Pollution of surface and groundwater by inorganic nitrogen (N) was assessed in farm villages of northern Vietnam. The pH and ammonium-N and nitrate-N concentrations were monitored at three communes near Ha Noi in the Red River Delta in March and September 2003, following monitoring in October 2002. In each monitoring time, ammonium-N concentrations exceeded the Vietnamese water standards appli-cable to surface and groundwater, whereas the nitrate-N concentrations were relatively low and below the corresponding standards. Neither spatial nor temporal variation was found in the ammonium-N and nitrate-N concentrations of the surface water. A temporal increase in the ammonium-N concentration was statistically confirmed in the groundwater. Intensively applied fertilizer-N and disposed animal/human wastes were thought to be sources of ammonium-N. Ammonium-N originating from those sources was seen to accumulate steadily in the groundwater through percolation from the surface water and surrounding land. A spatial variation of the nitrate-N concentration was observed in the groundwater. The nitrate-N concentration in the groundwater was particularly low in communes located in lowland areas with alluvial soils. The suppression of nitrification or promotion of denitrification was considered to be responsible for the low nitrate-N concentration. A spatial variation was also found in the groundwater pH. Therefore, variation of the pH may be linked to variation of the nitrate-N concentration in the groundwater.
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