I OBJECTIVES

To assess the effects on the environment of discharge of sewage to an estuary via a correctly designed sea outfall by a study at the Pyewipe outfall on the Humber estuary.

II REASONS

The use of the sea for sewage disposal has come under increasing attack from the public and from other countries bordering the North Sea. There is a need, therefore, to understand and quantify the environmental impact of sewage discharges from modern long sea outfalls. This report describes the study undertaken at the Pyewipe outfall, on the Humber estuary, in July 1988 and is presented as a supplementary report to the major case studies at two coastal outfalls, Weymouth and Tenby.

III CONCLUSIONS

1. The discharges from the Pyewipe outfall were having only minimal impact on the sediments immediately around the outfall.
   
2. There was an increase in the concentration of coprostanol and organic content, and in numbers of E. coli, within the sediment in the area immediately around the Pyewipe outfall: the greatest concentrations and numbers being found within 25 m of the outfall. There was no detectable spatial impact on the sediment particle size distribution around the outfall.
   
3. The concentrations of metals in the sediments within 100 m of the outfall were within the range reported for the Humber estuary. For some metals concentration increased with distance from the outfall (over the 100 m of the transect) particularly in the 110" direction. Microtox toxicity showed a similar trend along the 110" transect but no significant correlation could be detected between Microtox data and metal concentrations, or between these and any other physical, chemical or biological characteristics of the sediment.
   
4. The macrobenthic community within 100 m of the outfall was found to be impoverished and typical of that found in this part of the estuary. However, there was a relative increase in numbers of macrobenthic animals (as exemplified by the oligochaete, T ubificoides swirencoides) along the 100 m transects towards the outfall, with the highest numbers occurring within 5 m of the outfall. This may be an indication of organic enrichment close to the outfall but this was not confirmed by the spatial pattern of organic content of the sediments.
   
5. Results of a bivalve larvae bioassay on the Pyewipe sewage suggested that, at the dilutions expected in the receiving waters, the sewage discharges from the outfall were unlikely to have any acute toxicological impact.

IV RESUME OF CONTENTS

Previous research undertaken at two modern coastal long sea outfalls had indicated that there was only minimal environmental impact and this was confined to the area immediately around the outfall - in one case within 50 m. The study at the Pyewipe outfall was, therefore, based on sampling by divers along two 100 m transects that followed the dominant tidal directions. Sediment samples were taken and analysed for E. coli, coprostanol, organic content, particle size distribution, heavy metals and macrobenthic community structure. In addition, Microtox bioassays were undertaken on organic extracts from the sediments and the acute toxicity of the Pyewipe sewage was assessed using an oyster larval development bioassay.