Changes in Assemblages of Infaunal Organisms Around Wastewater Outfalls in Santa Monica Bay, California

The City of Los Angeles' Hyperion Treatment Plant presently operates two submarine outfalls: the 1-Mile outfall was used until 1959, but now is used only during emergencies; the plant's main outfall, the 5 -Mile, presently discharges about 1,211 X lO^ld"' of mixed primary and secondary effluent at a water depth of 60 m. A third outfall, the now defunct 7-Mile sludge line, operated from 1957- 87, discharging about 15 x 10^ 1 d"' of digested sludge into 100 m of water at the head of the Santa Monica Canyon. Solids discharged from Hyperion into the Bay have been greatly reduced since the mid-1980's; sludge disposal was termi- nated, and levels of suspended solids in the 5 -Mile effluent diminished from peak levels of 155 mg/L in 1985 to historically low levels averaging around 30 m.g/L in recent years. Analyses often years of semiannual benthic monitoring data demonstrated that assemblages of benthic organisms near the outfalls have shifted in composition from a polluted condition to one more typical of cleaner reference areas. This trend correlated with reductions of solids discharged from the plant. The infaunal assemblage in the old sludge field has changed from a highly degraded to an early transitional assemblage in terms of species composition and abundance, while those near the 5 -Mile outfall have changed from polluted to a more natural assemblage. Continued recovery is expected, but at a slower rate, if the plant maintains its present effluent quality as it moves towards full secondary treatment in 1998.