Soil-air relationships for toxaphene in the southern United States

Volatilization of toxaphene residues from agricultural soil was investigated at farms in the southern United States by collecting air samples 40 cm above the soil. The concentration of total toxaphene ranged over several orders of magnitude, from <3 to 6,500 ng g(-1) dry weight in soil, and <0.3 to 42 ng m(-3) in air. A log-log plot of total toxaphene concentrations in soil and overlying air showed a significant (p < 0.001) positive relationship, with r(2) = 0.73. The soil/air fugacity ratio (FR) for 26 events ranged from 0.4 to 238, exceeded FR = 1.0 (soil/air equilibrium) in 24 events, and exceeded FR = 10 in 17 events. This indicates that toxaphene in air sampled at 40 cm generally was not at equilibrium but undersaturated with respect to the soil. Compared to a technical toxaphene standard, chromatographic profiles of toxaphene residues in soil and air showed alterations due to preferential degradation and volatilization of the components. Peaks matching the retention times of labile octachlorobornanes B8-531 and B8-806 + B8-809 were depleted in both soil and air relative to the more recalcitrant B8-1413 + B8-1945 and B8-2229. For each event, log-log plots were made of the dimensionless soil/air concentration quotient (Q) versus liquid-phase vapor pressure (P-L, Pa) for 10 toxaphene components (peaks containing coeluting congeners) that spanned the volatility range of hepta- to nonachlorobornanes. Statistically significant linear relationships were obtained with r(2) values for most events ranging from 0.54 to 0.96. Slopes for all but one event ranged from -1.01 to -1.53 and averaged -1.28 +/- 0.20. When regressions were carried out for only components one to nine, which cover the vapor pressure range of most components reported in ambient air, the average slope was reduced to -1.02 +/- 0.15. Previous models of toxaphene emission, transport, and deposition have considered only total toxaphene. These results provide a basis for modeling soil/air exchange on the basis of single congeners or groups of congeners having similar volatilities.