Nitrate Concentrations and δ^<15>N Values of Groundwater in the Miyakojima Island

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  • 宮古島の地下水中の硝酸態窒素濃度とδ^<15>N値
  • ミヤコジマ ノ チカスイチュウ ノ ショウサンタイ チッソ ノウド ト デルタ

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Nitrate concentration and δ^<15>N value in the groundwater in Miyakojima islands, Okinawa, were measured during 1992-1993. Waters from the shallow and the deep wells at ten separate sites were sampled. Mineral contents and natural nitrogen isotope abundance (δ^<15>N) were analyzed using liquid chromatography and the mass spectrometry (Finnigan MAT 252). Except for the waters which were directly influenced by sea water invasion, most of the groundwater showed small variations among their mineral contents and δ^<15>N values. Average nitrate nitrogen concentrations were 1.4-l1.5 mg L^<-1> and average δ^<15>N values were+4.3-+9.7‰. From the nitrate concentration and δ^<15>N value observed, the type of groundwater could be categorized into four groups, such as high δ^<15>N and high nitrate, high δ^<15>N and medium nitrate, low δ^<15>N and medium nitrate, and low δ^<15>N and low nitrate, reflecting the main source of nitrate contamination, such as animal and domestic waste, animal waste and soil organic matter, soil organic matter and chemical fertilizer, and chemical fertilizer, respectively. It was discussed that the lowest δ^<15>N value was higher than the δ^<15>N value of the chemical fertilizers used in this islands ( -3.9- -1.4‰). Therefore, considerable amounts of nitrogen must be lost by ammonium evaporation or denitrification after fertilization.

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