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Volatility of Organochlorine Insecticides from Soil: II. Effect of Relative Humidity and Soil Water Content on Dieldrin Volatility¹

ABSTRACT

The influence of soil water content and soil water loss on the volatilization of dieldrin, a representative weakly polar insecticide, was investigated to test the "codistillation" theory of pesticide volatility. Dieldrin volatilization from Gila silt loam was dependent on soil water content, but was not dependent on rate of water loss from the soil. Air relative humidity indirectly influences dieldrin volatilization by influencing soil water content. Dieldrin volatilization from moist soil was always at a maximum when soil water loss was zero. With air at 100% relative humidity, volatilization remained high, and only decreased gradually as the dieldrin concentration at the soil surface decreased. When air was at less than 100% relative humidity, volatilization decreased rapidly as the soil water content decreased and was essentially zero below a soil water content of 2.8%. The influence of soil water content and soil water loss on dieldrin volatility was similar at 20 and 30C, at 2 and 8 ml/sec flow rate and at 5 and 10 µg/g dieldrin concentration. The results show that soil water content influences pesticide volatilization by competing for soil adsorption sites and thereby controlling the vapor density of the pesticide. These experimental results offer conclusive evidence that for dieldrin and probably for all pesticides, the term "codistillation" does not apply to volatilization from soils.

¹ Contribution of the Dep. of Soil Science and Agr. Eng., Univ. of Calif., Riverside 92502, and the Soil and Water Conser. Res. Div., ARS, USDA. This work has been supported by USDA Cooperative Agreement no. 12-14-100-9016(41). Presented in part before a joint meeting of Div. S-2 and S-3, Soil Science Society of America, Nov. 12, 1969, Detroit, Michigan.

² Former Graduate Student, Assistant Professor of Soil Science, University of California, Riverside, Soil Scientist, USDA, and Professor of Soil Science, University of California, Riverside, respectively. The senior author is now Associate Soil Chemist, The Inter-American Institute for Agricultural Sciences, Turrialba, Costa Rica.

Received for publication August 16, 1971. Accepted for publication January 17, 1972.