Determining the source of nitrate contamination through monitoring of geochemistry and delta 15N value in spring of the Shimousa upland area ?[Japan]

The Shimousa upland area of Chiba prefecture, where both crops and livestock are raised, is one of the most intensively managed agricultural areas in Japan. The uplands are located near Tokyo, where large amounts of agricultural products are consumed. Recently, nitrate contamination in groundwater, caused by over application of chemical fertilizers and inappropriate disposal of manure, has become a matter of great concern to many residents as well as farmers in this area. In order to accurately characterize nitrate contamination in groundwater and determinate its sources, we carried out geochemical monitoring and analysis of deltasup(15)N for 5 years (1999-2003) at 11 springs that are discharged under the uplands. Our results are summarized below. 1. NO3sup(-)-N concentrations varied among the monitoring points and ranged from 0 to 48 mg/L. NO3sup(-)-N concentrations at six springs exceeded the environmental standard for water (10 mg/L). All the springs except Ohoji and Koseki showed small seasonal fluctuations in NO3sup(-)-N concentrations during the 5 year period. 2. NO3sup(-)-N concentrations in springs are affected by the land uses in their catchment areas: they are , generally highest in agricultural fields, less in mixed fields and forests, and still less in forests. This trend suggests that agricultural fields comprise the primary source of nitrate contamination in groundwater in the Shimousa uplands. 3. There is a high correlation at all springs between NO3sup(-)-N concentrations, electrical conductivity, and concentrations of Clsup(-), SO4sup(2-)-S, Casup(2+), Mgsup(2+), and Nasup(+). The relationship between NO3sup(-)-N and Mgsup(2+) concentrations seems to be particularly characteristic of the Shimousa uplands, where the Jyousou marine clay layer, which includes clay with high Mg ion exchange capacity, is present. Clsup(-) and Nasup(+) ions are generally found in manures, and SO4sup(2-)-S and Casup(2+) are generally present in chemical fertilizer. Therefore, agricultural activities are likely the source of nitrate contamination in the springs. 4. deltasup(15)N values in six springs with NO3sup(-)-N concentrations over 10 mg/L are classified into two groups: group 1 with deltasup(15)N values about 4 permille and group 2 with deltasup(15)N values about 9 permille. Nitrate in group 1 and group 2 waters may be derived from chemical fertilizers and from manure, respectively, on the basis of the categorizing of deltasup(15)N values by Kumazawa (2000). Average NO3sup(-)-N concentrations in three springs of group 2 were higher (34.5 mg/L) than those of group 1.