How can we resolve conflicts in definition, regulatory requirements, and significance?
B O N D I G E VAO KEVIN C. JONES K I R K T. S E M P L E LANCASTER UNIVERSITY (UNITED KINGDOM) A N D R E W C R AV E N PESTICIDES SAFETY D I R E C T O R AT E (UNITED KINGDOM) PETER BURAUEL JÜLICH RESEARCH CENTRE (GERMANY)
Nonextractable
PESTICIDE
Residues in Soil
isk assessment and regulatory management should minimize the impacts of chemicals on human health and the ecosystem. As Calderbank noted in 1989, the basis for regulatory decision making regarding pesticides needs to address four issues (1). These are carryover of soil residues into succeeding crops, their consequent effect on biota in terrestrial and aquatic systems and their potential for food chain transfer, potential to contaminate groundwater, and long-term effects on the quality and fertility of soils. These issues should form the basis for pesticide registration, rather than persistence and the ambiguous “nonextractable” (or soil-bound) residues. The persistence of a pesticide in soil should not be the determining factor in registration decision making. In actuality, bioavailability is more important to human and environmental health than the amount that is nonextractable. With this in mind, understanding the processes that govern pesticide bioavailability is critical. This article considers if current analytical approaches used in nonextractable pesticide residue studies of soil are suitable and discusses the potential biological significance of residual concentrations. We present the case that current regulatory requirements are not based on our understanding of the nature of bound residues and overestimate the risk from soil-bound residues of pesticides. Certain regulatory criteria in this area of risk assessment are arbitrarily chosen, especially with regard to the role of metabolites and those compounds giving high incorporation rates into soil organic matter.
STEVEN HOLT
R
© 2003 American Chemical Society
APRIL 1, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 139 A
Regulatory framework and related issues
compound chemistry, use patterns, and fate in the Registration requirements for plant protection prodenvironment vary, the ideal scenario might be to deucts and written guidelines for conducting studies on velop sound guidelines for the conduct of studies these compounds have evolved gradually over the that can be reliably used for environmental risk aspast 30 or so years (2, 3). In most countries, but parsessments on a case-by-case basis. ticularly in Europe, North America, and certain Asian For example, currently required solvent extraction countries, regulatory requirements were developed procedures, aimed at retrieving the “total” pesticide first in the human health arena and tended to mirror concentration from soils and sediments, are known those previously developed for human medicines, to be inappropriate for predicting the bioavailable thus focusing on consumer safety and then operator proportion of the chemicals (7–8). Extraction methprotection. Regulation of plant protection products in ods provide little or no information on likely comthe environmental area followed later, largely as a repound bioavailability as they tend to overpredict sult of the huge increase in public awareness of genbioavailability and, hence, the potential for biodegraderal environmental issues ability and toxicity. For in the past 10 or 20 years. example, the extracted Assuming all extractable 14C residue may be the At a national level, regulatory requirements parent compound, its residues are bioavailable around the world for metabolites, or small plant protection products molecular fragments exoverestimates the risk to in the environmental area isting in forms that pose have generally tended to ecological risk (9–11). humans, animals, and plants. no reflect the national imHowever, in the regulatoportance attached to agriry guidance on how to culture compared to environmental protection. conduct studies (Annexes of the EU Directive Before the introduction of harmonized guidelines in 91/414/EEC) for the registration of pesticides, for exthe European Union (EU), the study and risk assessample, it is required that “soil is exhaustively extractment methodologies used to evaluate a pesticide ed by appropriate methods. Such methods should could vary dramatically between countries and cominclude extraction with water and organic solvents, pounds, reflecting differences in national agriculturusing appropriate hydrolytic and reflux techniques.” al priorities and practices, study temperatures, In the uniform principles for decision making (Annex duration, and efforts to identify the metabolites or VI) of this harmonized EU legislation, authorization degradation products. In addition, the lack of mass for the use of pesticides is declined if the active subbalance requirements made preparing an accurate stance “persists in soil with a DT50 (time taken for picture of the fate of a pesticide difficult (4–6). 50% of the compound to disappear) of >3 months and a DT90 (time taken for 90% of the compound to Adopting regulatory guidelines and the principles disappear) of >1 year”, or “forms bound residues in of good laboratory practice (GLP) has helped stanamounts >70% after 100 days with a mineralization dardize data collection and evaluation procedures. In rate
