Colour problems in drinking water: Dissolved impurities of which elements typically impart a reddish-brown colour to water supplies?

Introduction / Context:Water utilities frequently address aesthetic issues such as colour and staining that, while not always health hazards, affect consumer acceptance. A characteristic reddish-brown hue is a common complaint. The question asks you to identify the dissolved species most associated with this colour.

Given Data / Assumptions:

• Colour observed is reddish-brown in distributed water or household fixtures.
• Oxidation of dissolved metals during treatment or in the network is possible.
• Focus is on typical groundwater or distribution system chemistry.

Concept / Approach:Dissolved ferrous iron and manganous manganese are colourless to slightly tinted in reduced states. Upon exposure to oxygen or oxidants (e.g., chlorine), they convert to ferric and manganic oxides/hydroxides, forming visible particulates that give water a reddish-brown or blackish colour and cause staining. Carbonates/bicarbonates are generally colourless. Arsenic, while toxic, does not impart a distinctive colour at typical concentrations.

Step-by-Step Solution:Identify metals known for staining: iron (reddish-brown), manganese (brown to black).Connect oxidation during treatment/distribution to visible colour formation.Select “iron & manganese.”

Verification / Alternative check:Standard design guidance recommends oxidation–filtration for Fe/Mn removal to prevent colour and staining complaints, validating this linkage.

Why Other Options Are Wrong:Carbonates/Bicarbonates: Control alkalinity and hardness; do not cause reddish colour.Arsenic: Toxic trace metalloid; lacks a diagnostic colour at low levels.

Common Pitfalls:

• Confusing turbidity from particulates with true colour from dissolved organics; here the colour is from oxidized metal particulates.
• Ignoring distribution system corrosion as a source of iron.

Final Answer:iron & manganese