J.H. Smelt

Emission of pesticides into the air

During and after the application of a pesticide in agriculture, a substantial fraction of the dosage may enter the atmosphere and be transported over varying distances downwind of the target. The rate and extent of the emission during application, predominantly as spray particle drift, depends primarily on the application method (equipment and technique), the formulation and environmental conditions, whereas the emission after application depends primarily on the properties of the pesticide, soils, crops and environmental conditions. The fraction of the dosage that misses the target area may be high in some cases and more experimental data on this loss term are needed for various application types and weather conditions. Such data are necessary to test spray drift models, and for further model development and verification as well. Following application, the emission of soil fumigants and soil incorporated pesticides into the air can be measured and computed with reasonable accuracy, but further model development is needed to improve the reliability of the model predictions. For soil surface applied pesticides reliable measurement methods are available, but there is not yet a reliable model. Further model development is required which must be verified by field experiments. Few data are available on pesticide volatilization from plants and more field experiments are also needed to study the fate processes on the plants. Once this information is available, a model needs to be developed to predict the volatilization of pesticides from plants, which, again, should be verified with field measurements. For regional emission estimates, a link between data on the temporal and spatial pesticide use and a geographical information system for crops and soils with their characteristics is needed.

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.

Development and duration of accelerated degradation of nematicides in different soils

Abstract The development of accelerated degradation of nematicides was studied in incubation experiments with soils collected from trial plots at three long-term experimental fields. The plots had been treated annually for periods of 3–10 years with aldicarb, oxamyl, ethoprophos, fenamiphos or 1,3-dichloropropene (1,3-D). At one site (sandy soil; pH 7.3) highly accelerated degradation rates were measured for aldicarb, oxamyl and ethoprophos in soil from plots treated three times. After 5 years without further treatments, these high degradation rates were still present for aldicarb and oxamyl, but not for ethoprophos. At another site (sandy soil; pH 5.6) little or no increase in the degradation rates of aldicarb, oxamyl and ethoprophos was measured in soil from plots treated 10 times. Three annual applications of fenamiphos to a loamy soil (pH 7.7), doubled the rate of loss of the total toxic residue compared with the untreated soild. Fenamiphos and oxamyl did not show cross-enhancement when incubated in ethoprophos-adapted soils. After a lag-time of 4–9 days degradation of 1,3-D was very rapid in all soils including controls which had been treated only once or twice well before the sampling dates. Seven annual applications of 1,3-D resulted in only slightly shorter lag-times. An interval of 5 years without treatment resulted in only slightly longer lag-times. Simultaneous incubation of a commercial dose of the fumigant methyl isothiocyanate inhibited the fast degradation of 1,3-D.

Transformation of 14C-labelled 1,2-dichloropropane in water-saturated subsoil materials

Abstract 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).

Uptake of DDT and related compounds from soils into forage crops

Abstract The uptake of DDT and related compounds ( p , p ′-DDT, p , p ′-DDE, p , p ′-TDE and o , p ′-DDT, hereafter expressed as DDTR, in alfalfa, maize, sugarbeet, mangold, potato and ryegrass was studied in field trials on loamy soils that had previously been used for fruit trees. Residues in these soils result from repeated use of DDT in these orchards over the last 20 years. Contents of DDTR in soil ranged from 0.7 to 2.5 mg/kg dry soil. The value of the quotient “content of DDTR in crop/content of DDTR in soil” (both contents expressed on a dry matter basis) for the foddered part of alfalfa and maize and for sugarbeet and mangolds was less than 0.03. The highest values were found for potatoes with an average quotient for all sizes of 0.09. The DDTR residues in the tubers were mainly in the peelings; residues in peeled potatoes were below limits of determination. Ryegrass also took up measurable amounts of DDTR residues from soil. The value of the quotient content in crop/content in soil for the grass blades from the first three cuts after sowing was lower than 0.05. For grass, mown during a whole season on a permanent pasture, an average value of 0.044 for p , p ′-DDE was found. In most instances, no significant differences in the uptake of the four compounds were found. Contamination of crops with soil during harvesting can considerably enhance contents of DDTR, thus decreasing usability for animal feeding.

Field Test of The Pestla Model For Ethoprophos on a Sandy Soil

The nematicide ethoprophos was applied to a sandy soil (5% organic matter) in autumn. Concentration profiles in field soil were measured after 103 and 278 days. In the field, rainfall and soil temperature were continuously recorded. In the laboratory, the transformation rate and adsorption of ethoprophos were measured using soil sampled in the field before application of ethoprophos. The results were used to test an existing mathematical model (PESTLA) which is based on the convection/dispersion equation and which assumes equilibrium sorption and first-order transformation kinetics. During the first 22 days about 50% of the dose disappeared whereas the model calculated a loss of 4% in this period. The measured loss is probably the result of volatilisation which is not accounted for by the model. Measured and calculated persistence in soil corresponded well between 22 and 103 days but after 278 days the calculated remaining amount was about 1.5 times the measured amount. Measured and calculated concentration profiles corresponded reasonably well after 103 days. However, after 278 days the model overestimated leaching out off the top 4 cm.

Effects of ensilage methods and field drying on residues of DDT in grass and alfalfa

Abstract The behaviour of DDT and its analogues in ensiled grass was studied in the laboratory in containers of 1 dm 3 volume. After a storage period of 60 days at 20 and 35°C, no significant decrease in the contents of p , p ′-DDT, p , p ′-TDE, p , p ′-DDE and o , p ′-DDT was found. The results suggest that ensilage with desirable fermentations is not practical to reduce DDT residues in pasture grass. Grass and alfalfa contaminated with low contents of DDT and its analogues, resulting from uptake from soil and traces of spray drift, were field-dried to hay. This drying did not result in significant changes in the contents of p , p ′-DDT, p , p ′-DDE, p , p ′-TDE and o , p ′-DDT.