Centre for Contaminant Geoscience and Silja Kuerzinger – Former Centre for Contaminant Geosciences
Contamination of soil, ground and surface waters by pesticides is a huge environmental issue, and a thorough understanding of chemical and biological properties of pesticides is necessary to assess and solve the mechanism of off-target movement and problems of environmental persistence.
Pesticides with extremely long half-lives, such as DDT (up to 10 years) and other organochlorine pesticides, have been banned in Australia since the early 1970s as the build-up of bio-accumulation has been detected up the food chain, both in aquatic and terrestrial systems.
The general accepted view within the community is that the faster a pesticide breaks down in the environment, the better that pesticide is, however there is a slight misconception in this thought process: most agricultural pesticides must have some level of persistence and stability in the intended environment or host material in order to be effective. For example, if long term weed or insect control is required, it is inappropriate to apply a chemical which will last only one day or until the first shower of rain. Whether a compound lasts in the environment for two days or two years is not an environmental issue unless the compound is no longer required or the compound is capable of moving to another location or environmental compartment.
Another issue is that the prediction of half-life or degradation rates of pesticides involves a complicated interplay of variables, and therefore specific time frames are almost impossible to determine. Environmental factors such as soil type, soil water content, pH, temperature, clay, organic matter content and oxygen levels influence the course and rate of pesticide degradation. Molecular factors include chemical structure, molecular weight, functional groups (CH3, COOH, OH), concentration, toxicity and solubility. Depending on conditions, various processes can occur, including retention (adsorption desorption), transformation (microbial or chemical degradation), volatilisation and transport (overland flow, leaching).
An example relevant to most people is in the use of chlorpyrifos, an organophosphate insecticide which is not only widely used in agriculture, but also under homes. It has a reported half-life in soil varying from less than 10 days to greater than 120 days depending
on the different mechanisms involved in the pesticide degradation process in protected areas such as under buildings or slabs.
The trend now is for the synthesis of new pesticides which have the desired characteristics of sufficient longevity to destroy the pests without remaining persistent in the environment. Also, less halogenated compounds are being produced with the aim of non-biological factors initiating the breakdown mechanism, including ultra-violet light and chemical hydrolysis.
Organophosphate pesticides (OPPs) have been used extensively in agriculture as replacement chemicals for organochlorine pesticides (OCPs) as they are more readily biodegradable than the their more persistent chlorinated counterparts. Under aerobic conditions, concentrations of organophosphate pesticides in soils are often non-detectable within a year of application.
The organophosphates are characterised as having the general formula of the type shown in Figure (a). Examples include diazinon, disulfoton, malathion, parathion, chlorpyrifos, trithion, fenthion and phorate. The time for 75-100% degradation of these chemicals is between 7 days and 12 weeks, which is substantially faster than for chlorinated pesticides, where DDT requires 4 years and chlordane 5 years to undergo the same levels of breakdown.
Breakdown rates of pesticides relate to reactions that occur in the transformation of individual chemical groups of pesticides. Relatively short-lived pesticides, such as organophosphates, are attacked initially by hydrolysis of the aryl phosphate bond with subsequent ring-cleavage of the benzene ring. One of the major ways this breakdown pattern occurs, is through bacterial metabolism, usually though a consortium of microbes rather than a single species. Some of the microbes involved in organophosphate breakdown include Flavobacterium, Bacillus and Pseudomonas diminuta, which can hydrolyse and cleave organophosphates such as diazinon and parathion to form DETP (diethyl thiophosphoric acid) see Figure (b).
Malathion is an example of an organophosphate chemical which has a wide range of applications, including use as an insecticide against sucking and chewing insects and spider mites on vegetables, fruits, field crops, greenhouses, gardens and forestry. In soils, malathion undergoes mainly microbial breakdown with degradation in organic-rich soils being up to six times faster than in soils without organic matter. When exposed to air, malathion can undergo volatilisation as well as be degraded by photolytic processes from UV light.
Although generally considered to be more environmentally friendly than their chlorinated counterparts, organophosphate pesticides can still have detrimental effects. Symptoms of human exposure to malathion arise from neurotoxic properties and include headache, nausea, diarrhoea, ataxia, eye and skin irritation. The LD50 limits for malathion, lethal dose to kill 50 per cent of a population, range from 600 ppm for chickens to as little as 100 ppb for rainbow trout.
As organophosphate pesticides will continue to be used in Australia in the foreseeable future, it is important that they are managed so as to protect the environment. This involves having a thorough understanding of their persistence and breakdown patterns in soils and waters, as well as minimising misuse of these chemicals.
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