At the end of 1995 work was completed on a low level intersecting foul sewer for the upper Pymme' s Brook catchment (north London), known as the East Barnet foul water sewerage scheme. Commissioned by Thames Water Utilities Limited (TWUL), it was intended that this would both resolve flooding problems in the area, and address environmental concerns raised by the Environment Agency (EA). The key element of the scheme was the removal of seven combined sewer overflows (CSOs) that the EA had defined as 'unsatisfactory'. Consequently, the present study assesses the scheme's impact on the brook's environmental status, and considers the results in light of the pollutant generation, transport and dispersal properties of the catchment. The pollutant generation, transport and dispersal processes operating in the catchment were explored at a range of spatial and temporal scales, in order to assess the contributions made by a range of urban non-point sources of pollution (CSOs, misconnections and urban runofl), under differing weather conditions, and to determine the way in which they interacted to control water quality. Considerable temporal and spatial variability was identified in the quality of both the brook, and the effluents discharging to it. A first flush of contamination was noted for both solid and dissolved pollutants, during many of the studied storm events; although the studied determinants (pH, conductivity, suspended solids, biochemical oxygen demand, dissolved oxygen, ammonia-N, chloride and E. coli count) responded to storm driven processes in different ways. A holistic approach was adopted to define the environmental status of the studied watercourse; incorporating its benthic macro-invertebrate community structure (BMWP score and ASPT), bacteriology (E. coli count) and water chemistry. Temporal change was then identified in each data set by performing an ANOVA between years, and between the periods prior to, during and after the scheme's construction. The scheme's impact on catchment hydrology was also explored by assessing temporal changes in the catchment's unit hydrograph parameters, using both linear regression for, and ANOVA between the periods related to the scheme's construction. In addition, regression analysis was used to explore the relationship between climatically induced hydrological change and both BMWP score and water column E. coli count, in which both variables were related to the mean discharge recorded at the EA's Silver Street gauging station on a range of temporal scales. It was concluded that climatically driven hydrological change was the major factor in determining the environmental status ofPymme's Brook, whereas the East Barnet foul water sewerage scheme produced only a limited improvement. This was because as well as removing several pollutant sources, the scheme had a hydrological effect that negated some of the expected improvements in water qUality. In addition, the large number and variety of pollutant sources operating in the catchment meant that a scheme designed to address just one element of the problem was unlikely to have a wtifonnly positive effect. Consequently, the magnitude of the temporal changes observed varied between the eight sites sampled in a way that was determined by a combination of the sensitivity of the benthic macro-invertebrate community inhabiting a site, the contamination processes prevalent within its local catchment area and its location within the catchment as a whole. Methodological recommendations for the future are made.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:323428 |
| Date | January 2000 |
| Creators | Green, Andrew |
| Publisher | University of Hertfordshire |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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