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Effect of Flooding and Organic Matter Applications on DDT Residues in Soil¹

ABSTRACT

A field in Coachella Valley, California which had received repeated applications of technical DDT to sweet corn (Zea mays L.) for ear worm [Heliothis yea (Boddie)] control was flooded 48 days, with and without organic matter amendments, to determine whether DDT soil concentrations could be effectively reduced. Applications of 180 and 45 metric tons/ha of feedlot manure and 18 and 5.6 metric tons/ha of alfalfa (Medicago sativa L.) meal were disked to a depth of 15 cm. Redox potentials in all flooded plots rapidly dropped below –150 mV at both the 7.5- and 15-cm soil depths. All flooded plots including control plots showed a rapid decrease in soil levels of DDT isomers accompanied by a rapid accumulation of the respective DDD isomers in the top 15 cm. Flooding had little effect on the DDE isomers. By the end of the flood period p,p'-DDT had decreased from 8.1 µg/g to 0.5 to 1.6 µg in flooded plots receiving organic amendments while the p,p'-DDD level increased from zero to 4.2 to 5.6 µg/g. The rates of disappearance of DDT and the formation of DDD were significantly lower in the flooded plots receiving no organic amendments. After 18 days flooding the rates of p,p'-DDT losses and DDD gains were in the order of 180 metric tons/ha manure 18 metric tons/ha alfalfa > 5.8 metric tons/ha alfalfa 45 metric tons/ha manure > control. The transformation of DDT isomers to DDD isomers occurred to a lessor extent at 15–30, 30–45, and 45–50 cm soil depths. The data indicate that flooding organic matter-amended soil effectively reduced DDT concentrations to very low levels. The fate of DDD in flooded soils is unknown. However, through the formation of DDD, the conversion of DDT to the potentially hazardous DDE is prevented.

Key Words: pesticides • pesticide residues • DDD • DDE • redox potentials • anaerobic decomposition

¹ Contribution from the Department of Soil Science and Agricultural Engineering, Univ. of California, Riverside, and the Western Region, ARS, USDA. This work was supported by USDA Cooperative Agreement No. 12-14-100-10, 433 (41).

² Associate Professor of Soil Science, Univ. of California, Riverside; Soil Scientist, USDA; Staff Research Associate, Univ. of California, Riverside; and Chemist, USDA.

Received for publication October 20, 1973.