The benthic macroinvertebrate fauna and the physico-chemical properties of the sediments were described from 66 sites in the vicinities of the Rhymney Valley, Ystradyfodwg and Pontypridd (Y & P) and Cardiff East outfalls discharging to the Severn Estuary. Except in an area close to the Y & P outfall, where redox potential was relatively low (<100 mV at 4 cm depth), the discharges had no perceptible effect on local sediment characteristics. Carbon concentrations were similar (50 mg g-1) to those found generally in this area of the Severn Estuary. A total of 26 taxa were identified and total invertebrate density ranged from 110 to 46,000 m-2. Except for an area close to the Y & P outfall, there was no evidence that the effulent discharges affected the distribution of the benthic macroinvertebrate assemblages around the outfalls were similar to those found generally in this area of the Severn Estuary. Classification analysis suggested that shore height was the environmental feature most closely associated with the distribution of the fauna.
