Limitations of Applying Diffusive Gradients in Thin Films to Predict Bioavailability of Metal Mixtures in Aquatic Systems with Unstable Water Chemistries

Abstract

The present study accessed the use of diffusive gradients in thin film (DGT) as a surrogate for estimating the bioavailability and bioaccumulation of copper (Cu) and zinc (Zn) in a freshwater mussel. We coupled DGTs with mussels and deployed them in a constructed wetland. Water quality parameters were measured for a 4-d period on 3 continuous occasions during 12-d trials in the summer and winter; metal speciation was modeled for each occasion. Higher cumulative rainfall and water turbidity during the summer trial resulted in higher particulate metal concentrations compared to the winter trial. Mussel accumulated metals did not correlate with DGT-measured metals but positively correlated with particulate metals in the summer because filtering particulate food mainly contributed to the bioaccumulation. In contrast, the winter trial suggested a positive correlation between metal bioaccumulation and DGT-measured metals because uptake of dissolved organic matter (DOM) from water mainly contributed to the bioaccumulation, and the labile metal species complexed with DOM generally overlapped with DGT-targeted metals. Though Cu has a higher affinity for organic ligands than Zn, the interactions between Cu and Zn in the mixture did not impede their uptake and bioaccumulation. The deployment duration when DGTs and mussels are coupled to compare metal bioavailability should be no less than 12 d so that mussels have enough time to accumulate contaminants from the environmental media. In summary, DGT is a convenient surrogate for biomonitoring, but it may not fit the real environment such as the aquatic system with unstable water chemistries. Geochemical modeling is good at calculating metal speciation but inferior to DGT in predicting bioavailability and mimicking bioaccumulation.

Mercury Accumulation in a Stream Ecosystem: Linking Labile Mercury in Sediment Porewaters to Bioaccumulative Mercury in Trophic Webs

“…Considering the disadvantages of biomonitoring in contamination monitoring, such as the expenses for collecting and processing procedures that pose limitations on sampling a normal population and excluding the influences of complicated biotic factors [93,94], alternative approaches such as passive samplers and geochemical speciation models have been adopted. This study applied WHAM to explore the speciation of labile Hg in sediment porewaters and collected labile Hg species with peepers.…”

Diffusive gradients in thin films for the measurement of labile metal species in water and soils: a review

“…The theoretical foundations for the use of DGT technique in aqueous solutions has been extensively documented (Zhang and Davison 1995;Lehto et al 2006;Fernández-Gómez et al 2012b;Shiva et al 2016;Xu et al 2020). The DGT technique is based on Fick's first law of diffusion, which relates the diffusive flux to the gradient of the concentration.…”