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 distributions in proximity to wastewater treatment plants (WTP's) in the upper Clinch River and to test the tolerance of two mollusk species to monochloramine and unionized ammonia, the major toxicants in domestic effluent. River reaches up to 3.7 km downstream of WTP's were devoid of freshwater mussels (Unionidae), and tolerance to effluents varied among snails, sphaeriid clams, and the asian clam Corbicula fluminea. Residential communities with septic systems had no measurable impact on mollusk 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, respectively. Glochidia rank among the most sensitive invertebrates in their tolerance to these toxicants. The snail Pleurocera unciale unciale was moderately sensitive, with 96 h LC50 values of 0.252 mg l-1 monochloramine and 0.742 mg l-1 unionized ammonia. Monitoring of monochloramine and unionized ammonia concentrations 0.1 km below WTP outfalls indicated that monochloramine was the toxicant likely inhibiting mollusk recovery below these plants.