A Simulation Model for the Fate of Polycyclic Aromatic Hydrocarbons in a Field Soil.

Sources of polycyclic aromatic hydrocarbons (PAHs) in soils are both natural and anthropogenic. The objectives of this study were to determine the concentration of important PAHs compounds in contaminated soils (0-15 cm depth) and to find out through a partially stochastic model to what extent field heterogeneity and irrigation management will influence the PAHs concentration distributions. The total concentration of PAH compounds were 1.03 mg kg^-1 and 2.43 mg kg^-1 at 500 and 1500 m from a point source, respectively. As the distance from the point source increased the concentration of benzoflouranthenes/benzopyren increased from 0.31 mg kg^-1 to 0.91 mg kg^-1 while chrysene/benzathracenes decreased from 0.80 mg kg^-1 to 0.35 mg kg^-1, respectively. The simulation model assumed a constant pore-water velocity (v), linear-equilibrium adsorption, and first-order degradation. The adsorption parameter (k_d) was calculated from molecular structure with the help of a first-order molecular connectivity index. Concentration profiles for anthracene, benz(a)anthracene, and benzo(a)pyrene were obtained by employing the mode, median and mean values of (v, D) while the values of k_d and t_1/2 were kept constants; where D and t_1/2 were the apparent diffusion coefficient and the degradation half-life, respectively. A contaminated depth variable ξ was defined so that the soil beneath it would be practically free of PAHs during its entire life-time. The values of (ξ₁, ξ₂, ξ₃) under continuous leaching were (1, 5, 9 cm), (2, 7, 18 cm), and (8, 30, 70, cm) for benzo(a)pyrene, benz(a)anthracene, and anthracene, respectively; where  ξ₁, ξ₂, and ξ₃ were values of ξ calculated by employing the mode, median, and mean values of (v, D), respectively. The implication of the results for the assessment of PAHs contamination of groundwater was discussed.