Mercury contamination of sediments in the Mersey Estuary

Figure 1. Manual coring of sediment in the Mersey estuary

Recent investigations on the distribution of mercury in estuarine sediments from the Mersey estuary of north west England revealed evidence for contamination from past industrial activities in the heavily urbanised catchment. Concentrations of mercury above 2 mg/kg were found in several samples. The upper metre of sediment in core materials from salt marsh deposits was found to be most contaminated (ca. 4 mg/kg) and represents a potential threat to aquatic organisms.

Environmental significance of mercury contamination

Mercury is highly toxic to organisms and accumulation of methyl mercury up the food chain means the potential exposure of humans via consumption of fish and shellfish. The Mersey estuary is an industrialised and urbanised drainage basin (area some 5 000 km2) with a high proportion of chemical factories. The alkali, bleaching and detergents industries developed particularly in the Widnes–Runcorn area during the 19th century. These industries used the Castner-Kellner process for production of sodium hydroxide and bleach, a process which involved use of a flowing liquid mercury cathode in an electrolytic reaction cell. Such processes could have released contaminants including mercury to the local environment.

Core sampling

Sampling of core sediments in the estuary took place over 30 months during 2000—2002. Sampling methods included use of a manual corer, Marlow corer, Mackereth corer and day grab samples. In all, 203 sediment samples were analysed for mercury by atomic fluorescence spectrometry following pretreatment with aqua regia. Sediment loss on ignition was also measured as a proxy for organic matter concentration.

Mercury distribution in the Mersey sediments

The average mercury concentration in the sediment cores and grab samples was 2.03 mg/kg and that from depths 10—50 cm below surface was 3.16 mg/kg. Lower concentrations overall (average 0.84 mg/kg) were observed in surface sediments (0—10 cm depth). By contrast, surface sediments from salt marsh cores on the south bank of the estuary (Ince Banks) had an average concentration of 5.13 mg/kg; surface sediments from a site on the northern margin of the estuary were also relatively high.

Lower overall concentrations in surface sediments are attributed to a recent diminution in manufacturing and more stringent controls on the release of industrial effluent since the late 1970s. Higher values in surface sediments at two of the sites investigated suggest that some erosion and redeposition of sediments (and associated mercury) may have taken place.

The strong relationship between mercury and both organic matter and particle size likely reflects sediment properties and dynamics rather than localised point sources of contamination.

Of the 203 samples analysed, 169 exceeded the published effects range median (ERM) threshold of 0.17 mg/kg, taken as the approximate threshold above which adverse effects on sensitive biota can occur. Using the probable effect concentrations (PEC) threshold of 1.06 mg/kg, 142 sediment samples were in excess. Mercury concentrations may therefore have an adverse effect on ecosystems in the Mersey estuary.

Further information

Vane, C H, Jones, D G and Lister, T R. 2009. Mercury contamination in surface sedimentns and sediment cores of the Mersey estuary, UK. Marine Pollution Bulletin, 58, 940—946.

Contact

Please contact Dr Chris Vane for further information