Controlling Arsenic Mobilization during Managed Aquifer Recharge: The Role of Sediment Heterogeneity

Abstract

Managed aquifer recharge (MAR) enhances freshwater security and augments local groundwater supplies. However, geochemical and hydrological shifts during MAR can release toxic, geogenic contaminants from sediments to groundwater, threatening the viability of MAR as a water management strategy. Using reactive transport modeling coupled with aquifer analyses and measured water chemistry, we investigate the causal mechanisms of arsenic release during MAR via injection in the Orange County Groundwater Basin. Here, injection water is oxygenated, highly purified recycled water produced by advanced water treatment. Injection occurs via a well screened at several depth intervals ranging from 160−365 m, allowing recharge into multiple confined horizons (zones) of the aquifer system. However, these zones are characterized by varying degrees of prior oxidation due to historic, long-term infiltration from the overlying aquifer. The resulting sediment geochemical heterogeneity provides a critical control on the release (or retention) of arsenic. In zones with prior oxidation, As mobilization occurs via arsenate desorption from Fe-(hydr)oxides, primarily associated with shifts in pH; within zones that remain reduced prior to injection, As release is attributed to the oxidative dissolution of As-bearing pyrite. We find that As release can be attributed to various geochemical mechanisms within a single injection well owing to geochemical heterogeneity across the aquifer system.

Geophysical mapping of hyporheic processes controlled by sedimentary architecture within compound bar deposits

“…Conversely, lower-K silty sands present a more favorable environment for enhanced microbial activity (Graham et al, 2017;Newcomer et al, 2018;Wallace et al, 2020;Wallace & Soltanian, 2021) due to their relatively long residence times and high OM contents which provide conditions favorable for denitrification (Cardenas et al, 2008;Gomez-Velez et al, 2014;Yabusaki et al, 2017 In addition to geochemical activity, metal-oxides (e.g., iron, aluminium, manganese, phosphorous) within cross-bar channel fill sediments serve as sorption sites for trace metals and other potential fluvial contaminants (Bryant et al, 2020;Fuller & Bargar, 2014;Triska et al, 1993). Under anaerobic conditions, the reduction of iron oxyhydroxides could bind nutrients such as phosphorous (Carlyle & Hill, 2001;D'Angelo & Reddy, 1994;Richardson, 1985) and metals such as arsenic (Fakhreddine et al, 2020) and zinc (Fuller & Bargar, 2014) to sediment particles. Trace metals (e.g., arsenic, zinc) could then continuously accumulate until they are mobilized at high pH ($8-8.5 pH) conditions (Fakhreddine et al, 2020;Fuller & Bargar, 2014).…”

Assessing and Managing Large‐Scale Geochemical Impacts From Groundwater Replenishment With Highly Treated Reclaimed Wastewater

“…The aforementioned cases also highlight that the planning and implementation of MAR schemes requires a thorough, site-specific geochemical understanding, as well as the necessary tools for predicting the potential for groundwater quality risks and identifying technical options to mitigate these risks. Such a site-specific understanding is typically developed by a combination of hydrogeological and geochemical characterization activities (e.g., Rathi et al, 2017), laboratory tests (e.g., Fakhreddine et al, 2015;Schafer et al, 2018) and short-term field injection trials (e.g., Seibert et al, 2014Seibert et al, , 2016, or push-pull tests (e.g., Fakhreddine et al, 2020;Prommer et al, 2018;Rathi et al, 2017). Short-term field experiments can often provide some early warning signs of geochemical disequilibrium, and the interpretation of these results can be used to identify the geochemical mechanisms that control the water quality evolution as the injectant migrates through the native aquifer sediments (Rathi et al, 2017).…”

Contemporary practices in groundwater arsenic remediation and wastewater management in West Bengal, India: a systematic review

“…Artificially recharging an arseniferous aquifer by rain water or surface water [66,67], or injecting oxygenated water [68,69], have been found to drastically dilute and reduce the As(III) concentrations in the aquifers. But such a contamination regulation technique does not give much success, because of inadequate understanding of all the ecohydrological processes which govern the geochemical behaviour of these aquifer systems [28,70].…”