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

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

8 Methods for measuring the influence of sorption on the degradation

As it emerges from the chapters above, there has in recent times been a great deal of work going on carrying out studies where measurements of sorption are linked to measurements of the degradation, either through model descriptions of individual test readings of sorption and degradation, respectively, or through practical experiments where the sorption and the degradation are measured in the same series of experiments and a model is developed that describes the experimental results. The latter, by some called "reverse modelling", must be assumed to be the best solution, if the sorption- and degradation module in a leaching model such as MACRO is to be made more complex than it is in the present version, which uses K_d-value determinations for sorption specifications and simple first-order half-lives for specification of the degradation rate (Jarvis, pers. comm.).

As mentioned in the part "Bound residues", spectroscopic methods that identify the type of binding between foreign chemicals and the soil organic matter are becoming wide-spread. A further development in this area is to be expected. Moreover, there have been attempts at studying the amount of dissolved substances in the soil water under circumstances where the ratio between soil and water was natural to be able in this manner to describe the amount of pesticide in the soil water over time. For this purpose a high-speed centrifugation technique (Ronday, 1997) and a technique using glass microfibre filters for collecting small amounts of soil water have been used (Gaillardon et al., 1991).

The difficulties of deducing from chemical measurements on residues of pesticides and other chemicals whether there will be an undesirable effect on the soil biota (earthworms, insects, plants, and microorganisms) and presumably also on humans are obvious. Therefore, there is at present a large f_OC us on developing methods and descriptions that can relate these to one another. Alexander (2000) stressed that, as chemical analytical methods used at present overestimate both exposure and risk of foreign chemicals in soil, it is important to develop methods for description of bioavailability. Such methods have so far predominantly been biological, but it is also necessary to develop chemical methods that describe bioavailability, as chemical methods are easier to handle than biological (Alexander, 2000). Kelsey et al. (1996) compared the amount of atrazine and phenanthrene that could be extracted by various methods of extraction with the amounts that could be absorbed by earthworms and/or degraded by bacteria. For one of the extraction agents, a good correlation was found. White et al. (1999) showed that in an aged soil with residues of phenanthrene the mineralisation curve and the desorption curve had the same slope, which indicated that the desorption was determinative for the mineralisation rate and thus for the bioavailability. Kelsey and Alexander (1997) compared the amounts of atrazine, phenanthrene, and naphthalene that were assimilated by earthworms with the amounts that could be extracted from soil with mild and strong extraction agents and found the best correlation when using a mild extraction agent. Kelsey and Alexander (1997) therefore stated that strong extractions should not be used if one wishes to estimate exposure and risk for selected organisms. Kelsey et al. (1997) presented selected chemical extraction agents to be used for predicting the bioaccessibility of chemicals in aged soil. Szmigielska et al. (1998) showed that anion interchange membranes could extract amounts of metsulfuron from soil that could be related to the biological effects on roots from lentil. At present Koskinen (pers. comm.) is working on studies of the application of genetically manipulated bacteria for determination of bioavailability of foreign chemicals in soil. The bacteria have had a gene incorporated, which means that they have a very well-developed ability to degrade a specific substance. Degradation experiments with the specific substance in a radioactive version will then provide a measure of the availability of the substance to degradation.

For the present the National Academy of Science in the USA (2001) has set up a group of experts (the National Research Council Committee on Bioavailability of Contaminants in Soils and Sediments), which is to evaluate the scientific status concerning processes that influence whether chemical pollutions in soil and sediment are available to humans, animals, and plants. The group of experts will also evaluate the existing methods and methods which are being developed for the determination of bioavailability as well as assess how the bioavailability influences the effectiveness of bioremediation techniques.