Polycyclic aromatic hydrocarbons (PAH) are among the major contaminants in the terrestrial environment. The background level in normal agricultural land has increased for many years and it is expected to further increase in the future. Because of the very low water solubility and high K_ow values, PAHs tend to sorb to the organic carbon (OC) in the soil.

This study aims to understand the contribution to phenanthrene sorption of various soils fractions such as organic carbon, clay, silt and sand.

In 24 hours equilibrium sorption experiments, we determined the phenanthrene partition coefficient, K_D for more than one hundred Danish and European agricultural top and sub soils (122 topsoils and 28 subsoils) as well as the normalized distribution coefficient of the organic carbon content (K_OC), through single point isotherm measurements.

Possible effect of clay-complexed organic carbon was analyzed, as derived from the Dexter et al. (2008) n-index (ratio of clay to organic carbon of 10 kg kg^-1), on K_OC, but we did not find it to markedly influence K_OC nor be useful to better predict K_OC for cultivated soils. Globally, the soils split into two groups with one group above and the other below the saturation line as defined by Dexter.

Top soils and subsoils showed different sorption behavior, with typically higher Koc for topsoils, likely due to different organic matter quality related to soil management and hydrological impact. Topsoils generally exhibited Koc values between the traditionally applied models of Abdul et al. and Karickhoff et al. These two models were documented useful to predict maximum and minimum Koc for agricultural topsoils, for example in regard to predicting long-term PAH leaching from cultivated areas. Furthermore, we suggest a new Koc model in between Abdul and Karickhoff for predicting average Koc for agricultural topsoils. Subsoils mostly followed Abdul, in agreement with that this model was developed for deeper soils and groundwater sediments lower in organic carbon (0.4 - 2%).

This study only concerns solid-liquid partioning and thus the probability for PAH adsorption onto assumingly non-mobile soil particles. In perspective, the combined risk of dissolved PAH leaching and colloid-facilitated PAH leaching from cultivated land areas should be considered.