5.2 Redox Processes and Contaminant Transformation in Aquifers

Groundwater systems are dynamic environments where chemical reactions shape water quality. Redox processes, driven by electron transfers, control the fate of contaminants and nutrients. These reactions, often catalyzed by microbes, create distinct zones with unique chemical characteristics.

Understanding redox in aquifers is crucial for managing water resources and cleaning up pollution. Different redox conditions transform contaminants in various ways, affecting their mobility and toxicity. This knowledge helps us predict how pollutants will behave and design effective cleanup strategies.

Redox Processes in Groundwater Systems

Redox processes in groundwater

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• Reduction-oxidation reactions transfer electrons between chemical species
• Control chemical speciation of elements influence solubility and mobility of contaminants
• Affect biogeochemical cycling of nutrients (carbon, nitrogen, phosphorus)
• Electron acceptors in groundwater include oxygen, nitrate, manganese oxides, iron oxides, sulfate, carbon dioxide
• Redox potential (Eh) measures system's tendency to accept or donate electrons expressed in volts or millivolts
• pH interacts with Eh to determine dominant redox species affects reaction rates and equilibria

Microbial activity in aquifer redox

• Microbial catalysis accelerates redox reactions enables thermodynamically favorable but kinetically slow processes
• Electron donors include organic matter (dead biomass, hydrocarbons) and reduced inorganic compounds (hydrogen sulfide, ferrous iron)
• Microbial respiration oxidizes electron donors reduces electron acceptors generates energy for growth
• Biogeochemical cycles driven by microbial activity include carbon (CO2 fixation, methanogenesis), nitrogen (nitrification, denitrification), sulfur (sulfate reduction, sulfur oxidation), iron (iron reduction, iron oxidation)
• Microbial communities adapt to available electron acceptors form succession based on redox conditions (aerobes to anaerobes)

Contaminant Transformation in Aquifers

Major redox zones and contaminants

• Oxic zone contains dissolved oxygen supports aerobic respiration oxidizes organic contaminants (BTEX, phenols)
• Nitrate-reducing zone denitrifies NO3- to N2 transforms nitrate-based contaminants (agricultural runoff)
• Manganese-reducing zone reduces Mn(IV) to Mn(II) mobilizes manganese affects metal contaminants
• Iron-reducing zone reduces Fe(III) to Fe(II) transforms iron-bound contaminants (arsenic, phosphates)
• Sulfate-reducing zone produces sulfide precipitates metal sulfides (lead, zinc, copper)
• Methanogenic zone generates methane reductively dechlorinates organic contaminants (TCE, PCE)

Impact of redox on contaminants

• Organic contaminants biodegradation rates vary with redox conditions

1. Reductive dechlorination in anaerobic zones (PCE to TCE to DCE to VC)
2. Oxidative degradation in aerobic zones (BTEX compounds)

• Inorganic contaminants speciation changes affect mobility

1. Arsenic: As(V) to As(III) under reducing conditions increases solubility
2. Chromium: Cr(VI) to Cr(III) under reducing conditions decreases mobility

• Redox-sensitive processes include sorption/desorption, precipitation/dissolution, complexation affect contaminant transport
• Natural attenuation driven by redox transformations immobilizes contaminants in reducing zones
• Plume behavior shows redox zonation along flow path leads to sequential degradation of contaminants
• Remediation strategies manipulate redox conditions through biostimulation (electron donor addition) or bioaugmentation (microbial inoculation)