Diffuse urban pollution of surface and ground waters is a growing concern in many cities and towns. Traffic-derived pollutants such as salts, heavy metals and polycyclic aromatic hydrocarbons (PAHs) may wash off road surfaces in soluble or particulate forms which later drain through soils and drainage systems into surface waters and groundwater. In Brighton, about 98% of drinking water supply comes from groundwater (derived from the Brighton Chalk block). In common with many groundwater sources, the Chalk aquifer has been relatively extensively monitored and assessed for diffuse rural contaminants such as nitrate, but knowledge on the extent of contamination from road run-off is currently lacking.
This project examines the transfer of traffic-derived contaminants from the road surface to the Chalk aquifer, via urban drainage systems. Dissolved phase and sediment bound samples have been collected from road drainage systems (gully pots) and analysed for heavy metals and organic contaminants. A transect of five Chalk boreholes have also been sampled on a monthly basis for a period of 18 months, and groundwater samples analysed to examine the concentrations of key, mainly road run-off derived, hydrocarbon and heavy metal contaminants in groundwater across the Brighton area.
Results show the presence of heavy metals (Zn, Cu & Pb) in gully pot sediments with concentrations observed above guideline values. Their spatial distribution shows a complex trend. However, hot spots emerge around busy roads and junctions. The sediment bound Pb within the gully pots shows Pb-isotope ratios (206Pb/207Pb 1.098 – 1.114) closer to the historical isotope ratios of Pb from gasoline sources (206Pb/Pb2071.079), rather than from other anthropogenic or background sources. Leachability studies reveal that only Zn and Cu have the potential for remobilisation into the environment with the other elements trapped within immobile phases. Polycyclic aromatic hydrocarbons (PAH) in the dissolved phase gully pot samples collected appear mobile. The PAHs within both phases share a
hotspot along a major road junction. Furthermore, traces of organics have been recorded in the urban borehole monitoring site at Preston Park. Electrical conductivity changes in groundwater have also been used to assess local changes in ionic strength which may be associated with road-derived contaminants. There is a clear seasonal variation in the tracers identified in groundwater, with higher concentrations observed during summer (low rainfall) than winter (high rainfall). Results obtained indicate transfer of urban diffuse pollutants to groundwater. However this depends on a number of factors such as; redox conditions for heavy metals to mobilise, levels of heavy metals and PAHs released in the environment, and amount of rainfall available to transport the contaminants which depends upon season. A conceptual model has been developed to explain the processes involved.
|Date of Award
|Martin Smith (Supervisor) & Andrew Cundy (Supervisor)