Influence of Geochemical Fractionation of Fulvic Acid on its Spectral Characteristics and its Protection Against Copper Toxicity to Daphnia magna.

Dissolved copper (Cu) can contribute to toxicity in aquatic systems impacted by acid mine drainage (AMD), and its bioavailability is influenced by aqueous complexation with organic ligands that predominantly include fulvic acids (FAs). Because the geochemical fractionation of FAs that accompanies sorption to hydrous aluminum oxides (HAOs) and hydrous iron oxides (HFOs) can alter Cu complexation with FA, we investigated FAs isolated from three categories of water (pristine, AMD, and in situ-fractionated mixtures of pristine and AMD collected at stream confluences) in three mining-impacted alpine watersheds in central Colorado, USA. We also conducted geochemical fractionation of field-collected FAs and Suwannee River FAs by precipitating HAOs and HFOs in the laboratory. Spectral properties of the FAs (e.g., UV-VIS absorbance) were altered by geochemical fractionation, and in acute toxicity tests with an aquatic invertebrate (Daphnia magna) Cu was more toxic in the presence of in situ- and laboratory-fractionated FAs (median effect concentration [EC50] 19-50 µg Cu L-1 ) than in the presence of nonfractionated FAs (EC50 48-146 µg Cu L-1 ). After adjusting for the strain-specific sensitivity of our D. magna, we improved the accuracy of Biotic Ligand Model predictions of Cu EC50 values for AMD-related FAs by using an "effective dissolved organic carbon" based on spectral properties that account for among-FA differences in protectiveness against Cu toxicity. However, some differences remained between predicted and measured EC50 values, especially for FAs from AMD-related waters that might contain important metal-binding moieties not accounted for by our measured spectral indices. Environ Toxicol Chem 2023;42:449-462. © 2022 SETAC.

Dissolution of Mn-bearing dolomite drives elevated Cr(VI) occurrence in a Permian redbed aquifer.

Municipalities in central Oklahoma, U.S.A. increasingly rely on water drawn from the Central Oklahoma Aquifer (COA) as surface water resources have not grown in proportion to population and current water demands. However, water drawn from certain regions of the COA frequently contains elevated levels of naturally occurring hexavalent chromium. Rock samples from the Norman Arsenic Test Hole Core (NATHC) were investigated to identify the mineralogic host(s) of Cr and mechanisms of Cr(VI) release via bulk mineralogy and chemistry measurements, selective chemical extractions, and microscale elemental analyses. Results demonstrate most COA Cr is contained in Fe oxides and clays as isomorphic substitutions for Fe(III). Analyses of regional groundwater data, including hierarchical clustering methods and GIS, demonstrate the most intense Cr(VI) occurrence is linked to cation exchange with Na-clays at depth. Cation exchange allows dissolution of Mn-bearing dolomite, which in turn produces Mn oxides in otherwise dolomite-saturated groundwaters. Mn oxides in turn are known to oxidize Cr(III) to Cr(VI). In general, co-occurrence of Mn-bearing carbonates and exchangeable clays in any aquifer, particularly those with Cr(III) present in iron oxide cements, serve as ingredients for groundwater occurrences of oxidizable trace metals.