USE OF GASTIGHT PLASTIC FILMS DURING FUMIGATION OF GLASSHOUSE SOILS WITH METHYL BROMIDE - II. EFFECTS ON THE BROMIDE-ION MASS BALANCE FOR A POLDER DISTRICT

Methyl bromide is used on a large scale for disinfestation of glasshouse soils in the Netherlands. The introduction of gastight plastic films for covering the soil surface during fumigation has led to a reduction of the dose from an average of 100 g. m^-2 in 1979–1980 to an estimated average of 40 g. m^-2 in 1982–1983. At the same time, the mandatory cover-time of the soil surface with the plastic film was extended from three to ten days. Both the reduction of the dose and the extension of the cover-time were intended to reduce the emission of methyl bromide into air but will also influence the quantitative transformation of methyl bromide to bromide.

The transformation of methyl bromide under field conditions was evaluated by a quantitative analysis of the bromide-ion mass balance of a polder district in the western part of the Netherlands during a twelve months period in 1982–1983. The total bromide input by precipitation, by water pumped into the canal system of the district for water level control and by seepage was in the order of 1.0 x 10⁵ kg. The total output, primarily by discharge of excess precipitation, was in the order of 2.7 x 10⁵ kg. The difference of 1.7 x 10⁵ kg between input and output was attributed to the transformation of methyl bromide. This indicated that approximately 40% of the total use of 4.9 x 10⁵ kg of methyl bromide during the twelve months period was transformed in the soil. Emission to the air would account for the remaining 60%.

A similar mass balance computation for a twelve months period in 1979–1980 indicated a transformation of only 15% under the then prevailing conditions, i.e. a dose of 100 g. m^-2 use of low density polyethene and a cover-time of three days. Comparing the 1979–1980 and the 1982–1983 data, it is concluded that the decrease of the dose from 100 to 40 g. m^-2 is counteracted by an increase of the transformation of methyl bromide from 15 to 40%. Consequently, the bromide discharge to ground- and surface waters per unit area of glasshouse soil has not changed significantly. The emission of methyl bromide to the air, however, would have been reduced by approximately a factor five.