Controlling persistent organic pollutants-what next?

Within the context of current international initiatives on the control of persistent organic pollutants (POPs), an overview is given of the scientific knowledge relating to POP sources, emissions, transport, fate and effects. At the regional scale, improvements in mass balance models for well-characterised POPs are resulting in an ability to estimate their environmental concentrations with sufficient accuracy to be of help for some regulatory purposes. The relevance of the parameters used to define POPs within these international initiatives is considered with an emphasis on mechanisms for adding new substances to the initial lists. A tiered approach is proposed for screening the large number of untested chemical substances according to their long-range transport potential, persistence and bioaccumulative potential prior to more detailed risk assessments. The importance of testing candidate POPs for chronic toxicity (i.e. for immunotoxicity, endocrine disruption and carcinogenicity) is emphasised as is a need for the further development of relevant SAR (structure activity relationship) models and in vitro and in vivo tests for these effects. Where there is a high level of uncertainty at the risk assessment stage, decision-makers may have to rely on expert judgement and weight-of-evidence, taking into account the precautionary principle and the views of relevant stake-holders. Close co-operation between the various international initiatives on POPs will be required to ensure that assessment criteria and procedures are as compatible as possible.

An empirical map of critical loads of acidity for soils in Great Britain.

The method used to produce a critical load map of acidity for soils in Great Britain is described. Critical loads were assigned to the dominant soil in each 1 km grid square of the UK national grid. Mineral soils were assigned a critical load based on mineralogy and chemistry, using approaches appropriate to UK conditions. Critical loads for peat soils are based primarily on a maximum acceptable reduction of peat pH, and results from laboratory equilibration studies. The map shows that soils with small critical loads (<0.5 kmol(c) ha(-1) year(-1)) i.e. highly sensitive to acidic deposition, dominate in the north and west of Britain; the south and east are dominated by soils with large critical loads, with small areas of more sensitive soils associated with sandy soil-forming materials. A modified critical load map illustrates the potential impact of agricultural liming on soil critical loads.

The sensitivity of surface waters of Great Britain to acidification predicted from catchment characteristics.

Using a combination of soil, land use and geological information, a map of Great Britain has been derived which indicates the sensitivity of surface waters to acidification. For the geological information, a slightly modified version of an available map was used which indicated the sensitivity of groundwaters to acidification. For soils, 1-km databases of soil information for England and Wales and for Scotland were employed to map the soil sensitivity as determined by buffering capacity. The derived soils map was modified to take account of agricultural liming in arable and managed grassland areas using the ITE Land Classification. The final map of surface water sensitivity was obtained by using a geographic information system overlay procedure which enabled each combination of soil and geology sensitivity to be uniquely defined. The final sensitivity classification was based upon expert knowledge and the experience of a similar sensitivity mapping exercise for Wales.

An introduction to critical loads.

The critical loads approach to emission controls of gaseous pollutants is a concept with a short but eventful history. Despite difficulties with definitions and agreed values, its acceptance within the UN-ECE Convention on Long Range Transboundary Air Pollution has provided the impetus for developing methods to put critical loads to a practical use-the revision of the UNECE emission protocols for sulphur and nitrogen. Methodologies first focus upon quantifying a pollutant threshold at which harmful effects occur on particular sensitive receptors (usually biological species). This threshold is known as the critical load for deposited pollutants, and as the critical level for gaseous pollutants acting on receptors. To calculate a critical load, biological effects are usually 'translated' to critical chemical values, e.g. harmful effects on fish 'translate' to alkalinity or aluminium concentrations in water; thus, critical load calculations may be based upon the chemistry of a system. Such calculations may be performed using simple, steady-state models, whilst the use of more complex, dynamic models provides an insight into the past and future trends. Maps of critical loads can be drawn using calculated values, and maps of pollutant deposition data will then show geographical areas where critical loads are exceeded. Spatial emission-deposition models can identify sources contributing to areas of excess loads and quantify necessary emission reductions. Optimization procedures applied to such models can derive abatement strategies related to economic costs and critical load effects. The critical load calculations may also be used to underpin the setting of target loads; these are pollutant loads, determined by political agreement, which take account of social, economic and political considerations.

A classification of acidified catchments in Britain.

To compare catchments of possible acidified waters in Britain an extensive database was compiled for nearly 600 catchments of lakes and streams for which chemical data were available. Information on the database included map-derived data, pollutant deposition estimates and hydrological characteristics. To stratify the catchment database a randomly selected subset of 328 catchments was used to derive eight classes of catchments using the classification program TWINSPAN. Several classifications were generated by this method using an increasing number of catchment parameters. TWINSPAN classes were defined on the basis of some catchment parameters (indicators) but were shown to be associated with other variables. A general linear model (GLM) analysis of the available chemical data was used to test the different classifications. A TWINSPAN classification which excluded soil and geology characteristics gave the best GLM model because soil and geology types were poorly correlated with other catchment parameters. However, because of the recognized importance of soils and geology in acidification processes, these parameters were introduced into the GLM model as separate GLM classes. The resulting three-class model (TWINSPAN class, soil type, geology type) proved superior to both the one-class model and to two-class models which included either soil or geology. It was demonstrated that the TWINSPAN classes were associated with particular geographical areas and particular monitoring programmes. The TWINSPAN classification was used to evaluate a set of catchments selected as long-term monitoring sites for Britain. In general, the set covered a wide range of catchment types as defined by the classification. It was possible to identify minor shortfalls in classes of catchment selected and suggest possible additions to the sampling programme. Whilst the classification procedure has been applied to possible acidified waters, the method is of general relevance to all catchments and waters of more diverse nature.

The critical loads/levels approach to gaseous pollutant emission control.

There is increasing pressure to control gaseous acidifying emissions to prevent harmful environmental effects; many governments are looking for a more effective and scientific means of assessing effects and planning emission controls compared with arbitrarily uniform percentage reductions. The critical loads/levels concept may offer the basis for a solution which is acceptable internationally. There is already agreement to map the critical loads and levels for countries in Europe and North America as a first step towards assessment. Recent developments in defining and quantifying critical loads and levels show the approach could be sufficiently adaptable to apply to the effects of different pollutants on a range of environmental receptors. Whilst it is difficult to arrive at an agreed single definition, various international groups have arrived at definitions which suit their own particular purposes. Despite a variety of different approaches to arriving at estimates of critical loads, there is broad agreement at the figures calculated by the different methods. An extension of the concept, the target load/level, provides a practical solution for implementing control strategies to prevent or minimize environmental effects.

Egg mass and shell thickness in dippers Cinclus cinclus in relation to stream acidity in Wales and Scotland.

The shell thickness and mass of eggs of the dipper (Cinclus cinclus) collected on streams of different pH in Wales and Scotland were measured. The aluminium, phosphorus and calcium content of their invertebrate prey were also measured. In a regression analysis, significant at p<0.05, stream pH accounted for up to 7% of the variance in shell thickness, with shells decreasing by 2.5% of the overall mean with each unit of pH decline. In the Welsh sample, differences in shell thickness due to pH were small compared with differences between years across all sites. In data pooled between Scotland and Wales, pH accounted for 17% of the variance in egg mass, but a greater proportion (25%) in Scottish eggs alone. Aluminium concentrations in invertebrates showed no relationship with stream pH, but calcium levels in two insect orders increased significantly with pH. Calcium rich prey, such as Gammarus, were found only in circumneutral streams. The importance of calcium in the diet of dippers before and during egg formation is discussed. No evidence that aluminium in prey adversely affected dipper eggs was found.