environmental sciences research institute - University of Ulster

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The effects of transboundary air pollution The deposition of chemicals from the atmosphere on to the surface of the earth is less obvious than the emission of chemicals in waste water, solid waste or as a result of accidents. We can see emissions from chimney stacks in power stations and smelters and from the spraying of agrochemicals but the chemicals are deposited onto land and water many tens to hundreds of kilometres away and this is less noticeable. When these emissions cross national boundaries, this is called transboundary air pollution and the chemicals involved are carbon, sulphur, nitrogen, heavy metals and persistent organic pollutants (POPs). The heavy metals that are subject to transboundary air pollution are mercury, cadmium, lead, arsenic and zinc and the groups of POPs are the organochlorine pesticides (OCs), polychlorinated biphyenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) and because emissions in one country can affect the quality of the environment in another these chemicals are regulated by international treaty. The United Nations Economic Commission for Europe has five such treaties and the Convention on Long-range Transboundary Air Pollution is the main way that emissions and effects of transboundary air pollutants are managed; the 1998 Protocol on Heavy Metals and 1998 Protocol on Persistent Organic Pollutants (POPs) are concerned with heavy metals and POPs. The Department for the Environment and Rural Affairs (DEFRA) is responsible for implementing the Convention on Long-range Transboundary Air Pollution in the United Kingdom and they funded an investigation into the risks to lakes from heavy metals and POPs deposited from the atmosphere. As most of these pollutants are concentrated in lake sediments, they were the focus of the investigation. The work was published in 2007 (Rippey et al. 2007). The PhD of Sarah Travers increased the reliability of the assessment, by doubling the number of lakes involved, and providing independent evidence on the relevance of one of the toxicity tests used. The Tier I risk assessment was completed as follows. Eighteen lakes that covered the range of contamination by heavy metals and POPs deposited from the atmosphere in the United Kingdom and Ireland were selected (Fig. 1, previous page) and samples of the sediment retrieved. The concentrations of seven heavy metals and three groups of POPs, OCs, PCBs and PAHs, were determined and three sediment toxicity tests completed. The toxicity tests were reproduction and survival with Daphnia magna, growth and emergence with Chironomus riparius and growth and survival with Gammarus pulex (Fig. 2). The first two species are well established in sediment toxicity tests, but the latter much less so. Although there was evidence that Gammarus should be particularly sensitive to heavy metals, Sarah showed that this is not the case and that the Chironomus riparius test is the most relevant and efficient way of establishing sediment toxicity. Figure 2. Sediment toxicity tests. 14
A comparison of the sediment toxicity tests results with the concentrations of the chemicals showed that only the heavy metals are a risk to the animals that live in and around the sediment in lakes and not the OCs, PCBs or PAHs. This was an unexpected finding, given the widespread concern about POPs. Further analysis showed that it is lead and arsenic amongst the heavy metals that pose the greatest risk. This work also produced a chemical risk index for DEFRA and others to use to identify the lakes at risk from heavy metals deposited from the atmosphere and a map of the general risk is shown below. Figure 3. Three sub-regions of high, medium and low risk of biological effects in lake sediment due to the deposition of heavy metals from the atmosphere. The lakes are identified by a lake number. Not all lakes in the high risk region would be expected to have toxic sediments as a result of the deposition of metals from the atmosphere. This happens because lakes with a high accumulation rate of sediment will dilute the metals deposited from the atmosphere and so the concentrations would not be as high and the sediment not as toxic as expected. On the other hand, some lakes in the medium risk region would have toxic sediments, if they had low sediment accumulation rates; in this case, the metals would not be diluted much by the sediment and so the concentrations would be higher and the sediment more toxic than expected. This investigation is a good example of where sponsored research and research completed as part of a PhD can come together and make the overall contribution greater than the sum of the parts. 15
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environmental sciences research institute - University of Ulster

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