Ammonia and toxicity criteria in polluted Onondaga Lake, New York

Temporal patterns of total ammonia (T-NH3) in the upper layers of an ammonia-polluted, saline, hypereutrophic urban lake, Onondaga Lake, New York, and its status with respect to ammonia toxicity criteria are documented and evaluated for the spring to fall interval over 10 years (1989 to 1998). Implications of revisions (n = 5) in the criteria (1985 to 1999) and the omission of ionic strength effects in the most recent revisions for this saline system are considered. The implications of diurnal variations in pH for application of both chronic and acute criteria are illustrated based on 14 days of hourly pH data. Concentrations of T-NH3 in the upper layers of this lake were extremely high because of discharges received from a 3.5 m(3)/s (80 mgd) wastewater treatment plant; the average concentration during the April to June interval for the 10 years was 2.3 mg/L. Substantial interannual and seasonal variations were observed in the T-NH3 pool of the lake. April concentrations were largely regulated by the dilution provided by antecedent (i.e., December to March) tributary flow. Revisions of the chronic criteria have been progressively less stringent for low ionic strength conditions. However, under the saline conditions of this lake, the most recent (1998 and 1999) revisions can be more stringent than the earlier criteria. The 1984 chronic criterion (published in 1985) was exceeded by a wide margin annually (average factor of 2.2 for April to June) and for durations greater than 75 days. Widely different diurnal patterns in pH and the occurrence of an exceedance of the 1998 acute criterion during a pH maximum are documented. These diurnal variations are an important factor in the design of programs to assess status and in application of criteria for productive systems.