Donald G. Crosby Department of :Snvironmental Toxicology University of California Davis, California
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lU WHAT HAPPENS TO HERBICIDES? Donald G. Crosby Department of :Snvironmental Toxicology University of California Davis, California Pesticide use data from the California Department of Agriculture a?Xl the University's San Joaquin Valley Project indicate that, over the past decade, at least 150 million pounds of synthetic pesticides have been applied here in the San Joaquin Valley (1). Herbicides make an important contribution to this remarkable total: in 1970 alone, over one million pouoos of just the top six organic weed killers - 2,4-D, trifluralin, DNre, MSMA, simazine, am. dalapon - left the bins, spray tanks, and drills to enter some phase of the Valley environment. Now, discounting formulation carriers, an accumulated 100 million'. pounds.of dry materials spread evenly over this agricultural area can be calculated to produce a visible layer, and residue analysis could almost be made with a butcher's scales. Obviously, such is not the case. In fact, depending upon when samples were taken, residue analyses most often reveal only minute traces of the common pesticides at best. Herbi cides usually don't persist. Historically, and with full force into the l970's, pesticide residues seem to provide the focus for concern; low residues mean 11 nonpersistence," and everyone from the President's Scieooe Advisory Committee to the corner 11 ecologist11 knows that nonpersistence is GOOD. In high school, most of us had to memorize the Law of Conservation of Mass: "Matter is neither created nor destroyed, but it may be changed from one form to another." However, some spokesmen (other than nuclear physicists) are talking as though that law had been repealed. The fact remains that nonpersistent pesticides don•t just "disappear." Herbicides redistribute themselves in the environment according to the response of their particular chemical construction to basic physical laws. They rapidly become bound to soil particles from which they may be leached by water or vaporized into the atmosphere or, still bound, transported as dust or silt. To an 'undisclosed extent, then, herbicides are ''lost" by environ mental dispersion and dilution. However, whether they exist in soi 1, air, water, or living matter, they undergo continual and o~en rapid transfor mation into other substances. Plants, animals, and microorganisms share powerful transformation mechanisms~ oxidation, reduction, hydrolysis, and certain synthetic reactions. Considering the environioontal reagents available, it should not be surprising that nonbiological forces -- atmos pheric oxygen, hydrogen-donating solvents, water, and sunlight~ also provide the sa~ transformation products in the absence of living things (Fig. l (2). A chemical's outdoor life, however useful or notorious, is a downhill process aimed eventually at simple inorganic products such as water, carbon dioxide, and chloride ions. 113 OCHO OH 0 H ) 0 Cl Cl y Rl ~ OH2COOH 2COOH 6H 0 0 . R > > > ---~ Polyrn er ~ ~ 6 Cf 0OH H\. y ?CHO of.12 COOH 0 ? ~ 0 OH OH Fig. 1. Environmental transformations of p..chlorophenoxyacetic acid (4-CPA) in light (3). 0 =oxidation, R =reduction, H = hydrolysis. Considering this inevitable end, of what consequence are the inter mediate transformations? A few examples of recent research from our labora tory will indicate some of the possibilities. I'-bst often, the environ mental transformation products of herbicides show greatly reduced phytotoxicity; that is, they are detoxication products -- for plants. Unfortunately, this does not insure that they necessarily are detoxication products for animals also. For illustrati::>n, monuron [l,1-dimethyl-J (p-chlorophenyl)ureaj in water is converted !!.§ p..chloroaniline into p-chloronitrobenzene by light or certain aquatic animals such as frogs. Whereas even relatively large amounts of the herbicide are harmless to other animals such as fish, chloronitrobenzene is much more poisonous to them; harmful levels of this toxicological "activation product" could re sult from the photolysis or metabolism of non-toxic doses of monuron. Actually, transient activation products probably are responsible for the selective herbicidal activity of many chemicals. For example, we have evidence that 2,4-D (2,4-dichlorophenoxyacetic acid) and perhaps other 4-chlorophenoxyacetic acids are oxidatively decomposed (:netabolized) in 114 sensitive plants to produce the actual herbicidal chemical, chloroacetio acid (Fig. 2) (4). Resistant species or individuals also metabolize the phenoxy herbicides, but the process results in harmless products instead. zCOOR AlOOR 0 OH > > y 0 ~o Cl OCl Ct 2,4-D Chloroacetic Acid Fig. 2. ~tabolism of 2,4-D in sensitive plants (4). The hypothesis of herbicidal action based on a balance of detoxi cation and activation provides an understanding of selectivity, helps to explain some rather confusing structure-activity relations (e.g., that 2,4-D and 4..CPA regulate plant growth while 2,6-D and 2-CPA are inactive), and underscores and toxicological importance of "minor" metabolites which actually may appear to exist at only low levels because of their extrelll:3 reactivity. Nonbiological forces can cause other toxicologically significant changes. In the presence of sunlight, aqueous solutions of phenoxy herbicides were converted into the corresponding chlorophenols which, in turn, were predicted to generate the highly poisonous chlorinated dioxins found as impurities in certain lots of 2,4,5-T (2,4,5-trichlorophenoxyacetic acid), PCP (pentachlorophenol), and other herbicides (5). Although dioxin for.nation indeed was detected when solutions of sodium PCP were exposed to ultraviolet light, the dangerous tetrachlorodibenzodioxin could not be detected in irradiated 2,4,5-T or trichlorophenol solutions, apparently because of its own rapid photodecomposition to nontoxic products (Fig. J) (6) • 115 too "V 80 80 1 -0 ~ ),. ~ r: u ., 60 92 C t :$ ~ +.; Ill Q) '-' 'l.. ~ 4-0 & 8 0 17. (0 24 Irradiation (hours) Irradiation (hours) Fig. J. Photolysis rate of 2,3,7,8- Photolysis rates of tetrachlorodibenzo-p..dioxin (5 mg/liter chlorinated dibenzo-p..dioxins in in methanol) in sunlight. !IJ:3thanol under ultraviolet light: 2, 7-dichlorodibenzo-p..dioxin ( III) (5 :ng/liter); 2,3,7,8-tetrachlorodi benzo-p..dioxin (I) (5 mg/liter); l,2,3,4,6,7,8,9-octachlorodibenzo p-dioxin (IV) 2.2 mg/liter). Perhaps we need to be re:ninded again that almost all toxicity measure ments are made indoors under highly artificial coniitions and that even sub stances determined to be extremely poisonous under these conditions may become environmantally insignificant when exposed, at appropriate levels, to nature's degradative forces. Herbicide decomposition can have immediate practical effects. Although NU (1-napthaleneacetic acid) has been especially important as a fruit thin ner for the California olive industry, its effectiveness often has been irregular. NAA now is recognized to be very unstable to light (7), am its utility is affected by how much sunlight a sprayed leaf receives. (Note the rapid loss of NA.A. from the olive surface indicated in Fig. 4) • other herbicides such as nitrofen (8) and trifluralin (9) also can be rapidly de composed by sunlight under practical conditions. Once absorbed, the NAA socn is converted to other compounds by plant '.118tabolism (Fig. 4) (10). Some of us believe that the relative rates of the environmental transformations of herbicides (and other pesticides), as they lead to de toxication or activation, actually form the basis for the selective use of chemicals. Future exploitation and !?18nipulation of these rates and processes could lead to rather close control of persistence and biological action. 116 4.0 FIGURE. 4 i;,<trctcta.bl e R.ud ioa.c:.tlv\ty frorn NAA- frectted Ol'i\/e.5 6 TotQ\ Ra.<1ioa.ct1"ity (a.s NA~) 3.0 0 Surta.ce R.o.di OQd.i '4 it)' (Q..5 NA f\) ~ E 0. 0 Pu.Ip Ra.dioa.c:tiv1t>' (as NA A) Q. ._J <( <r. 2 2...0 l.O - 0 I I ~ 2 4 6 8 10 12. f 4- I~ \8 20 TIME lWEEKS) 117 For example, the vast energy of sunlight might be employed to decontami nate accidental over-application, spills, or wastes (11), and our labora tory's research in this area now appears quite promising. Adjuvents such as polymers already have been shown to allow control over the transport and deactivation of insecticides (12); a synthetic enzyme inhibitor (FC!oe, or p..chlorophenyl N-methylcarbamate) is used to slow the deactivation of certain herbicides (lJJand Dr. Kay's paper describes the application of carbon to intentionally deactivate others. Sufficient understanding of the mechanisms which underlie chemical and biochemical transformations in the environment can lead to deliberate variation of the selectivity or specificity of almost any herbicide. In fact, the future utility of herbicides will depend in many instances upon this understan:iing. While future control over the environ.inental breakdown of herbi cides is m:>stly speculative, and even the significance of the photode composition of dioxins or nitrofen can be argued, at least one aspect of the subject has become very real. The federal government now requires extensive (and expensive) information about the environmental fate and movement of every new product seeking registration: dissipation (such as soil leaching and runoff), photodecomposition, degradation in water, and the metabolism by microbes, plants, and animals are representative. It seems inevitable that the same intimate knowledge of every pesticide eventually must be secured. The resulting financial as well as toxicologi cal problems already have resulted in the witlxirawal of promising compounds and even of entire agricultural che:n:ical programs. We are having to seek the real consequences of pesticide breakdown whether we wish to or not. What happens to herbicides? Except for relatively isolated examples such as those DSntioned above, no one really knows. There is no instance in which the environmental movement and fate of a herbicide is completely known; we have been very tardy.