National survey finds fewer toxic chemicals in soil

Richard Van Noorden/London, UK

Concentrations of dioxins in the UK’s soil and plants have fallen by about 70 per cent since the late 1980s, according to a major nationwide survey conducted by the Environment Agency. The results suggest that industrial emissions are no longer a significant source of persistent organic pollutants (POPs) in the country, thanks to restrictions enforced under the Stockholm Convention. 

Declan Barraclough, geosystems science manager at the Environment Agency, said the survey was the first major stocktaking of significant environmental contaminants in UK soils. Researchers measured concentrations of metals and bioaccumulative chemicals in over 200 sites across the UK. Though the production of many industrial chemicals, like polychlorinated biphenyls (PCBs), has been banned for decades, their resistance to bacterial breakdown means they still persist in soil and vegetation. Carcinogenic byproducts of combustion processes, such as dioxins, have been shown to be steadily increasing in soil samples up to 1980. 

The UK’s industrial legacy lives on in its soil, the survey found. Urban and industrial areas were more heavily contaminated than rural areas with metals and POPs. Polycyclic aromatic hydrocarbons (PAHs) such as benzoapyrene had a particularly clear industrial ’footprint’, occurring at five to seven times the soil concentration in rural areas. The particular PAHs found suggested that road traffic was a major source - which might be expected to rise.    

But the chemicals audit showed that tighter industrial regulation had dramatically cut down on dioxins in soil. Dioxin concentrations in industrial soils were still higher than in soils from rural areas, reflecting past industrial emissions. But dioxins in plants - which are constantly growing, so reflect current conditions - were at very similar levels in all areas.  ’These findings suggest that in many cases major industrial sites are no longer the main factor determining dioxin concentrations in their immediate area because of better regulation, legislation and business practices,’ said Barraclough. 

Though it was encouraging that industrial sources of POPs had been successfully controlled, agreed Jane Stratford, of the UK Department for the Environment, Food and Rural Affairs, a number of smaller sources remained which would be harder to weed out. 

Comparisons with previous scattered soil surveys showed, for example, that though levels of PCBs had fallen 800-fold since their ban in the 1970s, an obstinate steady remainder persisted to the present day. These were probably slowly leaked from pre-1970s transformers, building materials, and possibly landfills, said Stuart Harrod, an environmental chemist at Birmingham University, UK. 

The survey would act as a reliable baseline from which future trends could be analysed, said Barraclough. A quick examination of the soil around Buncefield after the 2005 oil depot fire, for example, showed contaminant levels were not particularly high compared to the national trend. 

Since only a few samples were taken from each area, the researchers couldn’t say which cities were most polluted. But differences in metal concentrations across the UK might be more to do with underlying geology than human input, they pointed out: high concentrations of titanium in Northern Ireland and Scotland reflected the titanium-rich basaltic rocks in the area, not man-made pollution. 

Harrod suggested the study, which took just over two years, might be usefully revisited in 10 years. He said that other chemicals, such as fluorinated and brominated flame retardants, should also be monitored. 

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