7‰ in agricultural areas and a part of natural, undeveloped areas suggest that nitrogen in groundwater was supplied by commercial inorganic fertilizers. Low nitrate concentarations and low values of δ15N in residental areas were caused by dilution with groundwater influenced by commercial fertilizers flowing into residental areas. High δ15N values in agricultural areas suggested that denitrification occurred actively.

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