Bioavailability assessment of endocrine-disrupting chemicals in soil and sediment
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There are many methods currently available to assess the risk of chemical bioaccumulation in an organism. Many of these methods are either very difficult to implement, being costly and time-consuming, or contain flaws which may affect the final result. In this study we used semipermeable membrane devices (SPMDs) containing cod liver oil to assess the bioavailability of lipophilic and hydrophilic organic contaminants. This SPMDs proved to be an excellent method for this study. Atrazine is a hydrophilic endocrine-disrupting chemical and is likely to be taken up by SPMDs. Atrazine congeners were accumulated far less than the parent compound. The uptake of atrazine by SPMDs from pure water was rapid and reached equilibrium within 48 hours. The study also showed low bioaccumulation (0.05 - 13.5%), which is consistent with living organisms. Consequently, the SPMD method was appropriate for assessing atrazine. Organochlorine pesticide DDT was readily taken up by the SPMDs. Approximately 76% of the total DDT from spiked water was accumulated by SPMDs after 180 days of exposure. However, only 5% of total DDT was taken up from field-collected contaminated soil and only 10% from a synthetic spiked soil after 35 days of exposure. Based on the percentage uptakes, o,p' & p,p'-DDD congeners were more bioavailable than any other DDT congeners (such as o,p' & p,p'-DDE, and o,p' & p,p'-DDT). Up to 10% of o,p' or p,p'-DDD was taken up from the field-collected soil and 20% was taken up from freshly spiked soil. Kinetic uptakes of total DDT and six congeners by cod-liver-oil-containing SPMDs and earthworms were compared both in pure water and from soil and sediment. The correlation coefficients (r) between the SPMDs' uptake and the earthworms' uptake (Eiseniafoetida) at 14 days of o,p'-DDE,p,p'-DDE, o,p'-DDD,p,p'-DDD, o,p'-DDT, p,p'-DDT, and total DDT were 0.96, 0.74, 0.80, 0.98, 0.95, 0.81, and 0.99, respectively. Unexpectedly, the kinetic uptake rate by earthworms in the aquatic system was 1. 7 times faster than the uptake rate for the SPMDs. However, kinetic uptake rate by earthworms in soil was 1 to 4.3 times slower than the uptake rate for the SPMDs. The key advantage of SPMDs is 1) their ability to predict long term accumulation of the chemicals, 2) they provide more precise estimates of uptake than the earthworms, and 3) SPMDs require only simple preparation and give clean samples for chromatography. Even though earthworms can be cultured in the laboratory under controlled conditions, and can be tested in a variety of soil types, earthworm uptake rates were variable and experiments repeatedly failed. The available ΣDDT and congeners in the contaminated dip soil decreased over time as they were sequestered into the SPMDs. The uptake was greatest at the first exposure and decreased with subsequent exposures. The bioaccumulation factors of DDT were in the range of 157 to 2,125 during the first 35 days of exposure and decreased over subsequent sampling periods. The non-linear regression model was used to predict the maximum uptake of DDT by SPMDs. The percentage DDT uptake of the two spiked soils and field-collected sandy soil reached asymptote after 150 days, with 11% to 13% of maximum uptake-that is the amount of chemical taken up as a proportion of the initial soil/sediment. After 70 days of exposure, 3.5% of DDT was predicted its maximum uptake in heavy clay and clayey sand soils. Of all the DDT congeners, p,p '-DDD was the most bioavailable. Approximately 30% of p,p '- DDD in freshly spiked soil was taken up by SPMDs. The initial risk of studied EDCs added to the environment is high because these chemicals may be readily bioavailable, but this risk decreases over time. A mathematical model was developed to enable eventual inclusion of the DDT in environmental risk assessments and it was effectively used to explain changes in DDT bioavailability over a one-year exposure period. Soil with a higher clay proportion or with higher organic carbon was shown to have a lower environmental risk. For example, clay soil exhibited the risk at the commencement of the incubation with 3.3% of available DDT residue. As the sediments aged, either under aerated or non-aerated conditions, the bioavailable DDT fraction decreased in all soil types, following first-order exponential decay.
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