Regional-scale investigation of dissolved organic matter and lead binding in a large impacted lake with a focus on environmental risk assessment

Environmental risk assessment (ERA) increasingly relies on speciation modeling of bioavailability. Heavy metals are the most prevalent pollutants globally, and dissolved organic matter (DOM) plays an important role in speciation and bioavailability of heavy metals. Due to the variation of DOM properties in natural aquatic systems, improvements to the standard one-size-fits-all approach to modeling metal-DOM interactions are needed for ERA. In this study, we investigate variations in DOM and lead (Pb)-DOM binding in Lake Tai (Taihu), a large, impacted lake in eastern China that is characterized by a complex drainage network and is an important water resource at a regional level, and we assess implications of our findings within the context of ERA needs. In our study, DOM in water samples collected from across the 2,400 km² area of Taihu was characterized using three-dimensional excitation-emission matrix and synchronous fluorescence spectroscopy spectra, the latter being used to calculate conditional stability constants for metal binding. Parallel factor analysis and peak picking were used to assess contributions of protein- and humic-like components of DOM, and fluorescence indices indicative of diagenetic processes were calculated. These quantities calculated from spectroscopic studies, in addition to water quality parameters, were analyzed by bivariate and multivariate analysis. Results show that different DOM components are highly variable across different regions of Taihu, and bivariate and multivariate analyses confirm that water quality and DOM characterization parameters are strongly interrelated. This reflects the different inputs, diagenetic and transport processes across the large expanse of Taihu. We find that the conditional stability constant of Pb-DOM binding is strongly affected by the water chemical properties and composition of DOM, though the conditional stability constant is not itself a parameter that differentiates lake water properties in different regions of the lake. The variability of DOM composition and Pb-DOM binding strength across Taihu is consistent with prior findings that a one-size-fits-all approach to metal-DOM binding may lead to inaccuracies in commonly used speciation models, and therefore such generalized approaches need improvement for regional-level ERA in complex watersheds. The approach taken here to obtain site-specific metal-DOM conditional stability constants for use in increasing the accuracy of speciation modeling is fit-for-purpose for ERA applications at regional levels because the approach is relatively simple, inexpensive, and amenable to high throughput analysis.