increase the spacing between soil particles so that the soil can absorb
and hold more moisture. This in turn reduces runoff and the
damaging effects of excessive runoff on local streams. The amendment of
soils changes various other physical, chemical and biological
characteristics so that the soils become more effective in maintaining
Wood Treating Site:
On-Site Thermal Desorption of Contaminated Soils,
Final Grading and LeafGro Placement (Source:
amendments help to provide water quality benefits, not only
by increasing the infiltration capacity of the soil, but also
and breaking down potential pollutants.
Immobilizing and degrading pollutants by holding potential
pollutants in place so that soil microbes can decompose them.
the need for fertilizers, pesticides and irrigation by
supplying more nutrients and a slow-release of them to plants.
more rainwater on-site, decreasing runoff, and providing
increased soil moisture and infiltration capacity.
soil stability, leading to less potential erosion.
added protection to groundwater resources, especially from
heavy metal contamination.
thermal pollution by maintaining runoff in the soil and
A study on the
effectiveness of compost amended soils for stormwater management
benefits was undertaken in Richmond, Washington in 1995.2
The objective of the study was to examine the use of compost as an
amendment to increase the water holding capacity, reduce peak flows,
and to decrease phosphorus in both surface runoff and subsurface
flows in the local Alderwood soil series. Seven 8-foot by 32-foot
beds plywood beds were constructed with mixtures of soil and
compost. Surface and subsurface flows were obtained over a
three-month period from March to June, 1995 using a series of seven
simulated rainfall events.
achieved a number of promising results. It was determined that
amendments to previously compacted urban soils not only increased
infiltration rates substantially, but also provided significant
treatment for both surface runoff and local groundwater. Relative
to unamended soils, water conveyed through compost amended soils had
70% less total
soluble reactive Phosphorous
7% less total
In a study
undertaken in Seattle on the use of soil amendments for stormwater
management , compost was found to have a significant sorption and
ion exchange capacity that was responsible for pollutant reductions
in the infiltrating water.3
results of both tests are very promising, they did find that compost
amendments may initially cause increased nutrient discharges into
surface and subsurface waters until the material have become
stabilized. However, related tests in the Seattle area found much
less pronounced degradation with aging of the compost amendments.
Both studies exhibit the positive impacts that soil amendments can
make on the local water quality resources of an area.
One of the
primary objectives of LID site design is to minimize, detain, and
retain post development runoff throughout a site so as to replicate
the site's predevelopment hydrologic functions.4
By restoring or improving the physical and therefore hydrological
characteristics of a soil, that soil can then best be utilized for
stormwater management purposes. Compared to compacted, unamended
soils, amended soils provide greater infiltration and subsurface
storage and thereby help to reduce a site's overall runoff volume,
helping to maintain the predevelopment peak discharge rate and
timing. The volume of runoff that needs to be controlled in order
to replicate natural watershed conditions changes with each site
based on the development's impact on the site's curve number (CN), a
measure of infiltration based upon soil type and landuse. Soil
amendments, with their added storage volumes, are just one LID tool
that can be utilized to help reach this goal.
amendments provide stormwater management, quantity control,
more rainwater on-site, attenuating peak flows and decreasing
maintain base flow to local waterways, especially during dry
increased groundwater recharge through better infiltration and
by maintaining the water on-site longer.
soil structure and stability, while increasing infiltration
capacity and available storage within the soil.
paving and compaction of highly permeable or problem soils
through a site fingerprinting approach.
soil stability, leading to less runoff and erosion through
improved cover conditions.
Washington 1995 stormwater management study also yielded a number of
promising results in regards to the effectiveness of compost amended
soils for stormwater quantity control. It was determined
that soil amendments made on previously compacted urban soils
significantly increased infiltration rates. The following points
summarize some of the findings and conclusions from the study in
regards to the control of stormwater.
test plots the water holding capacity of the soil was doubled with
a 2:1 compost to soil amendment.
for compost amended soils increased by about 65% from those of
unamended soil values.
runoff rates were moderated with the compost amended soils. The
amended soils showed a greater lag time to peak flow at the
initiation of a rainfall event and attributed to an overall
In short, the
results of the study exhibited that compost amended soils
consistently had longer lag times to response, longer times to peak
flow, higher base flow, higher total storage, and smaller total
runoff than unamended soils.2
1 Soils for Salmon, 1999: The
Relationship Between Soil and Water, How Soil Amendments and
Compost Can aid in Salmon Recovery. Seattle, Washington.
Harrison, R.B., M.A. Grey,
C.L., 1997: Field Test of compost Amendment to Reduce Nutrient
Runoff. Prepared for the City of Richmond, College
of Forestry Resources,
University of Washington,
Robert and S. Chen, 1999: Compacted Urban Soils Effects on
Infiltration and Bioretention Stormwater Control Designs.
Department of Civil and Environmental Engineering, The
University of Alabama at
Coffman, L.S., R. Goo and R. Frederick, 1999: Low impact
development an innovative alternative approach to stormwater
management. Proceedings of the 26th Annual Water Resources
Planning and Management Conference ASCE, June 6-9, Tempe,