FEATURE
Using Enantiomers To Trace Pesticide Emissions This novel approach provides clues to sources of pesticides found in the atmosphere. TERRY F. BIDLEMAN AND RENEE L. FALCONER lthough they have been deregistered in industrialized countries for years and in some instances for decades, organochlorine (OC) pesticides are still routinely found in the atmosphere throughout the world, even in Arctic regions (i). OC pesticides are of concern because they bioaccumulate through the food chain to top predators, including people, and have endocrinedisrupting and other toxic properties. Last July, a negotiating committee of the United Nations Environmental Programme (UNEP) met for the first time to seek worldwide elimination of 12 persistent organic pollutants (POPs), 9 of which are OC pesticides: DDT, chlordane, heptachlor, aldrin, dieldrin, endrin, toxaphene, mirex, and hexachlorobenzene (HCB)—the latter is both a pesticide and an industrial byproduct. Criteria are under development that will address additional substances in the future. The ban is contentious for DDT, which is still used in tropical countries for control of malaria. Worldwide sales of chlordane and heptachlor were halted only in 1997, and use of existing stocks can be anticipated for some time. For the decision makers to effectively eliminate OC pesticides and other POPs from the world's atmosphere, they must know the origins of the compounds. Presently, sources that contribute to observed global atmospheric distributions are not fully understood. At issue is a central question: Are OC pesticides transported from regions where they are currently applied, or are they instead ghosts of the past— recycled by evaporation of residues from previously contaminated soils and bodies of water? One novel approach for investigating pesticide recycling from soil and water involves using enantiomers as tracers. This is feasible because several classes of insecticides and herbicides have members that are chiral—compounds having right- and
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left-handed molecular configurations, or enantiomers (see figure on top of next page). Four of the eight OC pesticides on the UNEP list (o,p'-DDT, chlordane, heptachlor, and toxaphene) are chiral. Pesticide enantiomers are useful as tracers of soilair and water-air exchange processes for the following reasons. Although a few chiral pesticides are manufactured as single-enantiomer products, most are racemic mixtures having a (1:1) enantiomer ratio (ER). Enantiomers have the same physical and chemical properties. As a result, transport processes (leaching, volatilization, and atmospheric deposition) and abiotic reactions (hydrolysis and photolysis) do not discriminate between the enantiomers—such processes leave ERs unaffected. In contrast, metabolism of pesticides by microorganisms in water and soil and by enzymes in higher organisms often proceeds enantioselectively, leading to nonracemic residues and an alteration of the original ER (2-6) (see sidebar on next page). By examining ERs, it is possible to differentiate new sources of OC pesticides from ghosts of the past. The technique provides a sensitive indicator of biological degradation and clues about the origins of pesticides found in the atmosphere. Work performed by ourselves and other investigators demonstrates the potential usefulness of this approach.
New sources, old sources OC pesticides are still being applied or have been used in the recent past in many tropical and subtropical countries. This has led to especially high levels of some compounds in the ambient air of eastern and southern Asia (7). The conclusion that less developed countries are solely responsible for global contamination is, however, overly simplistic. Historically, large quantities of OC pesticides were also used in industrialized countries. © 1999 American Chemical Society
A recent worldwide survey found that residues of HCB, dieldrin, and the technical chlordane component trans-nonachlor in tree bark, which integrates atmospheric exposure over a 3- to 5-year period, were positively correlated with a country's gross national product per person and Human Development Index (8). Tree bark sampled in the poorest countries generally showed the least amount of contamination, whereas the highest levels occurred in the United States and countries of northern Europe. No significant correlations were found between socioeconomic indicators and residues of hexachlorocyclohexanes (HCHs) or DDT. One explanation for the pesticide distribution picture provided by the tree bark survey is that emissions from previously contaminated soil and water may be entering the atmosphere in significant amounts. Soil, especially agricultural soil, is likely the largest reservoir of OC pesticides and a major source of emissions. For example, toxaphene was heavily used in the southern United States on cotton and soybeans before being deregistered in 1982. Toxaphene concentrations measured in South Carolina air during the mid1990s were 10 times higher than levels in the Great Lakes region and showed no relationship to air transport direction, suggesting volatilization from regional soils (9). Moreover, DDT residues in air above soil at a California farm, where DDT had been applied 23 years previously [10), were 2 to 3 orders of magnitude higher than in the Great Lakes region {Environ. Sci. Technol. 1998, 32,1920-1927). Long-term monitoring on the shores of the Great Lakes reveals that airborne pesticides are declining only slowly, with "virtual elimination" dates (when levels fall below detectability) ranging from 2010 to 2060 (Environ. Sci. Technol. 1998, 32, 1920-1927). Concentrations of the OC pesticides and other POPs in air and surface water are now nearly in steady state with respect to gas exchange (Environ. Sci. Technol. 1998,32, 2216-2221). Evidently, certain processes are acting to stabilize atmospheric concentrations. This finding has significant consequences for cleanup of the lakes, because improvement of water quality is pegged to longterm atmospheric trends (11). Understanding how POPs migrate through the global environment is therefore critical to developing control strategies, and as a result, tracing the cycling processes of POPs among air, soil, water, and vegetation has become an area of growing importance (Environ. Sci. Technol. 1996,30,390A396A). Methods that can investigate emissions from soil and water as well as discriminate between "new" and "old" sources of contamination contribute to our understanding of these issues. Techniques for directly measuring the flux of pesticides from soil are well established (Envion. Sci. Technol. 1993,27,121-128) (11), but they have been largely applied to determining postapplication volatilization rates rather than emission of in-place residues. Models can predict pesticide volatilization based on soil properties, residue levels, meteorological conditions, and physicochemical properties of chemicals; modeled and measured pesticide fluxes have agreed well in field trials (12). However, application of models to predicting emissions of formerly used pesticides is
Pesticide enantiomers Enantiomers such as those of trans-chlordane, one of the many chiral compounds in technical chlordane (5), can be used to investigate the origins of pesticide residues found in ambient air.
Enantioselective metabolism Enantioselective metabolism of a-HCH, the only chiral HCH isomer, takes place in lakes and marine waters. Nonracemic residues of a-HCH, o,p'-DDT, chlordane, and the metabolites heptachlor epoxide and oxychlordane were found in agricultural soils (3). Preferential accumulation of pesticide enantiomers and alteration of enantiomer ratios (ERs) have also been observed in biota (2, 5). Metabolic processes do not always favor the same enantiomer, as exemplified in studies with a-HCH (2). The (+) enantiomer is preferentially degraded in some cases and the (-) enantiomer in others (signs refer to the direction of optical rotation). Residues of a-HCH in biological tissues show preferential accumulation of either enantiomer, depending on the organism (2, 5), and strong preferential accumulation of (+) a-HCH in seal brain tissue appears to be due to its ability to selectively penetrate the blood-brain barrier (2). Ambivalence was also seen for o,p'-DDT in midwestern U.S. soils, with depletion of the (+) enantiomer in some soils and the (-) enantiomer in others (3). Reasons for these reversals are not known but may be related to different microbial populations in environmental media. Enantioselective metabolism takes place not only for 0C pesticides, but for other chiral compounds as well.
limited by the paucity of residue data for soils. Unlike programs for air and biota, there is not an organized effort to monitor soils. Another problem is that residues in agricultural soils are highly variable (3), which creates difficulties in assessing the reservoir of pesticides in soil and selecting representative concentrations for modeling emissions. Identifying sources Ratios of parent compounds to metabolites and proportions of components in technical mixtures have been used to infer sources. Total DDT residues in the upper slices of peat cores from the Great Lakes and eastern Canada regions contained a high proportion of parent DDT relative to the metabolite DDE, leading to the hypothesis that "new" DDT continued to be atmospherically transported from Mexico, Central America, and Caribbean countries for years after the 1972 U.S. ban (13). Atmospheric measurements taken in the mid-1990s at Integrated MAY 1, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 2 0 7 A
Tracing pesticide origins Enantiomer ratios (ERs) can be used to trace chlordane emissions from agricultural and nonagricultural (e.g.,termiticide usage in cities) sources: (top) chromatographic profiles of nonracemic trans-chlordane (TC), cischlordane (CO and MC-5 (an octachlordane) enantiomers in the soil of an Ohio farm; and (bottom) average ERs of TC and CC in ambient air of Columbia, S.C. and the Great Lakes, and in midwestern U.S. soils.
pure y-HCH (lindane), but large quantities of technical HCH were used in Asia throughout the 1980s and to a lesser extent into the 1990s (14). The atmospheric signal today consists of lindane superimposed on a background of technical HCH, and elevated ratios of y-HCH/a-HCH indicate episodic transport of lindane from regions of current use (2). A difficulty with interpreting this ratio is mat the two isomers are removed from die atmosphere at different rates during transport, possibly due to differences in air-sea exchange or photolysis rates (2). Technical chlordane is anodier mixture containing trans-chlordane (TC), cis-chlordane (CC), transnonachlor (TN), and heptachlor as major components. In the United States, chlordane and heptachlor were used in agriculture until the mid-1970s and as termiticides until 1988 when registrations were cancelled. The proportions of TC:CC:TN in the ambient air of Columbia, S.C, where chlordane was used for termite control, were quite close to diose in the technical product after accounting for differences in volatility (9). Residues in agricultural soil vary in TC:CC:TN (3) due to differences in rates of metabolism and physical dissipation. Selective removal of TC takes place during atmospheric transport, possibly by photochemical reactions, so that ratios of TC/CC are generally
