Dissolved organic carbon (DOC) characteristics in metal-rich waters and implications for copper aquatic toxicity

Abstract

Acid Mine Drainage (AMD) originating from abandoned hardrock mines characteristically has high concentrations of potentially toxic metals, such as Cu, as well as dissolved and particulate forms of Al and Fe. Free copper (Cu2+) is a well-known contributor to heavy metal toxicity in aquatic systems. The bioavailability of Cu2+ is influenced by aqueous complexation, with humic (HA) and fulvic acids (FA) being especially important ligands. This research focused on examining changes in the binding affinity of fulvic acid (FA) that result from its chemical fractionation, which occurs by FA sorption to hydrous iron and aluminum oxides (HFO and HAO). FAs used in this study were collected from three alpine watersheds in Central Colorado (Upper Snake River, Colorado Gulch, and St. Kevin’s Gulch). Variability in spectroscopic properties of SUVA254 and fluorescence index (FI) was a result of chemical fractionation, watershed source, and season. Graphs of SUVA254 verses FI revealed FA from pristine waters shifts from characteristics of being aromatic, allochthonous in the spring/summer to less aromatic, autochthonous in the fall/winter. In the confluence where AMD impacted streams mix with the pristine streams, SUVA254 verses FI graphs show the effect of fractionation resulting in less aromatic FA. Measurements of Cu2+ in solution by an ion selective electrode (ISE) and acute copper toxicity tests (D. magna) revealed that FAs remaining in the water column after fractionation have less binding affinity and/or capacity than DOC in aquatic systems without the presence of HFO and HAO. FA fractionated in the laboratory, and under natural conditions (confluence sites) had lower acute copper EC50 values (20 to 96 µg Cu/L) than unfractionationed FA, suggesting a decrease in Cu binding affinity and/or capacity associated with fractionated FA. Site FA from pristine tributaries also had variability in EC50 values (48 to 146 µg Cu/L) that is likely related to variations in source and seasonality. The binding affinity of DOC is related to its aromaticity, which is characterized by SUVA254 for which low SUVA relates to lower binding capacity/affinity of DOC and a resulting higher concentration of Cu2+ in solution, as measured with a cupric ion specific electrode (ISE). Variability in DOC-Cu binding affinity in alpine AMD impacted watersheds is likely a significant factor in copper toxicity in aquatic systems and should be included in toxicological modeling programs such as the Biotic Ligand Model (BLM). Findings from this research suggest a further need to characterize DOC due to the inherit source variability and fractionation processes in AMD impacted watersheds. Therefore, additional work would better refine the BLM in order to accurately predict acute copper EC50 values, which are necessary to formulate water quality criteria in freshwater environments.