INFLUENCE OF PH ON THE TOXICITY OF AMMONIA TO CHIRONOMUS-TENTANS AND LUMBRICULUS-VARIEGATUS

The U.S. Environmental Protection Agency water-quality criteria establ ish permissible levels of ammonia in the nation's fresh waters. These criteria are based on accumulated research suggesting that, for most a quatic species (primarily fishes), the toxicity of un-ionized ammonia predominates over that of the ammonium ion. The development of a sedim ent-quality criterion for ammonia requires evaluation of the relative toxicity of the two ammonia forms to benthic and epibenthic macroinver tebrates to determine whether the water-quality toxicity model can be applied to sediments. Flow-through ammonia toxicity tests were conduct ed over 10 d with the oligochaete Lumbriculus variegatus and the larva l midge Chironomus tentans at four pH values (6.3, 7.2, 7.8, and 8.6) using a unique pH control system. Total ammonia was more toxic at elev ated than at low pH to both species, suggesting that un-ionized ammoni a (more prevalent at high pH) is important in determining the toxicity of ammonia to these two species. Hardness or alkalinity differences i n the range of 30 to 200 mg/kg (as CaCO3) did not appear to affect the toxicity of ammonia to the two species in separate 4-d tests. Based o n results of the 10-d tests, the joint toxicity/pH model that establis hes the water-quality criterion value for ammonia appears to be suffic iently protective of L. variegatus and C. tentans, by factors of at le ast 3 and 10, respectively. However, this study did not address potent ial differences in exposure of benthic organisms to ammonia in sedimen ts vs. that in the water column; nor were the chronic effects of ammon ia on these species measured in this study.