1 Deposition and dissipation

I F Grant

Spray drift
Residue levels on bark and in soil
Dissipation of residues from bark and soil
References

Spray drift

DDT droplets were collected on water-sensitive papers placed on the ground around sprayed trees (Figure 1.1). Under a prevailing wind speed of 1.5 m/s the heaviest droplets fell immediately around the tree and most of the medium and fine droplets (100-400 um VMD) landed within two metres of the target (Figure 1.2). Gusts of wind up to 3.5 m/s carried some droplets up to 4 m from the target. The finest droplets (<100 um VMD), vapour and dust can be carried much further, accounting for the contamination of remote, unsprayed areas). Residues transported from the target by rain and animal activity add to residues in the near vicinity over a longer period of time.

Residue levels on bark and in soil

DDT residues on mopane bark showed wide variation in sprayed areas, occasionally reaching very high levels. This reflects the contagious distribution of DDT resulting from the spraying technique. There was no evidence of accumulation of residues on trees sprayed more than once. Residue levels on bark in unsprayed areas were generally close to detection limits.

Samples of bark, from approximately 1 m above ground, were removed by hammering a hollow steel soil corer into the tree trunk to the depth of the cambium. Average EDDT residues on mopane bark from sprayed areas ranged from 0.5-93 ug/cm2, compared with <0.01-0.03 ug/cm2 on bark from the unsprayed area.

Contamination of soil around sprayed trees could be high but generally, residue levels from both sprayed and unsprayed sites in the study area were at the low end of the recorded range for natural land. Untreated soils from deserts, tundra and prairies typically contain 0.1 to 2 ppm S DDT dry weight, while agricultural soils may average 2 ppm, although this varies with the crop (<100 ppm in orchards) and cultural practice2.

Soil samples to a depth of 5 cm were collected from a variety of sites and analysed for DDT residues. Samples from the unsprayed area contained between <0.01 and 0.1 ppm S DDT dry weight. Similar levels were found in samples from the sprayed area, unless the soil was taken below a sprayed tree. In the latter case, samples contained between 1 and 200 ppm S DDT dry weight.

Dissipation of residues from bark and soil

Residues of DDT dissipated rapidly from mopane bark, having a half-life of about 50 days (Figure 1.3). Dissipation from soil surfaces was also rapid, with half-lives of about 90 days on loamy sand and 125 days on silt loam; however 15-21% of the initial deposit remained on the soil surface one year after treatment.

Dissipation over the first 100 days was not affected by rainfall, suggesting that dry season radiation levels promoted volatilization of DDT and its more volatile photolytic product DDE. Photolysis and volatilization can account for losses of 80-100 % of DDT applied to tropical soil and explain the relatively rapid dissipation of DDT under tropical conditions, in the order of 2-9 months as compared to 3-10 years in temperate zones

The shorter half-life of S DDT on mopane bark than in soil is consistent with a greater opportunity for volatilization from bark, where the deposit is fully exposed to the air. The shorter half-life of S DDT in sandy as compared with silty soil is consistent with the relative adsorptive capacities of insecticides on sand and dry clay. Compared with the work of Wessels et al. (unpublished) on the dissipation of DDT from cultivated soils in Zimbabwe, the half life of DDT at Siabuwa was roughly doubled. Wessels et al., recorded half-lives of about 50 days in all but one of seven soil types, but rain had fallen at all sites within the first 100 days, and physical desorption and microbial degradation of S DDT usually increase with soil moisture. Moreover, cultivated soils have microbial populations adapted to pesticides, while woodland soils have more organic matter, perhaps the most significant factor influencing persistence.

References

1. Cohen, J.M. and Pinkerton, C. (1966) Widespread translocation of pesticides by air transport and rainout. Advances in Chemistry Series, 60:163-176.

2. Edwards, C.A. (1973) Persistent Pesticides in the Environment Cleveland, Ohio: CRC Press.

3. Cliath, M.M. and Spencer, W.F. (1972) Dissipation of pesticides from soil by volitilisation of degradation products, 1. Lindane and DDT. Environmental Science and Technology, 6: 910-914.

4. Yeadon, R. and Perfect, T.J. (1981) DDT residues in crop and soil resulting from application to cowpea Vigna unguiculata (L.) Walp. in the sub-humid tropics. Environmental Pollution Ser. B, 2: 275-294.

5. Edwards, C.A. (1966) Insecticide residues in soils. Residue Reviews, 13: 83-132.

6. FAO/IAEA (1986) Report on the fate of persistent pesticides in the tropics, using isotope techniques. Second FAO/IAEA Research Co-ordination Meeting, Quito, Ecuador 24-28 February 1986.

7. Hassan, A. (1991) Summary and Report of Second FAO/IAEA Research Co-ordination meeting on 'Behaviour of DDT in Tropical Environments', Jakarta, Indonesia 4-8 November 1991, Vienna: FAO/IAEA.

8. Wiese, J.H. and Basson, C.J. (1966) The degradation of some persistent chlorinated hydrycarbon
insecticides applied to different soil types. South African Journal of Agricultural Science, 9: 945-970.

9. Wessels, C.L., Tanock, J. and Phelps, R.J. (unpublished report) Longevity of some chlorinated hydrocarbon insecticide residues on the surface of different soils.