Source identification of nitrate on Cheju Island, South Korea

Abstract

Stable isotopes of nitrogen were used to identify sources of nitrate contamination to groundwater on Cheju, a subtropical island off the southernmost tip of the Korean peninsula. The δ¹⁵N ranges of potential animal waste and fertilizer N sources on the island were similar to those previously reported in the USA, Europe, and Africa. A total of 108 soil pore water samples were collected between January and October 1998 from fertilized soils below soybean fields and citrus groves. Low concentrations of nitrate below fertilized soybean fields indicated that it is highly unlikely that these fields contribute significant N to the groundwater problem on Cheju. The low average δ¹⁵N value of +1.9 ± 2.1‰ in pore-water nitrate and the even lower δ¹⁵N values after the fertilizer flush suggest that low levels of mineralized N are released from the bean roots or nodules. Located in the western region, the bean fields received less rainfall than the citrus groves in the southern region. Pore-water below citrus trees contained considerably higher nitrate levels, and the δ¹⁵N values became cyclically enriched after the initial fertilizer flush. Although denitrification can be expected in warm, wet soils high in organic-C content in the southern region of Cheju, it was not supported by pore-water or groundwater chemistry. Isotopic enrichment in soil pore-water is caused primarily by volatilization of ammonium-based fertilizers. Since isotopic fractionation in the soils did not exceed +4‰, source identification was possible. The dominant sources of nitrate contamination to Cheju groundwater were identified as commercial N-fertilizer applications to citrus, and, in the Seogwipo municipality, human or animal wastes.