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The Code of the King County Board of Health Title 13, (the Code), requires that an OSS site design application include verification that an approved domestic water supply source is/will be available and is adequate to serve the subject site prior to application approval. Per Code, there are two water source options available:

  • Public water supply source connection. Availability and adequacy is demonstrated by a water availability letter issued by a public water utility.

  • Individual private water source (private well or private spring). Availability and adequacy must also be demonstrated prior to site design approval (Section 13.04.070 B and C).

There are potentially three scenarios apparent for demonstrating availability/adequacy for a proposed individual well source.

  • Well is drilled prior to application submission. The well is drilled in an approved location, and the resultant water quantity and quality documentation are included in the site design application submission.

  • Well is not drilled at time of application submission. The applicant specifies a request to determine the outcome of OSS design review prior to investing in installation of a new individual private well. Additionally, the application does not include any "feasibility" information related to potential water availability for the subject site. The site design application review may proceed; however, the application will be disapproved on the basis of "an approved water source not demonstrated." The applicant could then proceed with the private well development and resubmit, within 90 days for approval, the OSS design application which now includes the installed well's water quantity and quality data.

  • Well is not drilled at time of application submission. However, the application includes well information from "neighboring wells." Under this option the application could demonstrate feasibility of adequate water quantity by "providing adequate information to the satisfaction of the health officer to demonstrate the aquifer's capability to provide 400 gallons per day. This information may include well logs or pumping reports from neighboring wells utilizing the same aquifer."

    In lieu of well logs or production reports, an hydrogeological evaluation and report by a qualified hydrogeologist, geologist or engineer may be submitted for review. In any case, if OSS staff are unsure about evaluating the material submitted, they should consult with the Senior EHS's. The OSS application review may proceed to the approval stage.

When not installed at time of application approval, a proposed individual well must be subsequently installed and documented to be satisfactory prior to issuance of any oss installation permit.


Under option 3 above, the question is: What constitutes "adequate information" and "neighboring wells"? The following guidance is intended to assist in determining whether information about "neighboring wells" is sufficient to reasonably indicate availability and adequacy of a proposed individual private well location and not to predict actual future production capability of a proposed well that is yet to be installed. Regulatory review, as with other aspects of the OSS application review, consist of evaluation of information submitted about existing conditions in the context of the design application.


King County geology consists of four major parent material types:

  • Bedrock
  • Glacial deposits
  • Mud flow deposits
  • Alluvium

Each parent material is typically associated with different types of aquifers, as shown in Table 1. In some cases (such as Osceola), there may be different geology below the surficial materials that may have aquifer properties that the overlying material doesn't.

The soil types associated with the parent materials are shown on Table 2. Regulatory staff and design professionals are familiar with soil types so this table is intended to help make a connection between parent (geological) material and soil types. A general geological map of King County is attached with this guidance to assist in determining the general geology type of a particular area in King County.

Table 1 - Surficial Geology and Aquifers
Surficial Geologic
(Parent) Material
Aquifer Properties Distance from site allowed for information Number of well logs
Bedrock Very poor yields; often flow occurs only through fractures in rock; subject to large seasonal fluctuations Less than 1/4 mile; prefer adjoining property At least two on opposite sides of the parcel
Glacial deposits include:

"Layer-cake" system: aquifers interspersed among confining units

Till ("hardpan") has poor yields (may have outwash below)

Outwash usually has best yields; may have protective soil layer (if not at surface itself)

Within one mile; must show similar soils, water level and production.3 Ideally, at least three on different sides of the parcel. Most aquifers in these areas have been drilled into, information should be readily available.
Mudflow Deposits Poor yields unless aquifer below Same as for glacial deposits Same as for glacial deposits
Alluvium Variable: high yields in gravelly layers; low yields in silty units Up to 2 miles; should show similar production and water level. At least two.

Production information from well logs over 10 years old, or in areas of reported water level decline, should be supported by current (within two years) production information.

Table 2 - Parent Material and Soil Series Association
(Parent) Material
Soil types (series) Possible underlying geology (e.g. other aquifers Where found
Bedrock Beausite, Ovall Only bedrock Hills, mostly in eastern part of County
Bedrock Alderwood, Arents, Kitsap, Orcas, Seattle May be (advance) outwash, or bedrock Upland plains
Glacial deposits include:

Till or lake deposits
Everett, Indianola, Klaus, Neilton, Ragnar


May be till Lower elevations of upland plains
Mudflow Deposits
(Osceola Mudflow)
Buckley May be glacial deposits, or alluvium White River Valley and Enumclaw plains
Alluvium Bellingham, Briscot, Earlmont, Edgewick, Newberg, Nooksack, Norma, Oridia, Pilchuck, Puget, Puyallup, Renton, Salal, Sammamish, Seattle, Si, Snohomish, Sultan, Tukwila, Woodinville Glacial deposits, bedrock (in river valleys in Cascades) Major river valleys

Wording for OSS Approval Addendum sheet:

"Acceptance of the proposed well is based on production data supplied by the applicant. This approval does not guarantee that the well will produce the same amount of water as in the neighboring wells. Production from the well to serve this property must be determined before the sewage system installation permit will be issued."