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Soil organic matter controls Pb release during redox cycles in floodplain soils

Soil organic matter controls Pb release during redox cycles in floodplain soils

Lead released from mineral p

hases during hydrologic redox cycles is retained by soil organic matter, limiting dissolved Pb concentrations

Dynamic hydrology drives redox changes in floodplain soils, dissolving and reprecipitating PbS and Pb adsorbed on Fe(III)-(hydr)oxides. Particulate organic matter retains released Pb prior to reprecipitation, limiting dissolved Pb concentrations.

 

The Science                                 

Lead contamination in soils is a major threat to water quality. Although Pb tends to occur in sparingly soluble minerals, changes in dissolved oxygen concentrations can promote dissolution of these minerals, potentially causing spikes in dissolved Pb concentrations and transport of dissolved Pb to drinking water sources. We examined the fate of Pb during changes in oxygen concentrations in contaminated floodplain soils. We found that Pb released from mineral phases is retained by soil organic matter (SOM). Thus, SOM limits spikes in dissolved Pb concentrations and prevents transport of dissolved Pb.

 

The Impact

Lead is highly toxic, and its consumption in any amount is considered unsafe. As a result of mining activities and leaded gasoline, soil lead contamination is widespread. It is critical to understand the fate of Pb in soils to assess the risks it presents to freshwater quality. Dissolved Pb is particularly dangerous, as it is easily transported and consumed. Our findings reveal that although common solid Pb phases are dissolved during changes in water levels in floodplain soils, released Pb is immediately retained on soil organic matter, and dissolved Pb remains low. Thus, although the soils we studied contain appreciable Pb, it likely does not pose a threat water quality in dissolved form.

 

Summary

Our objectives were to resolve Pb speciation and partitioning across hydrologically controlled redox transitions and to determine the extent of Pb release during these transitions. To examine the effects of soil redox transitions on Pb partitioning, we tracked solid-phase Pb speciation and dissolved Pb concentrations in mining-affected floodplain soils near Crested Butte, CO. Groundwater levels at our site varied seasonally, driving changes in soil redox conditions. We collected depth-resolved soil and porewater samples at 2-4 week intervals between June 2 and October 26, 2018, while monitoring groundwater levels hourly. We determined solid phase Pb speciation using Pb L3-edge EXAFS measurements. We found when water levels were high in June and early July, iron and sulfate reducing conditions developed in the soils, dissolving Fe(III)-(hydr)oxides and releasing associated Pb, and promoting PbS formation. As water levels declined into August, oxygen was reintroduced to the soil profile, and Fe(III)-(hydr)oxides precipitated while PbS was dissolved. A beaver dam was built near our site in late August, which caused water levels to rise again, resulting in Fe reducing conditions. As reducing conditions transitioned to oxidizing conditions and vice versa, we observed an increase in Pb adsorbed on particulate organic matter. We also did not observe increases in dissolved Pb concentrations. Taken together, this indicates that particulate organic matter retains Pb released during dissolution of Fe(III)-(hydr)oxides and PbS, thereby limiting its dissolved concentrations in porewater.

 

Contact

Christian Dewey

Stanford University

cwdewey@stanford.edu
 

Funding

This research was supported by the SLAC Groundwater Quality SFA program of the US Department of Energy, Office of Biological and Environmental Research, Subsurface Biogeochemistry Program (SBR), and by the SBR Project Award Number DE-SC0016544. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract No. DE-AC0276SF00515.

Publications

Dewey, C. et al. “Porewater Lead Concentrations Limited by Particulate Organic Matter Coupled with Ephemeral Iron(III) and Sulfide Phases during Redox Cycles within Contaminated Floodplain Sediments.” Environ. Sci. Technol. 9, 5878-5886 (2021).

 

Related Links

https://dx.doi.org/10.1021/acs.est.0c08162