THAMES ESTUARY - ENVIRONMENTAL STATUS WITH RESPECT TO SLUDGE DISPOSAL - FINAL REPORT
Report No FR0074

T ap Rheinallt, S C Nixon, I Codling and J Orr

October 1990

SUMMARY

I OBJECTIVES

1. To assess the dispersion of sludge after discharge and identify any areas where sludge material accumulates;

2. To determine the effect of sludge disposal on the concentration of contaminants in the sediments;

3. To investigate the diversity and nature of benthic communities in order to resolve the difference in results obtained by TW and MAFF from earlier studies, and to assess MAFF's statement (Norton et al 1981), that "current rates of dumping have somewhat exceeded the dispersive capacity of the area... these have placed the natural fauna under stress and promoted the growth of pollution indicator species... suggest that further increases in dumping may lead to more widespread, significant and readily identifiable effects";

4. To assess the effects of sludge disposal on sub-lethal stress in organisms and disease prevalence in fish populations within the study area;

5. To define the area of impact resulting from sludge disposal and develop a monitoring strategy for the future management of the disposal site.

II REASONS

Concerns among certain EEC member states and environmental pressure groups over the effects of sewage sludge disposal to sea were threatening to curtail or prevent this activity. The cost implications of this for the UK water industry are considerable. A case study has been undertaken into the effects of sludge disposal at the UK's largest discharge site - the Barrow Deep region of the outer Thames estuary. This study will provide data to enable decisions over the future of marine disposal of sewage sludge to be made on a scientific basis.

III CONCLUSIONS

• The Barrow Deep disposal site is highly dispersive: tracer studies have shown sludge to be widely dispersed over the northern part of the estuary within days of release.

• Traditional techniques of impact assessment (such as physicochemical measurements) have shown only limited effects of sludge disposal. Sensitive new field and laboratory bioassay techniques have detected effects which may be useful for the future monitoring of the site.

• Studies of the prevalence of disease in fish populations from the Thames estuary have not shown a direct link with sludge disposal.

IV RECOMMENDATIONS

The effects of sewage sludge disposal at dispersive marine sites are subtle and difficult to detect and interpret by traditional means. The sensitivity of new bioassay techniques offers promise for future monitoring of marine disposal operations. Further work should be undertaken to develop these tests so that observed bioassay responses can be related to likely ecosystem effects.

The occurrence of disease in fish populations is affected by many natural factors, which make the detection of pollution effects difficult. Further research is required to obtain a clearer understanding of the ways in which natural and anthropogenic factors influence fish health.

V RESUME OF CONTENTS

This report describes work undertaken on the Thames estuary sludge disposal project over a four year period (1985-89). The main elements of the project were:

• A study of all inputs of contaminants to the tidal Thames.

• Dispersion studies to quantify the distribution of sludge solids.

• Studies of sediment quality with respect to the distribution of metals and organic contaminants.

• Assessment of sublethal stress and contamination levels in mussels after set periods of exposure at key sites in the estuary.

• Statistical analysis of ecological survey data collected by WRc, Thames Water and MAFF.

• A study of the prevalence of external and internal abnormalities in fish.

• Laboratory-based bioassay tests on sewage sludge and sediments.

• Monitoring of indigenous species.

Copies of the Report are available from FWR, price £35.00 less 20% to FWR Members