Bioavailability-informed sediment assessment in the transboundary Begej Canal: A comparative evaluation of active and...

Sediments act as both sinks and potential secondary sources of heavy metals in aquatic systems, playing a key role in ecological risk dynamics and contaminant cycling. This study assessed sediment quality in the Begej Canal (Serbia) by combining conventional (pseudo-total and sequential extraction) and passive sampling with multiple contamination and risk indices. Pseudo-total analyses revealed elevated chromium, cadmium, lead, and zinc. Hakanson's framework indicated a moderate cumulative ecological risk (RI = 270.8), dominated by cadmium (high ERⁱ), while consensus PEC-Q values highlighted chromium, lead, and zinc as principal contributors to biological-effect potential. Sequential extraction confirmed enhanced mobility of cadmium and lead. In contrast, SPeeper™ passive samplers measured consistently lower freely dissolved concentrations of metals. The interstitial water criteria toxic unit (IWCTU) index (0.01117) remained well below threshold, and the Single Factor Pollution Index (SFPI) classified sediments as unpolluted based on bioavailable fractions (0.54), versus moderate pollution from pseudo-total content (3.22). These results emphasize that pore-water extraction and bulk analyses can overestimate hazard by disrupting equilibria, whereas equilibrium-based passive methods yield a more ecologically realistic assessment. Overall, although the Begej Canal remains impacted by heavy metals, the continuously bioavailable fraction poses negligible ecological risk. The combined application of active and passive sampling with complementary indices provides a transferable framework for distinguishing between potential mobility and actual exposure. This approach supports balanced sediment management and contributes to EU Water Framework Directive efforts to integrate bioavailability into sediment quality assessment across the Danube Basin.