| Observations on the behavior of pesticides in field-weathered soils have demonstrated that increased soil-chemical contact time (aging) may lead to increased persistence of contaminants in the environment. Aged chemicals can act as long-term sources of groundwater contamination, and aging may reduce the effectiveness of soil remediation technologies such as soil washing, pump and treat, and in-situ bioremediation. Environmental fate characterization based on laboratory studies of freshly added chemicals fail to consider the effects of aging and hence may underestimate the persistence of organic compounds in the environment. For example, although the soil fumigant ethylene dibromide (EDB) is readily biodegradable, relatively water soluble, and volatile, EDB residues have been found in field soils 20 years after its last known application. However, the causes of increased persistence due to aging are not well understood. One possible cause of increased persistence is a reduction in the bioavailability of contaminants to degrading microorganisms. This decrease in bioavailability is likely due to changes in the physical/chemical disposition of the contaminants due to aging. For example, an increase in sorption to soils due to aging can limit the extent of biodegradation of organic contaminants. Also, desorption from soils, which may decrease with aging, is thought to be the limiting factor controlling the rate of degradation of organic contaminants by degrading microorganisms.; This study describes three experiments designed to test the role of aging and its effect on the sorption/desorption dynamics and bioavailability of chemicals in soils. One experiment examined the sorption/desorption behavior of chlorobenzene on four soils (soil organic carbon content from 0.4–7.8%) after soil-chemical contact times (aging) of 24 hours and 14 months. Another experiment examined the sorption/desorption behavior of four compounds (EDB, chlorobenzene, 2,4-D, and atrazine) on one soil type after aging times of 24 hours, 1 month, and 14 months. Lastly, the sorption/desorption behavior and bioavailability of biphenyl to Pseudomonas putida P106 was studied after aging times of 24 hours and 8 months.; Increases in sorption due to aging were found for some chemicals (2,4-D, EDB) but not others (chlorobenzene). Large differences in sorption/desorption behavior for similar compounds (EDB and chlorobenzene) were observed, indicating that generalizations about compound behavior in soils are inappropriate. Desorption profiles were consistent in that aged chemicals showed a much larger desorption resistant fraction after long aging periods and this was true for all soil types. Desorption rates were unaffected by aging time. Biodegradation of biphenyl decreased with increased aging time and the desorption resistant fraction (observed in the desorption profiles) appeared to be inaccessible to microbes. |
