Excessive level of inorganic nitrogen in groundwater in the intensively farmed areas of northern Vietnam

Ammonium-nitrogen (N) and nitrate-N concentrations in groundwater were monitored at three farming villages in northern Vietnam during 2002 and 2006 with 6-month intervals, where 380 to 420 kg/ha of chemical fertilizer N have been applied annually. With reference to the δ15N value, the source of N in the groundwater was identified as chemical fertilizer in the two villages and as animal waste and chemical fertilizer in the one village where animal waste fertilizer had been applied additionally. Ammonium-N concentration was alarmingly higher than the drinking water standards, whereas the nitrate-N concentration was lower. These concentrations did not increase over time. The effect of the natural groundwater recharge diluting the concentrations was considered as a potential reason, and such trends are expected to continue. Spatial variation in the ammonium-N and nitrate-N concentrations was recognized as being due to differences in the applied amount and the source of fertilizer N, respectively.