Effect of residence time and environmental properties on the bioavailability and extractability of organic compounds in soil

Phenanthrene, naphthalene, and atrazine that had been previously added to sterile soil became progressively unavailable for uptake by earthworms and for mineralization by bacteria with persistence of the compounds. The availability of 2,4-dichlorophenoxyacetic acid to bacteria was similarly diminished with aging of the compound in soil. Recovery of each of the compounds by mild extraction procedures also declined with persistence in soil. However, vigorous extraction procedures recovered approximately 100% of the compounds regardless of residence time in soil, and the results were not correlated with bioavailability. These data suggest that bioavailability of persistent compounds in soil declines with time and that predictions of exposure that are based on the total amount of chemical extracted from soil do not appropriately estimate exposure or risk to susceptible populations.; The feasibility of using selective mild extractants to predict the bioavailability of soil-aged atrazine and phenanthrene to earthworms and bacteria was determined. The recovery by these extractants of atrazine and phenanthrene freshly added to soil varied appreciably among individual solvents, but the quantity extracted by each solvent declined as the test compounds persisted in soil. When the data are compared to the 0-d values for extractability and bioavailability, the percentage of the persistent compounds recovered by some extraction procedures approximated either the percentage of the persistent compounds taken up by earthworms or mineralized by bacteria.; The effect of environmental properties on extractability and bioavailability was also determined. The recovery with mild extractants and uptake by earthworms of phenanthrene was greater in soils with higher organic matter contents than in soils with lower percentages of organic matter, whereas earthworm uptake was greater in soils with higher clay contents than in soils with lower clay contents. Extractability was unaffected by clay content. Wetting and drying of soil after the addition of phenanthrene reduced the recovery by a mild extractant and the extent of mineralization by bacteria of the compound. Extractability of atrazine decreased with wetting and drying, but mineralization was unaffected. Thus, predictions of bioavailability based on extractability need to consider the effects of these environmental variables.