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All On-site Sewage Program Staff, On-site Sewage System Designers, Professional Engineers, Installers, Real Estate Professionals, Well Drillers, and On-site Sewage Maintainer Professionals
Darrell Rodgers, PhD, Assistant Division Director
Lynn Schneider, Supervisor, On-site Sewage Program

Treatment to reduce arsenic levels in individual drinking water wells located in King County with arsenic levels equal to or below 50 parts per billion

Arsenic is a naturally occurring contaminant in some groundwater in King County, most frequently found in bedrock aquifers in the central part of the county.  Map of Arsenic Detections in Washington Public Water Supplies (PDF). Drinking water from bedrock wells, also called drilled or artesian wells, and less frequently from shallow or dug wells, may contain arsenic.

Arsenic (chemical symbol As) occurs naturally in many parts of the United States, including parts of King County, Washington.  As ground water flows through soil formations including rocks and soil that contain arsenic, some of the arsenic dissolves into the water.  Drinking water in Washington typically contains less than 3 parts of arsenic per billion parts of water.  Arsenic has no smell, taste, or color when dissolved in water, even in high concentrations, and therefore only laboratory analysis can determine the presence and concentration of arsenic in water.  To learn more about arsenic in well water, read the Washington State Department of Health's pamphlet.

Arsenic ingestion can result in both chronic (long-term) and acute (short-term) health effects. Acute effects can include nausea, vomiting, neurological effects such as numbness or burning sensations in the hands and feet, cardiovascular effects and decreased production of red and white blood cells which may result in fatigue. Chronic effects include changes in skin coloration and skin thickening and small corn-like growths that can develop especially on the palms of the hand and soles of the feet. Chronic exposure to arsenic is also associated with an increased risk of skin, bladder, and lung cancer. There is also evidence that long-term exposure to arsenic can increase risks for kidney and prostate cancer. Your health risks are determined by the following factors:

  • The concentration of arsenic in your water
  • The amount of water you consume each day
  • The length of time you have been consuming the water
  • Your dietary intake of arsenic (in the foods that you eat)
  • Your individual sensitivity to arsenic.

1 Drinking Water Arsenic Rule History and 40 CFR 141.154(b)(1))

The current drinking water standard or Maximum Contaminant Level (MCL) set by the U.S. Environmental Protection Agency (EPA) is 0.010 mg/L or parts per million (ppm). This is equivalent to 10 ug/L (micrograms per liter) or parts per billion (ppb). In 2001, the U.S. Environmental Protection Agency (EPA) reduced the regulatory MCL from 50 ppb to 10 ppb on the basis on bladder and lung cancer risks. The MCL is based on the average individual consuming 2 liters of water a day for a lifetime. Long term exposure to drinking water containing arsenic at levels higher than 10 ppb increases the chances of getting cancer.

If your water has arsenic levels above 10 ppb, you should obtain drinking water from another source or install a home treatment device. Concentrations above 10 ppb will increase the risk of long-term or chronic health problems. The higher the level and length of exposure, the greater the risk. It is especially important to reduce arsenic concentrations in drinking water concentrations if you have children or are pregnant. Children are at greater risk (to any agent in water) because of their greater water consumption on a per unit body weight basis. Pregnant women should reduce their arsenic exposures because arsenic has been found at low levels in mother's milk and will cross the placenta, increasing exposures and risks for the fetus.

If you are developing property in King County to be served by an individual well with arsenic levels between 10 and 50 ppb, treatment is required to reduce arsenic levels to less than 10 ppb.

If the arsenic level in your well water is 50 ppb or less, there are a number of treatment methods available. However, before selecting a treatment method, there are a number of factors that need to be considered in determining the appropriate treatment methodology.

First, there are typically two varieties, or species, of arsenic in water: "arsenic 3" and "arsenic 5." This distinction is significant because "arsenic 3" is very difficult to remove from water and must be changed or "oxidized" to "arsenic 5" before it can be removed. A laboratory can determine how much of each kind of arsenic is in your water with a method called "speciation." Speciation will likely add additional cost to the analysis but should be considered to determine the proper treatment methodology. Ask the laboratory or your professional engineer what they require for this process as it may require additional samples. To learn more about oxidation methods, please contact a licensed engineer with knowledge of drinking water treatment systems.

The second factor is deciding what type of filter system is preferred depending on the well water quality and household considerations. The treatment options are either a point-of-use (POU) system or whole house system. A POU treatment device is installed under the kitchen sink and at the refrigerator/ice maker. A whole house treatment (point-of-entry) system treats all the water that enters the house.

The third factor is the possible presence of other constituents in the water, such as phosphate, silica, organic compounds, iron manganese, and others which might hinder the effectiveness of arsenic removal and will need to be removed before the arsenic treatment step. Arsenic removal methods or systems include anion exchange, reverse osmosis, adsorption, and co-precipitation of arsenic with iron followed by filtration. Each method has its advantages and disadvantages. Information on some of the more common treatment methods is listed below:

  • Anion exchange units operate using the same principle as a water softener. In this case the arsenic is exchanged for chloride. The systems are generally used to treat water for the entire house and generally require other pre and post treatment equipment to work properly.

  • Reverse osmosis (RO) is generally installed as a point-of-use treatment system and usually requires pre-filtration to reduce membrane fouling and convert the arsenic to the +5 valence form. RO is considered ineffective at removing arsenic 3.

  • Adsorptive filter media are used in both point-of-use and whole house treatment systems. There are several varieties of adsorptive media available granular ferric oxide, titanium oxide, iron coated anion media and others. Adsorptive media generally work best at removing arsenic 5, but in some cases will remove all types of arsenic. The efficiency of removal is dependent on the pH of the water, the presence of competing ions or interfering ions, therefore, pretreatment may be required.

Because the quality of the well water affects the efficacy, complexity, and how much maintenance is required for treatment systems, a professional engineer is required to design and verify the performance of the water treatment system.

Requirements for waiving King County Board of Health Code (BOH) 13.04.070.B.4.b Domestic water supply source, primary maximum contaminant level arsenic requirements.

For property development in King County that proposes the use of an Individual Well, arsenic treatment is allowed when:

  • The On-site Sewage System Designer/Engineer applies for a waiver from BOH 13.04.070.B.4.b (Domestic water supply source) with a two hour review fee. See fee schedule. The arsenic treatment allowance waiver application is available.

  • To learn about hiring an engineer, you may wish to visit the Washington State Department of Health's website. While this information is about hiring an engineer for a Group B well, much of it is applicable to a property owner wishing to hire an engineer to design a treatment system for a private individual well.

  • Treatment is required for wells with arsenic levels between 10 – 50 ppb.  All wells with untreated arsenic levels less than or equal to 50 ppb may be approved for treatment.  Note, some property owners have tried flushing the well to reduce arsenic levels.  This not only wastes water and energy, it may also misrepresent the actual arsenic level.  The arsenic concentration reported to Public Health – Seattle & King County must validly and accurately represent the well water.

  • If the well water is turbid due to the recent well construction, sampling after settling may better reflect the actual arsenic level of the well water.  Check with a water system designer to learn more about arsenic levels in well water.

  • All analysis of water quality must be sampled pre-treatment using chain of custody methodology conducted by the on-site sewage system designer, well driller or a licensed engineer with expertise in drinking water treatment systems and analyzed by a Washington State Certified laboratory.

  • A licensed engineer with expertise in drinking water systems designs the treatment system and shall at a minimum include each of the following:
    • Expected yield of treated water.
    • Justification for the proposed treatment system.
    • Locations at which treated water will be supplied.  For whole house systems, a record drawing of the drinking water treatment system, with the design engineer's stamp, shall be submitted to the Health Department.
    • Other water quality parameters the engineer considered in the design of the treatment process.
    • Method of treatment process residuals management.
    • Minimum ongoing testing requirements.

  • Adequate water quality after treatment is demonstrated by submitting results of water samples collected post treatment, sampled using chain of custody conducted by the on-site sewage system designer or a licensed engineer with expertise in drinking water treatment systems and analyzed by a Washington State Certified laboratory. Post-treatment arsenic levels must be below 10 ppb.
    • For a whole house or building treatment system, adequate water quality shall be demonstrated prior to approval of the Record Drawing.
    • For a point of use treatment system, adequate water quality shall be demonstrated prior to approval of Record Drawing.

A notice to title recorded with the King County Recorder's Office (Records and Licensing Services Division) is required. This notice shall include a statement containing the common and legal address of the property, the property legal description and tax parcel number, and the present owner's name. The notice must also contain the following additional information (see template):

  • The concentration of the arsenic that exceeds the MCL, both before and after treatment.

  • The brand name and model number of the treatment product and required maintenance interval.

  • Detailed description of the location(s) of the treatment product(s).

  • That the treatment device must be properly maintained, along with periodic sampling, to ensure continued safety of the water supply.  (NOTE: Specific sampling frequency is to be set in accordance with the designing engineer's recommendations).

  • A written statement of understanding and acknowledgement, by the property owner, that failure to sample and maintain the treatment system may result in adverse health effects to the users of the water supply and that any untreated water is considered unsafe for consumption.

  • The owner's obligation and responsibility to notify future property owners, heirs, successors, or tenants, about the treatment device, proper maintenance and operations, sampling requirements, potential health risks, and most recent sample results of the water supply, both before and after treatment.

  • The following US Environmental Protection Agency (EPA) "Informational Statement" is required to be recorded with this property: "While your drinking water meets EPA's standard for arsenic, it does contain low levels of arsenic.  EPA's standard balances the current understanding of arsenic's possible health effects against the costs of removing arsenic from drinking water.  EPA continues to research the health effects of low levels of arsenic which is a mineral known to cause cancer in humans at high concentrations and is linked to other health effects such as skin damage and circulatory problems" (40 CFR 141.154(b)(1)).
  • Property owners are responsible for operating and maintaining the system per manufacturer requirements and recommendations.

  • For properties developed using the arsenic treatment allowance: At the time the building is remodeled or expanded, if it is not connected to public water and the notice on title required by this section has not been recorded or needs to be updated to reflect design alterations with the treatment system, the owner shall:
    • Record the notice to title as described above; and
    • Demonstrate adequate water quality by submitting results of water sample(s) collected post treatment, sampled using chain of custody and analyzed by a Washington State Certified laboratory to demonstrate the arsenic level is below 10 ppb within the previous 12 months.

Contact us

If you have questions regarding arsenic in individual wells, contact Lynn Schneider at King County Environmental Health at 206-477-2124 or email