Volatilization of pesticides from the bare soil surface: evaluation of the humidity effect

Volatilization of pesticides from soils under dry conditions (water content below the permanent wilting point) can be significantly influenced by sorption to hydrated mineral surfaces. This sorption process strongly depends on the water activity, expressed as equilibrium relative humidity in the pore space of the soil, and on the available surface area of the hydrated minerals. In this study, the influence of different humidity regimes on the volatilization of two pesticides (triallate and trifluralin) was demonstrated with a bench-scale wind tunnel system that allowed the establishment of well controlled humidity conditions within the soil. In the experiment starting with very dry conditions, increasing the relative humidity in the adjacent air from 60 to 85% resulted in an up to 8 times higher volatilization rate of the pesticides. An additional strong increase in volatilization (up to 3 times higher) was caused by a simulated rain event, which eliminates all sorption sites associated to mineral surfaces. In agreement with this interpretation, the comparison of two soils suggested that mineral surface area was the soil property that governs the volatilization under dry conditions, whereas soil organic matter was the controlling variable under wet conditions. In contrast to expectations, the use of a novel capsulated suspension for triallate showed the same humidity effects and no substantially lower volatilization rates in comparison to the regular formulation. This study demonstrated that humidity effects on pesticide volatilization can be interpreted via the mechanism of sorption to mineral surfaces under dry conditions.