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Ponar, a r t i f i c i a l s u b s t r a t e , and d r i f t s a m p l e s were c o l l e c t e d t o d e f i n e b e n t h i c i n v e r t e b r a t e community c o m p o s i t i o n i n Pool 26 a t t h e s u b s t r a t e s were dominated by Potamyia flava, Hydropsyche o r r i s , and Chironomidae. I l l i n o i s R i v e r a r t i f i c i a l s u b s t r a t e s were dominated by C y r n e l l u s f r a t e r n u s , Chironomidae, and O l i g o c h a e t a . A r t i f i c i a l s u b s t r a t e s i n t h e M i s s i s s i p p i R i v e r were c o l o n i z e d by t w i c e a s many t a x a and 23 t r m e s more o r g a n i s m s t h a n t h o s e i n t h e I l l i n o i s R i v e r . S i l t a t i o n appeared t o be a major f a c t o r l i m i t i n g a r t i f~c i a l s u b s t r a t e c o l o n i z a t i o n i n t h e I l l i n o l s R i v e r . Composit i o n o f t h e d r i f t was d e p t h dependent and g r e a t e s t numbers o f o r g a n i s m s and :axa were c o l l e c t e d i n t h e two upper s t r a t a ( 0 . 3 m and 3 m). M i s s i s s i p p i R i v e r d i e 1 d r i f t samples were dominated by Hydrops y c h i d a e , Ephemeridae, Ch i r o n o m i d a e , and Chaoborus, w h i l e I l l i n o i s R i v e r d r i f t was dominated by Chaoborus. G r e a t e s t d r i f t d e n s~t i e s g e n e r a l l y o c c u r r e d a t 2200 and 0100 h.

ABSTRACT/Since 1972, 10 benthic surveys and 9 static fish bioassays have been conducted to assess the impact of AVTEX Fibers, Inc. effluent on the lower South Fork of the Shenandoah River. AVTEX (formerly FMC Corp.) is a rayon and polyester fibers plant located in Front Royal, Virginia. Benthic samples were taken at four stations, one above and three below the plant discharges. Single surveys in 1972 and 1973 indicated a severe impact on the benthic community along the right side of the river, below the plant, as a result of the channelized effluent. Diversity values (~ were low (0-2.42) and numbers of taxa and organisms were reduced. A fish bioassay in 1973 indicated the effluent to be acutely toxic at the 34.5% level (mixture of effluent and river water). In early 1974, FMC Corp. constructed an activated sludge treatment system to reduce BOD and supplement the neutralization and chemical precipitation (zinc hydroxide and liquid-solid separation) facilities that had been used to treat waste waters since 1948. After the new equipment was placed in operation, the previously stressed area became more stable. In 1975 and 1976 the stressed area exhibited greater ) and an increased number of taxa and organisms. Bioassays showed the effluent to be acutely toxic to fish only once since 1973. The major improvements in the effluent were a 70% reduction in BOD 5 and a 60% reduction in the amount of zinc entering the river. Community conditions in 1977 indicated a partial remission of improvement, probably due to drought conditions.

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