Sabine Beulke

Sources of uncertainty in pesticide fate modelling

There is worldwide interest in the application of probabilistic approaches to pesticide fate models to account for uncertainty in exposure assessments. The first steps in conducting a probabilistic analysis of any system are: (i) to identify where the uncertainties come from; and (ii) to pinpoint those uncertainties that are likely to affect most of the predictions made. This article aims at addressing those two points within the context of exposure assessment for pesticides through a review of the different sources of uncertainty in pesticide fate modelling. The extensive listing of sources of uncertainty clearly demonstrates that pesticide fate modelling is laced with uncertainty. More importantly, the review suggests that the probabilistic approaches, which are typically being deployed to account for uncertainty in the pesticide fate modelling, such as Monte Carlo modelling, ignore a number of key sources of uncertainty, which are likely to have a significant effect on the prediction of environmental concentrations for pesticides (e.g. model error, modeller subjectivity). Future research should concentrate on quantifying the impact these uncertainties have on exposure assessments and on developing procedures that enable their integration within probabilistic assessments.

Impacts of Climate Change on Indirect Human Exposure to Pathogens and Chemicals from Agriculture

Objective Climate change is likely to affect the nature of pathogens and chemicals in the environment and their fate and transport. Future risks of pathogens and chemicals could therefore be very different from those of today. In this review, we assess the implications of climate change for changes in human exposures to pathogens and chemicals in agricultural systems in the United Kingdom and discuss the subsequent effects on health impacts. Data sources In this review, we used expert input and considered literature on climate change; health effects resulting from exposure to pathogens and chemicals arising from agriculture; inputs of chemicals and pathogens to agricultural systems; and human exposure pathways for pathogens and chemicals in agricultural systems. Data synthesis We established the current evidence base for health effects of chemicals and pathogens in the agricultural environment; determined the potential implications of climate change on chemical and pathogen inputs in agricultural systems; and explored the effects of climate change on environmental transport and fate of different contaminant types. We combined these data to assess the implications of climate change in terms of indirect human exposure to pathogens and chemicals in agricultural systems. We then developed recommendations on future research and policy changes to manage any adverse increases in risks. Conclusions Overall, climate change is likely to increase human exposures to agricultural contaminants. The magnitude of the increases will be highly dependent on the contaminant type. Risks from many pathogens and particulate and particle-associated contaminants could increase significantly. These increases in exposure can, however, be managed for the most part through targeted research and policy changes.

Nanopesticides: State of Knowledge, Environmental Fate, and Exposure Modeling

Published literature has been reviewed in order to (a) explore the (potential) applications of nanotechnology in pesticide formulation, (b) identify possible impacts on environmental fate, and (c) analyze the suitability of current exposure assessment procedures to account for the novel properties of nanopesticides within the EU regulatory context. The term nanopesticide covers a wide variety of products and cannot be considered to represent a single category. Many nanoformulations combine several surfactants, polymers, and metal nanoparticles in the nanometer size range. The aims of nanoformulations are generally common to other pesticide formulations, these being to increase the apparent solubility of poorly soluble active ingredients, to release the active ingredient in a slow/targeted manner and/or to protect against premature degradation. Nanoformulations are thus expected to (a) have significant impacts on the fate of active ingredients and/or (b) introduce new ingredients for which the environmental fate is still poorly understood (e.g., nanosilver). Therefore, it seems that adaptations of current exposure assessment approaches will be necessary, at least for some nanopesticides. The present analysis provides a useful framework to identify priorities for future research in order to achieve more robust risk assessments of nanopesticides.

Evaluation of methods to derive pesticide degradation parameters for regulatory modelling

Abstract. Models to simulate the fate of pesticides in the environment are frequently used for risk assessments within the registration process. An adequate description of pesticide degradation in soil is important to provide input for these models. Often, DT50 values (time required for 50% dissipation of the initial concentration) are used as model input, but there is no widely agreed methodology to derive DT50 values from experimental data. DT50 values are often obtained by fitting first-order kinetics to observed degradation patterns. The result depends on the handling of pesticide data (e.g. logarithmic transformation) and initial concentrations (variable or fixed). Kinetics other than first-order may be more suitable to describe the decline of measured concentrations, but the derived DT50 values are then not appropriate as input for many simulation models. Field or laboratory DT50 values can be used for modelling and this has consequences for model parameterisation. Degradation parameters derived from static laboratory experiments may not be applicable to pesticide behaviour under flow conditions in the field. Several methods to simulate the fate of metabolites and to evaluate experimental data are available. The methodology used to derive model input parameters must be consistent with the approach used within the simulation model.

Nanopesticides: guiding principles for regulatory evaluation of environmental risks

Nanopesticides or nano plant protection products represent an emerging technological development that, in relation to pesticide use, could offer a range of benefits including increased efficacy, durability, and a reduction in the amounts of active ingredients that need to be used. A number of formulation types have been suggested including emulsions (e.g., nanoemulsions), nanocapsules (e.g., with polymers), and products containing pristine engineered nanoparticles, such as metals, metal oxides, and nanoclays. The increasing interest in the use of nanopesticides raises questions as to how to assess the environmental risk of these materials for regulatory purposes. Here, the current approaches for environmental risk assessment of pesticides are reviewed and the question of whether these approaches are fit for purpose for use on nanopesticides is addressed. Potential adaptations to existing environmental risk assessment tests and procedures for use with nanopesticides are discussed, addressing aspects such as analysis and characterization, environmental fate and exposure assessment, uptake by biota, ecotoxicity, and risk assessment of nanopesticides in aquatic and terrestrial ecosystems. Throughout, the main focus is on assessing whether the presence of the nanoformulation introduces potential differences relative to the conventional active ingredients. The proposed changes in the test methodology, research priorities, and recommendations would facilitate the development of regulatory approaches and a regulatory framework for nanopesticides.

Effects of the herbicides metazachlor and dinoterb on the soil microflora and the degradation and sorption of metazachlor under different environmental conditions

Abstract. Environmental conditions may modify pesticide effects on non-target soil micro-organisms due to their influence on pesticide persistence, bioavailability and interactions with microbial metabolism. This study investigated the relationship between effects of the herbicide metazachlor and its degradation and availability in soils with different organic C contents (soil A=1.3%, soil B=7.1%), incubated at 20°C and 30°C. Relative differences between dehydrogenase activity (DHA) in soil treated with metazachlor and untreated soil ranged from –16.6 to +18.9% with a greater impact for soil A, probably due to greater pesticide availability and potential for the microflora to recover from adverse effects. Relative effects on substrate-induced, short-term respiration (SIR, –11.8 to +6.0%) and N mineralisation (Nmin, –24.4 to +35.8%) were similar in both soils. Soil temperature did not consistently influence the effects of metazachlor on DHA and SIR. Nmin was more strongly affected at 20°C than at 30°C, which was potentially caused by larger pesticide concentrations at 20°C or a smaller potential for the microflora to reproduce destroyed biomass. Additional studies with the herbicide dinoterb showed stronger effects on DHA, SIR and Nmin than for metazachlor (–57.7 to +78.2%) and no clear influence of soil and temperature. Results showed that environmental factors may influence pesticide effects on micro-organisms. Clear relationships are not always found due to interactions between these factors, pesticide persistence, bioavailability and microbial metabolism.

The Kinetics of Sorption by Retarded Diffusion into Soil Aggregate Pores

This study investigates time-dependent sorption of pesticides in soil aggregates. We tested if the sorption kinetics of pesticides in soil aggregates can be described by modeling diffusion into aggregates for a range of soils and pesticides. Our hypothesis is that the rate of sorption is negatively related to sorption strength due to retardated diffusion. Natural aggregates of 3-5 mm diameter were separated from three soils: a clay, a silty clay loam, and a clay loam. The aggregates were stabilized with alginate gel, and adsorption of azoxystrobin, chlorotoluron, and atrazine was measured in batch experiments with eight equilibration times up to 28 days. Equilibrium sorption appeared to be reached within the 28-day period for each pesticide. An intra-aggregate diffusion model was employed to describe the increase of sorption with time. The model describes diffusion of the dissolved pesticides through the pore space inside the aggregates and sorption on internal surfaces. Sorption could be described by pore diffusion into the aggregates with diffusion coefficients between 0.5 x 10(-10) and 1.5 x 10(-10) m(2) s(-1). The model fits support the theory that pore diffusion is the rate-limiting process for sorption of pesticides in aggregates, although the diffusion coefficients were a factor 3-10 smaller than the theoretical diffusion coefficient for diffusion in water. Comparing the results from the different pesticide-soil combinations showed that the extent of nonequilibrium increased with increasing sorption strength. This confirmed that sorption takes longer to reach equilibrium for pesticides and soils with stronger sorption. The differences between the different pesticides and soils were fully accounted for in the model by stronger retardation of the more strongly sorbed pesticides. The results imply that diffusion into aggregates may be the major time-limiting process for sorption of pesticides in structured soils. Commonly performed sorption experiments with sieved soil fail to account for this process.

User subjectivity in Monte Carlo modeling of pesticide exposure

Monte Carlo techniques are increasingly used in pesticide exposure modeling to evaluate the uncertainty in predictions arising from uncertainty in input parameters and to estimate the confidence that should be assigned to the modeling results. The approach typically involves running a deterministic model repeatedly for a large number of input values sampled from statistical distributions. In the present study, six modelers made choices regarding the type and parameterization of distributions assigned to degradation and sorption data for an example pesticide, the correlation between the parameters, the tool and method used for sampling, and the number of samples generated. A leaching assessment was carried out using a single model and scenario and all data for sorption and degradation generated by the six modelers. The distributions of sampled parameters differed between the modelers, and the agreement with the measured data was variable. Large differences were found between the upper percentiles of simulated concentrations in leachate. The probability of exceeding 0.1 microg/L ranged from 0 to 35.7%. The present study demonstrated that subjective choices made in Monte Carlo modeling introduce variability into probabilistic modeling and that the results need to be interpreted with care.

Impacts of climate change on indirect human exposure to pathogens and chemicals from agriculture

Climate change is likely to affect the nature of pathogens/ chemicals in the environment and their fate and transport. We assess the implications of climate change for changes in human exposures to pathogens/chemicals in agricultural systems in the UK and discuss the effects on health impacts, using expert input and literature on climate change; health effects from exposure to pathogens/chemicals arising from agriculture; inputs of chemicals/pathogens to agricultural systems; and human exposure pathways for pathogens/chemicals in agricultural systems. We established the evidence base for health effects of chemicals/pathogens in the agricultural environment; determined the potential implications of climate change on chemical/pathogen inputs in agricultural systems; and explored the effects of climate change on environmental transport and fate of various contaminants. We merged data to assess the implications of climate change in terms of indirect human exposure to pathogens/chemicals in agricultural systems, and defined recommendations on future research and policy changes to manage adverse increases in risks.

How does crop type influence risk from pesticides to the aquatic environment

National-level risk mapping was undertaken to identify specific situations within England with the greatest potential for impacts on aquatic biodiversity from normal agricultural use of pesticides. Calculations of exposure via spray drift and drainflow were differentiated by landscape type, region, and crop and then compared with toxicity to the indicator organisms Daphnia magna and algae. The approach incorporated regional-level information regarding pesticide usage derived from farm visits. Risk was calculated for individual water bodies and then aggregated and mapped for each of 5,760 individual catchments ranging in area up to 248 km2. Type of crop adjacent to water was the major driver for risk, and orchards were identified as the crop associated with the greatest potential risk to the aquatic environment. Crops such as cereals, oilseeds, and potatoes are more widely grown in England but have potential risk an order of magnitude smaller than that for orchards. Several of the pesticides that contribute most to risk have been withdrawn from use since collection of the most recent usage data. Driven by crop distribution, surface waters adjacent to orchards in the midwest and southeast of England are predicted to be most at risk of ecological impacts from agricultural pesticide use. This information can be used in targeting monitoring campaigns designed to protect the aquatic environment.