Land and River Sediment Quality
Overall, despite extensive exploitation for public and industrial supply and waste disposal, the River Wandle has enjoyed a long historic reputation for the unusual clarity and quality of its chalk stream water. However, by 1905 the River Wandle in response to new sewage treatment works which provided continuous source of nutrients and chemicals that have not removed by sewage treatment processes, along with abstraction, road run-off and general urbanisation of the catchment has placed considerable pressure on the health of the River Wandle.
There is potential for made ground, containing a range of contaminants, which if disturbed could re-mobilise during flood events and enter the River Wandle, although based on the sediment samples recovered from the bed of the Carshalton and Croydon branches, the chemical determinants overall do not exceed the Generic Assessment Criteria (GAC) limit for Public Open Space, in particular Priority Hazardous Substances, including heavy metals; and the majority Polycyclic Aromatic Hydrocarbons (PAHs), with the exception of one of the PAHs, which does fail – benzo(b)fluoranthene. As such, there is potential that the sediment if removed from the Shepley Mill study area could be re-used for landscaping or habitat restoration work on site, although a risk assessment may be required regarding the re-use of benzo(b)fluoranthene.
There is potential for made ground, containing a range of contaminants, which if disturbed could re-mobilise during flood events and enter the River Wandle, although based on the sediment samples recovered from the bed of the Carshalton and Croydon branches, the chemical determinants overall do not exceed the Generic Assessment Criteria (GAC) limit for Public Open Space, in particular Priority Hazardous Substances, including heavy metals; and the majority Polycyclic Aromatic Hydrocarbons (PAHs), with the exception of one of the PAHs, which does fail – benzo(b)fluoranthene. As such, there is potential that the sediment if removed from the Shepley Mill study area could be re-used for landscaping or habitat restoration work on site, although a risk assessment may be required regarding the re-use of benzo(b)fluoranthene.
However, based on the Canadian Sediment Quality Guidelines for the Protection of Aquatic Life, there are some metals and hydrocarbons which are within a range which could have adverse effects to aquatic life. Thus, any direct habitat restoration works which may involve the substrate of the Carshalton and Croydon branches of the River Wandle, will need careful consideration regarding potential remobilisation of the contaminants.