ORGANIC POLLUTANTS: AN ECOTOXICOLOGICAL PERSPECTIVE - CHAPTER 13 pps

PART 3 Further issues and future prospects © 2001 C. H. Walker CHAPTER 13 The ecotoxicological effects of herbicides 13.1 Introduction Chapters 5–12 deal with groups of pollutants that have been studied in some depth and detail, largely because they have appreciable – sometimes very high – mammalian toxicity and are perceived as human health hazards. Some of them are markedly persistent and undergo biomagnification with passage along food chains. As has been explained, individual compounds, or mixtures of related compounds, have sometimes been shown to cause adverse ecological effects. Although it was necessary to take such pollutants as examples, it is important now to consider the more complex situation in which organisms are exposed to mixtures of compounds differing in their chemistry and/or mode of action. The present chapter is the first of the final part of the text, in which the emphasis moves on from the detailed descriptions of particular types of pollutants given in the foregoing chapters to address some wider issues. Apart from the question of the complexity of pollution in the real world, certain other issues arise directly from the findings of ecotoxicological studies on individual pollutants reported in the earlier text. Herbicides constitute a large and diverse class of pesticides that, with a few exceptions, have very low mammalian toxicity and have received relatively little attention as environmental pollutants. Paraquat and other bipyridyl herbicides have appreciable mammalian toxicity and will be discussed in Chapter 14. Dinoseb and © 2001 C. H. Walker 226 Further issues and future prospects related dinitrophenols, which act as uncouplers of oxidative phosphorylation in mitochondria, are general biocides that are little used today because of their hazardous nature. They will be mentioned, briefly, in section 14.2. Herbicides are, in general, readily biodegradable by vertebrates and are not known to undergo substantial biomagnification in food chains. Their principal use has been weed control in agriculture and horticulture, although they have also been used as defoliants in forests (e.g. in the Vietnam war), for controlling weeds on roadside verges and in water courses and as management tools on estates and nature reserves. This chapter will be mainly concerned with their impact on the agricultural environment. A brief mention will be made of their wider dispersal in the aquatic environment. 13.2 Some major groups of herbicides The following brief account identifies only major groups of herbicides not mentioned elsewhere in the text and is far from comprehensive. For a more detailed account see Hassall (1990). The phenoxyalkane carboxylic acids are among the most successful and widely used herbicides. They act as plant growth regulators and produce distorted growth patterns in treated plants. Compounds such as 2,4-D, MCPA, and mecoprop (Figure 13.1) are used as selective herbicides to control dicotyledenous weeds in monocotyledenous crops such as cereals and grass. They are formulated as water- soluble potassium or sodium salts, or as lipophilic esters, and they are frequently sprayed in combination with other types of herbicides that have different modes of action and patterns of weed control. They are applied to foliage and are not soil acting. Ureides (e.g. diuron, linuron, isoproturon) and triazines (e.g. atrazine, simazine, ametryne) all act as inhibitors of photosynthesis and are applied to soil (see Figure 13.1 for structures). They are toxic to seedling weeds, which can absorb them from soil. Some of them (e.g. simazine) have very low water solubility and, consequently, are persistent and relatively immobile in soil (section 4.3). Sulphonylurea herbicides such as chlorsulfuron and sulfometuron are also soil acting, affect cell division and have very high phytotoxicity. Indeed, they can be toxic to plants when present in soil at levels low enough to make chemical analysis difficult. Carbamate herbicides constitute a relatively diverse group. Some, like barban (Figure 13.1), are applied to foliage, whereas others (e.g. chlorpropham) are soil acting. The latter type affect cell division. Other important herbicides, or groups of herbicides, include glyphosate, aminotriazole, chlorinated benzoic acids (e.g. dicamba) and phenolic nitriles (e.g. ioxynil, bromoxynil). © 2001 C. H. Walker 2,4-D O Cl Cl CH 2 COOH MCPA O Cl Cl CH 2 COOH CH 3 CH 3 Mecoprop O CHCOOH CH 3 (a) Phenoxyalkane carboxylic acids (b) Ureides (d) Triazines (e) Carbamates (c) Sulphonyl urea Cl Cl NH CO CH 3 CH 3 N 1 4 2 3 2 1 3 Diuron Cl Cl NH CO CH 3 OCH 3 N Linuron NH CO CH 3 CH 3 N CH(CH 3 ) 2 Isoproturon N 6 N Cl SO 2 CH 3 OCH 3 Chlorsulphuron NH C O NH 23 41 56 32 14 5 N 1 N Simazine N Et HH Cl 5 N 2 N N Et 64 3 N Atrazine N Et HH Cl NN N isoPr N Ametryne N Et HH SCH 3 NN N Et Chlorpropham NH CO O CH Cl CH 3 CH 3 1' 2' 3' 4' CH 2 Cl Cl NH CO CH 2 O 1 4 3 2 Barban CC Figure 13.1 Structures of some herbicides. © 2001 C. H. Walker 228 Further issues and future prospects 13.3 Agricultural impact of herbicides Since the Second World War herbicides have come to be widely used in agriculture and horticulture in the developed world. Frequently, they have been used in ‘cocktails’ containing several ingredients of contrasting modes of action, thus giving control over a wide range of weed species. The effectiveness of the application of herbicides together with cultivation of the land is evident in many agricultural areas in Western Europe and North America, where few weeds are seen. It is easier to control plants, which are stationary, than to control mobile insect or vertebrate pests. Weed species have been very effectively controlled over large areas of agricultural land. In Britain concern has been expressed over the near extinction of certain once common farmland species that are of botanical interest, e.g. corn cockle and pheasants eye. Ecologically, such a large reduction in weed species represents a major change to farmland ecosystems, and may be expected to have knock-on effects upon other species. Certain problems have come to light with the investigation of the status of birds on farmland. In one study, the Game Conservancy Council investigated the reasons for the severe and continuing decline of the grey partridge (Perdix perdix) on farmland in Britain. The study commenced in the late 1960s, and established that the decline was closely related to increased chick mortality (Potts, 1986, 2000; also Chapter 12 in Walker et al., 2000). The chick mortality was largely explained by a shortage of their insect food (e.g. sawflies) due, in turn, to the absence of the weeds upon which the insects themselves feed. An effect at the bottom of the food chain led to a population decline further up. It is worth reflecting that such an effect by herbicides could not have been forecasted by normal risk assessment (see Chapters 14 and 15). The herbicides responsible are, in general, of very low avian toxicity, and ordinary risk assessment would have declared them perfectly safe to use, so far as partridges and other birds are concerned! Subsequent work has shown that partridge populations can continue to survive on agricultural land if headlands are left unsprayed, thereby allowing weeds to survive, which will support the insects upon which young partridges feed. This study helped to ring the alarm bells about possible other indirect effects of the wide variety of herbicides used in agriculture. More recently, further evidence has been gained of the reduction in populations of insects and other arthropods on farmland that may relate, at least in part, to the removal of weeds by the use of herbicides. A survey of farmland birds in Britain has established the marked decline of several species in addition to the grey partridge, which may be the consequence of indirect effects of herbicides and other pesticides (Crick et al., 1998; also Chapter 12 of Walker et al., 2000). Species affected include tree sparrow (Passer montanus), turtle dove (Streptopelia purpur), spotted flycatcher (Musciapa striata) and skylark (Alauda arvensis). A study is currently in progress to attempt to establish the cause of these declines. Recently, concern about the side-effects of herbicides used on agricultural land has intensified with the development of genetically manipulated (GM) crops. Some GM crops are relatively insensitive to the action of herbicides, thus permitting the application to © 2001 C. H. Walker The ecotoxicological effects of herbicides 229 them of unusually high levels of certain herbicides. The advantage of increasing dose, from the agricultural point of view, is the control of certain difficult weeds. From an ecotoxicological point of view, increasing dose rates of herbicides above the currently approved levels may cause undesirable ecological side-effects. It is very important that any such change in practice is rigorously tested in field trials, as part of environmental risk assessment, before approval is given by regulatory authorities. Such new technology, based on GM crops, should only be introduced if it is shown to be environmentally safe. One problem that has arisen with the use of herbicides in agriculture is spray or vapour drift. When fine-spray droplets are released, especially if applied aerially, they may be deposited outside the target area because of air movements and cause damage there. In the first place, this is a question of application technique. Herbicides, like other pesticides, should not be applied as sprays under windy conditions. In most situations, herbicides are not applied aerially because of the danger of drift. Where herbicides have appreciable vapour pressure, there may be problems with vapour drift. Under hot conditions, volatile herbicides may go into the vapour state, and the vapour may drift further than the spray droplets. Such was the case with early volatile ester formulations of phenoxyalkanecarboxylic acids (Hassall, 1990). Nowadays, formulations are of less volatile esters or of aqueous concentrates of sodium or potassium salts (which are of low volatility). Spray drift of herbicides can cause damage to crops and wild plants outside the spray area. The cause of such damage can be hard to establish with highly active herbicides (e.g. sulphonylureas), where the phytotoxic concentrations are low enough to make chemical detection difficult. 13.4 Movement of herbicides into surface waters and drinking water As discussed earlier (section 4.3), pesticides have a very limited tendency to move through soil profiles into drainage water because of the combined effects of adsorption by soil colloids (important for herbicides such as simazine, which have relatively high K ow values), metabolism (important for water-soluble and readily biodegradable herbicides such as 2,4-D and MCPA) and in some cases volatilisation. In reality, however, there are complications. In the first place there may be run-off from agricultural land into neighbouring water courses after heavy rainfall. Soil colloids, with adsorbed herbicides, can be washed into drainage ditches and streams. There is an additional problem with certain soils high in clay minerals (Williams et al., 1996; and section 4.3). During dry periods these soils shrink and develop deep cracks. If heavy rains follow, free herbicides located near the soil surface, and colloids with adsorbed herbicides, can be quickly washed down into the drainage system without passing through the soil profile. In the Rosemaund experiment, the herbicides atrazine, simazine, isoproturon, trifluralin, and MCPA were all detected in drainage water after © 2001 C. H. Walker 230 Further issues and future prospects heavy rain. The respective maximum concentrations in micrograms per litre (ppb) were – 81, 68, 16, 14 and 47 (Williams et al., 1996). These levels were reached after normal approved use of the herbicides and raise questions about possible effects on aquatic plants growing in receiving waters. As mentioned elsewhere (section 10.3.4) the level of carbofuran found during the same study was sometimes high enough to kill freshwater shrimps (G. pulex) used as a bioindicator (Matthiessen et al., 1995). Recent surveys have been providing more information on the levels of herbicides in rivers. In one study a number of different herbicides were detected in the River Humber, UK (House et al., 1997). A number of triazines were detected in the Rivers Aire, Calder, Trent, Don and Ouse, the most abundant of them being atrazine and simazine. The results for simazine showed peaks in the spring and again in the early autumn of 1994 for some rivers, the latter coinciding with the first major storm of the year (Figure 13.2). The maximum level of simazine recorded was 8 µg/L. This was high enough to be toxic to phytoplankton and algae, but was not sustained. Phenyl ureas and phenoxyalkanoic acids were also detected. Concentrations were generally low, but levels of the following herbicides were detected up to the maximum value (µg/L) given in brackets: diuron (< 8.7), chlortoluron (< 0.67), mecoprop (< 8.2). These high levels were sporadic and transitory. However, they were sometimes high enough to cause phytotoxicity, and more work needs to be carried out to establish whether herbicides in contaminated streams and rivers are having adverse effects upon populations of aquatic plants. With the acceptable concentrations of herbicides in drinking water being taken to very low levels by some regulatory authorities (e.g. the EC), there has been interest in low levels of atrazine present in groundwater and in drinking water. This finding illustrates the point that mobility of pesticides becomes increasingly evident as the sensitivity of analysis improves. 13.5 Summary As the first chapter in the final part of the book, contamination by herbicides is taken as an example of the complexity of pollution in the real world. A wide variety of compounds of diverse structure, chemical properties and mechanism of action are used as herbicides. Very few of them have appreciable toxicity to animals, and they do not usually undergo significant biomagnification with movement along food chains. Important groups of herbicides are phenoxyalkane carboxylic acids, ureides, triazines and carbamates. Herbicides are often applied as mixtures of compounds with contrasting properties. The successful use of herbicides and associated cultivation procedures has greatly reduced the populations of weed species in many agricultural areas, sometimes bringing species of botanical interest to near extinction. Intensive weed control in cereal farming has been shown to cause the reduction of certain insect populations and the reduction © 2001 C. H. Walker The ecotoxicological effects of herbicides 231 of the grey partridge. The decline of some other insectivorous birds on agricultural land may have a similar cause. The introduction of GM crops with high tolerance to herbicides may lead to increases in dose rates of herbicides on agricultural land with attendant ecotoxicological risks. Significant levels of herbicides have also been detected in rivers, although these are usually transitory. Heavy rainfall can move herbicides from agricultural land to nearby ditches and streams because of run off and percolation of water through deep fissures in certain soils that are high in clay. Figure 13.2 Atrazine levels in the Humber River area. Comparison of the concentration of simazine and river discharge over one annual cycle for (a) River Trent at Cromwell Lock and (b) River Aire at Beale. ɂ, Simazine concentration; ɀ, river discharge. From House et al. (1997) with permission. (a) (b) Concentration (µg/L)Concentration (µg/L) Discharge (m 3 /s) Discharge (m 3 /s) Date 0.6 0.5 0.4 0.3 0.2 0.1 0 07/03/94 26/04/94 15/06/94 04/08/94 23/09/94 12/11/94 01/01/95 20/02/95 0 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0 350 300 250 200 150 100 50 11/04/95 Date 07/03/94 26/04/94 15/06/94 04/08/94 23/09/94 12/11/94 01/01/95 20/02/95 11/04/95 250 200 150 100 50 0 © 2001 C. H. Walker 232 Further issues and future prospects 13.6 Further reading Ashton, F. M. and Crafts, A. S. (1973) Mode of Action of Herbicides. This book describes the mode of action of major types of herbicides. Hassall, K. A. (1990) Includes a readable account of the biochemistry of herbicides. Potts (1986) The Biochemistry and Use of Pesticides. An authoritative account of the factors responsible for the decline of the grey partridge on agricultural land. © 2001 C. H. Walker . PART 3 Further issues and future prospects © 2001 C. H. Walker CHAPTER 13 The ecotoxicological effects of herbicides 13. 1 Introduction Chapters 5–12 deal with groups of pollutants that have been. mammalian toxicity and have received relatively little attention as environmental pollutants. Paraquat and other bipyridyl herbicides have appreciable mammalian toxicity and will be discussed in Chapter. (important for herbicides such as simazine, which have relatively high K ow values), metabolism (important for water-soluble and readily biodegradable herbicides such as 2,4-D and MCPA) and in