Iron and Manganese

In California, the State Water Resources Control Board, Division of Drinking Water sets Secondary Maximum Contaminant Levels (MCLs), also known as “Consumer Acceptance Contaminant Levels” for those constituents in water that may adversely affect the aesthetic qualities of drinking water, such as taste and odor.

The table below is excerpted from Division 4, Chapter 15, Article 16, Section 64449 of the California Code of Regulations, Title 22.

Constituent Maximum Contaminant Level/Units 
Aluminum 0.2 mg/L 
Color  15 Units 
Copper  1.0 mg/L 
Foaming Agents (MBAS)   0.5 mg/L
Iron  0.3 mg/L 
Manganese  0.05 mg/L 
Methyl-tert-butyl ether (MTBE)  0.005 mg/L 
Odor—Threshold  3 Units 
Silver  0.1 mg/L 
Thiobencarb  0.001 mg/L 
Turbidity  5 Units 
Zinc  5.0 mg/L 


Iron and Manganese can be problematic at levels above Secondary MCLs because they both impart color to the water, which can in turn stain plumbing fixtures with rusty, reddish-brown deposits.  In addition, Manganese at very high levels can pose a neurotoxic risk. Additional information about Manganese in drinking water can be found using the following links:

The District pumps groundwater from the Purisima Formation and the Aromas Red Sands aquifer. The Purisima Formation is naturally high in Iron and Manganese.

The District operates nine treatment plants to reduce the amount of Iron and Manganese that is delivered to customers. The treatment plant technology is based on oxidation with sodium hypochlorite (chlorine bleach) followed by filtration through filter media containing anthracite and sand, pyrolusite, or greensand.

The District’s Iron and Manganese treatment plants are successful in achieving roughly 95 to 99 percent removal of the iron and manganese content of the raw groundwater.  The color value  of water that enters the distribution system is considered to be Not Detectable (ND).

  • Main Street Water Treatment Plant

  • O'Neill Ranch Water Treatment Plant

  • Polo Grounds Water Treatment Plant