MODELLING THE PESTICIDE CONCENTRATION IN A RICE FIELD BY A LEVEL IV FUGACITY MODEL COUPLED WITH A DISPERSION-ADVECTION EQUATION Equilibrium Thermodynamics group 2 Allie Ibarra  A01373959          Pamella Beltrán A01167262 Brian Morteo A01373400         Yibrahán Jiménez A01167436

AIM To study the models use to simulate the environmental concentration of pesticides. Such as a level IV fugacity model, represented by a system of ODE. Also, model applied to transportation of pesticides in a soil column.

Level IV fugacity model (FUGIV) Estimates the fugacities and the concentrations of the pesticide in air, water, plants rice and sediment.

ki j = Ci /C j Ci and C j concentrations of the substance in each one of the compartments.[mol m-3] Ci = Zi fi  relationship between fugacity and concentration. Zi [mol m-3 Pa−1]  capacity of fugacity. . Fi [Pa] fugacity and the proportionality constant.

system modelled air (i = a), water (i = w), rice plants (i = r) and sediment (i = s) Za = 1/RT R = 8.314[ m3 Pa mol−1 T−1], Za[mol m−3 Pa−1] Zw = 1/H H [m3 Pa mol−1] is the Henry’s constant for the pesticide (H = pm pv/sw) Pm [g mol−1] molar mass of the pesticide, Pv [Pa]vapour pressure of the pesticide Sw [g m−3] aqueous solubility of the pesticide.

Zr =(XwZw +xlkowZw)(ρr/ρw) ρw [kg m−3]water density, Kow  octanol–water pesticide partition coefficient. ρr [kg m−3] rice plants´ density. xw  rice plants’ water volumetric fraction. Xl rice plants’ lipids volumetric fraction. Zs = ρsocskoc /H Ocs sediment’s organic carbon volumetric fraction. Koc [m3 kg−1] soil organic carbon partition coefficient of the pesticide.

FUGIV Generally, this model allows to describe the unsteady state behaviour of a substance in the environment, which permits the observation of substances whose emission and fugacity vary with time, and to determine the time in which the system reaches the steady state condition.

Equation of the dispersion-advection (EDA) Advection: passive transport by the moving fluid that contains the substance. (Cushman) Simulates the pesticide leaching in the water-saturated soil profile; the water-saturated soil profile. Considers the hydrodynamics dispersion, the rate degradation of the pesticide in floor of the aquifer, and the coefficient of absorption of the pesticide.

These models allows the calculation of the distribution of the mass of a pesticide among compartments. Also facilitates the estimation of concentration of the pesticide in each. The EDA model includes processes of evaporation to a chemical free atmosphere, leaching via percolating water, diffusion, absorption and reaction.

Conclusions Results suggest how this can be used to determine the environmental compartment that is more vulnerable to a chemical compound or the risk of the groundwater contamination by a determined pesticide; more extensively, how to decide among many compounds, which one needs a better environmental analysis.

The results suggest that the level IV fugacity model coupled with a dispersion-advection equation is appropriate and permits estimation or anticipation of the pesticide fate and exposure in an environmental compartments system for a screening level risk assessment.

References Cushman-Roisin, B. (s.f.). Definitions Advection, Diffusion. Recuperado el 27 de agosto, 2015 de Dartmouth College: http://engineering.dartmouth.edu/~d30345d/courses/en gs43/Definitions.pdf Contreras, W., Ginestar, D., Paraíba, L. and Bru, R. (2008). Modelling the pesticide concentration in a rice field by a level IV fugacity model coupled with a dispersion-advection equation. Recuperado el 27 de agosto, 2015 de Science Direct: http://www.sciencedirect.com/science/article/pii/S0898 122108000564