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Location
All buffers can
provide some measure of protection of water bodies if they are sited
between pesticide-treated fields and water. Physical separation
of spraying operations and water reduces the chances for direct
application to water when spray booms overhang water when turning
at field ends and can reduce spray drift into water. However, to
act in trapping pesticides contained in runoff and drift, buffers
must be sited so that water runs over or wind passes through the
buffer area. Often, by the time field runoff reaches stream banks,
concentrated flow is prevalent. Natural berms may develop along
banks preventing overland flow into streams. This phenomenon was
illustrated by a study in Nebraska, where water runoff patterns
were characterized between field edges of watersheds and streams
(Eisenhaurer et al., 1997). In one watershed, about 51% of the area
had runoff pathways that experienced sheet flow, but only 22% of
the area had sheet flow distances greater than 10 feet. Thus buffers
adjacent to water may be limited in their ability to trap pesticides
unless land can be shaped to encourage sheet flow.
Buffers are most
effective in trapping pesticides when located as close to treated
fields as possible. Contour buffer strips and vegetative barriers
are most effective in trapping pesticides because they are located
within fields and are on the contour, thus maximizing sheet flow
across the buffer. Herbaceous wind barriers and cross wind trap
strips perform the same close proximity function for wind eroded
particles that have attached pesticides. Typically the ratio of
runoff source areas to buffers is smaller for this type of buffer
than most edge-of-field buffers, also increasing efficiency of trapping.
Grassed waterways intercept both sheet and concentrated flow from
fields and also have the opportunity to intercept pesticides close
to the source. Wider grass strips encourage more sheet flow and
infiltration as runoff enters the edges of waterways.
Stream networks
are designated by using stream orders. First order streams have
no tributaries. A second-order stream starts at the confluence of
two first-order streams. The confluence of two second-order streams
is a third-order stream and so on. Most experts conclude that stream-side
conservation buffers are most effective on first- and second-order
streams at the "top" of watersheds. The greatest volume
of runoff water and therefore pollutant volume enters most stream
systems from these small streams. Thus, ephemeral as well as first
and second order perennial streams require greater amounts of vegetative
buffer protection. Often there is little "new" water entering
third- and fourth-order streams over banks. Conservation buffers
along these larger streams have other benefits such as wildlife
habitat and streambank protection, but have less opportunity to
intercept pesticides and improve water quality. In watershed planning,
likely sources of pesticides based on cropping patterns can also
be identified in order to prioritize placement of conservation buffers.
Concentrated flow
is the nemesis of pesticide trapping by buffers. Natural berms often
develop along field edges from deposition of sediment. Such berms
become barriers to sheet flow off fields and should be removed when
possible. Land can also be shaped to encourage sheet flow. New techniques
have been developed to disperse concentrated flow. Level spreaders
are constructed to laterally disperse runoff uniformly across a
slope (Figure 1). They consist of a long, narrow trench with an
outlet lip of uniform elevation constructed in stable, undisturbed
soil. The outlet area should have uniform slope and be well vegetated.
Small berms or "water bars" (Figure 2) may be constructed
to break up concentrated flow and redirect it as sheet flow across
buffers. Strategically located vegetative barriers perpendicular
to the flow can serve the same purpose to slow runoff velocity and
redirect runoff across an associated grass buffer as shallow sheet
flow.
How Wide is
Wide Enough?
There is considerable
debate over appropriate widths for conservation buffers. Widths
are defined here as flow length across the buffer. Effectiveness
of buffers per unit area are affected by the flow rate and depth
of runoff as well as by conditions within the buffer such as soil
type and antecedent moisture, which affect water infiltration. Amount
of runoff is affected both by source area size and properties, as
well as rainfall intensity and quantity. Often selecting an appropriate
buffer size may involve consideration of several desired functions,
site conditions, and what is economically or politically practical.
Many studies have
investigated sediment trapping efficiency of grass buffers. Dillaha
et al. (1989) found that 30 foot and 15 foot strips of orchardgrass
trapped 84 and 70% of incoming solids, respectively. The source
area of runoff was 60 feet or 4 times as wide as the 15 foot buffers.
Magette et al. (1989) found that 30 foot and 15 foot strips of fescue
trapped 75% and 52% of incoming solids, respectively. The source
area was 72 feet deep or 4.8 times as wide as the 15 foot buffers.
Castelle et al. (1994) reviewed literature on buffer size requirements
and concluded that a range of buffer widths from 10 to 650 feet
was found to be effective, depending on site-specific conditions.
A buffer width of at least 50 feet was found to be necessary to
protect wetlands and streams under most conditions.
Adequate buffer
widths will depend on field slopes and source areas. A draft NRCS
Conservation Practice Standard for Filter Strips requires a minimum
flow length of 30 feet for the purpose of reducing sediment and
sediment-adsorbed contaminant loadings. It also sets ratios of filter
strip area to field areas based on Universal Soil Loss Equation
R factor values (rainfall amount and intensity) of regions: "The
ratio of the field or disturbed area to the filter strip area shall
be less than 70:1 in regions with USLE R factor values 0-35, 60:1
in regions with USLE R factor values 35-175, and 50:1 in regions
with USLE R factor values of more than 175." Be sure to consult
local NRCS Field Office Technical Guides for filter strip standards,
as these criteria will vary depending on local conditions. Additional
criteria may apply to reduce dissolved contaminants in runoff. The
draft national standard states: "Filter strip flow length required
to reduce dissolved contaminants in runoff shall be based on management
objectives, contaminants of concern, and the volume of runoff from
the filter strip drainage area compared with the filter strip's
area and infiltration capacity."
Several site characteristics
may dictate wider buffers, especially when trying to maximize water
infiltration and trapping of dissolved pesticides. Fine-textured
soils slow water infiltration rates. Or a high water-table underlying
buffers may limit infiltration. Iowa studies found that water infiltration
and trapping of dissolved herbicides by buffers was least effective
when previous rains saturated soils. Vegetation within the buffer
improves surface soil conditions, improving infiltration rates and
internal soil drainage.
NRCS guidelines
use Soil Hydrologic Groups to aid in sizing buffers. Soils are classified
into Hydrologic Groups based on speed of water infiltration and
transmission when soils are wet. Groups A and B have the fastest
infiltration and transmission. Groups C and D have slower infiltration
and transmission. NRCS recommends larger buffer areas for Group
C and D soils.
The specific pesticide
studies reported on in this publication found that buffers as narrow
as 1.6 feet were effective in trapping significant quantities of
pesticides. Increasing buffer width did not always significantly
improve pesticide trapping. Narrow buffers have sometimes been effective
in trapping pesticides. Tingle et al. (1998) compared tall fescue
buffers measuring 1.6, 3.2, 6.6, 9.8, and 13.1 feet wide placed
below 72 foot-long soybean plots. No significant differences in
pesticide trapping efficiencies were found between buffer widths.
Runoff of metribuzin was reduced by at least 73%, and runoff of
metolachlor was reduced by at least 67% by all buffer widths.
While site characteristics,
such as large source areas, or slow permeability soils, may dictate
larger buffers for high efficiency trapping of pesticides, relatively
small buffers should provide significant water quality benefits.
Typical buffer widths of about 50 feet can be effective in reducing
pesticide runoff by 50% or more if sheet flow occurs.
Species Selection
Conservation buffers
can be planted to perennial grasses, legumes and forbs, woody plants,
or a combination of the three. Some annually harvested crops such
as small grains or legume-grass forages can serve the purpose of
buffers, either when planted adjacent to water courses or in strip
cropping systems - alternating strips of row crop and densely planted
crops. In Texas (Hoffman, 1995), wheat was more effective in trapping
herbicides than bermudagrass when planted in contour strips below
a corn field.
Perennial grasses.
Many buffer studies have used common forage grass species such as
bromegrass, orchardgrass, fescue, and bermudagrass. While these
species have performed satisfactorily, researchers are investigating
other species, including native warm season grasses. To date few
studies have compared the effectiveness of grass species in trapping
pesticides. Rankins et al. (1998) compared giant reed (Arundo donax
L.), eastern gammagrass (Tripsacum dactyloides L.), big bluestem
(Andropogon gerardii Vitman), Alamo switchgrass (Panicum virgatum
L.), and tall fescue planted in filter strips below cotton treated
with fluormeturon and norflurazon. All species were similar in effectiveness.
The native warm season grass, switchgrass, was compared to cool
season grasses bromegrass, timothy (Phleum pratense L.), and fescue
in ability to trap sediment and nutrients in Iowa (Lee, 1997). Switchgrass
filter strips removed significantly more sediment, total N, nitrate-N,
total P, and PO4-P than cool season grass filter strips.
Ideally, buffer
grasses should produce dense vegetation with stiff, upright stems
near the ground level. Species that form sods rather than clumps
should provide more uniform coverage. Because increased infiltration
and percolation of water into buffers is an important mechanism
of removal of pesticides, species with deeper rooting patterns may
be more effective. Upright growth and stiff stems can slow velocity
of runoff and increase sediment drop and infiltration. Weak-stemmed
species may be pushed over by runoff, mat on the soil surface, and
decrease infiltration. Because buffers will trap considerable quantities
of sediment, buffer species should be able to tolerate deposition
of sediment over crowns.
Stiff-stemmed grass
species have received recent attention for use as narrow hedges.
Meyer et al. (1995) found a 19 inch wide hedge of switchgrass or
vetiver [Vetiveria zizanioides (L.) Nash.] ponded runoff to a depth
of 10 inches and trapped more than 90% of sediment coarser than
125 mm (fine sands and coarser). Such hedges can be used in contour
strips. Over time trapped sediment will form natural terraces. Or
short hedges could be integrated with other buffers to break up
concentrated flow and direct it across buffers. Vetiver is not winter
hardy, but switchgrass is adapted to both northern and southern
climates and is being widely used as a general conservation buffer
planting, as well as for grass hedge applications. Warm season grasses
such as switchgrass and big bluestem are also tolerant to triazine
herbicides which may be present in field runoff.
Conservation buffer
grass species and varieties will need to be adapted to local conditions.
Check local information sources such as NRCS and Extension before
making selections. These sources can also provide guidance as to
seeding rates and procedures. Some cost sharing programs may also
have specific seeding requirements.
Woody species. Trees
and shrubs can aid in trapping sediment, nutrients, and pesticides,
as well as providing wildlife habitat and streambank protection.
The deep roots of trees also help to intercept subsurface water
flow containing nitrate and introduce organic matter into deep soil,
facilitating denitrification of nitrate and acting as a carbon source
for pesticide-degradiing microorganisms.
Trees and shrubs
are often used in combination with a grass buffer located adjacent
to crop fields. Schultz et al. (1995) describe a 3-zone buffer with
a 23 foot-wide strip of perennial grass (switchgrass preferred)
adjacent to the crop field, two rows of shrubs next downgradient,
and four or five rows of trees adjacent to the stream, for a total
width of 66 feet. Gilliam et al. (1997) describe a 50 foot wide
buffer with half in perennial grass and half forest species. Welsch
(1991) describes a 3-zone riparian buffer where Zone 1 is permanent
woody vegetation immediately adjacent to the stream bank, Zone 2
is managed forest occupying a strip upslope from Zone 1, and Zone
3 is an herbaceous filter strip upslope from Zone 2.
Selection of appropriate
shrubs and trees for these riparian buffers will depend highly on
climate and site conditions, including soil type and depth to water
table, as well as the intended uses (possible harvest), species
of wildlife desired, and tolerance of the vegetation species to
the pesticides contained in the runoff.
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