Idaho Admin. Code r. 58.01.03.008

Current through August 31, 2023

Section 58.01.03.008 - STANDARD SUBSURFACE DISPOSAL FACILITY DESIGN AND CONSTRUCTION

01.Standard Drainfield. A drainfield consisting of an effluent sewer, one (1) or more aggregate filled trenches and a gravity flow wastewater distribution system. These standards will be the basis of acceptable design and configuration. Overall dimensions of a specific facility will depend upon site characteristics and the volume of wastewater. (3-31-22)

02.Site Suitability. The area in which a standard drainfield is to be constructed must meet the conditions stated in this subsection: (3-31-22)

a. Slope. The natural slope of the site will not exceed twenty percent (20%). (3-31-22)

b. Soil types. Suitable soil types must be present at depths corresponding with the sidewalls of the proposed drainfield and at depths which will be between the bottom of the proposed drainfield and any limiting soil layer (effective soil depth).

Design Soil Group	Soil Textural Classification	USDA Field Test Textural Classification
Unsuitable	Gravel	10 Mesh
	Coarse Sand	10-35 Mesh	Sand
A	Medium Sand	35-60 Mesh	Sand
	Fine Sand	65-140 Mesh	Sand
	Loamy Sand		Sand
B	Very Fine Sand	140-270 Mesh	Sand
	Sandy Loam		Sandy Loam
	Very Fine Loamy Sand		Sandy Loam
	Loam
	Silt Loam		Silt Loam
C	Silt		Silt Loam
	Clay Loam		Clay Loam
	Sandy Clay Loam		Clay Loam
	Silty Clay Loam		Clay Loam
Unsuitable	Sandy Clay		Clay
	Silty Clay		Clay
	Clay		Clay
	Clay soils with high shrink/swell potential		Clay
	Organic mucks
	Claypan, Duripan,
	Hardpan

(3-31-22)

c. Effective Soil Depths. Effective soil depths, in feet, below the bottom of the drainfield must be equal to or greater than those values listed in the following table.

EFFECTIVE SOIL DEPTHS TABLE
Site Conditions	Design	Soil	Group
Limiting Layer	A	B	C
Impermeable Layer	4	4	4
Fractured Bedrock, Fissured Bedrock or Extremely Permeable Material	6	4	3
Normal High Groundwater Level	6	4	3
Seasonal High Groundwater Level	1	1	1

(3-31-22)

d. Separation Distances. The drainfield must be located so that the separation distances given be maintained or exceeded according to the following Table:

Feature of Interest	Soil Types All	A	B	C
Public Water Supply	100
All Other Domestic Water Supplies including Springs and Suction Lines	100
Water Distribution Lines: Pressure Suction	25 100
Permanent or Intermittent Surface Water other than Irrigation Canals & Ditches		300	200	100
Temporary Surface Water and Irrigation Canals and Ditches	50
Downslope Cut or Scarp: Impermeable Layer Above Base Impermeable Layer Below Base		75 50	50 25	50 25
Building Foundations: Crawl Space or Slab Basement	10 20
Property Line	5

(3-31-22)

03.Subsurface Disposal Facility Sizing. The size of a subsurface disposal system will be determined by the following procedures: (3-31-22)

a. Daily flow estimates should be determined in the same manner as are flow estimates for septic tank sizing in Subsection 007.08. (3-31-22)

b. The total required absorption area is obtained by dividing the estimated daily flow by a value below.

Design Soil Group	A	B	C
Absorption Area - Gallons/Square Foot/Day	1.0	0.5	0.2

(3-31-22)

c. Required Area. The size of an acceptable site must be large enough to construct two (2) complete drainfields in which each are sized to receive one hundred percent (100%) of the design wastewater flow. (3-31-22)

04.Standard Subsurface Disposal Facility Specifications. The following table presents additional design specifications for new subsurface sewage disposal facilities.

SUBSURFACE DISPOSAL FACILITY TABLE
Item	All Soil Groups
Length of Individual Distribution Laterals	100 Feet Maximum
Grade of Distribution Laterals and Trench Bottoms	Level
Width of Trenches	1 Foot Minimum 6 Feet Maximum
Depth of Trenches	2 Feet Minimum 4 Feet Maximum
Total Square Feet of Trench	1500 Sq.ft. Max.
Undisturbed Earth Between Trenches	6 Feet Minimum
Undisturbed Earth Between Septic Tank and Trenches	6 Feet Minimum
Depth of Aggregate:
Total	12 In. Minimum
Over Distribution Laterals	2 In. Minimum
Under Distribution Laterals	6 In. Minimum
Depth of Soil Over Top of Aggregate	12 In. Minimum

(3-31-22)

05.Wastewater Distribution. Systems shall be installed to maintain equal or serial effluent distribution. (3-31-22)

06.Excavation. Trenches will not be excavated during the period of high soil moisture content when that moisture promotes smearing and compaction of the soil. (3-31-22)

07.Soil Barrier. The aggregate will be covered throughout with untreated building paper, a synthetic filter fabric (geotextile), a three (3) inch layer of straw or other acceptable permeable material. (3-31-22)

08.Aggregate. The trench aggregate shall be crushed rock, gravel, or other acceptable, durable and inert material which is, free of fines, and has an effective diameter from one-half (1/2) to two and one-half (2 1/2) inches. (3-31-22)

09.Impermeable Surface Barrier. No treatment area trench or replacement area shall be covered by an impermeable surface barrier, such as tar paper, asphalt or tarmac or be used for parking or driving on or in any way compacted and shall be adequately protected from such activities. (3-31-22)

10.Standard Absorption Bed. Absorption bed disposal facilities may be considered when a site is suitable for a standard subsurface disposal facility except that it is not large enough. (3-31-22)

a. General Requirements. Except as specified in this section, rules and regulations applicable to a standard subsurface disposal system are applicable to an absorption bed facility. (3-31-22)

b. Slope Limitation. Sites with slopes in excess of eight percent (8%) are not suitable for absorption bed facilities. (3-31-22)

c. Vehicular Traffic. Rubber tired vehicles must not be driven on the bottom surface of any bed excavation. (3-31-22)

d. Distribution Lateral Spacing. Distribution laterals within a bed must be spaced on not greater than six (6) feet centers nor may any sidewall be more than three (3) feet from a distribution lateral. (3-31-22)

11.Seepage Pit. Seepage pit disposal facilities may be used on a case by case basis within the boundaries of District Health Department Seven when an applicant can demonstrate to the district director's satisfaction that the soils and depth to ground water are sufficient to prevent ground water contamination. The district director shall document all such cases. (3-31-22)

a. General Requirements. Except as specified in Subsection 008.11.b., rules and regulations applicable to a standard subsurface disposal system are applicable to a seepage pit. (3-31-22)

b. Other conditions for approval, sizing and construction will be as provided for in the seepage pit section of the Technical Guidance Manual for Individual and Subsurface Sewage Disposal, except that the site size restriction in condition two (2) of the Conditions for Approval will not apply. (3-31-22)

12.Failing Subsurface Sewage Disposal System. If the Director determines that the public's health is at risk from a failed septic system and that the replacement of a failing subsurface sewage disposal system cannot meet the current rules and regulations, then the replacement system must meet the intent of the rules and regulations by utilizing a standard subsurface sewage disposal design or alternative system design as specified by the Director. (3-31-22)

Idaho Admin. Code r. 58.01.03.008