EFFECTS OF BIOLOGICALLY TREATED BLEACHED-KRAFT MILL EFFLUENT BEFORE AND AFTER MILL CONVERSION TO INCREASED CHLORINE DIOXIDE SUBSTITUTION - RESULTS OF AN EXPERIMENTAL STREAMS STUDY

Large-scale outdoor experimental streams were used to determine the ef fects of effluent on stream biota before and after conversion of a ble ached-kraft mill to increased chlorine dioxide substitution. Data for long-term (6 to 9 months) exposures at 1.5 and 5.0% v/v effluent in pe riods before and after conversion were compared for effluent quality, rainbow trout (Oncorhynchus mykiss) production, growth, survival, and histopathology, and periphyton and macroinvertebrate community charact eristics. Data for cytochrome P4501A and ethoxyresorufin O-deethylase (EROD) biomarkers in trout were obtained in an additional 1.5-month po stconversion exposure. Postconversion effluent was characterized by re duced chlorinated organic constituents and adsorbable organic halides. Rainbow trout in both the pre- and postconversion periods indicated a pattern of greater size, fewer numbers, and equal or greater producti on in the effluent-exposed populations compared to unexposed controls. Fish health measurements, including histopathology, hematology, and l iver somatic indices, did not indicate significant negative effects, c ontrasting with the biomarkers, where there was a significant, positiv e dose-response relationship with similar effluent exposures. The lack of major differences in fish and other stream biota responses followi ng the reductions in effluent chlorinated organic compounds raises que stions about the ecological relevance of these compounds at the concen trations tested.