Report No FR0095
EFFECTS OF SEA OUTFALLS ON THE ENVIRONMENT -
SUPPLEMENTARY REPORT ON THE PYEWIPE OUTFALL
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.
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.
- The discharges from the Pyewipe outfall were having only minimal impact
on the sediments immediately around the outfall.
- 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.
- 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.
- 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.
- 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.
Copies of the report are available from FWR, price £15.00, less 20% to FWR Members.