Aquifer-Scale Observations of Iron Redox Transformations in Arsenic-Impacted Environments to Predict Future Contamination

Abstract

Iron oxides control the mobility of a host of contaminants in aquifer systems, and the microbial reduction of iron oxides in the subsurface is linked to high levels of arsenic in groundwater that affects greater than 150 million people globally. Paired observations of groundwater and solid-phase aquifer composition are critical to understand spatial and temporal trends in contamination and effectively manage changing water resources, yet field-representative mineralogical data are sparse across redox gradients relevant to arsenic contamination. We characterize iron mineralogy using X-ray absorption spectroscopy across a natural gradient of groundwater arsenic contamination in Vietnam. Hierarchical cluster analysis classifies sediments into meaningful groups delineating weathering and redox changes, diagnostic of depositional history, in this first direct characterization of redox transformations in the field. Notably, these groupings reveal a signature of iron minerals undergoing active reduction before the onset of arsenic contamination in groundwater. Pleistocene sediments undergoing postdepositional reduction may be more extensive than previously recognized due to previous misclassification. By upscaling to similar environments in South and Southeast Asia via multinomial logistic regression modeling, we show that active iron reduction, and therefore susceptibility to future arsenic contamination, is more widely distributed in presumably pristine aquifers than anticipated.

Abundant Fe(III) Oxide‐Bound Arsenic and Depleted Mn Oxides Facilitate Arsenic Enrichment in Groundwater From a Sand‐Gravel Confined Aquifer

“…Previous studies have shown that high‐As groundwater was typically associated with high contents of HCl‐extractable Fe(II) minerals (Glodowska et al., 2020; Kontny et al., 2021; Qiao et al., 2020) and adsorbed As (Jung et al., 2012; Postma et al., 2017; Swartz et al., 2004) in sediments at matched depths. These are mainly due to the fact that the pool of Fe(II) minerals reflected the amount of Fe(III) oxide reduction (Kontny et al., 2021; Nghiem et al., 2020; Stopelli et al., 2021), and the adsorbed As was relatively labile and was ready to release under changeable groundwater environments (Gao, Jia, et al., 2020; Jessen et al., 2012; Neidhardt et al., 2018; Radloff et al., 2017; Stolze et al., 2019; Zhang, Guo, et al., 2017). Higher contents of adsorbed As thus facilitate more As remaining in groundwater.…”

“…Although deep confined aquifers were commonly characterized by oxidized sediments with abundant Fe(III) oxides (Nghiem et al., 2020) and Mn oxides (Gillispie et al., 2019) and had low‐As groundwater (Gillispie et al., 2019; Khan et al., 2019; Ravenscroft et al., 2013), high‐As groundwater being irrelevant to the allogenic source has been universally found in other deep confined aquifers (Table S1 in Supporting Information ) from the Junggar Basin (Xinjiang, China; Zeng et al., 2018), the Huhhot Basin (Inner Mongolia, China; Smedley et al., 2003), the Yuncheng Basin (Shanxi, China; Currell et al., 2011), the Songnen Basin (Jilin, China; Guo, Zhang, et al., 2014), the Jianghan Plain (Hubei, China; Du et al., 2018), the Pearl River Delta (Guangdong, China; Wang et al., 2012), the Atlantic and Gulf Coastal Plain (USA; Degnan et al., 2020), and the Tabriz Plain (Iran; Barzegar et al., 2015). Sand‐gravel sediments, which were considered to host low‐As groundwater because of their high permeability and low As contents in sediments (Gao, Jia, et al., 2020; Han et al., 2013; Zhang, Ma, et al., 2017), are characterized as the primary lithology of those deep confined aquifers (Table S1 in Supporting Information ).…”

Surface Flooding as a Key Driver of Groundwater Arsenic Contamination in Southeast Asia

“…It is important to recognize that our RF model could improve with the inclusion of predictor variables that capture a more complete understanding of the surface/subsurface hydrology and other important geochemical considerations. Hydrologically, the recharge zone and flow path for specific wells is difficult to constrain for areas of low topographic relief (i.e., floodplains) and likely to vary over time. ,− Geochemically, the composition and reactivity of OM and Fe mineralogy are important factors in regulating aquifer redox processes. ,, The inclusion of relevant geomorphic variables partially overcomes this limitation by providing information on unique hydrologic and geomorphic environments that correlate differently with groundwater As. Hydrologic and biogeochemical gradients are clearly difficult to describe across scales, but are invariably similar for adjacent wells at a given aquifer depth using this approach that generalizes the recharge environmental around the well.…”

“…9,38−40 Geochemically, the composition and reactivity of OM and Fe mineralogy are important factors in regulating aquifer redox processes. 9,27,63 The inclusion of relevant geomorphic variables partially overcomes this limitation by providing information on unique hydrologic and geomorphic environments that correlate differently with groundwater As. Hydrologic and biogeochemical gradients are clearly difficult to describe across scales, 9 but are invariably similar for adjacent wells at a given aquifer depth using this approach that generalizes the recharge environmental around the well.…”

Arsenic Accumulation in Hydroponically Grown Schizachyrium scoparium (Little Bluestem) Amended with Root-Colonizing Endophytes

“…Widespread arsenic (As) contamination of plants and sediment due to natural and anthropogenic processes has been extensively documented worldwide. − This study was conducted within the context of previous work, which identified As contamination in sediments from the Cheyenne River Watershed, and there are existing community concerns about As accumulation in local plants. Many As uptake and accumulation studies have focused on rice ( Oryza sativa ) not only because of its agronomic importance, but also because rice is cultivated in regions where soils contain natural As concentrations and in flooded conditions where As can be readily mobilized from soil. − However, the watershed is characterized by semiarid grassland and 90% of the economy depends on grasses for agricultural and livestock purposes. , Understanding the potential mechanisms for tolerance and As accumulation in these plants can aid in risk reduction strategies for nearby communities.…”