1. (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. (2) When conditions indicate a greater potential for contamination or pollution, the local health officer may increase the minimum horizontal separations. Examples of such conditions include, but are not limited to, excessively permeable soils, unconfined aquifers, shallow or saturated soils, dug wells, and improperly abandoned wells.
3. (3) The local health officer may allow a reduced horizontal separation to not less than two feet from where the property line, easement line, or building foundation is up-gradient.

4. (4) The local health officer may require an applicant to demonstrate the OSS meets (a), (b), or (c) of this subsection when determining if a horizontal separation to a minimum of 75 feet between an OSS dispersal component and a water well, spring, or surface water that is not a public water source is allowed:

   1. (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, an aquatard that separates potable water from the OSS treatment zone, excessive depth to groundwater, down-gradient contaminant source, or outside the zone of influence; or
   2. (b) Design and proper operation of an OSS with enhanced treatment performance beyond that accomplished by meeting the vertical separation and effluent distribution requirements described in Table VI in WAC 246-272A-0230; or
   3. (c) Evidence the OSS satisfies the requirements of (a) and (b) of this subsection.

5. (5) Persons shall design or install a soil dispersal component only if:

   1. (a) The slope is less than 45 percent or 24 degrees;

   2. (b) The area is not subject to:

      1. (i) Encroachment by buildings or construction such as placement of power poles and underground utilities;
      2. (ii) Cover by impervious material;
      3. (iii) Vehicular traffic; or
      4. (iv) Other activities adversely affecting the soil or the performance of the OSS.
   3. (c) Sufficient reserve area for replacement exists to treat and dispose one hundred percent of the design flow;
   4. (d) The land is stable; and
   5. (e) Surface drainage is directed away from the site.
6. (6) The local health officer may approve a sewer transport line within ten feet of a water supply line if the sewer line is constructed in accordance with section C1-9.1 of the department of ecology's "Criteria For Sewage Works Design," 2008.

[Statutory Authority: RCW 43.20.050(3), 43.20.065, chapters 70A.105 and 70A.110 RCW. WSR 24-06-046, § 246-272A-0210, filed 3/1/24, effective 4/1/25. Statutory Authority: RCW 43.20.050. WSR 05-15-119, § 246-272A-0210, filed 7/18/05, effective 7/1/07.]