(1) Persons shall design and install OSS 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, and nonperforated distribution pipe |
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 mark | 200 ft. | | 200 ft. | | 100 ft. | |
Spring or surface water used as drinking water source measured from the ordinary high-water mark1 | 100 ft. | | 50 ft. | | 50 ft. | |
Pressurized water supply 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 | |
Interceptor/curtain drains/foundation drains/drainage ditches | | | | | | |
| Down-gradient2: | 30 ft. | | 5 ft. | | N/A | |
| Up-gradient2: | 10 ft. | | N/A | | 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 | |
Other adjacent soil dispersal components/subsurface stormwater infiltration systems | 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. |
(2) If any condition indicates a greater potential for contamination or pollution, the local 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 feet where the property line, easement line, in-ground swimming pool, or building foundation is up-gradient.
(4) The horizontal separation between an OSS dispersal component and an individual water well, individual spring, or surface water that is not a public water source can be reduced to a minimum of seventy-five feet, by the local 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 hydro-geologic 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 groundwater, 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 WAC
246-272A-0230 Table VI; or
(c) Evidence of protective conditions involving both (a) and (b) of this subsection.
(5) Persons shall design and/or install a soil dispersal component only if:
(a) The slope is less than forty-five percent (twenty-four degrees);
(b) The area is not subject to:
(i) Encroachment by buildings or construction such as placement of 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.
(c) Sufficient reserve area for replacement exists to treat and dispose one hundred percent of the design flow;
(d) The land is stable; and
(e) Surface drainage is directed away from the site.
(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 of the department of ecology's "Criteria For Sewage Works Design," December 1998.