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Summary Soils represent a major sink for organic xenobiotic contaminants in the environment. The degree to which organic chemicals are retained within the soil is controlled by soil properties, such as organic matter, and the physico‐chemical properties of the contaminant. Chemicals which display hydrophobic and lipophilic characteristics, as well as a recalcitrant chemical structure, will be retained within the soil, and depending on the ‘strength’ of the association may persist for long periods of time. This review describes the behaviour of hydrophobic organic contaminants in soils, focusing on the mechanisms controlling interactions between soil and contaminants. The bioavailability of contaminants in soil is also discussed, particularly in relation to contact time with the soil. It considers the degradation of organic contaminants in soil and the mechanisms microbes use to access contaminants. Finally, the review discusses the ‘pros’ and ‘cons’ of chemical and biological techniques available for assessing bioavailability of hydrophobic organic chemicals in soils, highlighting the need to quantify bioavailability by chemical techniques. It concludes by highlighting the need for understanding the interactions between the soil, contaminants and biota which is crucial to understanding the bioavailability of contaminants in soils.

Traditionally, soil extraction techniques have been concerned with the determination of "total" organic contaminant concentrations, following an "exhaustive" extraction. However, in light of the increasing body of knowledge relating to organic contaminant availability and aging, such methods have little relevance to the amount of contaminant that may pose an ecological risk i.e., the "bioavailable" portion. Less exhaustive techniques have therefore been the subject of more recent approaches in the hope that they may access the "labile" or bioavailable pool. The use of an aqueous-based extraction technique utilizing hydroxypropyl-β-cyclodextrin (HPCD) is presented here for the extraction of PAHs from soil. The optimization of the method is described in terms of HPCD concentration, extraction time, and solution buffering. The procedure is then tested and validated for a range of 14 C-labeled PAHs (phenanthrene, pyrene, and benzo[a]pyrene) added at a range of concentrations to a range of soil types. The amounts of soilassociated phenanthrene mineralized by catabolically active microorganisms were correlated with total residual phenanthrene concentrations (r 2 ) 0.889; slope of best fit line ) 0.763; intercept ) -5.662; n ) 24), dichloromethane (DCM)-extractable phenanthrene concentrations (r 2 ) 0.986; slope of best fit line ) 0.648; intercept ) 0.340; n ) 24), butan-1-ol (BuOH)-extractable phenanthrene concentrations (r 2 ) 0.957; slope of best fit line ) 0.614; intercept ) 0.544; n ) 24), and HPCD-extractable phenanthrene concentrations (r 2 ) 0.964; slope of best fit line ) 0.997; intercept ) 0.162; n ) 24). Thus, in this study, the microbially bioavailable concentrations of soil-associated phenanthrene were best predicted using the optimized HPCD extraction technique. In contrast, the DCM Soxhlet extraction and the BuOH shake extraction both overestimated phenanthrene bioavailability by, on average, >60%.

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