Persistent contamination, limited accumulation: In-situ isotope tracing reveals fast thallium turnover in natural waters.

Although thallium (Tl) is one of the most toxic non-essential metals, environmental concern for aquatic organisms is often driven by its bioaccumulation rather than acute toxicity. However, current understanding of Tl bioaccumulation is derived largely from laboratory experiments, and little is known about its toxicokinetics under environmentally realistic field conditions. In this study, we employed an in-situ stable isotope tracing strategy to resolve the bioaccumulation kinetics of Tl in the freshwater bivalve Corbicula fluminea within an agricultural activity-impacted river. By pre-labeling organisms with enriched 203Tl, we decoupled the uptake and elimination kinetics under environmentally realistic conditions. Our results show that while dissolved Tl concentrations consistently exceeded water quality standards, the steady-state tissue Tl remained within a narrow range (0.1-0.28 µg g-1), only two- to three-fold higher than background levels. This limited bioaccumulation risk is mechanistically explained by the high in situ turnover of Tl; specifically, while the aqueous uptake rate constant (ku) in the field was consistent with laboratory observations (kulab = 0.503 L g-1 d-1, kufield = 0.549 L g-1 d-1, deviating by <10%), the field elimination rate constant (ke) was approximately three times higher (kelab = 0.285 d-1, kefield = 0.857 d-1). The discrepancy suggests that the uncertainty in lab-to-field extrapolations arises from the unequal environmental sensitivity of different kinetic processes, with elimination exhibiting a stronger response to field conditions (likely temperature) than uptake. This study provides a process-level framework for in-situ toxicokinetic calibration, offering a more realistic basis for assessing the long-term internal loading of trace metals in contaminated riverine environments.