|
Effectiveness
of Grass Filter Strips
Grass filter strips are effective in removing both
agricultural and urban nonpoint source pollutants.
The following tables below are taken from research conducted by
Leeds
et. al (1994) which examined filter strips and their application, installation
and maintenance. The first table,
Figure 77, summarizes the amount of sediment removed from grass filter strips
versus that of land with no vegetative filter strip.
The overall sediment removed was between 65 and 97 percent with
increasing sediment removal percents as the filter strip width increased.
|
Figure
77: Average percent removal of pollutants in runoff1
|
|
Location
|
Rainfall Source
|
Soil Texture
|
Slope (%)
|
Flow Conditions2
|
Filter Strip Width (feet)
|
Captured
Sediment
|
Percent Removal Nitrogen3
|
Phosphorus
|
|
Indiana
(1979)
|
Rainfall Simulator
|
Silt loam
|
2
|
OLF
|
56
|
-4
|
-
|
-
|
|
4
|
70
|
-
|
-
|
-
|
|
8
|
94
|
-
|
-
|
-
|
|
12
|
95
|
-
|
-
|
-
|
|
Virginia
(1989)
|
Rainfall Simulator
|
Silt loam
|
11-16
|
OLF
|
15
|
70%
|
54%
|
61%
|
|
30
|
84%
|
73%
|
79%
|
|
CF
|
15
|
83%
|
83%
|
85%
|
|
30
|
93%
|
82%
|
87%
|
|
Maryland
(1989)
|
Rainfall Simulator
|
Sandy
loam
|
3-4
|
OLF
|
15
|
66%
|
0%
|
27%
|
|
30
|
83%
|
48%
|
46%
|
|
Iowa5 (1991)
|
Natural Rainfall
|
Silt loam
|
7
|
OLF
|
10
|
72%
|
-
|
-
|
|
20
|
83%
|
-
|
-
|
|
30
|
97%
|
-
|
-
|
|
12
|
OLF
|
10
|
88%
|
-
|
-
|
|
20
|
90%
|
-
|
-
|
|
30
|
96%
|
-
|
-
|
|
Virginia
(1992)
|
Natural Rainfall
|
Silt loam
|
4-12
|
OLF
|
13
|
65%
|
-
|
-
|
|
26
|
65%
|
-
|
-
|
|
Iowa
(1993)
|
Rainfall Simulator
|
Silt loam
|
3-6
|
OLF
|
15
|
72%
|
-
|
-
|
|
30
|
76%
|
-
|
-
|
|
1
Percent removal compared to similar conditions with no vegetative filter
strip.
|
|
2
Characteristics of runoff as it entered filter strip;
OLF – shallow uniform overland flow; CF – concentrated flow.
|
|
3
Values given are for total nitrogen and total phosphorus.
|
|
4
Data not collected in this study.
|
|
5
Demonstration sites were not replicated; sediment removal for 40- and
60-foot wide filters were generally same as for 30-foot width.
|
Adapted from:
Leeds
et. al., 1994.
Vegetative Filter Strips:
Application, Installation and Maintenance.
Ohio
State
University
Extension.
Columbus
,
Ohio
table 1
Additional information was included in the same research on
expected percentage of pollutants removed from grass filter strips in
Illinois,
Iowa
, and
Ohio
(Figure 78). Overall it is
difficult to determine whether the buffer width played a large factor in the
percentage of pollutant removal because other factors such as the filter strip
vegetation quality, slope, and soil texture could all play a determining role as
well. However, as with the research above it is clear that a vegetative buffer
(grass, shrubs, or trees) can remove
large percentages of sediment and pollutants from runoff.
Even a narrow grassed buffer along agricultural ditches can greatly
improve water quality in areas with narrow or no filter strips.
|
Figure
78: Predicted Pollutant Removal in Runoff 1
|
|
State/Site
|
Filter Width (feet)
|
Drainage Area2 (acre)
|
Slope (%)
|
Soil Texture
|
Vegetation Quality3
|
Percent Removal
|
|
Illinois
|
Sediment
|
Phosphorus4
|
Nitrogen
|
|
1
|
99
|
2.5
|
7.3%
|
Silt Loam
|
Fair
|
42%
|
36%
|
36%
|
|
2
|
99
|
28
|
4%
|
Silt Loam
|
Good
|
67%
|
68%
|
68%
|
|
Iowa
|
Sediment
|
Phosphorus4
|
Nitrogen
|
|
1
|
66
|
39
|
2.2%
|
Loam
|
Excellent
|
75%
|
71%
|
71%
|
|
2
|
99
|
30
|
3.8%
|
Silt Loam
|
Fair
|
61%
|
54%
|
54%
|
|
Ohio
|
Sediment
|
Phosphorus4
|
Nitrogen
|
|
1
|
66
|
9.9
|
2.6%
|
Silt Loam
|
Good
|
71%
|
72%
|
72%
|
|
2
|
99
|
32
|
0.6%
|
Silty Clay Loam
|
Good
|
71%
|
73%
|
73%
|
|
1
CREAMS model predictions on CRP sites in
Illinois
,
Iowa
, and
Ohio
; percent removal compared to similar conditions with no filter strip.
|
|
2
Land area draining into filter-strip area.
|
|
3
Poor, Fair, Good and Excellent relate to uniformity of cover and
resistance to flow.
|
|
4
Values are total phosphorus and nitrogen.
|
Source:
Leeds
et. al., 1994.
Vegetative Filter Strips:
Application, Installation and Maintenance.
Ohio
State
University
Extension.
Columbus
,
Ohio
pgs Table 2
  
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