Miljøprojekt, 902 – The Influence of Sorption on the Degradation of Pesticides and other Chemicals in Soil

The Influence of Sorption on the Degradation of Pesticides and other Chemicals in Soil

1 Introduction

Many reported studies in which the fate of pesticides or other chemicals in soil have been studied have shown that it is exceedingly difficult to draw general conclusions about the degradation of these substances in soil compared with the soil parameters. A number of indexing methods have been developed that from few parameters for each substances aim to rank the pesticides according to the risk of leaching to the groundwater. E.g. the GUS-index ranks the pesticides solely according to inherent properties, degradability (measured as DT₅₀-values), and sorption (measured as K_OC) and in this way provides a measure of the potential of leaching. Lindhardt et al. (1998) showed that a ranking according to the GUS-index with a starting point in the DT₅₀ and K_OC –values of 12 pesticides that had been reported to the Danish Environmental Protection Agency involved great unreliability stemming from the large variation in the data material. For 12 pesticides (atrazine, bentazone, diuron, glyphosate, isoproturon, MCPA, mecoprop, metamitron, metsulfuron-methyl, phenmedipham, propiconazole, and triasulfuron), data of degradability DT₅₀ and sorption K_OC were collected in order to evaluate the dispersion on these parameters. The calculated GUS_min and GUS_max show that 11 of 12 substances (glyphosate left out) for extreme combinations of degradability and sorption conditions, based on information in the material that underlay the approval of the substances by the Danish Environmental Protection Agency, must be rated as "probably leachable" and that 8 of the 11 pesticides under other circumstances must be rated "probably not leachable".

It has often been assumed that chemicals in soil are degraded according to simple first-order kinetics from which a half-life is deduced. However, it has emerged from many cases that the degradation does not follow simple first-order kinetics. The cause for these reported deviations from simple first-order kinetics may probably be found partly in the microbial environment in the soil, partly in the fact that the sorption over longer time (and the formation of bound residues) take place according to more complicated processes than generally assumed.

In some connections authors choose to distinguish between the meanings of the expressions "half-life" and "DT₅₀-value". "Half-life" is used when it is a question of first-order kinetics where the time for a number of halvings is the same, whereas "DT₅₀-value" is used about the time that can be stated for a single halving (frequently the first one) but where the time varies for the following halvings.

The present report does not distinguish between the meanings of these two expressions.

According to the Uniform Principles Directive, 94/43 (OJ L227 31-55), a pesticide - if it is to be approved - must have a half-life of less than 90 days unless it can be shown that it is very probable that there are no harmful effects. The Danish Environmental Protection Agency generally accepts half-lives of 3-6 months if supplementary ecotox studies are carried out showing that the substance has no undesirable adverse effects.

Glyphosate is an example of a substance which is bound extremely strongly to soil and which therefore has a very variable half-life. The assessment whether pesticides can be approved in spite of their long half-lives may depend on e.g. the type of binding.

The pesticides are the group of substances of which by far the most information is available as to studies of the fate of chemicals in soil. This goes for both earlier applied hydrophobic substances such as DDT and similar substances and all the later developed substance groups of pesticides. In order that pesticides may be used, they must pass the approval system of the Danish Ministry of the Environment. Even though the pesticides have been approved for use in practical farming, there has generally been large interest within the research community in carrying out supplementary studies of the fate of the substances in the environment. And, indeed, a number of cases are known where the approvals have been withdrawn because of published studies.

Within the last 10 years, there has been an interest in studying the fate of other chemicals in soil as the widespread practice of spreading sludge from sewage treatment works on to the agricultural soil has made it necessary not just to introduce limit values for the presence of chemicals in sludge but also to study the fate of the permitted concentrations of these chemicals in agricultural soil. The chemicals in question may be antibiotics, other medicinal substances, biocides, and detergents. Finally, f_OC us has been put on the fate in soil of other chemicals, which among other things are present in the soil as industrial pollutions, as it – in the last cases – frequently is a question of the need to throw light upon purification techniques such as bioremediation.

Just as pesticides are the most studied substances, pesticides are also the substances for which the most specific legislative requirements exist as regards their presence in and effects on the environment. In drinking water the concentration of a pesticide must not exceed 0.1 µg/l. Studies of the fate of the substances in soil have thus always been carried out with the aim of predicting whether the limit value for the presence in drinking water can be exceeded. But there are no legislative requirements concerning either the quantities of pesticides that may be present in soil or the form of the pesticides.

All known models used for describing the fate, transport or risk evaluation of chemicals in soil include terms describing binding and degradation, respectively. Thus, an understanding of the dynamics of these parameters is unavoidable if the models are to be used successfully.

One has to distinguish between chemicals that normally resist biological degradation (recalcitrant), such as PCBs and PAHs (polychlorinated biphenyls and polyaromatic hydrocarbons) and more readily transformable substances such as modern pesticides. PCBs and PAHs are hydrophobic, and their binding to the soil is caused by van der Waals forces or interaction with lipophile constitutents in the soil. Thus, these substances are not particularly available to microbial degradation. For the same reason a large part of the literature on bioremediation of these substances f_OC uses on how the substances can be made available.