Occurrence of Nitrite in the Lough Neagh River System

The Lough Neagh catchment area covers about one‐third of the land area of Northern Ireland. This report documents NO 2 − concentrations in the major rivers entering Lough Neagh, which are frequently in the range of 100 to 150 μg N L −1 and exceed the European Community (EC) water quality guide values. The contribution of land drainage to NO 2 − loads carried by these rivers was estimated to be about 40%. The remaining 60% of NO 2 − appears to originate from N transformations at the sediment‐water interface of the river system. The available evidence suggests that NH + 4 originating from agricultural pollution provides the N substrate for nitrification by Nitrosomonas to NO 2 − . What is anomalous is why this NO 2 − is not further oxidized rapidly to NO − 3 by Nitrobacter . A possible mechanism is that the Nitrobacter is retarded by the presence of free ammonia concentrations that can be predicted to be present in the range of 65 to 76 μg N L −1 .