Arsenic in Petroleum-Contaminated Groundwater near Bemidji, Minnesota Is Predicted to Persist for Centuries

Abstract

We used a reactive transport model to investigate the cycling of geogenic arsenic (As) in a petroleum-contaminated aquifer. We simulated As mobilization and sequestration using surface complexation reactions with Fe(OH)3 during petroleum biodegradation coupled with Fe-reduction. Model results predict that dissolved As in the plume will exceed the U.S. and EU 10 µg/L drinking water standard for ~400 years. Non-volatile dissolved organic carbon (NVDOC) in the model promotes As mobilization by exerting oxygen demand, which maintains anoxic conditions in the aquifer. After NVDOC degrades, As re-associates with Fe(OH)3 as oxygenated conditions are re-established. Over the 400-year simulation, As transport resembles a “roll front” in which: (1) arsenic sorbed to Fe(OH)3 is released during Fe-reduction coupled to petroleum biodegradation; (2) dissolved As resorbs to Fe(OH)3 at the plume’s leading edge; and (3) over time, the plume expands, and resorbed As is re-released into groundwater. This “roll front” behavior underscores the transience of sorption as an As attenuation mechanism. Over the plume’s lifespan, simulations suggest that As will contaminate more groundwater than benzene from the oil spill. At its maximum, the model simulates that ~5.7× more groundwater will be contaminated by As than benzene, suggesting that As could pose a greater long-term water quality threat than benzene in this petroleum-contaminated aquifer.

Understanding the Evolution of Groundwater‐Contaminant Plume Chemistry Emanating from Legacy Contaminant Sources: An Example from a Long‐Term Crude Oil Spill

“…Arsenic is released from iron oxyhydroxides, Fe(OH) 3 , during iron reduction coupled to the oxidative degradation of dissolved hydrocarbons. The dissolved As is transported downgradient where it resorbs to Fe(OH) 3 at the leading edge of the anoxic plume (Ziegler et al 2021). The presence of biodegradable organic carbon controls the release of As from the solid to the aqueous phase, a process that is predicted to pose a greater long‐term water quality threat than benzene in this petroleum‐contaminated aquifer (Ziegler et al 2021).…”

“…The inset shows average concentrations for selected aromatic compound groups for all data collected between 2010 and 2015 vs. distance from well 421B. The biogeochemical zones are drawn based on data from Cozzarelli et al (2016) and Ziegler et al (2021). For illustrative purposes only.…”

“…Predictions of some of the long‐term effects of these reactions include persistent contamination of groundwater by hydrocarbon partial transformation products, the oxyhydrocarbons (Bekins et al 2016; Podgorski et al 2018; Bekins et al 2020), and arsenic released from aquifer sediments (Ziegler et al 2021). These secondary effects of hydrocarbon degradation have been documented in this and other groundwater plumes containing hydrocarbon source zones (Stasik et al 2015; Geng et al 2017; Smith et al 2019; Schreiber and Cozzarelli 2021).…”

“…Arsenic (As) concentrations were high in most wells in the north oil pool region (Table 1), reaching maximum concentrations of 230 μg/L in the oil spill area. The downgradient spread and attenuation of this As plume is discussed in detail in Cozzarelli et al (2016) and Ziegler et al (2021). Arsenic is released from iron oxyhydroxides, Fe(OH) 3 , during iron reduction coupled to the oxidative degradation of dissolved hydrocarbons.…”

Attenuation of Barium, Strontium, Cobalt, and Nickel Plumes Formed during Microbial Iron Reduction in a Crude-Oil-Contaminated Aquifer

“…Downgradient from the methanogenic zone is an Fe­(III)-reducing zone with a diffuse interface between the two. The interface between the zones is a hotspot of reactivity as methanogenic groundwater enters a less reduced aquifer matrix . In the Fe­(III)-reducing zone, petroleum hydrocarbons are biodegraded coupled to Fe­(III)-reduction dominated by Geobacter . , pH values are ∼1 unit lower (∼6.0–6.5) in these zones compared to uncontaminated groundwater (∼7.5) due to the presence of organic acids and acidity produced via methanogenesis .…”