Pesticide biotransformation and fate in heterogeneous environments

The effects and relative impacts of environmental variables on the behaviour of pesticides, through the effect on pesticide-degrading microorganisms, was studied in a broad spectrum and covered the most relevant emission routes. It is shown that the effect of landscape geochemistry, which is a pre-set condition in an agricultural management, may be significant (chapter 2). Adjoining soil types, which occur within short distance in an agricultural unit, were characterized and tested on their pesticide leachability and potential risk to groundwater. The use of a ¹⁴C radio-labelled pesticide enabled accurate study of its movement in soil layers. Distinctly different pesticide behaviour in topsoil and subsoil layers were observed within this soil series sequence. An alternative of conventional (i.e. uniform) pesticide application rates, which generally results in over-treating some sites and under-treating others, is site specific treatment. In such managements, doses are tailored to individual soil types or soil properties. This may be beneficial from both an economical as an environmental point of view. A prerequisite in such pest managements is the accuracy and reliability in which the leaching potentials of pesticides in various soils are predicted. Of much concern is the fact that there appears to be little agreement between the existing pesticide leaching models in predicting the residence time in soil and subsequent leaching to groundwater (chapter 3). This may be especially valid in soils that display preferent flow patterns, in which solutes by-pass the soil matrix without being subjected to significant sorption and degradation. It was concluded that the introduction of a variety of conceptual descriptions, thus covering ranges in specific soil properties, may improve the applicability of such models. From an environmental point of view, it is necessary to approximate concentrations not only in terms of magnitude but also at the time and the duration at which these occur. With conventional application rates, distinctly different leaching behaviour was observed in a clay lysimeter for aldicarb, a carbamate nematicide, and simazine, an s- triazine herbicide. Aldicarb leached almost instantaneously, but its aerobe metabolites were found in very high concentrations which did not meet the EC-norm level for water over a period of 300 days. A mass balance for aldicarb showed that 0.35% of the initial dose had leached from the soil. However, when the two isosteric metabolites aldicarb-sulfoxide and aldicarb-sulfone were included in the mass balance, this percentage increased dramatically to almost 20%. The concentrations of both metabolites far exceeded those of the parent compound over a long period. Aldicarb is isosteric to acetylcholine, which plays an important role in the nerve system of organisms, and is therefore capable of inhibiting the performance of acetylcholine-esterase. Since aldicarb's metabolites are oxidation products, in which the critical molecular length is not altered, it may be assumed that aldicarb-sulfoxide and aldicarb-sulfone are capable of doing the same. Therefore, the toxicity of these metabolites may be at least additive. In contrast, simazine leaches in relatively low concentrations - only 0.11% of the initial dose was recovered - but these concentrations were measured over a very long period. The absence of a 'breakthrough behaviour' (peak exposure to aqueous environments), as was observed for aldicarb, implies long term delivery (chronic exposure) of simazine from the soil. To better quantify the effects on intrinsic pesticide behaviour, the relative impact of individual soil characteristics and environmental properties was discussed (chapter 6). For the herbicide metamitron, it is shown that transformation rates were dictated, in order of significance, by temperature, oxygen availability and sorption to organic carbon. Since these variables show large variation in depth of the soil column, the resulting biodegradation rates may change dramatically. Half lives increased from several days in the topsoil to over one year in subsoil layers. The underlying dynamics of microbial growth and development that are proposed (chapter 4 and 5) show that a multi-layered approach and the assignment of realistic concentrations and conditions to soil layers may improve approximations of microbial inhibition and growth in depth. These nonlinear dynamics can be linked, simultaneously, to pesticide metabolism and disappearance rates, using parameter estimation techniques.

Transformation rates and pathways that occur over the soil-aqueous transition zone (i.e. aerobic topsoil-subsoil- anearobic sediment) were tested extensively for four distictly different pesticides that represent their chemical group (chapter 7). It is shown that the prevailing redox conditions have a large impact on pesticide transformation rates. Some phenoxy-acetic compounds, which are considered improbable leachers based on their short aerobic half lives, appear to be persistent in low-oxygeneous conditions. The opposite effect was observed for aldicarb, in which chemical catalysis increased transformation rates when redox potentials decreased. It is shown that a temporal but severe period of oxygen inhibition can be survived by the microbial population. The involved microorganisms can temporarily decrease their activity and can recover within some days from a 109 day stress period. A dynamic chain reaction model is presented, which describes the formation of metabolites from the parent compound, and subsequent transformation, as an interactive, concentration-dependent process.

An attempt to identify the major discriminating variables that determine the fate of pesticides in surface waters was undertaken (chapter 8). A large set of environmental parameters, composed of physico-chemical, bio-chemical and chemical characteristics, was reduced to three major component groups, explaining the majority of variance of transformation rates of four pesticides that were observed in a variety of surface waters. The first component contains variables that promote biorespiratory processes. The second component is a macro/micro- nutrient group. The third component is the phosphorous group. It is shown that small, lotic systems such as field ditches have a larger potential to degrade specific compounds than large, lentic systems, such as channels and lakes. This effect is largely attributed to microbial activity and the possibility of a relevant community to develop. The specific role of Mg/Mn and phosphorus concentrations in nitrifying surface waters on biotransformation rates is identified (chapter 9). Large phosphorus concentrations favour bacterial growth, but a large fraction of less available phosphates may inhibit transformation rates of aldicarb. Addition of orthophosphate increased the residence time of dissolved Mn, which may under certain conditions promote biotransformation rates. Furthermore, PO ₄ enrichment may decrease concentrations of aldicarb's metabolites. In a mechanically aerated batch experiment, it was shown that simazine is virtually persistent except for a short period that coincides with the nitrification process in which NH ₄ dissipates and NO ₂ and NO ₃ are formed. Respectively, relationships of Mg/Mn concentrations with MCPA transformation rates, and P/PO ₄ concentrations with aldicarb transformation rates, are presented. These relationships may be used to assess these elements as environmental indicators for potential biotransformation of these compounds, or members of the chemical group, but only in combination with conditions that warrant the development and growth of a degrading population over a longer period of time. An illustration of the effects of the individual properties, that were identified in chapter 8 and 9, is given in figure 1. The relationships between these properties were previously discussed in detail. It is generally believed that the dissolved fraction of a compound, as opposed to the sorbed fraction, is much better available to microorganisms and is therefore degraded rapidly. For surface waters however, it is likely that sorption may in fact enhance biodegradation by concentrating the target compound, by concentrating nutrients, and by providing a large surface area for the attachment of bacteria.

It should be emphasised that the observed behaviour of the studied pesticides is not restricted to the individual compound, but may represent analogies for compounds within their chemical group. It is evident that laboratory breakdown tests, which are mostly conducted in a batch- type set up, should include system characterisation, and should recognise the influence of alterations in these characteristics. Co-precipitation, which is a common phenomenon in heterogeneous solutions, may lead to deficiencies of essential elements that are utilised by pesticide degrading microorganisms, and therefore affect the interpretation of such experiments.

A key issues in pesticide risk assessments is the fact that many compounds are readily transformed to compounds which are toxic to target and non-target organisms throughout the environment. Organophosphate and organosulfur insecticides commonly have initial transformation products with well-established insecticidal activity, often of greater potency than the parent compound. A common reaction observed in many sulfide- containing pesticides is oxidation to sulfoxides and sulfones which are usually active on a spectrum of pests similar to the parent compound. The formation of aldicarb sulfoxide and sulfone, which is described in chapter 7, is an example of this. The simultaneous occurrence of parent compound and oxides may even lead to an increased toxicity. Of much concern is the fact that these toxicologically active transformation products tend to be more mobile than the respective parent compound (Chapter 3). Thus, there is an underestimation of the environmental risks when the parent compound rather than the residue concentrations are used. It may well be stated that metabolite formation must be considered a key issue in pesticide risk evaluations that consider the terrestrialaquatic emission route. However, there is much hesitation to study pesticide transformation products. This is due to mainly four arguments:
1. High costs are associated with the analyses of the numerous possible compounds.
2. The increase in polarity makes the isolation and analysis of metabolites often more difficult than the analysis of the parent compound.
3. If a metabolite standard is not available, synthesis may be required.
4. The problem of which metabolites to identify, to prioritise, and to assign as important, environmental risk indicators.

The last argument is of particular importance, and future scientific efforts should focus on this issue. The recent advances in solid phase extraction (SPE) techniques, which have increased the ability to isolate metabolites from water, have attributed significantly to the feasibility of this goal.

A second key issue in risk assessments originates from the fact that the environment is subjected to a

range of pesticides that occur simultaneously. This is true for soil, but may be of particular importance for surface waters and aquatic sediments. According to the published literature, the toxicity of many pesticide combinations is at least additive. In some cases, pesticide mixtures - particularly those involving insecticides - have been shown to be synergistic. The bilateral effects in pesticide mixtures on sorption has also been discussed in chapter 7. The most appropriate approach to minimising risks for pesticide mixtures appears to be to assume additive toxicity in all cases, which may include the possible formation of specific metabolites. Still, the problem remains of identifying the environmental compartment that dictates the bottle-necks in risk assessments of a specific pesticide or chemical group.