Limiting and Restrictive Layers - MOWA

2102 – 2013 U of M CE workshops

Always two issues:
Treatment
Hydraulic

Limiting layer
Non-treating layer
Limited treatment layer

▪ Layer in which there is little to no
treatment at or below this layer
▪ Layer used to measure the vertical
separation distance

The zone beneath the infiltrative surface to
the limiting layer is the “treatment zone”

Treatment Zone

Periodically saturated soil
Bedrock
Seven feet from final grade
Height of ten-year flood elevation

"Periodically saturated soil" means the highest
elevation in the soil that is in a reduced
chemical state due to soil pores filled or nearly
filled with water causing anaerobic conditions.
………….. 7080.1100 subpart 59

Topsoil
distinct redox concentrations or depletions
Subsoil

Topsoil
“gray” redox depletions
Subsoil

Topsoil
matrix (background) color chroma of 2 or less (gray)
(likely will also have redox concentrations)
Subsoil

Topsoil
matrix color hue of 5Y chroma of 3 or less (gray)
(likely will also have redox concentrations)
Subsoil

Topsoil
“faint” redox concentrations or depletions in red subsoil
(subsoil matrix hue of 7.5YR or redder)
Subsoil

Topsoil
Black followed by gray
Subsoil

Topsoil
Subsoil

Black color (N 2/0)
Depressional landscape
Peat or Muck soil textures
Redox features
Wet vegetation

Once you reach the periodically saturated soil
level, you are done gaining any additional
vertical separation distance.

Artificial drainage is not allowed to lower the
periodically saturated soil.

Tile Drainage Not Allowed
for New Systems

May be prudent to account for groundwater
mounding for Type 4 system
Reduced separation distance
Heavier loading rate

Type 4 – Groundwater Mounding

The limiting layer is determined when 50% or
more of the volume is rock – either glacial or
true bedrock

Rock fragments – any material over 2 mm in
size (bigger than coarse sand – 12 grit sandpaper).

Two methods
Soil survey and auguring/probing
Dig pit

50% or more
is bedrock

Oxygen is needed for treatment
The oxygen in the soil becomes depleted (for
breakdown BOD and ammonia)
The oxygen gets replaced by exchange with
the atmosphere
The treatment zone then needs to be shallow

The rule has had a maximum depth of 4 feet
for a long time

Oxygen
3 feet
Must be less
than 7 feet

3 feet Must be less
than 7 feet

six inches

Sandy soil with 35% or greater coarse
fragments is equal to a percolation rate of
less than 0.1 MPI
Effluent moves through these soils too fast
for effective treatment to occur

3 feet
treatment zone
Sand with coarse
fragments provides less
treatment
(low surface area and short
residence time).

Any sandy soil layers in which
has 35 to 50% percent coarse
fragments is credited at 50%
treatment value
xx feet

Any soil layers in which has greater than 50%
percent coarse fragments is not credited for
treatment value

% Coarse
Fragments
Nonsandy
Textures
All
sandy
textures
0 - 35% 100% T 100% T
36 - 50% 100% T 50% T
>50% NT NT
100% T = Full Treatment 50% T = ½ Treatment Credit
NT = No Treatment

Example
40 %
Coarse
Fragments
1 foot = 6”
1 foot
1.5 feet
Must be
less than 7
feet

Example
More than
50 % Coarse
Fragments
1 foot = 0”
1 foot
2 feet
Must be
less than 7
feet

What you need:
# 10 Sieve
1,000 ml graduated cylinder
1000 mL or 2000 mL plastic container.
Prefer one that has a 3‐inch diameter
Water

Will not need a scale – will determine by
volume

Step One -
Fill plastic container with 1000 ml of the
sand/gravel sample
1000 ml

Sieve sample (break-up small clumps)
#10 Sieve
Bottom pan

Place the gravel back into the
plastic container.

Fill the graduated cylinder with 1000 mL of
water.
1000 ml

Pour water from the graduated cylinder to
the 1000 ml level of the plastic container.
1000 ml

Record the milliliters of water that remain in
the container.
1000 ml
600 ml

Calculation:
600 / 1000 * 100 = 60% gravel

The exact procedure (with all the fine points)
can be found at:
http://www.septic.umn.edu/prod/groups/cfans/@
pub/@cfans/@ostp/documents/asset/cfans_asset
_403465.pdf

Means different things for different purposes

A layer is the soil which will cause the effluent
to mound and/or move laterally

Change in the amount, size or continuity of
the pores

What are we looking for?
Change in texture
Change in structure
Change in consistence
Change in density

Soil Texture
Approximate
Natural
Recharge to
Groundwater
(ft/year)
Sand 1.0 30
Sandy Loam 0.5 19
Loam 0.4 15
Silt Loam 0.3 12
Recharge to
Groundwater from
absorption area* of
SSTS - based on ½ of
design flow (ft/year)
Clay Loam 0.25 11

However, the daily application rate is fairly
low as compared to the saturated hydraulic
conductivity

Soil Texture
Typical
Saturated
Hydraulic
Conductivity
(in/day)
Effluent Loading
rate from SSTS
- based on ½ of
design flow
(in/day)
Sand 960 1
Sandy Loam 30 0.6
Loam 10 0.5
Silt Loam 10 0.4
Clay Loam 1 0.36

Soil Texture
Typical
Saturated
Hydraulic
Conductivity
(in/day)
Effluent Loading
rate from SSTS
- based on ½ of
design flow
(in/day)
Sand 960 1
Sandy Loam 30 0.6
Loam 10 0.5
Silt Loam 10 0.4
Clay Loam 1 0.36

Sand
• Size on the sand = 1.2 gpd/ft2 =
1 inch/day (using ½ the flow)
• Sized on biomat in contact with
the sand
Clay Loam = can take 1 inch per day
of clean water (or treated effluent)

Wisconsin’s use of contour loading
rate:

Table 1 INFLUENT FLOWS AND LOADS Cont.
Linear loading rate for
systems with native soils
having an effluent
application rate of 0.3
gal/ft 2 /day within 12 inches
of distribution media or
clean sand fill of mounds
4.5gal/ft

1 foot
0.2 gpd/ft2

Wisconsin’s definition of a limiting layer
(=

Texture Structure
Sandy loam or finer
Moderate platy or
massive
Silt, Clay loam, Clay All structures

In addition:
Firm Consistence:
Cemented layers
Hard pans
Dense Layers
Compacted soil

Are there any special design standards that
can be used?
Yes

7080.1720 subpart 6
The loading rate based on an examination of soil
texture, undisturbed soil structure, and soil consistence
at the depth of either the proposed soil absorption area
or the most restrictive layer within three feet of the
proposed soil absorption area …………

Are you required to maintain a vertical
separation distance from a restrictive layer?
No
However check carefully for redox features
as restrictive layers can cause a periodically
saturated soil - which is limiting

3 feet?
Restrictive Layer

Restrictive Layer

Determine loading rate
in upper (good) soil and
use CLR
Restrictive Layer

Determine loading rate on restrictive layer