Development of guidelines for ammonia in estuarine and marine water systems

The water quality guideline trigger value for ammonia in estuarine and marine waters has been revised with the addition of 38 new results to the data set of 21 used in earlier guideline derivations. Using species sensitivity distributions, a new trigger value of 460 microg total NH(3)-N/L was derived for slightly to moderately disturbed systems (95% protection concentration, PC95), with a value of 160 microg total NH(3)-N/L applying to waters of high conservation value (PC99). For sediment pore waters, a guideline trigger value of 3.9 mg total NH(3)-N/L, derived from the 80th percentile of background data from Sydney Harbour, is recommended. This value is likely to be exceeded in degraded sediments subject to dredging; however, ocean disposal of such sediments results in rapid decreases in porewater ammonia and a guideline trigger value for dissolved ammonia during disposal of dredged sediments of 1550 microg total NH(3)-N/L is proposed.