Groundwater chemistry—common forms of iron and manganese In reduced groundwater conditions, iron and manganese pollutants are most commonly present in the soluble form of their __________.

Introduction / Context:
Groundwaters drawn from anoxic aquifers often contain dissolved iron and manganese. Knowledge of their chemical speciation informs the design of aeration, oxidation, and filtration steps in water treatment plants.

Given Data / Assumptions:

• Reduced conditions (low dissolved oxygen) prevail underground.
• pH is near neutral to slightly alkaline, with dissolved carbon dioxide present.
• Dominant soluble species for Fe and Mn are required.

Concept / Approach:
Under anoxic, carbon dioxide–bearing conditions, Fe^2+ and Mn^2+ tend to exist as soluble bicarbonate complexes (e.g., “ferrous bicarbonate,” often represented as Fe(HCO3)2 in water chemistry shorthand). Upon aeration/oxidation, these convert to insoluble oxides/hydroxides that can be filtered.

Step-by-Step Solution:

Verification / Alternative check:
Standard texts depict Fe^2+ and Mn^2+ stability fields showing solubility in reducing conditions and precipitation after oxidation to Fe(OH)3 and MnO2.

Why Other Options Are Wrong:

• Carbonates: typically sparingly soluble at neutral pH; metals tend to remain as bicarbonates in CO2-rich waters.
• Chlorides: may be present but are not the prevalent complexing form for Fe/Mn in natural groundwater.
• Sulfides: occur in strongly reducing, sulfidic environments, not the most common case.

Common Pitfalls:
Confusing solid carbonate minerals with aqueous bicarbonate species; the aqueous form dominates under typical aquifer conditions.

Final Answer:
Bicarbonates