Seattle Public Schools, Middle School Science
Investigating Wastewater: Solutions & Pollution (SEPUP) Curriculum Workshop
Reading: Healthy Soils and Stormwater Percolation
Reading
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 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
Seattle Public Schools, Middle School Science
Investigating Wastewater: Solutions & Pollution (SEPUP) Curriculum Workshop
Reading: Healthy Soils and Stormwater Percolation
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.metrokc.gov/dnr/swd/ResRecy/soil4salmon.htm

WSU Extension Rain Garden Handbook
http://county.wsu.edu/mason/nrs/water/Documents/Raingarden_handbook.pdf
Seattle Public Schools, Middle School Science
Investigating Wastewater: Solutions & Pollution (SEPUP) Curriculum Workshop
Reading: Healthy Soils and Stormwater Percolation