Transcript Soil Texture and Water

Lecture 2b
Soil Texture, Water & Septic Tanks Drain Fields
• Soil Texture
influences the
water content of
soils.
What happens to water when it is added to the soil?
Soil Permeability –
how we describe
water movement through the soil
• Hydraulic Conductivity – movement of
water through the soil = permeability
• Determines the suitability of the soil
for a septic tank drain field.
Permeability – Water Transmission
• Sandy soils will
have fast water
transmission
and low water
retention
• Clay soil will
have slow water
transmission
and high water
retention.
Porosity and Permeability
• Porosity is a measure of the open
space within soil or rock
• Pore spaces represent the
reservoir for holding water.
• Permeability is a function of the
sizes of particles, pores, and the
way they are arranged.
• Permeability is how quickly water
will flow through the soil
• The straighter and larger the
pores, the faster the
permeability.
• Clays tend to reduce the porosity
and permeability of soil material
due to the small pores.
• Generally, surface horizons have a
larger porosity and subsoils have
smaller porosity and reduced
permeability
Depth to Water Table
• Many landscapes have
soils that are going to
be in contact with the
water table.
• Soils will show “Wet
Soil” characteristics
when they develop in
this type of landscape
position.
• The depth to the
water table is
independent of soil
texture.
The soil here will be
Saturated when the
Water table is high
In areas with high water tables - water in the basement or the pit for
the basement may occur. Determine the oxidation-reduction status of
the soil for clues about this condition.
High chroma (bright) – well drained soil
Low chroma – gray, bluish, or gray green – anaerobic conditions & high
water table is likely
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Basements & Water Tables
• Water in basement : need to provide drainage tile
around the basement next to outer wall, water
collected by sump pump in basement
Basement
gravel
Tile
New House Construction
This
condition
May
eventually
lead to a
wet
basement
Sewage Disposal
• For cities and towns
sewage is collected
and treated in a waste
water treatment plant.
• The US has spent may
billions of dollars
upgrading and building
these facilities to
clean up our surface
waters.
• Some ocean
communities still use
ocean outfalls for
their waste.
Septic Tank Systems
• In rural areas not
serviced by sewer
systems, a method of
disposal of common
household and human
wastes is necessary.
• For these areas an
anaerobic treatment
process called a septic
system is employed.
Septic Systems
The septic system consists of a series
of four steps.
1. Septic Tank (collects the wastes from
the house)
2. Biological process occurring in the
settling tank. (converts solids to
liquids)
3. Dosing to the absorption field.
(effluent is leaked into the soil from
pipes with holes)
4. The absorption field. (soil converts the
waste water to clean water)
• Have you ever been to a place that used a
septic tank drain field for sewage waste?
• How did you know this?
Test to determine soil suitability for a septic tank drain field
• Percolation test :
calculate perc rate
by measuring time it
takes water to drop 1
inch in a pre-wetted
hole = time/drop in
water level = min/inch
• see Minnesota Guide
to onsite sewage
treatment systems
for details.
•
http://septic.umn.edu/publications/index.ht
ml
Soils and the Absorption Field
• Soil Texture - determines
permeability : too slow and field fails,
too fast and ground water polluted
• Depth to water table - less than 3
feet and effluent will pollute
groundwater.
• Depth to bedrock or impermeable
layer - water moves laterally on rock
and is not cleaned
• Slope - steep slopes allow effluent to
seep out hill side.
• Soil organisms decontaminate the
effluent by killing pathogens.
• Nutrients absorbed by soil and used by
organisms and plants growing on the
soil.
Septic Systems
• If the complete septic system is
working correctly the homeowner may
never have to think where the waste is
going.
• However, major failure of the system
could cause major problems and they all
fail with age. (20 to 30 years)
• Too slow percolation will cause the
sewage to back up into the house or
move over the surface of the soil.
• Too fast will allow insufficient time for
microbes in soil to clean water
Septic Systems
• They system can also fail if
the biological organisms
are no longer working to
metabolize the waste.
• Detergents and bleaches in
sufficient quantity can kill
off the organisms.
Septic Tank & Wells
Leaching Fields
• Leaching fields contribute Nitrogen to the surrounding
soil and eventually water table. As more fields are added
to the area, the water table’s Nitrate levels increase.
• This process is faster with soils that have rapid
permeability. The Anoka Sand plain is an example of this
kind of environmental problem.
Alternative Systems
• Traditional septic systems have
proved to be an effective way to
treat sewage when space and good
soil conditions exist.
• However -- When space is limited or
soil conditions are poor, homeowners
may need a modified treatment
system.
• Poor soil conditions include clay
textures, soils that are poorly
drained, soils shallow to bedrock or
soils that are sand and too
permeable.
Mound System – used when soils are not suitable
Mound System for Septic tank Drain field
Other Alternative Systems
a Sand Filter
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http://www.bae.umn.edu/~septic/LCMR/todd.html
http://www.extension.umn.edu/distribution/naturalres
ources/DD7672.html
The homeowners wanted to
try out a new system and
maximize their smaller lot by
avoiding the use of a mound.
A sand filter system was
chosen because of its
reliability, flexibility, and level
profile.
A single-pass sand filter
system pre-treats septic tank
effluent by filtering it
through sand before sending
it to a soil treatment system.
Alternative
to septic
tank
drain fields
- flush
toilets
that don’t
use
water.
Compost Toilets
Sun Mar & Envirolet
• A composting
toilet must
perform 3
completely
separate
processes
• 1) compost waste
and toilet
paper quickly and
odorlessly
• 2) ensure finished
compost
is safe
• 3) Evaporate
Liquid
Composting Units
• Separating solid wastes from the
toilet and delivering them to a
composting unit reduces
household water use up to 40
percent.
• Composting removes many
pathogens while nutrients are
stored in the safe compost which
can be used on the garden.
• Some systems use worms, while
others use bacteria and aeration.
The composted material must be
removed periodically.
• Cost – $1200 -3000