«: AGROCHEMICALS: FATE IN FOOD AND THE ENVIRONMENT PROCEEDINGS OF A SYMPOSIUM, ROME, 7 - 1 1 JUNE 1982 JOINTLY ORGANIZED BY IAEA AND FAO l^J I N T E R ...»
Attention has been drawn to the conventional use of isotopic tracers for studying the fate o f an agrochemical and to their use as monitoring tools, in short, to the use o f isotope techniques for studying interactions. This is particularly relevant to studies in less industrialized countries where isotope laboratory facilities tend to be concentrated in one 'nuclear' institute and to be used mainly as conventional tracers. However, as already illustrated, these same facilities can be used in the wider and more important context o f interactions between chemical and exposed biota. Some observations in this wider context are dealt with in the following sub-sections.
3.1. Physical and chemical fate
Isotopic tracer techniques provide the ideal method for the integrated type o f study , which is fundamental, for example, to the formulation o f realistic pesticide residue 'tolerances' or 'maximum residue limits' (MRLs), as they are now called. For such a study the labelled chemical is applied experimentally to the crop under the simulated conditions o f 'good agricultural practice' and its fate followed from the time o f application through harvest, post-harvest treatment, processing, cooking, etc., to the 'terminal residue' at the point o f human ingestion. Therefore, the study provides especially well for relating the residue at the most convenient point for sampling, analysis and M R L enforcement, and the terminal residue. The M R L is, o f course, designed to obviate the presence o f a terminal residue likely to exceed that implied by the maximum acceptable daily intake (ADI).
One o f the first, if not the first, o f such studies was made with a radioactive bromine analogue o f D D T more than 30 years ago . The residue was followed and chemically characterized from the point o f application to wheat grain through the authentically duplicated conditions o f milling, fractionation, baking and feeding to experimental animals, as well as one human volunteer (the present author), and finally to the point o f radiochemical analysis for excreted metabolites.
Any review o f the now very extensive literature on agrochemical residues is quite beyond the scope o f this contribution. The annual series o f 'Residue Reviews' and F A O / W H O Monographs on pesticide residues in f o o d, feed and the environment are but two key series o f publications. A series of concise summaries o f individual or grouped environmental chemicals and radioactive substances was also initiated under the Joint F A O / I A E A Division Chemical Residues Programme as an aid to scientists in developing countries [21 ].
The literature indicates that agrochemical residues invariably undergo some abiotic chemical modification and/or metabolism, some rapidly with disappearance half-times o f the order o f minutes, others slowly with half-times measured in years.
Halogenated unsaturated rings tend to be especially recalcitrant and persistent.
While only minor modification tends to destroy acute toxicity o f the parent molecule , some residual moieties, e.g. the hexachlorocyclopentadiene moiety o f the 'drin' insecticides, can be very persistent and there is as yet little information about their possible long-term ecotoxicological significance .
Disappearance rates depend greatly on the chemical structure and climate, and on both abiotic and biotic environment factors. Thus, rates determined under temperate conditions may not be applicable to tropical ones .
3.2. Side effects
Given the nature and magnitude o f an agrochemical residue as a function o f time the most important question remains: what is their significance in terms o f effective usage and possible hazards to health and environment?
The most serious side effect o f accepted pesticide usage has been the slow but ever widening appearance o f 'resistant' strains o f pest on the basis o f accelerated Mendelian selection . This has involved virtual physiological immunity o f entire field populations o f hitherto effective acaricides, insecticides, fungicides, herbicides and even rodenticides . These phenomena have implications for unrelated agrochemical effects and emphasize the difference between classical toxicology, which is mainly concerned, directly or indirectly, with human health, and the wider concept o f ecotoxicology . In the latter case it is the vast range o f microbiological, animal and plant species of the agroecosystem which is o f concern. Moreover, it is the longer term effects o f the likely exposure o f populations over several generations which will be more important than acute effects on individual organisms or initially exposed populations, except for target pest populations. Various compounds are under evaluation to improve the efficiency o f nitrogen fertilizer usage by reducing nitrogen volatilization and/or leaching losses. They are based on their ability to inhibit microbiological mineralization o f organic nitrogen, e.g. o f added urea nitrogen or nitrification o f ammonium, since both leaching and denitrification (volatilization) losses imply nitrate formation beyond the immediate needs o f the growing crop or retentive capacity o f the soil. However, the phenomenon o f resistance suggests that while such compounds might appear effective under short-term tests, they would lose effectiveness under field conditions due to the selection of resistant strains o f microorganisms .
Finally, it is important to consider any demonstrable side effects o f agrochemical residues in the context of agrochemical practices as a whole. Thus, the effects o f an agrochemical residue on some non-target microorganisms of the soil may be o f negligible significance compared with the slow but sure decline o f total soil organic matter (and its nitrogen) as a result o f clearance and/or intensive agricultural practices generally [ 1, 4 8 ]. Similarly, although DDT at extremely low concentrations can inhibit the growth o f some species o f marine phytoplankton, more significant interaction may well be the effective removal o f DDT by planktonic sedimentation .
facilities for handling radioactive and stable isotopically labelled chemicals also provide for studying the effects o f unlabelled chemicals on critical parameters of soil and aquatic ecosystems. They similarly provide for monitoring by using labelled reagents and enzyme substrates.
Methods o f labelled synthesis, application and assay are now well advanced.
However, as in other instrumental assay techniques the explosive development in applied micro-electronics promises a major impact on the isotope laboratory in terms o f programmed and automated analysis and assay, data storage, retrieval and processing .
Trends in land use, population and demands per capita imply a continuing rise in pesticide and fertilizer use, particularly in tropical areas. It is, therefore, important that the countries concerned have the full range o f isotope techniques available for the study and control o f their own agrochemical residue problems under their own local conditions. Wise use o f the information and data so acquired will surely facilitate optimal use o f agrochemicals, and obviate problems of toxicology and ecotoxicology [ 5, 1 1 ].
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PESTICIDES IN AQUATIC ECOSYSTEMS(Session III) Chairman
PESTICIDES IN LIMNIC METABOLISM: A REVIEW ON THE APPLICATION OF NUCLEARTECHNIQUES.
Pesticides in fresh water ecosystems are discussed with respect to their toxicity, bioaccumulation, degradability and transportation. Water quality problems centre around the balance or imbalance between oxygen supply, primary production and community respiration.