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The soil-acting pesticides comprise many compounds with divergent biological activities. This chapter illustrates key ideas, principally by reference to research results for the widely used soil-acting nematicides aldicarb, oxamyl, and ethoprophos. It discusses simulation of pesticide movement in soils. Pesticides need to be formulated to assure ease of application. For spraying, it is necessary to mix the products quickly throughout a comparatively large volume of water. The sorption of a pesticide on soil reduces its availability to the target organism, both in the soil solution and the soil-gas phase. Weakly sorbed compounds usually exhibit little effect of sorption on efficacy, thus they are usable for a wide range of soils. The persistence of a pesticide in soil is increased by applying it as a granular formulation. The chapter also discusses efficacy of a pesticide in relation to exposure of pests to the pesticide over time.

In the evaluation of the risk of pesticide leaching to groundwater, the soil surface is usually assumed to be level, although important crops like potato are grown on ridges. A fraction of the water from rainfall and sprinkler irrigation may flow along the soil surface from the ridges to the furrows, thus bringing about an extra load of water and pesticide on the furrow soil. A survey of the literature reveals that surface-runoff from ridges to furrows is a well-known phenomenon but that hardly any data are available on the quantities of water and pesticide involved. On the basis of a field experiment with additional sprinkler irrigation, computer simulations were carried out with the Pesticide Emission Assessment at Regional and Local scales model for separate ridge and furrow systems in a humic sandy potato field. Breakthrough curves of bromide ion (as a tracer for water flow) and carbofuran (as example pesticide) were calculated for 1-m depth in the field. Bromide ion leached comparatively fast from the furrow system, while leaching from the ridge system was slower showing a maximum concentration of about half of that for the furrow system. Carbofuran breakthrough from the furrow system began about a month after application and increased steadily to substantial concentrations. Because the transport time of carbofuran in the ridge soil was much longer, no breakthrough occurred in the growing season. The maximum concentration of carbofuran leaching from the ridge-furrow field was computed to be a factor of six times as high as that computed for the corresponding level field. The study shows that the risk of leaching of pesticides via the furrow soil can be substantially higher than that via the corresponding level field soil.

The volatilization of pesticides from crop canopies in the field should be modeled within the context of evaluating environmental exposure. A model concept based on diffusion through a laminar air-boundary layer was incorporated into the PEARL model (pesticide emission assessment at regional and local scales) and used to simulate volatilization of the pesticides parathion and chlorothalonil from a potato crop in a field experiment. Rate coefficients for the competing processes of plant penetration, wash off, and phototransformation in the canopy had to be derived from a diversity of literature data. Cumulative volatilization of the moderately volatile parathion (31% of the dosage in 7.6 days) could be simulated after calibrating two input data derived for the related compound parathion-methyl. The less volatile and more slowly transformed chlorothalonil showed 5% volatilization in 7.6 days, which could be explained by the simulation. Simulated behavior of the pesticides in the crop canopy roughly corresponded to published data.

Volatilization of pesticides from crops in the field can be an important emission pathway. In a field experiment with characterization of meteorological conditions, the pesticides chlorpyrifos and fenpropimorph were sprayed onto a potato crop, after which concentrations in the air and on/in the plants were measured. Rates of volatilization were estimated with the aerodynamic profile (ADP), energy balance (EB), relaxed eddy accumulation (REA), and plume dispersion (PD) methods. The volatilization rates obtained with the ADP and EB methods were similar, while some rates obtained with the REA and PD methods in the initial period were lower. Cumulative volatilization of chlorpyrifos during daylight hours (ADP and EB methods) was estimated to be about 65% of the dosage. By far the majority of this volatilization occurred in the first few days. Competing processes at the plant surface had a considerable effect on the dissipation of fenpropimorph, so cumulative volatilization during daylight hours was estimated to be only 7% of the dosage. Plant surface residues were higher than would correspond with the volatilization rate, indicating that penetration into the leaves had occurred. © 2006 American Chemical Society.

Depending on their vapour pressure, volatilisation can be one of the main pathways of emission of pesticides into the environment. The volatilisation of fenpropimorph was studied in a field experiment in which the fungicide was sprayed onto a sugar beet crop. Volatilisation rates were calculated by measuring the concentration gradient in air, using the Aerodynamic and Bowen Ratio methods. A simplified computation model was used to simulate pesticide volatilisation, together with the concurrent processes of penetration into the plant leaves and phototransformation. Input data for the model had already been obtained by carrying out a wind-tunnel study with fenpropimorph, whereby field conditions were imitated. The computations yielded a reasonable description of the level and rate of decline of fenpropimorph volatilisation in the first 4 h after spraying. The continued volatilisation 2 and 3 days after spraying could be described by assuming that a fraction of the deposit was poorly exposed with comparatively low rates of the decline processes. In the first 3 days, penetration of fenpropimorph into the plant leaves was computed to be the main route for the pesticide (52% of the dosage), with substantial contributions from volatilisation (12%) and phototransformation (11%). The computation model can be developed further as a tool for extrapolating results on volatilisation from small-scale experiments to field conditions, but this requires more information on the effect of environmental conditions on the model parameters.

Use of the insecticide lambda-cyhalothrin in agriculture may result in the contamination of water bodies, for example by spray drift. Therefore, the possible exposure of aquatic organisms to this insecticide needs to be evaluated. The exposure of the organisms may be reduced by the strong sorption of the insecticide to organic materials and its susceptibility to hydrolysis at the high pH values in the natural range. In experiments done in May and August, formulated lambda-cyhalothrin was mixed with the water body of enclosures in experimental ditches containing a bottom layer and macrophytes (at different densities) or phytoplankton. Concentrations of lambda-cyhalothrin in the water body and in the sediment layer, and contents in the plant compartment, were measured by gas-liquid chromatography at various times up to 1 week after application. Various water quality parameters were also measured. Concentrations of lambda-cyhalothrin decreased rapidly in the water column: 1 day after application, 24-40% of the dose remained in the water, and by 3 days it had declined to 1.8-6.5%. At the highest plant density, lambda-cyhalothrin residue in the plant compartment reached a maximum of 50% of the dose after 1 day; at intermediate and low plant densities, this maximum was only 3-11% of the dose (after 1-2 days). The percentage of the insecticide in the ditch sediment was 12% or less of the dose and tended to be lower at higher plant densities. Alkaline hydrolysis in the water near the surface of macrophytes and phytoplankton is considered to be the main dissipation process for lambda-cyhalothrin.

The sorption of nine pesticides to the aquatic macrophytes Chara globularis, Elodea nuttallii, and Lemna gibba was studied. A batch equilibrium method was used to study the sorption at five concentration levels to fresh shoots of the macrophytes. The results for the herbicides atrazine and linuron were described by nonlinear Freundlich equations, with Freundlich exponents ranging from 0.53 to 0.60. The results for the other compounds showed almost linear sorption isotherms, with Freundlich exponents ranging from 0.9 to 1.1. The highest sorption was measured for chlorpyrifos, with sorption coefficients ranging from 1,660 to 2,150 L/kg. Sorption coefficients for C. globularis tended to be lower than those for the other two macrophytes. Correlation (R(2) = 0.80) was found for the relation between the sorption coefficient (K(d)) of six pesticides and their solubility in water (S). The equation log K(d) = 3.20 - 0.65 log S can be used for a first estimate of the sorption coefficient of a pesticide to aquatic macrophytes.http://link.springer-ny.com/link/service/journals/00244/bibs/37n3p310.html

When pesticides leach through the soil to the upper groundwater zone, it is important to know whether further transformation occurs before the pumping wells for drinking water are reached. Atrazine and bentazone were incubated (at 10°C) in five water-saturated sandy subsoils (collected at depths between 1·5 and 3·5 m), simulating the conditions in the field. In three subsoils with comparatively low pH and intermediate to high redox potential, atrazine was transformed gradually, to leave 1·9%, 6·2% and 17·5% of the dose after about five years. In one of these subsoils, hydroxy-atrazine was detected; the amount corresponded to half of the dose of atrazine. In one anaerobic subsoil with high pH, the transformation of atrazine was comparatively fast (half-life about 0·15 year). Another anaerobic subsoil, with similar pH and a somewhat higher redox potential, however, showed hardly any transformation. Sterilization of the first anaerobic subsoil had no effect on the rate of transformation. In the course of about five years, bentazone in the first three subsoils was transformed gradually to leave

The performance of a newly developed column method for pesticide transformation rate measurements in the subsoil was tested using (Z)- and (E)-3-chloroallyl alcohol as model compounds. The subsoil columns were filled in situ. In the column experiment the half-life ranged from 0.5-5.2 d for (Z)-3-chloroallyl alcohol and from 1.0-5.5 d for (E)-3-chloroallyl alcohol. The transformation rates were compared with rates in laboratory incubation systems, in which the 3-chloroallyl alcohols were transformed gradually in the first three days, with a half-life of 1.9 d for both isomers. The two methods yielded similar results for these rapidly transforming compounds.

A sampling programme was set up to measure 1,3-dichloropropene and methyl isothiocyanate in air in a region with intensive agricultural use of these soil fumigants. In two consecutive autumns, 6-hour air samples were taken at two locations in that region with charcoal tubes using automatic samplers. Most (81%) of the 6-hour concentrations of 1,3-dichloropropene measured in both years were below 3.2 μg m−3 and a few percent could not be measured with a detection limit of around 0.3 μg m−3. Only 4% of the 6-hour concentrations exceeded 10 μg m−3, almost all of which were measured at a location where a field just upwind of the measuring site had been treated. For methyl isothiocyanate, 73% of the 6-hour concentrations of both years could not be measured with a detection limit in the two years of 1 and 2 μg m−3, respectively. A small fraction (3%) of the concentrations were in the range of 3.2 to 10 μg m−3 and only 1% exceeded 10 μg m−3.

Water flow and pesticide transport in the soil of fields with ridges and furrows may be more complex than in the soil of more level fields. Prior to crop emergence, the tracer bromide ion and the insecticide carbofuran were sprayed on the humic-sandy soil of a potato field with ridges and furrows. Rainfall was supplemented by sprinkler irrigation. The distribution of the substances in the soil profile of the ridges and furrows was measured on three dates in the potato growing season. Separate ridge and furrow systems were simulated by using the pesticide emission assessment at regional and local scales (PEARL) model for pesticide behavior in soil–plant systems. The substances travelled deeper in the furrow soil than in the ridge soil, because of runoff from the ridges to the furrows. At 19 days after application, the peak of the bromide distribution was measured to be in the 0.1–0.2 m layer of the ridges, while it was in the 0.3–0.5 m layer of the furrows. After 65 days, the peak of the carbofuran distribution in the ridge soil was still in the 0.1 m top layer, while the pesticide was rather evenly distributed in the top 0.6 m of the furrow soil. The wide ranges in concentration measured with depth showed that preferential water flow and substance transport occurred in the sandy soil. Part of the bromide ion distribution was measured to move faster in soil than the computed wave. The runoff of water and pesticide from the ridges to the furrows, and the thinner root zone in the furrows, are expected to increase the risk of leaching to groundwater in ridged fields, in comparison with more level fields.

The transformation of [1- 14 C]-1,2-dichloropropane in three water-saturated sandy subsoil materials (sampled at a depth of between 1.2 and 2.3 m) was studied in laboratory incubations at 10 °C. In the first material (methanogenic with a redox potential of 0.0–0.2 V and a pH of 7) no measurable transformation took place during the first 156 days but after 608 and 712 days more than 96% of the dose had been transformed into one or more highly volatile products. Analysis of the headspace above this subsoil material after 1059 days indicated that propene and propane are probably transformation products and excluded chlorinated hydrocarbons with one, two or three C atoms as significant transformation products. In the two other subsoil materials (with redox potentials of 0.0–0.4 V and 0.3–0.6 V and pH values between 4 and 5) no measurable transformation took place during the whole incubation period (1056 days).

Methods of sampling and analysis have been developed to measure concentrations of the soil fumigants 1,3-dichloropropene and methyl isothiocyanate (formed from metham-sodium) in air. We tested these methods under laboratory and field conditions. Air samples were taken with glass tubes containing charcoal as adsorbent. The charcoal was extracted with acetone and subsamples of the extracts were injected into a gas Chromatograph with a capillary column. 1,3-dichloropropene was determined by an electron capture (EC) detector and methyl isothiocyanate by a nitrogen-phosphorus (NP) detector. Concentrations of these fumigants in 40 L of air as low as 0.2 μg m−3 and 1.0 μg m−3, respectively, could be determined. A study on the influence of storage temperature and time on the recovery from charcoal showed that charcoal tubes can be stored for up to 4 d at −20 °C. A test program on the breakthrough of the charcoal tubes when sampling at different flow rates and air humidity revealed no significant breakthrough from the first to the second (check) tube. Similar results were obtained from sampling the air during fumigation of the soil in a greenhouse and in two fields.

The extent of the risk of (Z)- and (E)-1,3-dichloropropene and their transformation products (Z)- and (E)-3-chloroallyl alcohol causing a hazard to the quality of groundwater pumped up for public water supply from below flower-bulb fields has been evaluated. The 1,3-dichloropropenes were incubated at 10°C in water-saturated subsoil material from three such fields. A first stage with gradual transformation was followed by a second stage with comparatively rapid transformation and after three months negligible amounts remained. The 3-chloroallyl alcohols were incubated at 15°C in soils from the root zone of the same fields. Complete transformation had occurred in about a week or even less. The 3-chloroallyl alcohols were also incubated in the water-saturated subsoil material at 10°C. Again, a first stage with gradual transformation was followed by a second stage with fast transformation. In most instances, the concentrations had fallen to a very low level within three months. It was concluded that it is unlikely that residues in the upper groundwater would permeate into the deeper groundwater pumped up for public water supply.

BACKGROUND: Volatilisation of pesticides from crop canopies can be an important emission pathway. In addition to pesticide properties, competing processes in the canopy and environmental conditions play a part. A computation model is being developed to simulate the processes, but only some of the input data can be obtained directly from the literature. RESULTS: Three well-defined experiments on the volatilisation of radiolabelled parathion-methyl (as example compound) from plants in a wind tunnel system were simulated with the computation model. Missing parameter values were estimated by calibration against the experimental results. The resulting thickness of the air boundary layer, rate of plant penetation and rate of phototransformation were compared with a diversity of literature data. The sequence of importance of the canopy processes was: volatilisation > plant penetration > phototransformation. CONCLUSION: Computer simulation of wind tunnel experiments, with radiolabelled pesticide sprayed on plants, yields values for the rate coefficients of processes at the plant surface. As some input data for simulations are not required in the framework of registration procedures, attempts to estimate missing parameter values on the basis of divergent experimental results have to be continued.

When studying the effect of mixtures of toxic substances on soil organisms, attention must be paid to peculiarities in exposure to mixtures as opposed to that of single toxicants. The fungicides carbendazim and iprodione compete in the adsorption to soil. The presence of iprodione reduced the adsorption of carbendazim by 30%, while carbendazim reduced the adsorption of iprodione by 70%. Iprodione had little effect on the transformation rate of carbendazim in soil. However, carbendazim retarded the transformation of iprodione in soil by 26%. The concentration of the fungicides in pore water was found to be substantially higher for mixtures than when a fungicide alone was present in the soil. The effect of the additional fungicide on the concentration is especially apparent in the period following the first 1 to 2 weeks of the incubation. The inclusion of copper in the mixture has little additional effect on the concentration of the fungicides in pore water.

Calculation of pesticide volatilization from plants as an integral component of pesticide fate models is of utmost importance, especially as part of PEC (predicted environmental concentrations) models used in the registration procedures for pesticides. A mechanistic approach using a laminar air-boundary layer concept to predict volatilization from plant surfaces was compared to data obtained in a wind-tunnel study after simultaneous application of parathion-methyl, fenpropimorph, and quinoxyfen to winter wheat. Parathion-methyl was shown to have the highest volatilization during the wind-tunnel study of 10 days (29.2%). Volatilization of quinoxyfen was about 15.0%, revealing a higher volatilization tendency than fenpropimorph (6.0%), which is attributed to enhanced penetration of fenpropimorph counteracting volatilization. Predictions of the boundary-layer approach were markedly influenced by the selected values for the equivalent thickness of the boundary layer and rate coefficients, thus indicating that future improvements of the approach will require a deeper understanding of the kinetics of the underlying processes, e.g. phototransformation and penetration. The boundary-layer volatilization module was included in the European registration model PELMO, enabling simultaneous calculation of volatilization from plants and soil. Application of PELMO to experimental findings were the first comprehensive PEC model calculations to imply the relevant processes affecting the postapplication fate of pesticides.

Volatilisation of pesticides from plants is one of the main pathways for their emission to the environment. A simplified computation model was set up to simulate this volatilisation, including penetration into plants and photochemical transformation as competing processes. Previous wind tunnel experiments using plants sprayed with C-14-labelled fenpropimorph were simulated using the model. Volatilisation could be simulated by diffusion through a laminar air-boundary layer, with a thickness in the range of 0.5-1.0 mm. Rate coefficients of 1.7-4.8 d(-1) had to be used to simulate the penetration of fenpropimorph into different plant species. The rate of phototransformation was lowest when the incoming air stream was filtered through activated carbon, thus minimising the formation of hydroxyl radicals by sunlight. The simulations enabled us to estimate model parameters that could neither be derived from laboratory studies nor could be obtained with pesticide (non-labelled) in the field.

The rate of transformation of a pesticide as a function of the depth in the soil is needed as an input into computations on the risk of residues leaching to groundwater. The herbicide bentazone was incubated at 15 degrees C in soil materials derived from four layers at depths of up to 2.5 m in a humic sandy soil profile with a fluctuating water table (0.8 to 1.4 m), while simulating the redox conditions existing in the field. Gamma-irradiation experiments indicated that bentazone is mainly transformed by microbial activity in the soil. The rate constant for transformation was highest in the humic sandy top layer; it decreased with depth in the sandy vadose subsoil. However, material from the top of the phreatic aquifer had a higher rate constant than that from the layers just above. The presence of fossil organic material in the fluviatile water-saturated sediment probably stimulated microbial activity and bentazone transformation. The changes in the transformation rate constant with depth showed the same trend as those in some soil factors, viz organic carbon content, water-extractable phosphorus and microbial density as measured by fluorescence counts. However, the (low) concentration of dissolved organic carbon (DOC) in the top of the aquifer did not fit the trend. The rate constant for bentazone transformation in the layers was higher at lower initial contents of the herbicide.

Laboratory and field studies show that pesticides may be transformed in the groundwater zone. Possible reaction mechanisms are chemical hydrolysis, catalytic reduction and aerobic or anaerobic microbial transformation. Transformation in the groundwater zone can be an important element in the advanced evaluation of the potential risk arising from a pesticide in the public drinking water supply. However, rate and pathway of transformation can show large differences, depending on the bio-geochemical conditions in the groundwater zone. Knowledge of the reaction mechanisms and the effect of aquifer conditions would allow vulnerable and low-vulnerable application areas for a pesticide to be delimited. An outline is given of possible approaches to quantifying these transformation processes and using the results in registration procedures, especially in the EU and its member states. Furthermore, areas where there is need for continued research and better understanding are highlighted.

The behaviour of the herbicide metamitron and of the main transformation product, hydroxy-chlorothalonil (HTI), of the fungicide chlorothalonil was studied to assess the risk of leaching from low-humic sandy soil. The adsorption of metamitron corresponded to a Kom value of about 60 dm3 kg−1 (moderate adsorption). The half-life of metamitron in soil at 15 °C was only three days, presumably due to adaptation of the micro-organisms. In the autumn, the residue of metamitron in the soil profiles corresponded to less than 1% of the cumulative dosage. The half-life of chlorothalonil at 15 °C was about 12 days and about 45% of it was transformed to HTI. The adsorption of HTI to the soils corresponded to a Kom value of 260 dm3 kg−1. The incubation study (15 °C) showed the transformation of HTI in the soils to be very slow. The amounts of HTI remaining in the soil profiles in the autumn corresponded to 4 and 16% of the cumulative dosage of chlorothalonil. In winter, the HTI residue decreased by 40% relative to the autumn level. Occasionally, HTI could be detected in the upper ground-water level (at a depth of about 1 m), at an average concentration of 0.1 to 0.2 µg dm−3. © 1999 Society of Chemical Industry

Three parameters for indicating toxicologic risk of pesticides to soil organisms were compared: the traditional total content parameter, the equilibrium partitioning approach, and the pore-water concentration parameter. The relevance of each parameter was tested using different soil types and prolonged chemical–soil contact time by relating the results to bioavailability as measured in toxicity tests. The pesticide concentration measured in the soil pore water could be shown to correspond strongly with toxicologic effects on the soil organism Folsomia candida, irrespective of soil type and chemical–soil contact time. This relationship is predicted by the equilibrium partitioning theory and so this aspect of the theory could be validated in this study for pesticides in soil. However, the theory's assumption of equilibrium (which implies that the bioavailable fraction of the chemical is constant with time and can be calculated using one particular soil to pore-water partition coefficient) could not be justified in this study. Although organic matter normalization reduced soil-to-soil differences, it did not account for the time-related decrease in bioavailability that was found in two of the four soil–chemical combinations investigated. As a consequence, pore-water concentrations calculated from chemical content measurements in soil and a partition coefficient obtained in short-term laboratory experiments may not be a good estimate for sorption or toxicologic effects on a longer term. The suggestion is made to measure the concentration of chemical in pore water directly and to use water-only toxicity values as a basis for soil quality criteria.

This paper shows the results of experiments (adsorption, degradation and mobility) carried out to investigate the risk of pirimicarb leaching to groundwater, after its application to integrated fruit growing systems. Soil batches were collected from experimental fruit farms in Numansdorp and Zeewolde (The Netherlands). The distribution coefficients and the degradation rate coefficients for Numansdorp and Zeewolde soils, respectively, show a higher adsorption and degradation rate in the Zeewolde soil than in the Numansdorp soil. By leaching of pirimicarb in two soil columns, it was found that pirimicarb mobility is lower in the Zeewolde soil columns than in the Numansdorp ones but, in both cases, it is much lower than it would be expected from their adsorption coefficients. Pirimicarb is detected in the first effluents, however, the highest concentration remained in the upper 10 cm of the soil columns. A computation model simulating uniform water flow and instantaneous adsorption desorption equilibrium is not suitable to describe the leaching of pirimicarb in structured loamy soils at relatively high water flow-rates.

The transformation rates of methyl isothiocyanate and 1,3-dichloropropene were studied in four water-saturated sandy subsoil materials collected between 2 and 4 m below the soil surface, using laboratory incubation systems at 10 degrees C. In three samples, transformation of methyl isothiocyanate was rapid (< 10% remaining after 30 days), whereas in the fourth, a half-life of ~ 35 days was found. In the transformation of the (Z)- and (E)-isomers of 1,3-dichloropropene in the same materials, half-lives between 16 and 64 days were found. Incubation in systems where the subsoil materials had been exposed unintentionally to temperatures up to 30 degrees C before adding the compounds, resulted in much lower transformation rates for methyl isothiocyanate (half-lives ranging from 150 to 570 days) and in only slightly lower rates for (Z)- and (E)-1,3-dichloropropene (half-lives ranging from 38 to 76 days). (Abstract retrieved from CAB Abstracts by CABI’s permission)

In field trials, loamy soils were fumigated by injecting dichloropropene and metham-sodium with a horizontal-blade injector. Concentrations of cis- and trans-1,3-dichloropropene, and of methyl isothiocyanate were measured. The influence of different methods of tillage on fumigation effectiveness was compared by computing concentration-time products at the various depths in the soil. With a fine-structured top layer, there was no clear beneficial effect of shallow or deep rotary tillage before injection. If a dense top layer was not rotary-tilled beforehand, deep and wide cracks were formed during injection. Shallow or deep rotary tillage both left loose cereal stubble mixed in the surface layer, which was difficult to finish off. Concentration-time products could be substantially increased by finishing off the soil surface better after injection.

The rates of degradation and downward movement of ethoprophos (O-ethyl SS-dipropyl phosphorodithioate) were measured under field conditions in four soils in aluminium columns (40 cm long). A 10% granular formulation was incorporated in the top 10 cm at a rate of 10.0–10.5 kg a.i./ha. Under outdoor conditions during spring and summer, loss of ethoprophos approximated to first order kinetics; the half-life was about 87 days in a humic sand and a peaty sand, with pH values of 4.5 and 4.6, respectively. In a sandy loam and a loam soil with pH values of 7.2 and 7.3, respectively, the half-life ranged between 14 and 28 days. Under experimental conditions with fallow soils and 35.3 cm rainfall, the downward movement of substantial concentrations of ethoprophos by leaching and diffusion was restricted to a few centimetres.

The development, testing and application of computer models for the behaviour of organic chemicals, especially pesticides, in soil and ground water has been reviewed. Detailed data are needed on the structure and properties of the soil and ground-water systems, and on the flow of water through these systems. Adsorption and transformation of organic chemicals can be studied in the laboratory and the results introduced into the models. The mathematical techniques most frequently used for the solution of the differential equations are briefly discussed. Some models for the behaviour of pesticides in the root zone have been tested against results of field trials and some interesting deviations between computations and measurements emerged. Techniques for the simulation of the behaviour of organic chemicals in the ground-water zone are also available. However input data for the models are often lacking, as are also results of field studies for testing the models.

Soil fumigation with metham-sodium in the field was studied in detail by characterising soil and climatic conditions, and by measuring concentrations of methyl isothiocyanate. The effectiveness of two dosages in two contrasting soil profiles was compared on the basis of computed concentration-time products for methyl isothiocyanate in the water phase. Under wet conditions, the vapour diffusion of methyl isothiocyanate was very slow and resulted in an irregular distribution in the soil. Showers further decreased effectiveness in the upper part of the soil. A dried top layer was unfavourable for the concentration-time product near to the surface. Diffusion of methyl isothiocyanate to depths greater than about 30 cm was very slow.

Distribution and transformation of the nematicides aldicarb, oxamyl and ethoprophos was measured on annually treated plots and plots not previously treated on two potato fields. This trial plan was chosen because the effect of the nematicides decreased in the annually treated plots on these fields. In the field, the contents of the nematicides decreased more rapidly in soil of the annually treated plots than in the untreated ones. This rapid disappearance of the active compounds was found to be the cause of the decreased nematode control on the annually treated plots. The considerably more rapid transformation of the nematicides in treated soils than in untreated ones was also measured in incubation tests at 15°C. Sterilization of the treated soils drastically reduced the rates of disappearance. The repeated applications of the nematicides must have induced microbial adaptation, which resulted in accelerated transformation. The carbamoyloximes aldicarb and oxamyl were transformed at about the same rates in previously untreated soils and in soils from plots previously treated with ethoprophos. However, oxamyl was transformed very rapidly in soil previously treated with aldicarb and also aldicarb in soil previously treated with oxamyl: this indicates that cross-adaptation occurred for the two carbamoyloximes.

Aldicarb and its oxidation product aldoxycarb (aldicarb sulphone) were applied separately to columns of fallow, sandy loam soils under field conditions. The breakdown and movement of these compounds were monitored, as was the behaviour of aldicarb sulphoxide and aldoxycarb formed by oxidation of the applied aldicarb. The behaviour of these compounds was simulated by a computer model using laboratory data for adsorption and rates of degradation in soil. The model simulated the observed behaviour reasonably well, although redistribution of chemicals was often more rapid than predicted. Production of aldoxycarb from the sulphoxide was less in the field than was expected from the laboratory incubations. Accumulation of chemicals near the soil surface in dry periods was overestimated, indicating that the processes occurring under these conditions are not well described by the model. About 4 months after application, only aldoxycarb, in small amounts, remained in the soils.

2-Methyl-2-(methylsulphinyl)propionaldehyde O-methylcarbamoyloxime (aldicarb sulphoxide), aldoxycarb, oxamyl and methomyl were incubated at 10°C in soil samples taken from layers above and below shallow ground-water tables at four locations in the Netherlands. Soil samples from above the water table were incubated under moist and aerobic conditions. The anaerobic conditions below the water table were simulated by incubating the soil samples under 0.5-1 cm of ground water, and a nitrogen atmosphere. During incubation, the pH and redox potentials were measured. Less than 5% of the oxamyl and methomyl remained after one day in four water-saturated, anaerobic subsoils. The half-lives of aldicarb sulphoxide and aldoxycarb ranged from 5.1 to 131 days in the four anaerobic subsoils. Conversion rates in the aerobic soil layers above the water table were from 8 to more than 100 times lower than in the water-saturated layers in the same soil profile. Half-lives in the aerobic soils ranged from 26 days for oxamyl in loamy fine sand (pH 8.0), to 1100 days for aldoxycarb in fine sand (pH 5.0). When soil from below the water table was incubated aerobically, the conversion rates of oxamyl and aldoxycarb were drastically reduced. The opposite was found when an originally aerobic soil was incubated anaerobically. Autoclaving the incubation systems retarded the conversions.

Data were collected that are needed to simulate soil fumigation with metham-sodium with computation models. The rate of conversion of metham-sodium into methyl isothiocyanate was dependent on temperature and soil type, and conversion was usually completed within a few hours. In comparison with dichloropropene, there was a higher water/gas distribution ratio and thus a slower vapour diffusion. Adsorption from the water phase onto the solid phase was weaker. The first-order rate equation described the decomposition of methyl isothiocyanate and the half-lives varied from a few days to a few weeks according to temperature and soil type.

The loss of aldicarb sulphoxide was studied in incubation experiments with soil from four plough layers and two deeper layers. The loss during the 111 days of the experiment could be described by first-order kinetics. The half-lives at 15°C ranged from 20 days in a clay loam to 46 days in a peaty sand. The loss of sulphoxide in deeper layers was considerably slower than in the corresponding top layers of a soil profile. In soil from a silty layer at 70–90 cm depth the half-life was about 53 days. In soil from a sand layer at 90–110 cm depth a loss of only about 15% was measured after 111 days of incubation. First-order rate constants for sulphoxide conversion in a clay loam at 6, 15 and 25°C were found to be 0.009, 0.033, and 0.05 day−1 respectively; in a greenhouse soil these rate constants were 0.0052, 0.019 and 0.04 day−1 respectively. The fractions of aldicarb sulphoxide that were oxidised to sulphone at 15°C in soil from plough layers were computed to range from 0.52 to 0.76.

With a computer model, pesticide behavior in soil after spring application to a sandy loam field with a potato crop, was simulated. Special attention was paid to the risk of leaching through the upper meter of soil in catchment areas. Unsaturated water flow resulting from rainfall was modeled in some detail. Uptake of water and solute by the developing root system of the annual crop was included in the model. The computations were carried out for hypothetical pesticides with first-order decomposition rate constants in soil of 0.03, 0.01, 0.003, and 0.001 day–1 at 20°C. Adsorption decreased with increasing soil depth and the adsorption coefficient for the top layer ranged from 0.0 to 10.0. For 20 combinations of decomposition rates and adsorption strengths, the extent of leaching from a top layer 1 m thick was computed. With a decomposition rate constant at 20°C of 0.03 day–1 or higher, leaching was extremely low; with 0.01 days–1 leaching was 1.7% of the dosage or lower. For compounds with a high persistence and mobility, leaching from the soil ranged up to about 10% of the dosage or more. Besides decomposition, uptake by plants was an important factor in reducing leaching, particularly for the weakly adsorbed and comparatively persistent compounds.

The fumigant 1,3-dichloropropene was injected into the soil of three bulb fields in the summer. At various times after injection, the contents of the (Z) and (E) isomers (formerly called thecis andtrans isomers) were measured in the unsaturated and saturated zones down to a 3 m depth. Within a month, the contents decreased to less than 0.2 mg/kg and they only gradually declined further in the subsequent months. Occasionally, very low contents (less than 1 μg/kg) of the (Z) and (E) isomers were measured in the groundwater zone.

There are several reasons for making the behavior of pesticides in soils the subject of detailed investigations. A first type of problem in this field mainly concerns the effectiveness of applications, particularly in the case of the soil-applied pesticides. This group embraces many herbicides, fungicides, nematicides, and insecticides, all having the common characteristic that they have to be taken up from the soil system in adequate amounts and at a sufficient rate to be effective. The central question here is how the desired control effect can be achieved with the lowest possible dosage. This involves such related questions as formulation, method of application, application conditions, methods of culture, and the control period to be expected. A second type of problem is that the use of pesticides may be attended by undesirable side effects, both during and after the intended control period. One of the most marked is phytotoxicity, which may result in loss of yield or quality. The compounds that occur in soil may frustrate the aim of keeping their residues in food crops as low as possible. Soil fertility may be affected through, for example, the effect on the soil microbe population. Possible contamination of groundwater is receiving increasing attention at the present time, when high quality sources of drinking water are becoming scarce.

The movement and breakdown of the insecticide and nematicide oxamyl was monitored in fallow sandy loam soils under field conditions. Using measurements of rainfall and evaporation from a water surface, the water flow in soil was simulated by a computer model, and the results were compared with the measured soil-moisture profiles. The model was extended to simulate the behaviour of oxamyl, using laboratory data for adsorption and rates of degradation in soil. The model generally underestimated oxamyl movement in the first month, whereas it tended to overestimate later movement. The rate of breakdown of oxamyl, as affected by soil type, temperature and soil-moisture content, was fairly well described. After about 2 months only small amounts of oxamyl remained. Accumulation of oxamyl near the soil surface in dry periods was overestimated, indicating deficiencies in the modelling procedure under these conditions.