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How
well do lawns filter runoff?
Lawn
Care Links
City
of Plymouth
UM
Extension
Golden
Valley-
What
happens to all that phosphorus ?
2001-2002
Lawn
Fertilizer Experiment
USEPA
Report on
fertilizer use in the US
(PDF file)
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How well
do lawns filter runoff?
Dig deep for the answer
John Barten, Water Quality Mgr. Hennepin Parks.
Written for Focus 10,000-Minnesota's Lakeside Magazine
With the coming
of summer, local, regional and statewide publications have been
printing numerous articles extolling the virtues of lush green lawns
as filters for runoff water, and encouraging the use of fertilizers
and herbicides to grow these lawns. As concerned lake users, we
need to be aware of the limitations of lawns as runoff filters,
and make an informed assessment of what is really needed to maintain
an adequate lawn.
Research done
at Penn State University as well as other places shows that properly
established and managed turf on good soils can filter runoff water.
While this may be true, it is also true that most lawns are not
established on good soils.
During the home
construction process, most lots are repeatedly driven over with
earthmovers, bulldozers, and trucks loaded with concrete and other
building materials. After the building is completed, the compacted
ground is leveled, covered with one to two inches of black dirt,
and then seeded or sodded. In many suburban lots, the ground is
so hard that a sharp metal soil probe cannot be pushed more than
tow or three inches into the lawn. Unfortunately, neither grass
roots nor rainfall can easily penetrate the compacted ground. As
a result of the compaction, the typical residential lawn cannot
filter runoff like the test plots at research facilities.
Compounding
the compaction problem is the fact that turf grasses have a very
shallow rooting depth (two to four inches). Effective infiltration
of rainfall is limited to the top few inches of the turf, severely
limiting the ability of turf to filter runoff. Conversely, native
grasses, trees and shrubs have a rooting depth of two to three feet,
resulting in a much higher rainfall infiltration capacity, and more
effective filtering of runoff. In addition, the taller vegetation
in natural buffer areas and landscape beds slows down raindrops
and runoff water, allowing more time for infiltration. Therefore,
properly established and managed natural buffer areas and landscape
beds have the capacity to infiltrate runoff much more effectively
than turf areas.
Another important
point is that poor turf does not necessarily mean more pollutant
runoff. A study of lawn runoff quality by Hennepin Parks showed
that medium fertility lawns had a significantly higher suspended
solids concentration than high fertility lawns, but medium fertility
lawns had a much lower phosphorus concentration in runoff water.
This suggests that lush lawns are more of a water quality problem
than poorer turf lawns.
The reason that
the lush lawns had such a high phosphorus concentration is runoff
water is that they had phosphorus fertilizer applied to them. Most
lawns in Minnesota (approximately 80 percent) have soils that are
saturated with phosphorus, and do not need any more added to them.
Therefore, the only fertilizer that will not create a water quality
problem is a phosphorus-free product. It does not make any difference
what kind of phosphorous is applied; if your lawn has enough, whatever
is added will just be washed off with rainfall runoff water. Adding
phosphorus fertilizer to a lawn is like pouring coffee into a full
cup. It is going to spill over. The good news is that we do not
have to choose between poor lawns and clean lakes. Studies on golf
courses clearly show that turf does not necessarily have to degrade
water quality. The phosphorus concentration in golf course runoff
water was one-half of the concentration in medium fertility lawn
runoff, and one-fifth that from very high fertility lawns. The difference
is that golf course operators do not apply phosphorus unless a soil
test indicates a need for it. Many golf courses have very high quality
turf, and good runoff water quality. In order to maintain high quality
turf, golf course managers need to aerate the soil regularly to
reduced compaction, over-seed bare or thin turf areas, maintain
adequate organic matter in the soil, and established turf on till
subsoil to encourage a deeper rooting depth. By implementing these
practices, and raising the mower cut height to three inches or higher,
the impact of lawns on water quality can be significantly reduced.
However, the
most important thing you can do for our lakes is to insist on
a true phosphorus free fertilizer product. Remember, as golf
courses have shown, you can have good turf and protect water quality
by using a phosphorus free fertilizer.
Lawn
Runoff Experiment
Purpose
This study has been designed to assess the potential to reduce phosphorus
levels in residential runoff by restricting the use of lawn fertilizer
containing phosphorus.
Planned Monitoring
Period
Spring 2001 - Fall 2002
Study Design
Six small watersheds have been selected as study sites. Three of
these watersheds are located in Plymouth where the use of fertilizers
that contain phosphorus is restricted, and three are located in
Maple Grove, where there are no such restrictions. These watersheds
were carefully selected to include one newly developed area, one
middle-aged, and one older neighborhood within each of the cities.
All of these watersheds are located within 6 miles of each other
to minimize differences in precipitation patterns, soil types, and
aerial loading of phosphorus.
In each of these
watersheds, runoff flows to a single outlet pipe. Automated sampling
equipment was installed at each of these outlet points to collect
detailed flow information and flow-weighted composite water samples.
By determining the mean phosphorus concentration and the total flow
volume for each rain event, we will be able to calculate the total
phosphorus load from each watershed during the monitored period.
If restricting the use of fertilizers containing phosphorus reduces
the amount of phosphorus in residential runoff, we should see significantly
lower phosphorus loading from the Plymouth watersheds when compared
to runoff from the Maple Grove watersheds.
Additional information
has been collected to assess the potential role of other variables
in leading to differences in phosphorus loading between Maple Grove
and Plymouth. Each city has provided lot size information and percent
impervious area for the watersheds, and surveys will be administered
to all homes within each watershed to determine lawn-care practices
including fertilizer application, watering, mowing, and aeration.
The watersheds in Plymouth were surveyed in the spring of 2001,
but Maple Grove residents will not be surveyed until the fall of
2002 to prevent any changes in lawn-care practices during the planned
monitoring period. To maximize the number of Plymouth residents
using phosphorus-free fertilizer, Hennepin Parks has offered a free
bag of phosphorus-free fertilizer to a select group of homes that
indicated that they had been using fertilizer with phosphorus or
that did not respond to the survey.
Equipment
ISCO GLS Automated Water Samplers ISCO 4120 Submerged Probe Flowloggers
ISCO 4150 Area/Velocity Probe Flowloggers
Quality Assurance
and Quality Control
Samples analyzed under same QA/QC protocol as Hennepin Parks lake
samples. Blanks, replicates, duplicates, recovery of known additions,
as recommended in Standard Methods 1020 and 1030
Links
University
of Wisconsin Extension Yardcare
also see Land Owners
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