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The surfactant nonylphenol (NP) is an endocrine-disrupting compound that is widely spread throughout the environment. Although environmental risk assessments are based on total NP concentrations, only the bioavailable fraction possess an environmental risk. The present study describes the bioavailability and biodegradability of NP over time in contaminated river sediment of a tributary of the Ebro River in Spain. The bioavailable fraction was collected with Tenax TA(R) beads, and biodegradation was determined in aerobic batch experiments. The presence of NP was analyzed chemically using gas chromatography-mass spectrometry and indirectly as estrogenic potency using an in vitro reporter gene assay (ER(alpha)-luc assay). Of the total extractable NP in the sediment, 95%+/-1.5% (mean +/- standard error) desorbed quickly into the water phase. By aerobic biodegradation, the total extractable NP concentration and the estrogenic activity were reduced by 97%+/-0.5% and 94%+/-2%, respectively. The easily biodegradable fraction equals the potential bioavailable fraction. Only 43 to 86% of the estrogenic activity in the total extractable fraction, as detected in the ER(alpha)-luc assay, could be explained by the present NP concentration. This indicates that other estrogenic compounds were present and that their bioavailability and aerobic degradation were similar to that of NP. Therefore, we propose to use NP as an indicator compound to monitor estrogenicity of this Ebro River sediment. To what extent this conclusion holds for other river sediments depends on the composition of the contaminants and/or the nature of these sediments and requires further testing.

Solid-phase extraction with Tenax® is one of the most used methods for determining the fraction of a pollutant that desorbs rapidly from sediment and thus is available for living beings. In the present study, this technique has been applied to sediment contaminated in the laboratory with polybrominated diphenyl ethers (PBDEs) and dichlorodiphenyltrichloroethane, dichlorodifenyldichloroethylene, and dichlorodifenyldichloroethane (generically, DDXs). The amount of chemical retained in sediment during the time of the experiment fit well with a three-phase exponential desorption model. The ratios between the rapidly desorbing fraction and the fraction desorbed during a fixed time (6 or 24 h) were calculated. The fraction desorbed in 6 h was lower than the rapidly desorbing fraction for most of PBDEs, whereas the fraction desorbed in 24 h exceeded the rapidly desorbing fraction for both groups of compounds. However, variability of these data suggests, when possible, a long time measure of desorption in order to achieve a more accurate estimation. Both the extent and the velocity of desorption were inversely related with the bromination degree and, consistently, with hydrophobicity and molecular size. In this way, low brominated PBDEs and DDXs showed a high availability relative to high brominated PBDEs.