Section 24.17.120 Location.

24.17.120 Location.

(1) OSS must be designed and installed to meet at least the minimum horizontal separations shown in Table IV:

Table IV—Minimum Horizontal Separations
Items Requiring Setback	From Edge of Soil Dispersal Component and Reserve Area	From Sewage Tank and Distribution Box	From Building Sewer and Nonperforated Distribution Pipe
Well	100 ft.	50 ft.	50 ft.
Public drinking water well	100 ft.	100 ft.	100 ft.
Nonpublic drinking water well	100 ft.	50 ft.	50 ft.
Public drinking water spring or surface water measured from the ordinary high-water mark	200 ft.	200 ft.	100 ft.
Nonpublic drinking water spring or surface water measured from the ordinary high-water mark1	100 ft.	50 ft.	50 ft.
Nonpublic, in-ground, drinking water containment vessel3	20 ft.	10 ft.	10 ft.
Pressurized water supply line or easement for water supply line	10 ft.	10 ft.	10 ft.
Closed geothermal loop4 or pressurized nonpotable water line	10 ft.	10 ft.	10 ft.
Decommissioned well (decommissioned in accordance with Chapter 173-160 WAC)	10 ft.	N/A	N/A
Surface water measured from the ordinary high-water mark	100 ft.	50 ft.	10 ft.
Building foundation/in-ground swimming pool	10 ft.	5 ft.	2 ft.
Property or easement line	5 ft.	5 ft.	N/A
Lined5 stormwater detention pond6
Down-gradient7	30 ft.	N/A	N/A
Up-gradient7	10 ft.	N/A	N/A
Unlined8 stormwater infiltration pond6 (up or down-gradient)7	100 ft.	50 ft.	10 ft.
Irrigation canal or irrigation pond (up or down-gradient)	100 ft.	50 ft.	10 ft.
Interceptor/curtain drains/foundation drains/drainage ditches:
Down-gradient2	30 ft.	5 ft.	N/A
Up-gradient2	10 ft.	N/A	N/A
Subsurface stormwater infiltration or dispersion component6
Down-gradient7	30 ft.	10 ft.	N/A
Up-gradient7	30 ft.	10 ft.	N/A
Other site features that may allow effluent to surface:
Down-gradient2	30 ft.	5 ft.	N/A
Up-gradient2	10 ft.	N/A	N/A
Down-gradient cuts or banks with at least 5 ft. of original, undisturbed soil above a restrictive layer due to a structural or textural change	25 ft.	N/A	N/A
Down-gradient cuts or banks with less than 5 ft. of original, undisturbed soil above a restrictive layer due to a structural or textural change	50 ft.	N/A	N/A
Soil dispersal components serving a separate OSS	10 ft.	N/A	N/A

1 If surface water is used as a public drinking water supply, the designer shall locate the OSS outside of the required source water protection area.

2 The item is down-gradient when liquid will flow toward it upon encountering a water table or a restrictive layer. The item is up-gradient when liquid will flow away from it upon encountering a water table or restrictive layer.

3 Any in-ground containment vessel used to store drinking water.

4 A network of underground piping carrying fluid under pressure used to heat and cool a structure.

5 “Lined” means any component that has the intended function of detaining the stormwater with no intention of dispersal into surrounding soil.

6 OSS components take precedence in cases of horizontal setback conflicts between OSS and stormwater components.

7 “Down-gradient” means that subsurface water flows toward and is usually located lower in elevation. “Up-gradient” means subsurface water does not flow toward and generally flat, or flows away from and generally located higher in elevation.

8 “Unlined” means any component that has the ability to or intended function of infiltrating the stormwater.

(2) Where any condition indicates a greater potential for contamination or pollution, the health officer may increase the minimum horizontal separations. Examples of such conditions include excessively permeable soils, unconfined aquifers, shallow or saturated soils, dug wells, and improperly abandoned wells.

(3) The local health officer may allow a reduced horizontal separation to not less than two (2) feet from where the property line, easement line, or building foundation is up-gradient.

(4) The horizontal separation between an OSS dispersal component and an individual non-public water well, spring, or surface water may be reduced to a minimum of seventy-five (75) feet, by the health officer, and be described as a “conforming” system upon signed approval by the health officer if the applicant demonstrates:

(a) Adequate protective site-specific conditions, such as physical settings with low hydrogeologic susceptibility from contaminant infiltration. Examples of such conditions include evidence of confining layers and/or aquatards separating potable water from the OSS treatment zone, excessive depth to ground water, down-gradient contaminant source, or outside the zone of influence; or

(b) Design and proper operation of an OSS system assuring enhanced treatment performance beyond that accomplished by meeting the vertical separation and effluent distribution requirements described in Section 24.17.140(2), Table VI; or

(5) Persons shall design and/or install soil dispersal components only where:

(a) The slope is less than forty-five (45) percent (twenty-four (24) degrees);

(b) The area is not subject to:

(i) Encroachment by buildings or construction such as placement of swimming pools, power poles and underground utilities;

(ii) Cover by impervious material;

(iii) Vehicular traffic; or

(iv) Other activities adversely affecting the soil or the performance of the OSS;

(d) The land is stable; and

(e) Surface drainage is directed away from the soil dispersal area.

(6) New OSSs shall be located on the same lot as the buildings they are designed to serve. This does not apply to new OSSs that cannot be located on existing legal lots of record or to OSSs being repaired or replaced.

(7) The health officer may approve a sewer transport line within ten (10) feet of a water supply line if the sewer line is constructed in accordance with Section C1-9 of the WDOE’s “Criteria for Sewage Works Design,” December 1998. (Sec. 13 of Ord. 2007-10-01; amended by Sec. 9 of Ord. 2021-11-05; amended by Sec. 10 of Ord. 2025-03-12)