The Science

Groundwater quality is driven by complex biogeochemical processes determined by the chemistry and composition of both the groundwater and the aquifer. Many otherwise sandy aquifers contain abundant organic-enriched, fine-grained, and sulfidic lenses that   are important sources of organic carbon, Fe(II), and sulfur (S).  r i While these lenses are recognized as playing important roles in aquifer biogeochemistry and redox cycling, the specific reactive transport mechanisms by which these reactive species influence biogeochemical function in the surrounding aquifer are poorly understood. Numerical models of these processes generally have microbially driven reduction reactions occurring only inside the actual sediment lenses.  . However, in two experimental studies investigating reactive transport in and around these lenses, researchers showed that in addition to reduced aqueous species (e.g., Fe(II) and HS^–) that were produced by redox reactions inside the lenses, organic carbon is also exported from organic-enriched lenses into the sandy aquifer matrix. This stimulates microbial anaerobic reduction in the surrounding aquifer and creates a microbial redox-active zone around the lenses.

The Impact

Findings from these studies imply that an additional reactive transport mechanism and more long-lived pool of reducing equivalents controls redox cycling in oxic aquifers, identifying gaps in recent numerical models. The studies show that microbial redox cycling of micronutrients and contaminants that need anoxic conditions can be sustained within nominally oxic aquifers in the vicinity of organic-enriched sediment lenses. This means that a larger volume of the subsurface matrix is redox active. However, the redox conditions in these “reducing halos” in the surrounding sandy environment are far more sensitive to the influx of oxidants than are the lenses. Thus, the mobility of redox-sensitive micronutrients and contaminants can quickly change within this environment.

Summary

In these studies, researchers used natural floodplain sediments and examined the influence of organic-enriched, fine-grained lenses on redox conditions in surrounding sandy aquifer sediments, and they examined the consequential implications for speciation and mobility of zinc (Zn) (Engel 2021) and arsenic (As) (Kumar 2020). Synchrotron X-ray absorption spectroscopy at the Stanford Synchrotron Radiation Lightsource’s beam lines 4-3 and 7-3 showed that Fe(II) minerals, including FeS and elemental S, were present in the surrounding nominally oxic aquifer in abundances that exceeded what abiotic, aqueous-reduced products could explain. The research team concluded that, when sulfate concentrations in the groundwater are high, the export of reducing capacity (“exported reactivity”) from fine-grained, sulfidic lenses into aquifer sand can promote microbial Fe and sulfate reduction, which in turn leads to FeS precipitation and elemental S formation. Elemental S can then react with As to form thiolated As species, which appear to have a higher solubility and mobility than other As species. In contrast, when Zn(II) is present as a dissolved contaminant, it reacts strongly with dissolved HS^– and precipitates as ZnS, sharply limiting the export of HS^– and Zn (but not impacting Fe and organic matter export) from the organic-enriched lenses. Thus, the combination of high-sulfate groundwater and heterogeneous sediment composition (e.g., fine-grained, organic-rich/coarse interfaces) can locally promote severely elevated As concentrations, even when sediment As concentrations are below the global average. Conversely, Zn attenuation is amplified by the same sediment heterogeneities.

Principal Investigator

Kristin Boye
SLAC National Accelerator Laboratory
Kboye@stanford.edu

Funding

The project was conceived and supported by the SLAC Floodplain Hydro-Biogoechemistry Science Focus Area project funded by the Earth and Environmental Systems Sciences Division of the U.S. Department of Energy’s (DOE) Office of Science Biological and Environmental Research (BER) Program under contract no. DEAC0276SF00515SBR (SLAC National Accelerator Laboratory).

The United States-Israel Binational Agricultural Research and Development Fund (BARD; grant no. FI-569-2018) and the Israeli Council for Higher Education provided financial support for Engel through postdoctoral fellowships. Additional support (for Engel and Fendorf) was provided by SBR Project DE-SC0020205. Use of the Stanford Synchrotron Radiation Lightsource at SLAC National Accelerator Laboratory is supported by the DOE’s Office of Science Basic Energy Sciences Program (contract no. DE-AC0276SF00515).

References