ベトナム北部Red River Deltaにおける河川水・井戸水の窒素・リン濃度

抄録

Nitrogen and phosphorus concentrations were measured in rivers (15 sites) and domestic wells (10 sites) in the Red River Delta, north of Vietnam in two dry and two rainy seasons between 2002 and 2004. The average concentrations of total nitrogen (TN) including particulate forms in the water samples from the rivers were 2.1 mg N·l^-1 in the urban areas and 1.3 mg N·l^-1 in the rural areas, with little differences between the dry and rainy seasons. The stable nitrogen isotope ratios (δ¹⁵N) of nitrate were from 1.4 to 11.7 ‰ (5.9 ‰ on average) in the river water samples. Markedly high concentrations of TN (4.7 mg N·l^-1 in the dry season and 6.7 mg N·l^-1 in the rainy season) were observed in water samples from wells in the rural areas. In those water samples, considerably high ammonium N concentrations and high δ¹⁵N values (upto 31.6 ‰) were often observed. Particulate forms of P predominated in the river and well water samples. The average concentration of total phosphorus (TP) in the rainy season (0.24 mg P·l^-1 ) was twofold higher than that in the dry season (0.12 mg P·l^-1) in the river water samples. The average concentrations of TP and dissolved total phosphorus (DTP) in the well water samples were 0.09 and 0.02 mg P·l^-1, respectively, with little seasonal and areal differences. The relatively low nitrogen concentrations and intermediate δ¹⁵N values of the river water samples suggest that nitrogen pollution is not yet serious in rivers in the Red River Delta, despite the rapid increase in the use of chemical fertilizers and population there. On the other hand, contamination by wastes from human or livestock is suggested in domestic wells in the rural areas.