“Soils as Sponges?” Field Study
Reading #1
Where Does Water Go?
Puddles, streams, and lakes all have something in common. They collect water that
has drained from watersheds. Watersheds are like funnels; they are drainage basins
where surface water runs off and drains into a common collection site. Watersheds
are separated from each other by land forms (ridge lines or maintain divides). Water
falling on each side of the divide drains into different watersheds and collection sites.
Surface runoff flows over a school’s grounds on its way to the collection site (e.g., a
river); therefore, schoolyards are part of a watershed. (Puddles are the collection sites
of mini-watersheds: land surrounding puddles are the mini-drainage basins that
empty into the puddle.) When the puddles overflow or the soil becomes saturated,
water is released.
Often, materials carried by water to the school grounds (e.g., litter, twigs, leaves, oil)
are left behind. Surface water living the school grounds may carry materials to the
collection site of the watershed. These materials include soil, leaves, and twigs: litter;
oil and gasoline from parking lots; and fertilizer from lawns.
As water flows from the school grounds, it combines with runoff from other land areas
within the drainage basin. Materials from these other places are added to the water.
While some substances decompose, settle out, or are filtered by soil, other matter
continues to travel long distances downstream. Organic materials carried by the
water nourish aquatic life. Some substances are toxic, however, and can endanger
organisms consuming or living in the water.
Contaminants whose entry point into the watershed is difficult to locate are classified
as nonpoint source pollutants. Along with residential area, agricultural fields, and
paved parking lots, school grounds can contribute nonpoint source pollutants. The
school yard contributes source pollution when the source of the pollutant can be
traced back to a specific location on the school grounds (e.g., sewer, ditch, pipe).
From Tahoma School District’s Stormwater unit
Developed by Pacific Education Institute, Seattle Public Schools, and Seattle Public Utilities
Adapted from:
http://www.doe.virginia.gov/instruction/science/elementary/lessons_bay/lesson_plans/does_it_soak/index.shtml
“Soils as Sponges?” Field Study
Reading #2
Making Connections: Percolation and Pollution
What does the soil in your schoolyard have to do with pollution in the Puget Sound?
Think back to the stormwater video we saw. You learned that when rain falls, the
stormwater flows over the land picking up chemicals, garbage, pet waste, sediment
and other pollutants. The video illustrated how most of this polluted surface runoff
flows over “impervious surfaces”, such as driveways, roads, and parking lots, and
directly into storm drains that flow into Puget Sound, instead of soaking into the soil.
Also, excessive runoff, especially when it flows at high rates of speed, causes erosion
and flooding. Pervious (or permeable) surfaces are land surfaces that act like a
“sponge” and allow polluted stormwater to soak in (or “percolate”) instead of running
off into storm drains. Further, pervious surfaces allow stormwater to soak in (or
percolate) to slow the flow (speed) of water reducing erosion and flooding. The video
compared grass versus a driveway as an example of pervious and impervious
surfaces.
Some pervious surfaces are better than others at allowing water to percolate or soak
in. In this field study you will perform a percolation test to measure the rate (time) at
which water soaks into the soil. The rate of percolation is determined by how porous a
surface is (for example, sand v. clay), the presence of vegetation, the amount of
organic matter (humus), the amount of rock in the soil, and the amount of compaction
in the soil. All of these factors can affect the ability of soil to act as a sponge for
soaking up polluted stormwater and reducing the speed of water flow. Soil that has
less pore space does not allow water to soak in as easily (less pervious) as soils that
have more pore space and contain a greater amount of organic matter (more
pervious). When water can soak into a surface and travel through the ground slowly,
much of the pollutants are filtered out through the soil. Water that runs off the land
quickly, on the other hand, carries pollutants directly to the waterways.
Review
1. What does impervious mean?
2. Name some examples of impervious surfaces in the city?
3. What is it called when water can soak into the ground?
4. What are 2 ways impervious land surfaces affect stormwater runoff?
Developed by Pacific Education Institute, Seattle Public Schools, and Seattle Public Utilities
From The Relationship Between Soil and Water:
http://www.soilsforsalmon.org/pdf/RelationshipBetweenSoilandWater10-14-99.pdf
5.
“Soils as Sponges?” Field Study
Reading #3
6.
The Importance of Soil and its Effect on Water
Resources
PDF page 5 of “The Relationship Between Soil and Water”
http://www.soilsforsalmon.org/pdf/RelationshipBetweenSoilandWater10-14-99.pdf
Discussion Questions:
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Name 3 functions of soil
Name 3 ways that soil helps the problem of surface runoff.
What is the role of soil in the water cycle?
Why is it important to reduce the amount of runoff?
What is the role of vegetation in surface runoff?
Developed by Pacific Education Institute, Seattle Public Schools, and Seattle Public Utilities
“Soils as Sponges?” Field Study
Reading #4
7.
Healthy Soils and Stormwater Percolation
From Native Soil to Pavement
Native soils and forests of Western Washington store, filter, and slowly
release cool, clean water to streams, wetlands, and the largest estuary on the
west coast—Puget Sound. The rich diversity of life in marine and fresh
water, as well as on land, depends on clean water to thrive.
Native soils and forests are disappearing rapidly in the Puget Sound region.
As the region grows, native forests and soils are replaced with roads,
rooftops and other hard surfaces. Typical development practices remove forests and topsoil,
reducing the land's ability to hold and recycle rainwater. After development, precipitation rushes
swiftly off roofs, roads, and compacted soil. These surfaces are known as "impervious surfaces",
meaning that stormwater is unable to soak in, or absorb, into the ground. When it rains or snows,
more water flows from these surfaces than undisturbed areas, carrying oil, fertilizers, pesticides,
sediment and other pollutants. This rapidly-flowing stormwater causes flooding, erosion and
washes pollution and sediment into storm drains which empty into streams lakes and other bodies
of water, damaging essential habitat for salmon and other aquatic life. You have learned that 75%
of the water pollution in the Seattle area now comes from stormwater. Because salmon and other
fish species rely on clean, fresh water to survive, they equally need healthy soil in the watershed
above them.
How Healthy Soils Can Help!
A healthy soil does a number of important jobs including:
 storing water and nutrients
 controlling the flow of stormwater and allowing it to soak in like a sponge
 trapping and breaking down pollutants in stormwater
Adding decayed organic matter such as compost has the ability to make damaged urban soils
healthy again. Compost is the product resulting from the decomposition of organic waste (such as
yard debris, food waste, soiled paper, wood waste, and manures). Adding compost immediately
reduces soil compaction (compaction meaning squeezed together and hardened) and increases
stormwater percolation rates.
Much like a giant sponge, healthy soil acts as a storehouse for water
and nutrients. The slow release helps plants absorb the correct amount.
As a storage reservoir for both water and nutrients, healthy soil has a
Developed by Pacific Education Institute, Seattle Public Schools, and Seattle Public Utilities
greater holding capacity than soils that lack organic matter and pore spaces.
Compost also feeds and creates habitat for beneficial organisms in the soil – tiny living things such
as worms, insects, bacteria and fungi - restoring the soil. When the soil ecosystem is healthy, soils
begin to function more like native soils again. This complex food web of soil organisms actually
acts as an “environmental protection agent.” The diverse soil life breaks down pesticide and
hydrocarbon pollutants (found in motor oil and plastics), binds heavy metals so they stay in the
soil, and converts excess nutrients from chemical lawn fertilizers into natural organic forms that
can be stored in the soil until they are needed by plants. Breaking down, trapping and converting
pollution in the soil is known as “biofiltration” (meaning “a living filter”) and is what keeps these
pollutants from entering ground and surface waters.
The polluted stormwater flowing off of impervious and into our storm drains is damaging aquatic
ecosystems and harming aquatic life in Puget Sound. Healthy soils catch and hold stormwater and
act as a natural filter to remove pollutants. By constructing gardens with healthy soils and adding
compost to damaged soils around our homes, schools and businesses we can reduce flooding in our
neighborhoods, erosion in streams and even filter pollutants from stormwater before they enter the
Puget Sound and other water bodies.
RESOURCES:
 The Relationship Between Soil and Water: How Soil Amendments and Compost Can Aid in Salmon
Recovery, King County Department of Natural Resources. Fall 1999.

http://www.soilsforsalmon.org/pdf/SoilsforSalmonLIDrev9-16-04.pdf
WSU Extension Rain Garden Handbook
http://county.wsu.edu/mason/nrs/water/Documents/Raingarden_handbook.pdf
Developed by Pacific Education Institute, Seattle Public Schools, and Seattle Public Utilities
The Relationship Between Soil and Water: How Soil Amendments and Compost Can Aid in Salmon Recovery,
King County Department of Natural Resources. Fall 1999.
http://www.soilsforsalmon.org/pdf/SoilsforSalmonLIDrev9-16-04.pdf
Native Forest/Soil
Disturbed Soils after
Development
Developed by Pacific Education Institute, Seattle Public Schools, and Seattle Public Utilities