Variation of plant p,p′-DDT uptake with age and soil type and dependence of dissipation on temperature

Abstract

The extent to which DDT may be absorbed and translocated from 14C-p,p′-DDT contaminated soils into cowpeas plant tissues, and the variation of uptake of p,p′-DDT by the plants in relation to the dissipation of p,p′-DDT in the soils was studied using a radioisotope technique for three months (90 d). Substantial absorption and accumulation of residues was observed for the cowpeas grown in two different sites, namely, in the coastal province Mombasa and highland region, Nairobi. The degree of uptake varied with soil type and growing conditions. Total residue levels ranging from 0.945±0.040 mg/kg to 7.765±0.211 mg/kg were obtained for 2 to 12 week old Mombasa plants. However, Nairobi plant values fell in the range of 1.136±0.038 mg/kg to 3.239±0.007 mg/kg. The Mombasa plants gave a range of residue levels from 0.800±0.065 mg/kg to 6.110±0.038 mg/kg and 0.084±0.001 mg/kg to 1.390±0.003 mg/kg for extractable and non-extractable (bound) residue, respectively. The corresponding values for Nairobi samples were 1.034±0.011 mg/kg to 2.241±0.014 mg/kg and 0.080±0.002 mg/kg to 0.411±0.007 mg/kg, respectively. Further, results suggest that the higher the water retention by the soil, the higher the rate of evaporation of DDT. Extractable DDT in soils decreased with temperature while soil-bound DDT increased with temperature. Coastal region soil-p,p′-DDT samples indicated p,p′-DDE to be the major metabolite over a period of 292 d. The levels of DDT decreased with time. The reverse was true for DDE, as expected. First order rate constant for DDT dissipation in soils was confirmed by half-life measurements. The rate constants (k), calculated from the data, for Mombasa and Nairobi, were 5.6 × 10−6 min−1 and 5.1 × 10−6 min−1, respectively. Within a period of 90 d, DDT residue in the plants ranged from 0.94 to 7.73 mg/kg, while that in soils ranged from 88.9 to 32.0 mg/kg.