Assessment of diffusive gradients in thin films (DGT) for bioaccessible copper measurements in environmental water samples

Copper (Cu) is an essential trace element widely used in agriculture, industry, and consumer products, but its persistence and ecotoxicity at elevated concentrations require accurate environmental risk assessment. Traditional monitoring based on total or dissolved Cu concentrations often fails to capture the speciation and temporal variability that govern bioavailability and ecological effects. In contrast, bioavailability-based assessments focus on biologically available copper fractions, providing greater ecological relevance for regulation. This study evaluated Diffusive Gradients in Thin Films (DGT) passive samplers as an in situ method for quantifying bioaccessible Cu across five freshwater systems with contrasting geochemical conditions. DGT-measured Cu concentrations were 47–91% lower than dissolved Cu, reflecting reduced bioavailability due to complexation in natural waters. Comparison with Biotic Ligand Model (BLM) predictions showed broad agreement, while DGT values consistently exceeded WHAM-modelled free Cu2+ concentrations, indicating that DGT measures Cu species beyond the equilibrium free ion fraction. The difference between dissolved and DGT-Cu (ΔCu) was introduced as a metric of the complexed Cu fraction and showed strong relationships with dissolved organic carbon, Mn, and Si. These findings suggest that DGT is a practical field-based tool for assessing Cu bioavailability, supporting environmental monitoring, regulatory risk assessment, and improved characterisation of Cu behaviour in freshwater systems.