Emission of fumigants from soil and dispersion in air

In the Netherlands, soil fumigants are used on a large scale for nematode control in arable farming. After injecting the fumigant into the soil, a fraction of the dosage of 1,3-dichloropropene and methyl isothiocyanate (formed from metham-sodium) diffuses up to the soil surface and escapes into the air.The processes involved in fumigant behaviour in soil are described and the factors affecting these processes are discussed. A standard model including the most important processes is described and used to simulate the fumigation of fields injected with 1,3-dichloropropene or metham-sodium. It was computed that up to a few tens of percent of the dosage of the fumigant can be emitted into the air during the first three weeks after injection.The impact of several simplifications in the standard model on the rate and extent of emission of fumigant into the air was evaluated. The effect of a fumigant content- dependent rate coefficient for the transformation of fumigant in soil and that of changes in soil moisture content on the rate and extent of emission into the air was computed to be substantial. Although a diurnally changing soil temperature was computed to affect the rate of emission of fumigant into the air substantially, its effect on the cumulative emission in time, compared with a soil system at the average temperature, was computed to be negligible.Measurements on the one-hour concentrations of methyl isothiocyanate and 1,3-dichloropropene in air were done around two fields injected with metham-sodium and around two other fields injected with 1,3-dichloropropene. Concentrations measured in air were compared with those computed using a gaussian plume model. The comparatively large differences between the computed and measured concentrations in the air during the first few days after injection could be ascribed to an underestimation of the source strength of the emission. Improving the description of the pattern of concentrations of fumigant in air would require more detailed measurements on input parameters for the dispersion model.In two consecutive years, 6-hour concentrations of 1,3-dichloropropene and methyl isothiocyanate were measured at two locations in a region with intensive use of soil fumigants. For some weeks with many fumigations in this region, the concentration of fumigant in air at a receptor site, with representative fumigated fields at different upwind distances, was computed using a gaussian plume model. The computed and measured concentrations were of the same order of magnitude.