M. Leistra

Accelerated transformation of aldicarb, oxamyl and ethoprophos after repeated soil treatments

Abstract 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.

Behaviour of metamitron and hydroxy‐chlorothalonil in low‐humic sandy soils

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

Adsorption, transformation, and bioavailability of the fungicides carbendazim and iprodione in soil, alone and in combination

Abstract 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.

Movement and Transformation of 1,3-Dichloropropene in the Soil of Flower-Bulb Fields

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.The transformation 1 kinetics of 1,3-dichloropropene in soil material from the root zone was studied in more detail at 15°C in the laboratory. At higher initial contents (62 mg/kg), the rate of transformation was approximately first-order in the first period of about three weeks and this was followed by a rapid transformation. At lower initial contents (12 mg/kg), transformation was rapid from the beginning and only about 1% of the dose was left after two weeks.

Pesticide Leaching from Agricultural Fields with Ridges and Furrows

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.

Rate of transformation of atrazine and bentazone in water-saturated sandy subsoils

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 <0·25%, 11% and 25% of the dose. Bentazone transformation in the two subsoils with high pH and low redox potential was very slow, but the presence of oxygen in one of these subsoils speeded up the transformation.

Computations on the Volatilisation of the Fungicide Fenpropimorph from Plants in a Wind Tunnel

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 14C-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.

Contribution of leaching of diazinon, parathion, tetrachlorvinphos and triazophos from glasshouse soils to their concentrations in water courses

Abstract Data on the adsorption and transformation rates of diazinon, parathion, tetrachlorvinphos and triazophos in soils were collected from a survey of the literature. As little information is available on their mobility, the adsorption of tetrachlorvinphos and triazophos on three soils was measured in a slurry experiment. Properties of diazinon were introduced into a computer model simulating glasshouse soil systems in a simplified way. The leaching of diazinon from the root zone was calculated to be zero. The properties of the other three organophosphates indicate that in similar computations leaching from the root zone would have been even lower. Samples from tile drains and water courses in areas with many glasshouses were analysed by gas-liquid chromatography. The concentration of the four organophosphate insecticides in almost all of the samples of water from tile drains was below the detectable limit. However in samples from the water courses, pesticide residues were found regularly, sometimes at fairly high concentrations. Thus contamination of water courses would seem to be produced not by leaching of pesticides through the soil but by other pathways.

Measured and computed concentrations of 1,3-dichloropropene and methyl isothiocyanate in air in a region with intensive use of soil fumigants

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.The rates of emission of 1,3-dichloropropene and methyl isothiocyanate into air were estimated for weeks with many applications in the region studied. Using the PAL model, the concentration of fumigant in air at a receptor site was computed for representative fumigations at different upwind positions. The computed concentrations in air ranged up to 9.9 μg m−3 for 1,3-dichloropropene and up to 2.5 μg m−3 for methyl isothiocyanate.

Volatilisation and competing processes computed for a pesticide applied to plants in a wind tunnel system

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.