4: ENVIRONMENTAL FATE AND TRANSPORT OF PERSISTENT ORGANIC POLLUTANTS
5. USES, SOURCES, ALTERNATIVES
4. ENVIRONMENTAL FATE AND TRANSPORT OF POPs
By definition, POPs are likely to be more persistent, mobile, and bioavailable than other substances. These properties are conferred by the structural makeup of the molecules and are often associated with greater degrees of halogenation. Included in this group of substances are some older chlorinated pesticides like DDT and the chlordanes, polychlorinated biphenyls, polychlorinated benzenes, and polychlorinated dioxins and furans. The physico-chemical properties of these compounds are such that they favour sufficiently high atmospheric concentrations that result in global redistribution by evaporation and atmospheric transport.
4.1 Physiochemical Properties and Environmental Partitioning
The physical properties of greatest importance are water solubility, vapour pressure, Henry's law constant (H), octanolwater partition coefficient (KOW), and the organic carbonwater partition coefficient (KOC). Persistence in the environment is the other important property of a substance since transport can extend the range of exposure to persistent substances far beyond the immediate area of use and/or release.
4.2 Environmental Influences on Persistence, Movement and Deposition
Persistence can be reduced by environmental transformation processes. These are: biotransformation; abiotic oxidation and hydrolysis; and photolysis. The relative importance of these processes depends on the rates at which they occur under natural environmental conditions. These rates are, in turn, dependent on the chemical structure and properties of the substance and its distribution in the various compartments of the environment. As would be expected, environmental factors have little effect on the breakdown and transformation of POPs. In addition, those that might have some effect are less effective in polar regions. Given the continued use and release of POPs in other parts of the globe, the result of this is a net accumulation of POPs in the polar regions.
Some of the above physical properties are strongly dependent on environmental conditions. For example, temperature strongly affects vapour pressure, water solubility, and, therefore, Henry's law constant. The net exchange direction for substances in the open ocean also reflects differences in surface water temperature and atmospheric concentration. For example, net movement of POPs in the Bay of Bengal in the Indian Ocean is from the ocean to the atmosphere while that in polar regions is the reverse. Temperature may also affect deposition in other locations. The distribution of POPs is inversely related to vapour pressure, and thus to temperature. Lower temperatures favour greater partitioning of these compounds from the vapour phase to particles suspended in the atmosphere. This increases the likelihood of their removal and transport to the surface of the earth by rain and snow (Figure 3).
Countries in the tropics experience higher year-round temperatures than countries in the temperate and polar regions of the globe. The practice of using some pesticides in tropical agriculture during the warmer, wetter growing season may facilitate the rapid dissipation of POPs through air and water.
These and other observations suggest that inputs of POPs to tropical coastal water bodies through river discharge are less significant than in temperate zones. The residence time in the tropical aquatic environment is quite short and transfer to the atmosphere is greater in these areas. The relatively short residence time of POPs in the tropical water bodies might be viewed as favourable for local organisms. However, it does have more far-reaching implications for the global environment because these volatilized residues from the tropics then disperse through the global atmosphere.
The present-day distribution of POPs in the oceans is consistent with a major change in distribution pattern during the last decades. Until the early 1980s, there were higher concentrations of POPs (such as DDT, and PCBs) in the midlatitude oceans of the northern hemisphere, probably reflecting the large usage in developed countries such as Japan, Europe, and North America. This distribution has not been seen in the most recent samples.
Atmospheric transport and accumulation of POPs (PCBs, DDT, HCHs, and chlordanes) in the polar regions has been extensively documented. Accumulation in polar regions is partly the result of global distillation followed by cold condensation of compounds within the volatility range of PCBs and pesticides. These contaminants are continually deposited and reevaporated and fractionate according to their volatilities (Figure 3). The result is relatively rapid transport and deposition of POPs having intermediate volatility, such as HCB, and slower migration of less volatile substances such as DDT (Figure 4).
The characteristics of polar ecosystems intensify the problems of contamination with POPs. The colder climate, reduced biological activity and relatively small incidence of sunlight would be expected to increase the persistence of the POPs.
Considerable data on concentrations of POPs in samples from the Arctic and the Antarctic are available and are summarized in the companion document to this assessment. Most of these data are published in summary form as means or means with ranges. It was not possible to access the raw data from which these means were calculated, however, the range of concentrations are presented in Table 4-1 for information. Inspection of this data showed indications of declines in concentrations since some of these POPs were banned or restricted. The maintenance of a central database of all analytical data on the POPs would greatly aid in determining spatial and temporal trends in the data and linking these to changes in use pattern of these substances.
5. USES, SOURCES, ALTERNATIVES
The twelve POPs which are the subject of this report, are used in or arise from industry, agriculture and disease vector control; nine are pesticides used on agricultural crops and/or for public health vector control. By the late 1970 s, all of the nine pesticides and PCBs had been either banned or subjected to severe use restrictions in many countries. Current information indicates that some of these POPs are still in use in parts of the world where they are considered as essential for ensuring public health. In an effort to further reduce their use in these countries, it is important to understand what countries are using these POPs, and how they are applied. It was found that there is considerable information that describes the aggregate volume of POPs produced and used in the world, however, there is very little reliable data about the specific uses in each country. Although this lack of specific data makes it difficult to evaluate the rationale for the continued use of the nine pesticides, the available information still allows one to discuss the use patterns and barriers to adoption of alternatives in a generic fashion.
Most, if not all, of the nine pesticides in question are still in use or existing in many countries. However, the actual quantity that specific countries may be currently using is unknown. There are no central registers of individual country use, although some organizations, like the FAO, United Nations Economic Commission for Europe, and the World Bank have begun to assemble aggregate use data. The cumulative production of most of the compounds, as of approximately 1987, is outlined in Table 5-1. Thus, while country specific data was not found, the cumulative global (sometimes only US or "other" countries not defined) were identified. While this does not tell enough about usage to know specifically where and how much of these compounds are being used it does show that the compounds are in fact still in use and aids in forming a general picture of use patterns.
A variety of chemical and non-chemical alternatives are available for the POPs. Lists of alternative pesticides have been cited for use in developed countries and are described in Table 5-1. It is important to note that not all developing countries use POPs, and those countries that allow the use of certain POPs do not do so to the exclusion of alternatives. For example, in Honduras integrated pest management (IPM) systems are used in some areas that rely on the judicious use of newer and pest specific pesticides and biological control methods. In these same areas, there exists a well developed distribution network for both pest control technologies and information. In other areas of Honduras, where there are fewer producers operating smaller farms, the use of older compounds, including some POPs, is common for a variety of reasons, including:
* common social attitudes that foster the continued use of older products,
* poor dissemination of both alternatives and information,
* relatively high degree of illiteracy that constrains the dissemination of any information, and
* other production related factors that limit the practical adoption of alternatives.
5.4 Constraints to Adoption of Alternative Technologies
Why the alternatives that are available are not being used is an important issue. There are many barriers to the adaptation of these alternatives and to the adaptation of technologies in general especially in developing countries. Some of the alternatives are simply more costly both in price and in other resources required to apply them compared to the older more hazardous compounds. Some alternatives are believed to be more acutely toxic to the applicator than the POPs and therefore more hazardous to the individual, thus adding a human health cost dimension.
Other barriers to adoption include education and training. Education and training on both the older compounds as well as the possible alternatives is necessary for everyone in the production chain including the individual users and vendors. It may be that many individuals do not realize how hazardous the older chemicals are, what alternatives are available, and how to use these alternatives effectively.
The infrastructure and regulations that are needed to manage the use of pesticides, as well as educate and train individuals in the use of possible alternatives is not fully developed in all countries. Not all countries have the necessary infrastructure to implement effective management programs, nor do they have the infrastructure for the types of training that is described above.
The regulatory structure that some developing countries have adopted is based on the developed countries regulatory structure. This structure is often not adaptable or appropriate to the particular situation in the developing country. In addition, both financial and human resources needed to make such structures function effectively are often insufficient. Once a regulatory system is in place that is compatible with the resources available then, influence on the gradual elimination of older and hazardous compounds can be initiated.
The first initiative that is necessary to investigate these issues further is an in-depth inventory of the 12 compounds in individual countries, including a close examination of the amount used, the reasons for use, the alternatives available for the specific uses and the barriers that exist to the adaptation of alternatives specific to the country. Possibly a few case studies could be performed that would give a general idea of the answers to these questions. Once more quantitative data is available, then more meaningful work can be done in evaluating different alternatives and aiding in the implementation of these alternatives.