The survey was commissioned by the NRA from AEA Technology. River water and sediment samples were analysed from 40 sites which were chosen to represent background rural and urban areas. The site selection was also designed to investigate the impact of potential dioxin sources, such as chemical, steel, textile and sewage treatment plants.
The study found that concentrations in river water were always very low, below 6ng/kg, or below 0.08ng/kg expressed as a toxic equivalent (TEQ) of the most toxic dioxin 2,3,7,8- TCDD. The highly hydrophobic nature of the compounds means that they tend to be strongly absorbed by organic matter in sediments. AEA concluded that the most appropriate method of assessing dioxin contamination was through sediment analyses.
At six background sites, sediments contained 300-400ng/kg of dioxins and furans, or 2-4ng/kg TEQ, mainly as a result of atmospheric deposition and wash-off. In urban and industrial areas levels were higher, but there was a great deal of variation depending on the organic content of sediments and local sources.
The median level of contamination was about 3,580ng/kg or 16.8ng/kg TEQ, but sources of dioxins were not always apparent. Analyses upstream and downstream of a Schering Agrochemicals discharge to the river Cam, for example, found that levels were higher upstream.
Analyses upstream and downstream of textile plants, a sewage works and a colliery on the river Amber in Derbyshire found that these discharges increased the dioxin levels from 1147ng/kg to 10,390ng/kg (8.9ng/kg to 81.0ng/kg TEQ).
The highest dioxin concentration found at any of the survey sites was in the river Alt, Lancashire, where levels of 15,970ng/kg (122.8ng/kg TEQ) were measured downstream of various industrial sources.
The report also contains data from a known dioxin hotspot near Bolsover, Derbyshire, which has been investigated separately. The river Doe Lea received effluent discharges from Coalite Chemicals' hazardous waste incinerator. The plant was spotlighted in 1991 after the Ministry of Agriculture found elevated levels of dioxins in cows' milk from the area. The incinerator was closed, and Coalite eventually paid local farmers £200,000 in an out-of-court settlement (ENDS Report 226, p 4 ).
Investigations by the NRA have shown that the river sediments contain 20,269,000ng/kg of dioxins (45,300ng/kg TEQ) immediately downstream of the company's outfall, compared to only 2,030ng/kg upstream (9ng/kg TEQ). The impact of the site was clearly apparent 11 kilometres downstream on the river Rother, where sediments contained 110,000ng/kg of dioxins.
The NRA is now pursuing a civil action against Coalite to secure clean-up of the river. The NRA estimates that this will cost about £1 million. Coalite has dismissed the contamination as a minor environmental problem.
The dioxin levels in the Doe Lea are the highest reported in any aquatic sediments. The report says that the next highest were found in a Norwegian fjord which had been polluted by a magnesium plant, with levels of 749,000ng/kg.
The report concludes that levels of dioxins in UK river sediments are similar to those reported elsewhere in Europe. However, sources of dioxins were difficult to predict and further research is needed to follow up some unexplained high levels found during the survey.