Mobilization of Arsenic and Other Naturally Occurring Contaminants during Managed Aquifer Recharge: A Critical Review

Abstract

Population growth and climate variability highlight the need to enhance freshwater security and diversify water supplies. Subsurface storage of water in depleted aquifers is increasingly used globally to alleviate disparities in water supply and demand often caused by climate extremes including floods and droughts. Managed aquifer recharge (MAR) stores excess water supplies during wet periods via infiltration into shallow underlying aquifers or direct injection into deep aquifers for recovery during dry seasons. Additionally, MAR can be designed to improve recharge water quality, particularly in the case of soil aquifer treatment and riverbank filtration. While there are many potential benefits to MAR, introduction of recharge water can alter the native geochemical and hydrological conditions in the receiving aquifer, potentially mobilizing toxic, naturally occurring (geogenic) contaminants from sediments into groundwater where they pose a much larger threat to human and ecosystem health. On the basis of the present literature, arsenic poses the most widespread challenge at MAR sites due to its ubiquity in subsurface sediments and toxicity at trace concentrations. Other geogenic contaminants of concern include fluoride, molybdenum, manganese, and iron. Water quality degradation threatens the viability of some MAR projects with several sites abandoning operations due to arsenic or other contaminant mobilization. Here, we provide a critical review of studies that have uncovered the geochemical and hydrological mechanisms controlling mobilization of arsenic and other geogenic contaminants at MAR sites worldwide, including both infiltration and injection sites. These mechanisms were evaluated based on site-specific characteristics, including hydrological setting, native aquifer geochemistry, and operational site parameters (e.g., source of recharge water and recharge/recovery cycling). Observed mechanisms of geogenic contaminant mobilization during MAR via injection include shifting redox conditions and, to a lesser extent, pH-promoted desorption, mineral solubility, and competitive ligand exchange. The relative importance of these mechanisms depends on various site-specific, operational parameters, including pretreatment of injection water and duration of injection, storage, and recovery phases. This critical review synthesizes findings across case studies in various geochemical, hydrological, and operational settings to better understand controls on arsenic and other geogenic contaminant mobilization and inform the planning and design of future MAR projects to protect groundwater quality. This critical review concludes with an evaluation of proposed management strategies for geogenic contaminants and identification of knowledge gaps regarding fate and transport of geogenic contaminants during MAR.

Molybdenum Release Triggered by Dolomite Dissolution: Experimental Evidence and Conceptual Model

“…1−10 Among those, metal mobilization in conjunction with the redox shifts that occur during the injection of oxygenated or nitrate enriched water into anoxic aquifers during managed aquifer recharge (MAR) activities has been widely reported. 11 The current prediction is that at least 10% of the world's drinking water supply will be provided through MAR applications in the near future. 12 The release of arsenic (As) during aquifer storage and recovery (ASR) operations has been described for many sites in Europe, 13 North America, 14 and Australia.…”

Oxidative Dissolution of Arsenic-Bearing Sulfide Minerals in Groundwater: Impact of Hydrochemical and Hydrodynamic Conditions on Arsenic Release and Surface Evolution

“…The experiments and simulations conducted in this study highlight the complex interactions between multiple co-occurring geochemical mechanisms controlling the dissolution of As-bearing sulfide minerals and the transport of arsenic in groundwater. Such process-based understanding is of pivotal importance for the practical management of groundwater resources both under natural conditions and in the presence of engineered interventions such as managed aquifer recharge, MAR. − In particular, our results show that the precipitation of secondary mineral phases affects both the dissolution of the sulfide minerals and the mobility of arsenic by promoting the passivation of the sulfide mineral reactive surface and significant sequestration of arsenic. pH plays a key role in the mobilization of arsenic and mineral surface evolution by controlling the dynamics and extent of the secondary mineral phase precipitation.…”

“…Although the dissolution of As-bearing sulfide minerals occurs naturally in the subsurface, anthropogenic activities can exacerbate this contamination by enhancing the exposure of sulfide minerals to oxygen. For instance, the disposal of mine tailings and their exposure to atmospheric oxygen is known to potentially result in the generation of acid mine drainage, triggering the dissolution of a variety of mineral phases and the mobilization of contaminants. − Managed aquifer recharge (MAR) can also enhance arsenic release in groundwater from the dissolution of sulfide minerals naturally present in the subsurface due to the direct injection of oxic water in the subsurface. − Furthermore, seasonal fluctuation in water level and hydraulic fracturing have been shown to increase the exposure of sulfide minerals to atmospheric O 2 and, consequently, to enhance their dissolution. Thus, detailed understanding of the geochemical and hydrological processes controlling the dissolution of these As-bearing minerals is fundamental for the adequate understanding of natural and engineering systems and for improved management of groundwater resources.…”

The 21st century water quality challenges for managed aquifer recharge: towards a risk-based regulatory approach

“…Recharging a reduced, As-rich coastal aquifer in Khulna, Bangladesh, with pond water resulted in arsenic concentrations in recovered water of >100 μg/L (Sultana et al 2015 ) and was attributed to reductive dissolution of As-bearing Fe-oxyhydroxide. A recent critical review on mobilization of arsenic and other naturally occurring contaminants during MAR (Fakhreddine et al 2021 ) concludes that arsenic poses the most widespread challenge at MAR sites due to its ubiquity in subsurface sediments and toxicity at trace concentrations; other geogenic contaminants of concern include iron, molybdenum, manganese, chromium, and fluoride. Fortunately, the same review points out many approaches to mitigate MAR-induced arsenic problems in recovered water, but these need process understanding and predictive capability to ensure such risks are managed appropriately.…”