Water Testing 

Well water testing 

The Snohomish County Health Department no longer provides water testing services. Please see the list of private water testing labs in Snohomish County.

Regular water tests are recommended for all household water systems. Owners of private wells should test their drinking water based on the following recommendations:

  • Drilled wells: 1 sample every 12 months
  • Dug wells: 1 sample every 3 months
  • Springs: 1 sample every 3 months

If your water sample shows bacteriological contamination (unsatisfactory for coliform bacteria, fecal coliform, or E.coli), follow the CDC’s recommendations for disinfection of your well.

Understanding water testing results

The Snohomish County Health Department established primary drinking water requirements to help determine if your water supply is safe to drink. After your water samples are tested, you will receive an advisory in the mail if your well water has a detectable concentration of certain minerals and bacteria that may pose a risk to you and the community.


Maximum Contaminant Levels (MCLs) represent the level of risk a contaminant poses to your health and the environment. Secondary Maximum Contaminant Levels (SMCLs) are standards for contaminants that may make your water have an unpleasant taste, odor, or appearance.


To ensure that your water is safe to drink, it should meet the following minimum requirements:

  • Aluminum (proposed SMCL: 0.05 milligrams per liter (mg/L))
  • Arsenic (MCL: 0.010 mg/L) (required test)
  • Asbestos (proposed MCL: 7 million fibers/liter over 10 microns long)
  • Bacteria, coliform (required test) (MCL: 0 present utilizing the presence/absence methods)
  • Barium (MCL: 2.0 mg/L) (required test)
  • Cadmium (MCL: 05 mg/L) (required test)
  • Chloride (SMCL: 250 mg/L)
  • Chromium (MCL: 0.10 mg/L) (required test)
  • Copper (SMCL: 1.3 mg/L)
  • Fluoride (MCL: 4 mg/L, SMCL: 2 mg/L) (required test)
  • Hardness (No standard established)
  • Iron (SMCL: 0.30 mg/L)
  • Lead (Environmental Protection Agency action level: 0.015 mg/L) (required test)
  • Manganese (SMCL: 0.05 mg/L)
  • Mercury (MCL: 02 mg/L) (required test)
  • Nitrate (MCL: 10 mg/L) (required test)
  • Selenium (MCL: 0.05 mg/L) (required test)
  • Silver (MCL: 0.05 mg/L) (required test)
  • Sodium (no standard established) (required test)
  • Sulfate (SMCL: 250 mg/L)
  • Zinc (SMCL: 5 mg/L)

See the Environmental Protection Agency’s recommended water quality criteria for more information on each of the contaminants above.

If your water sample shows bacteriological contamination, follow the CDC’s recommendations for disinfection of your well.

Arsenic in your well

Arsenic is naturally occurring in the Snohomish County region, and measured arsenic levels in groundwater and wells vary over time. If your well water test shows that arsenic is present, but at acceptably low levels, you will be notified and advised to test regularly to make sure safe levels are not exceeded.

Well water that tests high for arsenic must have an approved treatment system installed before the well will be approved. An arsenic disclosure form will become part of the property's title record.

If you drink water containing arsenic in excess of the safe limits over many years, you could experience skin damage or problems with your circulatory system and may have an increased risk of cancer.

Refer to Snohomish County Health Department Code Chapter 4.40, Drinking Water Rules and Regulations - Arsenic Testing, for more information.