The pollution history of two urban lakes in Coventry, UK
Human society has influenced the environment for at least the last 15000 years but, since the Industrial Revolution, the resultant environmental impacts have been widely used in many studies for reconstructing this impact over medium timescales (tens to hundreds of years). Few long-term studies of hydrological change exist and sediments are useful as surrogates for direct monitoring since they are sensitive to change within the catchment. This study uses the properties of urban lake sediments in order to reconstruct environment pollution history. The two principal objectives of this study were the reconstruction of historical atmospheric, point source and diffuse heavy metal pollution in an urban environment and the evaluation of the lake-sediment record as a source of proxy hydrological data over the last 100-150 years. A paired lake-catchment study was undertaken by comparing the records contained in a closed and an open lake. The closed lake (Swanswell Pool) is situated in the centre of the city of Coventry where the main source of pollution is atmospheric. This site provides a contrast to an open basin (Wyken Pool) with a multi-source catchment in addition to atmospheric influx. Trends in urban lake sediment cores indicate increasing heavy metal concentrations upcore, with cultural enrichment factors for individual heavy metals of between 55.4 and 2.6. Storage of heavy metals in the catchment of the closed basin were significant, although it was found that up to 85% of the Zn and 90% of the Pb were actually stored in the lake sediments. Catchment sources contributed up to 5 times more than the atmosphere in the Wyken Slough catchment. Heavy metals budgets were calculated, and these showed that loadings of metals have increased by up to 7.5 times between 1850 and the present day. Sequential digestion of the lake sediments at both sites showed that the important fractions containing heavy metals were Fe and Mn oxides and organic matter. The heavy metals associated with these fractions could be remobilised with changing environmental conditions, but an analysis of contemporary water quality indicated that, at present, suitable Eh and pH conditions for remobilisation did not occur. It was concluded that these urban lakes do preserve the heavy metals record and can provide surrogate data on medium-term environmental change. However, the complex mixture of materials associated with urban sedimentation resulted in a lack of correlation between heavy metals and mineral magnetic properties in either lake, and in the catchment of Wyken Slough. Hence mineral magnetic properties of sediments in urban catchments do not appear to be a suitable surrogate for heavy metals analysis. Urban lakes appear to provide a much-neglected opportunity for palaeolimnological reconstruction over a period when little directly monitored data exists.