Section 24.17.120 Location.

24.17.120 Location.

(1) Persons shall design and install OSSs to meet the minimum horizontal separations shown in Table IV—Minimum Horizontal Separations:

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, Collection, and Nonperforated Distribution1
Nonpublic well or suction line	100 ft.	50 ft.	50 ft.
Public drinking water well	100 ft.	100 ft.	100 ft.
Public drinking water spring measured from the ordinary high-water mark2	200 ft.	200 ft.	100 ft.
Spring or surface water used as drinking water source measured from the ordinary high-water mark2	100 ft.	50 ft.	50 ft.
Pressurized water supply line3	10 ft.	10 ft.	10 ft.
Decommissioned well (decommissioned in accordance with Chapter 173-160 WAC)4	10 ft.	N/A	N/A
Surface water measured from the ordinary high-water mark:
Marine water	100 ft.	50 ft.	10 ft.
Fresh water	100 ft.	50 ft.	10 ft.
Building foundation/in-ground swimming pool	10 ft.4	5 ft.4	2 ft.
Property or easement line4	5 ft.	5 ft.	N/A
Interceptor/curtain drains/drainage ditches:
Down-gradient5	30 ft.	5 ft.	N/A
Up-gradient5	10 ft.	N/A	N/A
Other site features that may allow effluent to surface:
Down-gradient5	30 ft.	5 ft.	N/A
Up-gradient5	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
Other adjacent soil dispersal components/subsurface storm water infiltration systems	10 ft.	N/A	N/A

1 “Building sewer” as defined by the most current edition of the Uniform Plumbing Code. “Nonperforated distribution” includes pressure sewer transport lines.

2 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.

3 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 WDOE’s “Criteria for Sewage Works Design,” revised December 1998, or equivalent.

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

5 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.

(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 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

(4) Persons shall design and/or install disposal 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.

(5) 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.

(6) 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)