Idaho Administrative Code › Title IDAPA 58 - Environmental Quality, Department of › Rule 58.01.03 - INDIVIDUAL/SUBSURFACE SEWAGE DISPOSAL RULES AND RULES FOR CLEANING OF SEPTIC TANKS › Section 58.01.03.013 - LARGE SOIL ABSORPTION SYSTEM DESIGN AND CONSTRUCTION

Current through August 31, 2023
01.
Site Investigation
. A site
			 investigation for a large soil absorption system by a soil scientist and/or
			 hydrogeologist may be required by the Director for review and approval and
			 shall be coordinated with the Director. Soil and site investigations shall
			 conclude that the effluent will not adversely impact or harm the waters of the
			 State. (3-31-22)
02.
Installation Permit Plans
. Installation permit application plans,
			 as outlined in Subsection
			 005.04, for a large soil
			 absorption system submitted for approval shall include provisions for
			 inspections of the work during construction by the design engineer or his
			 designee and/or by the Director. (3-31-22)
03.
Module Size
. The maximum
			 size of any subsurface sewage disposal module shall be ten thousand (10,000)
			 gallons per day. Developments with greater than ten thousand (10,000) gallons
			 per day flow shall divide the system into absorption modules designed for ten
			 thousand (10,000) gallons per day or less. (3-31-22)
04.
Standard Large Soil Absorption
			 System Design Specifications
. (3-31-22)
a. All design elements and applications rates
			 shall be arrived at by sound engineering practice and shall be provided by a
			 professional engineer licensed by the state of Idaho and specializing in
			 environmental or sanitary engineering. (3-31-22)
b. Within thirty (30) days of system
			 installation completion the design engineer shall provide either as-built plans
			 or a certificate that the system has been installed in substantial compliance
			 with the installation permit application plans. (3-31-22)
c. Effective Soil Depths. Effective soil
			 depths, in feet, below the bottom of the absorption module to the site
			 conditions must be equal to or greater than the following table:
TABLE -- EFFECTIVE SOIL
							 DEPTHS
Site
							 Conditions
Design
Soil
Group
Limiting
							 Layer
A
B
C
Impermeable Layer
8
8
8
Fractured Bedrock, Fissured Bedrock or Extremely
							 Permeable Material
12
8
6
Normal High Groundwater Level
12
8
6
Seasonal High Groundwater Level
2
2
2
(3-31-22)
d.
			 Separation Distances. The disposal area absorption module must be located so
			 that the following separation distances given, in feet, are maintained or
			 exceeded as outlined in the following table:
TABLE -- SEPARATION
							 DISTANCES
Feature of
							 Interest
Design
Soil
Group
A
B
C
All Domestic Water Supplies
Sewage Volume - 2,500-5,000 GPD
250
200
150
Sewage Volume - 5,000-10,000 GPD
300
250
200
Property Lines
Sewage Volume - 2,500-5,000 GPD
50
50
50
Sewage Volume - 5,000-10,000 GPD
75
75
75
Building Foundations -
							 Basements
Sewage Volume - 2,500-5,000 GPD
50
50
50
Sewage Volume - 5,000-10,000 GPD
75
75
75
Downslope Cut or Scarp
Impermeable Layer - Below Base
100
50
50
Separation Distance - Between Modules
12
12
12
(3-31-22)
e.
			 No large soil absorption system shall be installed above a downslope scarp or
			 cut unless it can be demonstrated that the installation will not result in
			 effluent surfacing at the cut or scarp. (3-31-22)
f. A minimum of two (2) disposal systems will
			 be installed, each sized to accept the daily design flow, and a replacement
			 area equal to the size of one (1) disposal system will be reserved.
			 (3-31-22)
g. The vertical and
			 horizontal hydraulic limits of the receiving soils shall be established and
			 flows shall not exceed such limits so as to avoid hydraulically overloading any
			 absorption module and replacement area. (3-31-22)
h. The distribution system must be
			 pressurized with a duplex dosing system. (3-31-22)
i. A geotextile filter fabric shall cover the
			 aggregate. (3-31-22)
j. An in-line
			 effluent filter between an extended treatment system or lagoon system and the
			 large soil absorption area shall be installed. (3-31-22)
k. Observation pipes shall be installed to
			 the bottom of the drainrock throughout the drainfield. (3-31-22)
l. Pneumatic tired machinery travel over the
			 excavated infiltrative surface is prohibited. (3-31-22)
m. The drainfield disposal area shall be
			 constructed to allow for surface drainage and to prevent ponding of surface
			 water. Before the system is put into operation the absorption module disposal
			 area shall be seeded with typical lawn grasses and/or other appropriate shallow
			 rooted vegetation. (3-31-22)