EFFECTS OF WASTE-WATER TREATMENT-PLANT EFFLUENTS ON FRESH-WATER MOLLUSKS IN THE UPPER CLINCH RIVER, VIRGINIA, USA

Field and laboratory studies were conducted to determine mollusk distr ibutions in proximity to wastewater treatment plants (WTP's) in the up per Clinch River and to test the tolerance of two mollusk species to m onochloramine and unionized ammonia, the major toxicants in domestic e ffluent. River reaches up to 3.7 km downstream of WTP's were devoid of freshwater mussels (Unionidae), and tolerance to effluents varied amo ng snails, sphaeriid clams, and the asian clam Corbicula fluminea. Res idential communities with septic systems had no measurable impact on m ollusk assemblages downstream. Laboratory bioassays with glochidia of Villosa iris yielded the following results: 24 h EC50 and LC50 values of 0.042 mg l-1 and 0.084 mg l-1 monochloramine, respectively; and 24 h EC50 and LC50 of 0.237 mg l-1 and 0.284 mg l-1 unionized ammonia, re spectively. Glochidia rank among the most sensitive invertebrates in t heir tolerance to these toxicants. The snail Pleurocera unciale uncial e was moderately sensitive, with 96 h LC50 values of 0.252 mg l-1 mono chloramine and 0.742 mg l-1 unionized ammonia. Monitoring of monochlor amine and unionized ammonia concentrations 0.1 km below WTP outfalls i ndicated that monochloramine was the toxicant likely inhibiting mollus k recovery below these plants.