A preliminary monitoring of the NO₃-N concentration in the surface water in the paddy-cultivated area of the central Red River Delta, Viet Nam

The NO₃-N concentration and pH in the surface water in the paddy-cultivated areas of the central Red River Delta, Viet Nam were monitored at 24 sites of 5 districts in March 2002 after the spring rice was transplanted. In the areas, fertilizer N of 100-150 kg/ha has been conventionally applied in each of the three cultivations in a year. The measured NO₃-N concentration was in the range of 0.11-3.03 mg/L and was below the permissible level of 11.3 mg/L for drinking water, but some samples exceeded the standard for pH (8.5). The districts could be separated from one another according to the combination of the NO₃-N and pH levels, but this separation was not due to the comparative land elevation. Relevant factors such as the soil pH, the amount and time of N application in every cultivation, and the NH₄ level are to be monitored in the future study. The NO₃-N concentration was negatively correlated with the pH level in some districts, possibly impacted by nitrification. No remarkable difference was observed in both the NO₃-N and pH levels between the ponding water of the paddy field and the water of the neighboring irrigation/drainage canal.