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Title: Effects of soil moisture on the diurnal pattern of pesticide emission: Numerical simulation and sensitivity analysis

item REICHMAN, RIVKA - University Of California
item Yates, Scott
item Skaggs, Todd
item ROLSTON, DENNIS - University Of California

Submitted to: Atmospheric Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2012
Publication Date: 2/1/2013
Citation: Reichman, R., Yates, S.R., Skaggs, T.H., Rolston, D.E. 2013. Effects of soil moisture on the diurnal pattern of pesticide emission: Numerical simulation and sensitivity analysis. Atmospheric Environment. 66(2013):41-51.

Interpretive Summary: The use of pesticides in modern agriculture has led to large increases in crop production. However, pesticide volatilization is a primary mechanism leading to the dispersion and accumulation of toxic chemicals in the environment. To assess the effects of pesticide emissions upon risks to ecosystems and human health, accurate prediction of volatilization rates is critical. The soil water content influences the partitioning of a pesticide among the soil water, gas, and sorption phases. Studies have shown that pesticide adsorption increases dramatically when the water content decreases below a critical value, which signifies the point where the solid phase is no longer covered by at several molecular layers of water. Above this critical value the equilibrium vapor density is generally not affected by soil-water content. To accurately predict the volatilization rate, it is very important to account for the effects of the soil water content. This present paper describes a theoretical study focusing on the impact of soil water content on pesticide vapor adsorption to the soil particles and their effect on the diurnal pattern of pesticide emission. The results demonstrate that the daily peak emission rate depends on the soil water content, the threshold value and the vapor sorption process. Although additional research is needed, this research will one day provide a more accurate and reliable method to predict pesticide emissions and will be of great use to the scientific community, regulators, agricultural consultants and farm advisors.

Technical Abstract: Accurate prediction of pesticide volatilization is important for the protection of human and environmental health. Due to the complexity of the volatilization process, sophisticated predictive models are needed, especially for dry soil conditions. A mathematical model was developed to allow simulation of the diurnal variation of pesticide volatilization as affected by soil-water content, the air-solid interface partition coefficient, soil-water retention function and soil surface resistance processes. The model formulation considered two possible water retention functions and two soil surface resistance functions. To test the model, simulations were performed for ten successive days of drying under typical semi-arid summer conditions following application of the pesticide diazinon to either a loam or sand soil. Results showed that the temporal variation and magnitude of diazinon emission were strongly affected by the air-solid interface partition coefficient, soil-water content and the surface resistance function. The model was capable of simulating complex diurnal patterns in the peak emission rates which are caused by changes in soil water content and air-solid partitioning. The water retention function formulation had only a minor effect on the simulated water content and volatilization rates, whereas the soil surface resistance function significantly influenced the volatilization rate. Neither the water retention function nor the soil surface resistance formulation had a significant effect on the simulated soil temperature.