Porewater Lead Concentrations Limited by Particulate Organic Matter Coupled With Ephemeral Iron(III) and Sulfide Phases during Redox Cycles Within Contaminated Floodplain Soils

Abstract

Lead contamination in soils and sediments is a major threat to water quality. In surface and near-surface environments, Pb is not redox active; however, common Pb hosts, including Fe(III)-(hydr)oxides and sulfides, dissolve and precipitate as redox conditions change. Dissolution of Pb hosts may release Pb to porewater, leading to spikes in dissolved Pb concentrations and potential transport into surface or groundwater. Here, we examine the impacts of hydrologically coupled redox transitions on Pb partitioning in contaminated floodplain soils. We find that the affinity of Pb for particulate organic matter (POM), inclusive of mineral-associated organic matter, ensures that across redox transitions Pb is retained in the solid phase, despite host-phase (Fe(III)-(hydr)oxide and sulfide) dissolution. As seasonal hydrologic dynamics shift porewater redox conditions, Pb-bearing Fe(III)-(hydr)oxides (Pb-HFO) and sulfides (PbS) are dissolved and (re)precipitated. However, despite these shifts in redox conditions and associated host-phase transformations, Pb retention on POM, coupled with the formation of PbS and Pb-HFO, maintains dissolved Pb concentrations below 17 μg L −1 . Importantly, the predominance of Pb adsorbed on POM alongside low dissolved Pb concentrations indicates that Pb released from HFO and PbS is retained by POM. Thus, despite host-phase dissolution during redox transitions, partitioning of Pb to the aqueous phase is minimal and, therefore, transport of dissolved Pb is unlikely.

Redox Properties of Solid Phase Electron Acceptors Affect Anaerobic Microbial Respiration under Oxygen-Limited Conditions in Floodplain Soils

“…These could include sulfide species, manganese species, and/or quinone functional groups in organic matter. Mackinawite was observed at this field site, 34 and its presence is consistent with a rise in total sulfur concentrations around 150 cm (Figure 1d). Dewey et al 34 further observed elevated concentrations of reduced organic sulfur and elemental sulfur at this depth based on results from sulfur X-ray absorption spectroscopy, which was consistent with sulfide formation during microbial sulfate reduction.…”

Export of Organic Carbon from Reduced Fine-Grained Zones Governs Biogeochemical Reactivity in a Simulated Aquifer

“…Previous studies reported mackinawite (FeS) in a similar column system with Wind River fines 4 and in Slate River field samples. 46 Consistent with the formation of Fe sulfides, total S concentrations followed similar trends as total Fe(II) concentrations across the columns (Figure S13) and aqueous sulfide concentrations were close to or below the detection limit in all ports throughout the experiment (Figure S5).…”

“…Wind River fines are exposed to high sulfate levels in the field and, thus, contain a high abundance of sulfate-reducing microorganisms, 45 whereas Slate River fines are exposed to lower sulfate concentrations and have greater Fe(II) levels. 46 In the sand of both Wind River and Slate River columns, Fe(II) concentrations initially increased downstream of the lenses and remained stable at these elevated concentrations 21 d into the experiment. Measured Fe(II) concentrations in the ports located 3 to 6 cm downstream of the Wind River lenses (at 10.5 and 13.5 cm) were 14-fold higher at the end of the experiment than Fe(II) concentrations that we simulated using a model incorporating only transport of Fe(II) out of the lenses but no microbial or chemical reactions (Figure 1d,g).…”

“…In Slate River lenses, Fe­(II) concentrations increased quickly upon groundwater flow and were an order of magnitude higher than in Wind River lenses. Wind River fines are exposed to high sulfate levels in the field and, thus, contain a high abundance of sulfate-reducing microorganisms, whereas Slate River fines are exposed to lower sulfate concentrations and have greater Fe­(II) levels . In the sand of both Wind River and Slate River columns, Fe­(II) concentrations initially increased downstream of the lenses and remained stable at these elevated concentrations 21 d into the experiment.…”

“…It is likely that solid phase Fe­(II) occurred in the form of Fe sulfides, as evidenced from black coloration of the columns beyond 10 cm from the inlet (Figure S19) and the fact that the color disappeared upon reoxidation of the columns (not shown). Previous studies reported mackinawite (FeS) in a similar column system with Wind River fines and in Slate River field samples . Consistent with the formation of Fe sulfides, total S concentrations followed similar trends as total Fe­(II) concentrations across the columns (Figure S13) and aqueous sulfide concentrations were close to or below the detection limit in all ports throughout the experiment (Figure S5).…”