The impact of point source pollution on an urban river, the River Medlock, Greater Manchester

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Abstract

The River Medlock is a small (22km) urbanised river, and is one of the five main tributaries which forms part of the River Irwell Catchment in Greater Manchester, UK. The river has a legacy of pollution from the 18th century and continues to be affected by anthropogenic factors including point source pollution from waste water treatment works (WwTWs) and combined sewer overflows (CSOs). In order to investigate the impact of CSOs and the WwTWs on the river hydrology, water quality and ecology of the lower largely urbanised reach, data sets were obtained from the Environment Agency and from direct sampling of the river. Load estimations from continuous discharge records from the river’s gauging station plus estimates of sub-catchment area indicate the lower sites, classified as a “highly modified water body” and downstream of treatment works had had a higher load of discharge and phosphate-P linked to point sources and episodic discharges. Short term, continuous monitoring revealed that CSOs were active during high velocity, but increased concentrations of nutrients post high velocity indicate WwTW effects and possibly diffuse sources. This project reveals that the WwTW are a major source of phosphate-P and that the impact of CSOs on the river quality is short-lived and depends on the degree of precipitation. Other parameters indicate good water quality although the benthic macroinvertebrate community is degraded as a result of episodic increases in the quantity of water destabilising the river bed. Therefore, pollution from the CSOs, the WwTW and rapid changes in discharge are the reasons for the river’s failure to conform to EU's requirements of the Water Framework Directive.

Layman's Abstract

The River Medlock is a small (22km) urbanised river, and is one of the five main tributaries which forms part of the River Irwell Catchment in Greater Manchester, UK. The river has a legacy of pollution from the 18th century and continues to be affected by anthropogenic factors including point source pollution from waste water treatment works (WwTWs) and combined sewer overflows (CSOs).CSO is a collection system of pipes designed to collect surface runoff, domestic waste water and other waste water especially during rainfall. In order to investigate the impact of CSOs and the effect of the waste water treatment works on the river’s quality, datasets were obtained from the Environment Agency and from direct sampling of the river. The project showed that the wastewater treatment works are a major source of phosphate pollution and that the impact of combined sewer overflows on the river quality is short-lived. Therefore, the river cannot conform to the standard EU Water Framework Directive.

Keyword(s)

Combined sewer overflows, discharge, Water Framework Directive; River Medlock, urban water quality, waste water treatment works, benthic macroinvertebrates

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