§ 4.84.120. OWTS; minimum siting criteria.

Approval of all OWTS shall require compliance with the following minimum siting criteria:

a.

The septic tank, drainfield, and other components of the OWTS shall be located within the boundaries of the parcel upon which the structure requiring the system is built.

b.

Minimum depth of soil beneath the bottom of the proposed dispersal field shall be three (3) feet.

c.

No dispersal of effluent shall be allowed within soil fill. All dispersal systems shall have at least twelve (12) inches of soil cover, except for shallow subsurface drip dispersal systems, which must have at least nine (9) inches of soil cover.

d.

The average soil percolation rate in the proposed dispersal field area shall not be faster than twelve (12) inches per hour (five (5) minutes per inch) nor slower than 0.75 inches per hour (eighty (80) minutes per inch), determined by soil percolation testing in accordance with procedures prescribed by this chapter and the Onsite Systems Manual. Dispersal system areas with percolation rates between 0.75 inches per hour and 0.99 inches per hour must utilize pressure-dosing distribution consistent with article 5 of this chapter.

e.

Underground utility lines, conduits or trenches, including irrigation lines, shall not be installed in or across drainfields, nor shall they be located near drainfields so as to pose a potential preferential pathway for effluent.

f.

Minimum required vertical groundwater separation distance between trench/distribution system bed bottom and groundwater shall be at least five (5) feet. Vertical groundwater separation distance between trench bottom and groundwater may be reduced to three (3) feet by utilizing supplemental treatment or pressure-dosing distribution systems OWTS in compliance with article 5 of this chapter and the Onsite Systems Manual.

g.

OWTS shall not be located in areas subject to flooding, consistent with criteria as prescribed in the Onsite Systems Manual.

h.

OWTS must be located to be easily accessible for maintenance and repairs.

i.

OWTS shall not be located in areas subject to vehicular traffic or other areas subject to significant compaction, including areas of concentrated livestock use. Maximum ground slope in the dispersal field area shall not equal or exceed fifty percent (50%). Where ground slope in the dispersal field area is thirty-five percent (35%) up to fifty percent (50%), pressure-dosing distribution is required. For any site where the ground slope exceeds twenty percent (20%), approval shall be dependent upon completion of a geotechnical report as prescribed in the Onsite Systems Manual.

j.

Minimum horizontal setback distances from various site features to OWTS components shall be as follows:

Site Feature	Minimum Setback Distance (feet)
	To Septic Tank	To Dispersal Field
Building or foundation	5	10
Septic Tank	N/A	10
Property line  From properties served by wells or springs From properties served by approved public water supply	50 10	50 10
Swimming pool or spa	25	25
Road easement, pavement or driveway	5	5
Watercourses  General (from top bank) Between 1,200 to 2,500 feet from a public or state small water system intake Within 1,200 feet downstream from a public or state small water system intake	100 100   100	100  200    400
All wells and domestic/irrigation springs	100	100
Public and state small water system supply wells or springs	150	150
Reservoirs (from high water mark)  General Within 1,200 feet from a public or state small water system intake	200 200	200 400
Groundwater interceptor trench or drain	5	25
Stormwater infiltration trench, gallery or well	25	25
Drainage swale or ditch (from edge of flow path)  General, lined or unlined Evidence of sustained wet conditions or ponding	25 25	25 50
Cuts or embankments (from top of cut)	10	4 x h 2, 3
Steep slopes (lower in elevation than OWTS)  Height of steep slope less than 12 feet Height of steep slope greater than 12 feet	10 10	25 50
Unstable land mass	100	100

 

For areas tributary to and upstream of water supply intake, setback distance is measured from high water mark. Exceptions may be allowed per SWRCB OWTS policy, as follows:

(a)

For replacement OWTS, comply with the maximum extent practicable and incorporate supplemental treatment unless Environmental Health finds no impact or significant threat to water source; and

(b)

For new OWTS on pre-existing lot of record (pre-May 13, 2013), comply to maximum extent practicable and incorporate supplemental treatment for pathogens per article 5 of this chapter and the Onsite Systems Manual.

"h" equals the height of cut or embankment, in feet. The required setback distance shall not be less than twenty-five (25) feet nor more than fifty (50) feet.

Steep slope is considered to be land with a slope of greater than or equal to fifty percent (50%). Where there is clearly discernible embankment and/or break in slope, treat as cut or embankment. Setback of less than fifty (50) feet must be confirmed by appropriate geotechnical evaluation, including but not limited to investigation of slope stability, evidence of rock or impermeable soil layers intersecting slope, seeps, or springs.

Setback distance may be reduced in accordance with recommendations provided in a geotechnical report prepared by a California civil engineer or professional geologist consistent with guidelines prescribed in the Onsite Systems Manual.

k.

Additional OWTS-related setbacks may be included in other County policies or ordinances based on the specific land use, including but not limited to the County Confined Animal Policy and San Mateo County Ordinance Code, chapter 4.68, Wells.

(Ord. No. 04754, § 1, 1-5-2016)