ReviewThe mobility and degradation of pesticides in soils and the pollution of groundwater resources
Introduction
Any substance or mixture of substances intended for preventing, destroying, repelling, or mitigating any pest or weed is a pesticide. Pesticides can be classified according to their target, their mode or period of action, or their chemistry (Table 1). More than 500 different pesticide formulations are being used in our environment, mostly in agriculture (Azevedo, 1998), although the control of biological public health hazards also continues to be an important field of application. In the last 50 years, the use of pesticides has greatly increased the quantity and improved the quality of food for the growing world population. However, with increasing amounts used, concern about their adverse effects on nontarget organisms, including human beings, has also grown. Nontarget pesticide poisoning has been identified as the cause of fish kills, reproductive failure in birds, and illness in humans (Rao et al., 1993). In fact, it has been estimated that less than 0.1% of the pesticide applied to crops actually reaches the target pest; the rest enters the environment gratuitously, contaminating soil, water and air, where it can poison or otherwise adversely affect nontarget organisms (Pimentel and Levitan, 1986). Furthermore, many pesticides can persist for long periods in an ecosystem—organochlorine insecticides, for instance, were still detectable in surface waters 20 years after their use had been banned (Larson et al., 1997); and once a persistent pesticide has entered the food chain, it can undergo “biomagnification”, i.e., accumulation in the body tissues of organisms, where it may reach concentrations many times higher than in the surrounding environment (Brewer, 1979). It is presumably because of the similarity of the threats they pose to health and the environment that in U.S. law the term “pesticide” is defined to cover not exclusively pesticides, but also defoliants, desiccants and plant growth regulators used for different purposes than pest control.
Information on the actual input of pesticides into the environment is crucial for proper risk assessment and the rational design of risk reduction measures. The greatest concern regarding human exposure to pesticides is their presence in water (Younes and Galal-Gorchev, 2000). In 1999, the U.S. Geological Survey found widespread contamination of U.S. water resources; in particular, more than 95% of samples collected from streams, and almost 50% of samples collected from wells, contained at least one pesticide (Robert et al., 1999). The U.S. Environmental Protection Agency (EPA) had previously reported that normal agricultural use had led to the presence of at least 46 pesticides in groundwater and 76 in surface waterbodies (Larson et al., 1997, USEPA, 1998); the most frequently found was atrazine (Hallberg, 1989, Stoltenberg et al., 1990, Jayachandran et al., 1994), more than 4000 tonnes of which were used in North American agriculture in 1989 (Environment Canada, 1993). Atrazine was also the herbicide found most often in the period 1993–1995 in surface waters of central and north-western New South Wales, although the pesticide found most often was endosulfan, due to its widespread use in irrigated cotton production and oilseed crops; other pesticides detected included pronofos, dimethoate, chlordane, diuron, prometryn and fluometuron (Cooper, 1996). All pesticides in groundwater, and most residues present in surface water enter via the soil. There are two main routes by which pesticides enter the soil: spray drift to soil during foliage treatment plus wash-off from treated foliage (Rial-Otero et al., 2003) and release from granulates applied directly to the soil (López-Pérez et al., 2006) (Fig. 1). It is of paramount importance to study the dynamics of pesticides in soil: sorption–desorption (Arias-Estévez et al., 2005a, Arias-Estévez et al., 2005b), transport (López-Blanco et al., 2005), and the dependence of transport on entry dynamics and transformation processes. When studying these issues, especially in field experiments, it is essential to use robust analytical techniques allowing complete pesticide extraction and interference-free quantification (Rial-Otero et al., 2004).
The continually growing amount of information on the behaviour of pesticides in soil, and in the environment in general, has deepened our understanding of these phenomena. This has also allowed the parameterization and testing of increasingly sophisticated mathematical models and the corresponding computer simulation programs (Larson et al., 1997, Azevedo, 1998, Reichman et al., 2000, Chen et al., 2001, Li et al., 2001). Run-off models such as CREAMS (Knisel, 1980), AGNPS (Young et al., 1986) or RZWQM (USDA-ARS, 1995), and models of pesticide transport in soil such as SWACRO (Belmans et al., 1983), MACRO (Jarvis, 1991), LEACHP (Hutson and Wagenet, 1992) or PRZM-2 (Mullins et al., 1993), have been applied to many pesticides, though generally within limited spatial and temporal windows. Well-tested integral models of this kind are necessary if simulations are to lay a basis for the formulation of rational pollution control policies and regulations. The development of geographic information system (GIS) technology, and of remote sensing, offers hopes for the imminent evolution of comprehensive pesticide transport models.
The mobility of pesticides in soil, and hence their bioavailability and transfer to other environmental compartments (the atmosphere, waterbodies), depends on the mechanisms and kinetics of their sorption on and desorption from soil particles (Moorman et al., 2001). An understanding of these processes is essential for transport modelling and the rational design of remedial and measures against pollution (Struthers et al., 1998). The main subject of this review is to study the processes that determine pesticide sorption-mobility and persistence-degradation in soils; all this with the purpose of providing advice for the development of risk assessment tools in groundwater pollution and policies on pesticide use.
Section snippets
Degradation and sorption
The behaviour of pesticides in soils is governed by a variety of complex dynamic physical, chemical and biological processes, including sorption–desorption, volatilization, chemical and biological degradation, uptake by plants, run-off, and leaching (Table 2). These processes directly control the transport of pesticides within the soil and their transfer from the soil to water, air or food. The relative importance of these processes varies with the chemical nature of the pesticides and the
Pesticide fluxes in soil
There is evidence that chemicals applied to the soil surface may be transported rapidly to groundwater, bypassing the unsaturated soil zone (Johnson et al., 1995). The hypotheses proposed to explain this rapid transport include preferential flow (Elliott et al., 2000, Roulier and Jarvis, 2003), co-transport with colloidal matter (Worall et al., 1999, Hesketh et al., 2001), and a combination of both these processes (Williams et al., 2000). The rate and magnitude of rapid transport seem to be
Improved formulations and application techniques
Use of pesticides in agriculture will lead to their occurrence in non-agricultural environments. Therefore, the present environmental concerns over agrochemical residues in the atmosphere, water, soil and foodstuffs will not vanish. To guarantee minimal negative side-effects in ecosystems other than the soil–plant system, pesticides, whether natural or synthetic, should have no or low toxicity, except for the target organisms. There seems to be a great potential to develop microbially derived
Multidisciplinary approach to environmental research
An understanding of the fate of pesticides is essential for rational decision-taking regarding their authorization. The retention of a pesticide by soil can prevent its short-term access to ground or surface waters and its effects on nontarget organisms, but the persistence of the undegraded pesticide or of harmful metabolites constitutes an ever-present – and cumulative – risk to the environment and, eventually, to human health. Thus, it is necessary to understand both the processes involved
Acknowledgements
This work was supported by the Xunta de Galicia under contracts PGIDIT03PXIB38302PR, PGIDIT03RAG38301PR, PGIDT03PXIC20905PN and PGIDIT04TMT209003PR, and by the Spanish Ministry of Science and Technology under contracts AGL2003-02244, CTQ2005-04779, AGL2006-04231 and AGL2007-62075. We are also grateful by the Ramón y Cajal contracts awarded to Manuel Arias-Estévez and Eugenio López-Periago, and also by the Parga Pondal contract to Elena Martínez-Carballo.
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