DTA downstream demonstration

Direct Toxicity Assessment (DTA), or ecotoxicological assessment based on ecological quality rather than just chemical criteria, has been undergoing trial in the UK since October 1997 (see IEM, August 1998). Taking into account the additive, antagonistic and synergistic effects of all substances present in whole effluent or receiving water samples, DTA provides a more holistic measure of harm than current, substance-specific measures, and as such the Environment Agency, Scottish Environmental Protection Agency and the Department of the Environment in Northern Ireland are developing procedures for the employment of DTA as one aspect of water quality management, alongside substance specific measures and biological assessment techniques.

Last month, the Demonstration Programme Steering Group, consisting of representatives from both regulators and industry – independently chaired by Professor Peter Callow from the University of Sheffield – reported on progress so far.

Practical work to assess the quality of the receiving water environment and to screen discharges for acute lethal toxicity to Daphnia – stages one and two of the seven-stage DTA protocol – has been completed as planned in the Esk and Aire projects, despite a wet summer at both sites delivering less than worst-case scenario data. Derek Tinsley, technical secretary to the DTA Demonstration Programme Steering Group, explains: “Our contractors at the Esk site [in North West Scotland], a consortium headed up by Waterfront Technology, have effectively carried out stages one and two of the DTA demonstration protocol. The results of that work have indicated that the final effluent from Langholm sewage treatment works was found to be acutely lethal to Daphnia, one of the tests we feel we are happy with, in that it provides the best quality of data. Bearing in mind the wet conditions, we have not found acute lethal toxicity in the river; in addition to that, however, we have also been deploying longer term eco-tox assessment methods – specifically a test that looks at the inhibition of feeding of freshwater shrimp – both upstream and downstream of the discharge. Those deployments in-situ in the river have produced evidence of longer-term sub-lethal effects downstream.

“So we are seeing this very much as an opportunity, while setting up a project with a main core objective, to also be trialling out other, or related, methodologies, such that we can get a better understanding of what is going on in what are clearly very complex systems.”

At the Aire project site in Yorkshire, none of the river water samples taken, nor any of the discharges screened, were found to be acutely lethal to Daphnia. Again, however, longer term in-situ exposure tests using Gammarus indicated reduced feeding along the entire study reach.

A modified programme of work for the Esk project – including further acute lethal toxicity assessments of the Langholm discharge during periods of low flow in the river – have been approved by the Steering Group. If effluent toxicity is confirmed in these samples then river water samples taken at the same time will be assessed. A tracking and identification exercise and toxicity removal evaluation are also planned.

Specific methods

Work on the Aire project will stop, no lethal toxicity having been found, with funds set-aside to be re-allocated elsewhere in the programme.

“For the third programme, on the lower Tees, we are dealing with an estrine marine environment,” says Tinsley. “Here, partly driven by the results of the Esk and Aire projects, we have not gone through stages one and two of the DTA protocol, but instead tested selected elements of the protocol. What we plan to do is try and model the dilution and dispersion of the discharges from eight of 12 outlets screened, try to predict toxicity at different points within the receiving water, and go out and see if we can confirm that toxicity.

“These are elements of the protocol which are very important both in terms of testing the approach in general and the specific methods which are unique to the more marine and estrine systems, and in particular methodologies involving toxicity testing with oysters.”

Where the most important elements of the demonstration programme are the field projects and the results that they produce, two further elements, of methods and legalities, have been under consideration by respective working groups. Programme Chairman, Peter Callow, sums up: “Working in parallel with us, the Methods Working Group is looking at the kinds of methods available in terms of the base set systems, but also at broadening and refining those systems. The Legal Group is looking at the extent to which what is intended with regard to this instrument is compatible with existing legislation, because the way that the Environment Agency, at least at present, is thinking about using this instrument is not to write it directly into consents, but to use it to draw attention to problems.

“The objective is to tell the world what we are doing, get as many people involved as possible, and to get feedback from as broad a range of people as possible. In general I feel that we are happy with progress, and I think feel that we are going to be able to add value and benefit to this instrument in terms of its ultimate use. But at the end of the day, it will be the Agency that does or does not take on board and use the recommendations that we make, to develop the instrument to regulate.”