Technically speaking

WWT's technical editor, Peter Minting, looks at direct toxicity assessment.

Daphnia pulex - the miner's canary of aquatic pollution

Daphnia pulex - the miner's canary of aquatic pollution

The future of direct toxicity assessment (DTA) is to be decided this autumn, following the completion of a four-year research programme. The programme has been completed by the Environment Agency (EA), independent scientists and representatives from large companies such as ICI.

DTA is a means of measuring the actual toxicity of an effluent or receiving water sample, rather than the individual toxicity of the substances released. This has a number of advantages over substance-specific analysis when trying to assess harm to the environment.

Wildlife may respond quite differently to a combination of substances compared to one substance at a time. DTA should therefore allow the EA to make a better judgement over which discharges to tackle first and which to spend money on.

At a meeting in Torquay in 1996, the EA, Scottish Environmental Protection Agency (SEPA) and DoE (Northern Ireland) held a meeting chaired by Professor Peter Calow of Sheffield University, to launch DTA as an extra 'tool-in-the-toolbox' for those assessing pollution. Several industry representatives joined and a research programme was then funded by British industrialists (£0.65M) through the EnviroNet Foundation.

The steering group led by Prof Calow has, since 1996, been trying to establish the best way of carrying out DTA in the field. Four study rivers were initially selected for experimental trials. Only two of the selected study rivers were actually used throughout the programme, the lower Tees Estuary in Northumberland and the River Esk in south-west Scotland. Insufficient evidence for toxic discharges could be found in the River Aire in Yorkshire and work has been postponed on the River Spey in Scotland by the SEPA.

From the early stages of the programme it was made clear to the companies involved that DTA would at some stage become a legal reality and discharge consents would be set accordingly. This could in theory work in favour of a discharger, because although many pollutants are additive or synergistic in effect, some could partly cancel each other out.

The research programme was designed to be carried out in two phases. For the first phase in 1997-1998 Entec was chosen as the contractor for field work and the WRc chosen as technical auditor. Toxicity assessments of water and effluent samples were carried out using standard techniques such as bacterial bioluminescence, Daphnia acute lethality (freshwater) and oyster larval embryo (estuarine) tests. In addition two UKAS-accredited laboratories were commissioned to carry out algal growth studies, fish acute lethality and chemiluminescence tests. The purpose of these tests was to find out the toxicity of water and effluent to reliable test organisms, in order to choose locations and discharges for further examination.

In phase two of the programme more detailed work was carried out. In the case of the lower Tees Estuary, environmental consultant Andrew Girling worked with the Brixham Environmental Laboratory, WRc-NSF and ICI to develop a water quality model. The EA's national ecotoxicology lab provided additional support.

Several new rapid toxicity tests were tried out during the Tees project. The tests must be repeatable and comparable across locations, if DTA is to achieve credibility as an assessment technique;

  • electrical measurement of the metabolic status of bacteria (Cellsense),
  • nitrification inhibition using immobilised nitrifying bacteria (Amtox),
  • infrared invertebrate heart rate monitoring (Plymouth University),
  • bioluminescence using modified bacteria (Aberdeen University's NEWT),
  • a hand-held bacterial bioluminescence test (Merck's Toxalert).

Fortunately the results using these rapid and Daphnia toxicity tests were highly comparable between labs. The results were discussed with local dischargers, giving them an opportunity to address potential problems before legal action.

On the River Esk, sampling was carried out by Waterfront Technology. The sampling was carried during a period of low rainfall and river flow, i.e when discharges were likely to be present at the highest percentage of flow and have the greatest effect on wildlife.

Again Daphnia were used to assess toxicity along with crustaceans, freshwater algae and rainbow trout. Although trout are among the most sensitive fish to pollution, high temperatures and low oxygen levels, acute lethality was only seen in Daphnia when testing the main effluent discharge from Langholm STW. Sub-lethal effects were observed in the crustacean Gammarus, or freshwater shrimp. There was a poor correlation between the results of Daphnia and bio/chemiluminescence studies, suggesting the need for more extensive tests. Although data can be statistically manipulated to a degree in order to account for insufficient sampling, in this case neither a correlation or plausible explanation could be found. Despite such inconsistencies the scientists did find some evidence for pesticide pollution by textile manufacturers.

Puzzling results
Work on the Tees also produced some puzzling results during the latter stages of the test programme. For instance, no correlations could be found between the concentrations of chemicals in some of the samples and the toxicity results, although there were clear relationships between model predictions and the distribution of toxic discharge plumes. In the case of industrial effluents released into the Dabholm Gut, a tributary of the Tees, one of the effluents was found to be very toxic even after a 2,000-fold dilution. Identification work showed the toxicity to be caused by volatile organic carbons (VOCs) and cyanide in solution. Many of the discharges are now being diverted to Bran Sands STW, but there is still much work to be done in the area.

According to Ian Johnson, principal ecotoxicologist at WRC-NSF, the EA is currently writing standard procedures for DTA so that they can be incorporated into consent manuals. As DTA is relatively expensive, the EA plans to revert to single-limit tests after the initial consent has been set. The steering group, including the EA, will meet to discuss the results of the programme on September 26 at Warwick University. Legal implementation of the technique is unlikely to take place until early next year.



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