This research aims to identify and map mercury contamination and pollutants known as persistent organic pollutants (POPs) within sediment in the Mersey estuary and assess, where possible, where the pollutants have come from. These pollutants are found both at the surface and at depth in the sediment.
The Mersey estuary (north-west England, UK) has a heavily industrialised drainage basin, which dates back to the 18th century. It is approximately 5000 km2 and is widely regarded as one of the most polluted estuaries in Europe.
High concentrations of mercury (>2 mg/kg) in these sediments can be linked in part to the chemical factories in the Widnes–Runcorn area. In particular, the alkali, bleaching and detergent industries developed in this area in the mid-19th century used the Castner-Kellner process for large-scale sodium hydroxide and bleach production, which uses mercury.
This pollution, plus that from other industries and domestic coal-burning, has released many tonnes of mercury into the estuary over several decades. In addition to this, the sediments in the Mersey estuary are also contaminated with POPs from the dockyards, shipping, chemical works, oil refineries and sewage works situated in close proximity to the Mersey, its tributaries and the Manchester Ship Canal.
As sediment moves around the estuary, it accumulates in sediment sinks, which can also accommodate concentrations of various contaminants. These toxic substances can be released having a harmful impact on the ecosystem and biodiversity.
Four different methods of sampling were used: manual corer, Marlow corer, Mackereth corer and Day grab. Sampling sites in the middle estuary were accessed by hovercraft, using GPS to accurately locate each position. Shallow cores were collected by manually driving plastic tubes, fitted with a stainless steel basket core catcher at the base, into the exposed sediment.
Sediment cores and grab samples were then frozen and stored in the dark to avoid any chemical change and then transported to the laboratory. Core samples were defrosted and sampled. Grab samples were homogenised and prepared in an identical manner to the core samples. A total of 203 sediment samples were subsequently analysed for mercury content. Eight shallow cores were collected for POPs analysis.
This survey showed that mercury in sediments remain elevated (~2 mg/kg) throughout much of the Mersey estuary and that mercury is associated with the organic matter. Surface sediments showed a change from high mercury concentrations upstream and lower values in the outer estuary, in line with distance from major chemical works and the adjacent eroding salt marshes.
Mercury in surface sediments collected from the inner and middle Mersey estuary had higher mercury concentrations compared to the outer estuary. The distribution pattern can be explained in part by the presence of fine-grained muds along the margins of the upper estuary and fine to medium-grained sands, as well as gravelly sands, towards the outer estuary.
The upper metre of salt marsh sediments was most contaminated with mercury (~4 mg/kg) and represents a potential threat to near surface dwelling aquatic organisms existing in these environments. In the cores, a decrease in mercury content was observed at the surface indicating a possible recent decline in mercury contamination within the Mersey estuary since the 1980s.
The survey of surface sediments and sediment cores collected in the Mersey estuary has revealed mean PAH concentrations of 2316 µg/kg and mean PCB levels of 123 µg/kg, which reflects the industrial and urban history of the area.
In comparison to other UK estuaries, the sediments in the Mersey are less contaminated in PAHs than the Tyne and Tees estuaries but have higher concentrations of PCBs than the Humber or Thames estuaries.
This study has also shown that sediments of the inner Mersey estuary are approximately 30 times more contaminated in PCBs than the Liverpool Bay area into which the Mersey river discharges. However, no clear pattern of declining PCB concentration from inner to mid to outer estuary was observed. It is suggested that this is the result of sedimentary reworking, tidal scouring and the localised point source effects of industrial discharges.
Mercury (Hg) is one of the most important heavy metals in estuarine and coastal sediments. This is due to its toxic effect on marine invertebrates and tendency for the methyl-Hg to bio-accumulate up trophic levels, where it can enter humans via the ingestion of fish and shellfish.
Persistent organic pollutants are a suite of organic compounds which do not degrade. They can remain in the environment (e.g. the Mersey estuary) and can be transported long distances. They can be present in human and animal tissue and are thought to have impacts on human health.
PAHs can come from biological, geological or combustion sources such as wood, coal or oil burning. They are also found in traffic fumes and a high density of riverside industries, e.g. chemical works, power stations, sewage works, docks, boat dismantlers, oil refineries, paper works etc.
PCBs are man-made compounds, which were widely used as plasticisers and heat-transfer fluids in the manufacturer of such products as transformers. Their use is now banned in the UK due to their high toxicity if released into the environment, but residues persist and significant amounts continue to enter estuaries, such as the Mersey, through rivers, ocean currents and the atmosphere.
Cave, M.A., Wragg. J., Harrison, I., Vane, C.H., Van de Wiele, T., Nathanail, P., Ashmore, M. A., Thomas, R., Robinson, J., Daly, P. 2010. Comparison of a batch mode and dynamic bioaccessibility tests for PAHs in soil samples. Environmental Science and Technology, 44, 2654-2660.
Harrison I. and Vane C.H. 2010 Attenuation of TNT in seawater microcosms. Water Science & Technology.
Contact Dr Christopher Vane for more information.