Volatilization of Organic Chemicals from Soil as Related to Their Henry's Law Constants

The output from a pesticide screening model indicated that chemicals with low Henry's Law constants (KH) will tend to accumulate at the soil surface when water is evaporating, resulting in an increased volatilization with time. Volatilization losses and surface distributions of two pesticides with widely differing KH, were measured to test the predictions of the screening model, particularly with respect to the importance of KH in controlling relative volatilization and vapor behavior. Volatilization of prometon (2,4‐bis(isopropylamino)‐6‐methoxy‐s‐triazine], a Category III compound with low KH, increased with time with water evaporating and prometon accumulated at the soil surface. Volatilization of lindane (gamma isomer of 1,2,3,4,5,6‐hexachloro‐cyclohexane), a Category I compound with high KH, decreased with time with or without water evaporating and did not accumulate at the surface with water evaporating. The experimental data confirmed the model predictions that volatilization of chemicals with low KH is controlled by the air‐boundary layer above the soil surface, whereas control of volatilization of Category I chemicals with high KH is within the soil. Accumulation and increased volatilization of compounds with low KH are controlled by boundary layer thickness and water evaporation rate as well as KH. The phenomena of organic chemicals with low KH accumulating at the soil surface following convective movement in evaporating water could greatly enhance their volatilization and increase their availability for photolysis and runoff into surface waters. Sufficient accumulation can occur that changes in adsorption coefficients with concentration must be taken into account in modeling their volatilization.