23

United States

Department of

Agriculture

Natural

Resources

Conservation

Service

Conservation

Buffers to Reduce

Pesticide Losses

March 2000
24

The United States Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the

basis of race, color, national origin, gender, religion, age, disability, political beliefs, sexual orientation, and marital or

family status. (Not all prohibited bases apply to all programs.) Persons with disabilities who require alternate means

for communication of program information (Braille, large print, audiotape, etc.) should contact the USDA’s TARGET

Center at (202) 720-2600 (voice and TDD).

To file a complaint of discrimination, write USDA, Director, Office of Civil Rights, Room 326W, Whitten Building, 14th

and Independence Avenue, SW, Washington, DC 20250-9410, or call (202) 720-5964 (voice or TDD). USDA is an equal

opportunity employer.

Issued March 2000

Conservation Buffers to Reduce Pesticide Losses is a joint product of the United States

Department of Agriculture Natural Resources Conservation Service (USDA-NRCS)

National Water and Climate Center and the United States Environmental Protection

Agency (USEPA) Office of Pesticide Programs. It was conceived by an ad hoc joint

committee formed in 1994. Participants were Joe Bagdon, USDA-NRCS National Water

and Climate Center, Ron Parker, USEPA Office of Pesticide Programs, Tom Gilding,

American Crop Protection Association, Nick Poletika, Dow AgroSciences LLC, and

Dick Fawcett, Fawcett Consulting. The committee would like to express its thanks to

reviewers from the following organizations for comments on and additions to the

publication:

Iowa State University - Department of Agricultural Engineering

Michigan Agricultural Cooperative Marketing Association

New York State Department of Environmental Conservation

Rachel Carson Council, Inc.

United States Geological Survey

USDA Agricultural Research Service

USDA-NRCS Science and Technology Consortium and National Headquarters

USEPA Regional Offices

USEPA Office of Water

Virginia Polytechnic Institute - Dept. of Biological Systems Engineering

The committee hopes that the production of this document will demonstrate the

opportunities for intergovernmental as well as public - private cooperation in furthering

goals of common interest. While all of the above mentioned organizations share the

common goal of reducing pesticide losses, not all items in the publication necessarily

represent the official policy of each organization. The cooperation of the Federal

agencies does not imply endorsement of any commercial product or service mentioned

in this publication.
25

Executive Summary

Conservation buffers are areas or strips of land maintained in

permanent vegetation. These buffers can be used in a systems

approach to manage soil, water, nutrients, and pesticides for sustainable agricultural production, while minimizing environmental impact.

This publication summarizes recent research investigating the ability of

conservation buffers to trap and degrade pesticides carried in field runoff.

It is designed to be used by change agents, such as NRCS field staff,

Extension agents, certified crop advisors, land use planners, and agrichemical

sales and support personnel. When used in conjunction with local and

regional research and information sources, such as the NRCS Field Office

Technical Guide (FOTG), this publication should help planners design,

install, and maintain buffers for optimal pesticide trapping, while gaining

other buffer benefits, such as erosion control, nutrient and sediment trapping, wildlife habitat, and streambank protection.

 Some pesticides are highly adsorbed to soil particles. Because properly

designed buffers are effective in trapping eroded sediment, runoff losses of

this kind of pesticide have been consistently reduced by buffers. However,

a number of modern pesticides are only moderately adsorbed to soil

particles and are carried with runoff from fields primarily in the dissolved

phase. To be effective in trapping this type of pesticide, buffers must slow

runoff and increase infiltration so that pesticides can be trapped and

degraded in buffer soil and vegetation. Many studies have demonstrated

pesticide trapping efficiencies of 50 percent or more for this type of

pesticide, provided that sheet flow, not concentrated flow, occurs. Concentrated flow is the nemesis of pesticide trapping by buffers. Techniques to

encourage shallow sheetflow across buffers are described. Maintenance of

buffers is critical. Trapped sediment can change the profile of buffers,

increasing concentrated flow, and will need to be removed or leveled to

maintain buffer function. Soil conservation practices used above buffers

will prolong their effectiveness. Buffer location, size, and vegetation species

selection are described.

 Conservation buffers are not a substitute for careful pesticide selection

and use. They are a tool to further improve water quality and produce

additional environmental benefits when used along with other practices

described in the text.
26
1

Conservation Buffers to Reduce Pesticide Losses

these pesticides in the same way they trap sediment.

However, some pesticides are only moderately adsorbed

to soil particles and are carried off fields primarily dissolved in water. For buffers to be effective in trapping

this type of pesticide, either water must infiltrate into the

buffer, carrying the chemical into the soil, or chemicals

must be removed from solution flowing over the soil

surface by contact with soil or vegetation. Most studies

indicate that an increase in water infiltration is the most

important factor in trapping these pesticides.

This booklet examines current knowledge of how conservation buffers can be most effectively used to reduce

pesticide losses to water. Studies specifically measuring

pesticide trapping by buffers will be reported, as well as

relevant studies on effectiveness of buffers in trapping

sediment and increasing water infiltration. The effectiveness of buffers in reducing pesticide losses depends on

the properties of the specific pesticide, the design and

maintenance of the buffer, and local climate, weather,

and soil conditions. When combined with specific local

input from such sources as the Natural Resources Conservation Service (NRCS), Soil and Water Conservation

Districts, and the Extension Service, this booklet will

provide guidance to those advising and assisting farmers

and landowners installing conservation buffers. Other

Best Management Practices (BMPs) which improve management and reduce losses of pesticides and can be used

in combination with buffers, will also be described. This

publication does not comprehensively examine buffer

impacts on nutrient losses to water. However, buffer

impacts on nitrate loss are briefly described and contrasted to impacts on pesticide loss.

Literature Reviews

Gilliam, J.W., et al. 1997. Selected agricultural best management practices to control nitrogen in the Neuse River

Basin, North Carolina State University Technical Bulletin 311, Raleigh, NC.

Lowrance, R., et al. 1995. Water quality functions of

riparian forest buffer systems in the Chesapeake Bay

Watershed. U.S. EPA Publication EPA 903-R-95-004.

Bibliographies

Vegetated stream riparian zones: their effect on stream

nutrients, sediments, and toxic substances. Smithsonian

Environmental Research Center, Edgewater, MA. http:/

/www.serc.si.edu/documents/ripzone.html

Introduction

Conservation buffers are areas or strips of land maintained in permanent vegetation to help control pollutants and manage other environmental problems. These

buffers can be used in a systems approach to help

manage soils, water, nutrients, and pesticides for sustainable agricultural production while minimizing environmental impact. Buffers have long been a staple in

conservation systems designed to prevent erosion and

trap sediment and nutrients from field runoff. They also

provide other benefits, such as wildlife habitat improvement, streambank protection, and farming safety. Many

studies have been conducted to document these benefits

and to provide guidance in designing buffers for these

purposes. But do buffers work to reduce pesticide losses?

Rainfall or irrigation can cause pesticides to run off the

surface of treated fields. Edge-of-field losses can range

from less than 1 percent of the amount applied to as

much as 10 percent (Wauchope, 1978; Baker, 1983).

Losses are greatest when severe rainstorms occur soon

after pesticide application. Edge-of-field concentrations

of pesticides in surface runoff can range from less than

1 part per billion (ppb) or 1 microgram per liter (μ/L) to

1 part per million (ppm) or 1 milligram per liter (mg/L) or

more.

Soil erosion runoff

Until recently, few studies had been conducted to measure the effectiveness of buffers in trapping pesticides in

runoff. Physical and chemical properties of pesticides

affect pesticide behavior and transport. Some pesticides

are highly adsorbed to soil particles and are carried

primarily adsorbed to eroded sediment. Buffers trap
2

Function and design of vegetation filter strips: an annotated bibliography. Texas State Soil and Water Conservation Board, Temple, TX. http://waterhome.tamu.edu/

tsswcb/Projects/bibliography/index.html

NRCS Websites

National Handbook of Conservation Practices and Buffer

Job Sheets:

http://www.ncg.nrcs.usda.gov/nhcp_2.html

Natural Resources Conservation Service Homepage:

http://www.nrcs.usda.gov

Types of Buffers

Water buffers within fields

Grassed waterway—a natural or constructed vegetated

channel that is shaped and graded to carry surface water

at a nonerosive velocity to a stable outlet. Because of

concentrated flow that normally occurs in waterways,

sediment trapping and water infiltration can be minimal

with large runoff events, but substantial with smaller

events. Waterways are most effective in trapping sediment and dissolved chemicals when designed to spread

concentrated waterflow over a vegetated filter adjacent

to streams.

Contour buffer strips— strips of perennial vegetation

alternated with wider cultivated strips that are farmed

on the contour. Buffers are most effective in trapping

pesticides when runoff enters uniformly as sheetflow.

Contour buffer strips are one of the most effective

buffers to trap pesticides. There is less chance for

concentrated flow and smaller areas of cultivated field

deliver runoff directly to each strip within a relatively

short distance compared to some edge-of-field buffers.
3

Vegetative barriers—narrow, permanent strips of stiff

stemmed, erect, tall, dense, perennial vegetation established in parallel rows and perpendicular to the dominant slope of the field. These barriers function similar to

contour buffer strips and may be especially effective in

dispersing concentrated flow, thus increasing sediment

trapping and water infiltration.

Edge-of-field

Field borders—a band or strip of perennial vegetation

established on the edge of a cropland field. This buffer

reduces pesticide runoff only when runoff flows over the

strip. Even when no water flows over the strip, some

water quality benefit may be gained because spraying

operations are physically separated from adjacent areas,

reducing drift and direct application to riparian areas.

Filter strips—areas of grass or other permanent

vegetation used to reduce sediment, organics, nutrients,

pesticides, and other contaminants in runoff and to

maintain or improve water quality. Filter strips are

located between crop fields and waterbodies. More

pesticides can be removed by encouraging as much

sheetflow as possible across the strip and minimizing

concentrated flow. This may be accomplished by

combining filter strips with other conservation practices

that control concentrated flow, such as vegetative

barriers, level spreaders, or water bars.

Terrace tile inlet buffer—setbacks surrounding inlets

to tile-outlet terrace systems. Some herbicide labels

describe leaving untreated setbacks around these inlets

when tiles draining terraces outlet directly into streams.

These terraces are designed to cause water to pond over

areas adjacent to inlets following runoff. Therefore,

buffers may not increase sediment trapping or water

infiltration compared to cropped areas, but only reduce

total areas treated with pesticide.
4

Set back

Application area

Setbacks—untreated areas where surface runoff enters

streams. Some herbicide labels describe leaving these

areas untreated. Seeding these areas to perennial grass

improves herbicide trapping compared to trapping with

untreated row crop.

Constructed wetlands

Riparian forest buffer—an area of trees and shrubs

located adjacent to streams, lakes, ponds, and wetlands.

Forest buffers are often combined with perennial grass

buffers. Woody vegetation provides food and cover for

wildlife, helps lower water temperatures by shading the

waterbody, contributes energy sources to aquatic communities, protects streambanks, and slows out-of-bank

flood flows. Deep tree roots may intercept nitrate entering streams in shallow subsurface flow and provide soil

carbon for microbial energy. Microbes can degrade

pesticides and denitrify nitrate.

While pesticides are usually carried to streams in surface runoff, most nitrate is carried to streams in subsurface flow. Subsurface flow may also carry low concentrations of pesticides. Riparian buffers can intercept

shallow subsurface flow and cause either uptake of

nitrate and utilization by plants or encourage denitrification. Drainage tiles bypass buffers and deliver subsurface drainage directly to streams. Wetlands constructed

at tile outlets or as part of a riparian buffer system can

effectively degrade pesticides and denitrify nitrate.

Wind buffers

Windbreak/shelterbelts—plantings of single or multiple rows of trees established for environmental purposes. The primary purpose of such buffers is to protect

leeward areas from troublesome winds. They may also

separate spraying operations from adjacent areas, reduce drift resulting from lowered wind speed, and intercept spray drift. Taller plantings provide the most drift

protection.
5

Soil water

Soil particle

Kd = Conc. pesticide on soil

Conc. pesticide in H2O

P P

P

P

P

P

P

P P

Cross Wind Trap Strips—areas of herbaceous vegetation, resistant to wind erosion, and grown in strips

perpendicular to the prevailing wind direction. These

strips trap wind-borne sediment and nutrients and pesticides carried by sediment.

Pesticide Trapping

Pesticides may be held on the surface of material, such

as soil particles. This phenomenon is called adsorption.

Pesticides may also be taken inside a material, such as

being taken up by a plant. This process is called absorption. The term sorption is sometimes used to refer to

both processes. Pesticide interaction with soil is primarily an adsorption process.

Pesticides vary in how tightly they are adsorbed to soil

particles. Degree of soil binding is measured by binding

coefficients, or K values. Koc (K of organic carbon) is a

measure of adsorption to the organic matter or carbon

content of soil, with higher values indicating more binding. While pesticides are also bound to clay particles,

binding to organic matter is a useful predictor of pesticide behavior and movement in soil. Koc values can be

used to predict whether a specific pesticide will be

carried primarily in the sediment or dissolved phase of

field runoff. Example Koc values for specific pesticides

range from 2 for dicamba (which is held loosely in the

soil) to 1 million for paraquat (which is bound tightly to

soil). Koc values greater than 1,000 indicate that pesticides are highly adsorbed to soil. These pesticides tend

to be carried off fields on eroded soil particles. Thus, if

conservation buffers are effective in trapping the sediment particle sizes that transport the pesticides, they

effectively trap this type of pesticide. Pesticides with

lower Koc values (generally less than 500) tend to move

more in water than on sediment. Concentrations carried

on sediment are higher than concentrations in water,

but because water quantities running off fields are so

much greater than eroded soil quantities, water accounts for the majority of chemicals leaving fields. To

trap low Koc pesticides effectively, buffers need to increase water infiltration and maximize runoff contact

with soil and vegetation that may adsorb pesticides.

Herbaceous Wind Barriers—tall grasses, up to 5 feet,

and other non-woody plants established in 1- to 2-row,

narrow strips spaced across the field perpendicular to

the normal wind direction. These barriers reduce wind

speed and wind erosion and intercept wind-borne soil

particles that may carry pesticides and nutrients.

Other Barriers—other types of perennial vegetation

on the landscape can serve as a buffer. These barriers

include CRP fields, wood lots, terrace back slopes,

ditchbanks, and wildlife plantings.
6

Percent of pesticide trapped

Koc

In contrast to most pesticides, nitrate is water-soluble

and not readily adsorbed by soil particles. Usually nitrate is not in runoff because in enters the soil quickly.

Rather, nitrate that is not taken up by plants may leach

to ground water and be carried to streams by subsurface

flow. (Significant losses of nitrate in surface runoff can

occur in certain situations, such as heavy rainfall after

surface application of nitrogen fertilizer or manure.) To

trap nitrate effectively, roots of conservation buffer

plants need to intercept this subsurface flow. Conditions for denitrification present in this biologically active zone also reduce nitrate reaching streams. Similarly, some weakly adsorbed pesticides may leach to

shallow ground water in small amounts. Although subsurface flow may carry small quantities of pesticides to

streams, quantities present in surface runoff are usually

much greater. The NRCS maintains a current Pesticide

Property data base and can provide Koc values for specific pesticides.

Study Results

One of the earliest studies of the impact of buffers on

pesticide runoff investigated pesticide retention by

grassed waterways (Asmussen, et al., 1977). When runoff from a small plot was directed into an 80-foot-long

grassed waterway, 70 percent of the weakly adsorbed

herbicide, 2,4-D, was trapped. In a similar study (Rohde,

et al., 1980), 96 percent of the strongly adsorbed trifluralin was trapped when the waterway was dry before

runoff, and 86 percent was trapped when the waterway

was wet before runoff occurred.

Hall, et al. (1983) studied the impact of strip cropping on

atrazine runoff in Pennsylvania. Seventy-two-foot-long

plots were constructed up and down a 14 percent slope

with a 19.7-foot-wide area at the base of the slope seeded

to oats (Avena sativa L.). Runoff of atrazine applied to

corn was measured throughout the season, which included a severe, once-in-100-year frequency storm in

June. The oats strip trapped 91 percent of atrazine in

runoff when the herbicide was applied at a rate of 2

pounds per acre.

Results of these early studies surprised some scientists

who assumed that buffers would have minimal impact

on runoff of moderately adsorbed pesticides like atrazine and 2,4-D. However, significant infiltration of runoff water into buffers was identified in these studies as

the primary mechanism of pesticide removal.

In Mississippi, Webster and Shaw (1996) measured runoff of metolachlor and metribuzin from soybean plots

with and without tall fescue (Festuca arundinacea

Schreb.) filter strips. Plots were 13 by 72 feet with 3

percent slope. Filter strips were either 13 or 6.5 feet

wide. Width of the filter strip did not affect herbicide

trapping efficiency. Over 3 years, metolachlor loss was

reduced 55 to 74 percent by the filter strips, compared

to plots without filter strips, while metribuzin loss was

reduced by 50 to 76 percent. Much of the herbicide

trapping could be attributed to infiltration of runoff into

the filter strips. Using similar techniques in a later study

(Rankins, et al., 1998), tall fescue buffers reduced runoff

of fluormeturon and norfluorazon applied to cotton by

at least 60 and 65 percent, respectively.

Several studies in Iowa have investigated herbicide

trapping by smooth bromegrass (Bromus inermis

Leysser) filter strips using either simulated or natural

runoff. In some studies field runoff was simulated by

adding known concentrations of herbicide to water

released above filter strips while rainfall was simulated

(Mickelson and Baker, 1993). Simulated runoff solution

representing runoff from a 150-foot-long area was applied to the top of 15- and 30-foot-long filter strips (thus

representing source area to buffer ratios of 10:1 and 5:1).

The 15-foot strip reduced atrazine runoff by 35 percent,

while the 30-foot strip reduced atrazine runoff by 59

percent.

Other similar studies with smooth bromegrass buffers

(Misra, et al., 1996) showed less difference between

buffer sizes. When comparing 15:1 and 30:1 source area

to buffer ratios, atrazine removal was 31.2 and 26.4

percent, respectively. Herbicide concentrations in simulated runoff were applied at either 0.1 ppm or 1.0 ppm.

A higher percentage of herbicide was trapped by buffers

when inflow had higher concentrations. When inflow

had 1.0 ppm herbicide, 15:1 area ratio buffers trapped

50, 47, and 47 percent of atrazine, metolachlor, and

cyanazine, respectively. When inflow concentrations

were 0.1 ppm, these buffers trapped 31, 32, and 30

percent of atrazine, metolachlor, and cyanazine, respectively. Infiltration of runoff water into buffers accounted

for most herbicide trapping.
7

In other studies, natural runoff from a treated area was

collected and distributed to replicated smooth bromegrass buffers 66 feet long (Arora, et al., 1996). Runoff

was distributed to represent source area to buffer area

ratios of 15:1 and 30:1. Efficiency of herbicide trapping

was determined for six runoff events over a 2-year

period. Trapping of atrazine, cyanazine, and metolachlor

ranged from a low of 8 percent to a high of 100 percent,

depending largely on the timing and intensity of rainfall

and antecedent soil moisture conditions. Herbicide trapping was least efficient when soil was saturated from

previous rains. For most events there were only small

differences in herbicide trapping efficiency between

area ratios. Averaging results for all herbicides and area

ratios over the six events, 62 percent of herbicides

contained in runoff was trapped by buffers. Infiltration

of runoff into buffers was determined to be the major

mechanism of herbicide removal. While sediment retention ranged from 40 to 100 percent, only about 5 percent of

total herbicide retention resulted from sediment trapping.

Analysis of soil within the buffer strips confirmed that

herbicides were being trapped and held by soil within the

strips (Fawcett, et al., 1995). Concentrations declined

during the growing season, presumably because of degradation. No phytotoxicity was observed on buffer grasses.

Bermudagrass (Coynodon dactylon L.) and wheat (Triticum aestivum L.) contour buffers were studied in Texas

(Hoffman, 1995). Three 30-foot-wide buffers were equally

spaced within a 435-foot-wide watershed planted to

corn. Hydrologic data showed that water runoff was

reduced 57 percent by bermudagrass and 50 percent by

wheat. Total atrazine loss was reduced 30 percent by

bermudagrass and 57 percent by wheat in 1 year, and by

44 to 50 percent by all buffers in another year.

Patty, et al. (1997) studied ryegrass (Lolium perenne L.)

buffers in France under natural rainfall conditions. Buffer

widths varied from 20 to 59 feet Runoff volume was

reduced by 43 to 99.9 percent, suspended solids by 87 to

100 percent, lindane losses by 72 to 100 percent, atrazine

and metabolite losses by 44 to 100 percent, isoproturon

losses by 99 percent, and diflufenican losses by 97

percent.

The ability of bermudagrass buffers to trap runoff of turf

pesticides was studied in Oklahoma (Cole, et al., 1997).

Buffers 16 feet wide trapped from 90 to 100 percent of

dicamba, from 89 to 98 percent of 2,4-D, from 89 to 95

percent of mecroprop, and from 62 to 99 percent of

chlorpyrifos in runoff. In most instances buffer mowing

height (3.3 or 9.7 inches), buffer length (7.9 or 16 feet),

and tine aeration did not significantly affect pesticide

trapping efficiency.

The effectiveness of a three-zone riparian buffer in

trapping herbicide runoff was studied in Georgia

(Lowrance, et al., 1997). Total buffer width was 164 feet,

with a 26-foot-wide grass buffer adjacent to the crop

field, a managed pine forest downslope from the grass,

and a narrow hardwood forest containing the stream

channel. More than 90 percent of atrazine and alachlor

in field runoff was trapped by the buffer. Most herbicide

was trapped by the grass strip, with 60 to 70 percent of

herbicide in runoff trapped. Grass was a mixture of

bahiagrass (Paspalum notatum Flugge), bermudagrass,

and perennial ryegrass.

The impact of untreated setbacks around tile inlets in

tile-outlet terrace systems was studied in Iowa, Nebraska, and Missouri (Mickelson, et al., 1998; Franti, et

al., 1998). In all studies setbacks provided no reductions

in herbicide runoff into inlets beyond what would be

expected because of reduced area treated. This result is

not surprising since these terraces are designed to pond

water around inlets, causing sediment to settle and

increasing water infiltration. Much of the untreated

setback area is under water during runoff events and

cannot be expected to increase infiltration or sediment

trapping beyond that caused by normal functioning of

the terrace system. The studies’ authors concluded that

alternative BMPs, such as herbicide incorporation and

no-till production, were more effective in reducing herbicide runoff into inlets. Based on this research, USEPA

allowed label changes on atrazine-and cyanazine-containing products. Three alternative BMPs are now described on these labels for use in tile-outlet terrace

systems: (1) 66-foot untreated setback around inlet, or

(2) incorporation of herbicide in areas draining to inlet,

or (3) no-till production with high crop residue levels in

areas draining to inlet.

The importance of water infiltration as a mechanism of

trapping moderately adsorbed pesticides is illustrated

by some studies that have shown that buffers do little to

reduce concentrations of moderately adsorbed pesticide in runoff. In Nebraska (Yonts, et al., 1996), smooth

bromegrass or intermediate wheatgrass buffers did not

significantly reduce concentrations of alachlor,

cyanazine, 2,4-D, or atrazine present in furrow irrigation

runoff water, although concentrations of the strongly

adsorbed chlorpyrifos were reduced.

In contrast, some studies have shown significant reductions in concentrations of moderately adsorbed pesticides caused by buffers. In Mississippi (Tingle, et al.,

1998), tall fescue buffers as narrow as 1.6 feet at the base

of 72-foot plots reduced concentrations of metolachlor

and metribuzin in runoff by almost 50 percent.
8

Biological and physical conditions, which develop in

buffers planted to grasses and/or trees, favor increased

water infiltration and subsequent attenuation of nutrients and attenuation and degradation of pesticides.

Bharati (1997) compared infiltration and soil properties

under a multispecies riparian buffer in Iowa to adjacent

cultivated fields and a grazed pasture. Average cumulative water infiltration was five times greater under the

buffer than that under the cultivated field and pasture

sites. Soil bulk density was also consistently lower

under the buffer. Wood (1977) compared hydrologic

characteristics of forested land to adjacent sugarcane,

pineapple, or pastureland of the same soil series at 15

sites in the Hawaiian Islands. Infiltration rates were

higher under forest cover at 14 of the 15 sites. Mean

weight diameters of the surface soil aggregates were

larger for forested soils.

The extensive root growth in buffers and superior soil

structure most likely explain observed water infiltration

increases. This root growth also increases biological

activity by supplying an organic carbon energy source to

soil micro-organisms. These micro-organisms in turn

are responsible for degrading pesticides and denitrifying nitrate. Such untilled areas also attenuate atmospheric carbon dioxide through carbon sequestration in

tree and grass vegetation and soil organic matter. Tillage

and crop production often deplete soil organic matter

(Reicosky, et al., 1995), releasing carbon dioxide to the

atmosphere. Increased soil organic matter in buffers

serves to better adsorb pesticides in runoff. Nutrients

are taken up by vegetation and stored in living tissue.

Periodic harvest may be desirable to prevent later release. Pesticides are also taken up by roots and may be

metabolized in plants. In addition, vegetation at the soil

surface adsorbs pesticides during runoff events. In Iowa

(Fawcett, et al., 1995), atrazine concentrations in plant

residue collected in buffers ranged from 80 ppb to 740

ppb, depending on collection date, and were similar to

concentrations found in surface buffer soil.

Few studies have investigated pesticide trapping by

constructed wetlands. Some pesticides are relatively

short-lived in water and will degrade while sequestered

in wetlands, therby reducing contaminants reaching

streams. Wetlands can also serve to attenuate pulses of

concentrated runoff before it enters streams. Some

pesticides are relatively persistent once they reach water. However, the high organic matter content of wetland sediment binds these pesticides, removing them

from water. Matter (1993) used intact freshwater wetland sediment microcosms to study the behavior of

atrazine. Atrazine was removed from the overlying water column at a rate of 15.8 percent per day for 3 days

after introduction. After 10 months, 88 percent of the

applied atrazine was unextractable from sediment and

none was recovered from the overlying water.

Considering buffer research to date, buffers have been

effective under controlled conditions, in trapping highly

adsorbed and moderately adsorbed pesticides. Table 1

summaries buffer studies showing trapping efficiency

for specific pesticides and pesticide Koc values. Highly

adsorbed pesticides were trapped at rates of from 62 to

100 percent. Trapping of moderately adsorbed pesticides was more variable and ranged from 8 to 100

percent. Buffers retained the lowest percent of pesticide when buffer soil was saturated from previous rains.

Many studies found pesticide trapping efficiencies of 50

percent or more.

Do results of these controlled studies predict what will

happen in the real world? Nearly all of these studies (with

the exception of early grassed waterway studies) were

designed to encourage sheetflow across buffers. Therefore, they represent the maximum trapping that can be

expected. In the real world, concentrated flow often occurs across buffers, reducing their effectiveness. To maximize trapping of sediment-adsorbed and dissolved pesticides, sheetflow needs to be encouraged through proper

buffer design and maintenance.

Dillaha, et al. (1989) analyzed 33 existing buffers in

Virginia for sediment trapping efficiency. They found

sediment trapping was often poor because of either

concentrated flow where topography was hilly or sediment that accumulated in the buffer, causing runoff to

flow parallel to the buffer until a low point was reached

where concentrated flow occurred.

Excessive sediment load in runoff may not only change

flow patterns caused by accumulation in buffers, but

may also reduce water infiltration, making buffers less

effective in trapping dissolved pesticides. In an Iowa

simulation study (Misra, 1994), runoff with and without

suspended sediment was introduced into buffers. In

absence of sediment, buffers removed over 80 percent

of atrazine, cyanazine, and metolachlor. When sediment

at 10,000 mg/L was included in runoff, trapping of the

three herbicides fell to about 50 percent. Accumulation

of sediment apparently caused soil surface sealing, reducing total water infiltration from 83 percent, in the

absence of sediment, to 30 percent with sediment. It is

thus critical that soil conservation methods be used

above conservation buffers to reduce the amount of

sediment entering buffers.
9

Pesticide Koc Study reference Percent pesticide trapped

Highly adsorbed pesticides

Chlorpyrifos 6,070 Boyd, et al., 1999 57–79

Cole, et al., 1997 62–99

Diflufenican 1,990 Patty, et al., 1997 97

Lindane 1,100 Patty, et al., 1997 72–100

Trifluralin 8,000 Rhode, et al., 1980 86–96

Moderately adsorbed pesticides

Acetochlor 150 Boyd, et al., 1999 56–67

Alachlor 170 Lowrance, et al., 1997 91

Atrazine 100 Arora, et al., 1996 11–100

Boyd, et al., 1999 52–69

Hall, et al., 1983 91

Hoffman 1995 30–57

Lowrance, et al., 1997 97

Mickelson and Baker 1993 35–60

Misra, et al., 1996 26–50

Patty, et al., 1997 44–100

Cyanazine 190 Arora, et al., 1996 80–100

Misra, et al., 1996 30–47

2,4-D 20 Asmussen, et al., 1977 70

Cole, et al., 1997 89–98

Dicamba 2 Cole, et al., 1997 90–100

Fluormeturon 100 Rankins, et al., 1998 60

Isoproturon 120 Patty, et al., 1997 99

Mecoprop 20 Cole, et al., 1997 89–95

Metolachlor 200 Arora, et al., 1996 16–100

Misra, et al., 1996 32–47

Webster and Shaw 1996 55–74

Tingle, et al., 1998 67–97

Metribuzin 60 Webster and Shaw 1996 50–76

Tingle, et al., 1998 73–97

Norflurazon 600 Rankins, et al., 1998 65

Table 1 Summary of buffer studies measuring trapping efficiencies for specific pesticides. Koc values listed for each pesticide

are from the NRCS Field Office Technical Guide, Section II Pesticide Property data base.
10

Adehesion

Ponded flow

Sediment deposition

Infiltration

Sorption

1

2

3

3

3

3

3

3 3

4 4

4

4

4

5

5

5

5

2

2

2

2 2

2

2

1

1

1

1

1

1

1

1

1 1

Designing Buffers for

Maximum Pesticide Trapping

Efficiency

Location

All buffers can provide some protection of waterbodies

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 where spray booms overhang water when turning

at field ends. It can also reduce spray drift into water.

However, to trap the pesticides in runoff and drift,

buffers must be sited so that water runs over, or wind

passes through, the buffer area. Concentrated flow is

often prevalent by the time field runoff reaches

streambanks. 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 percent of the area

had runoff pathways that experienced sheetflow, but

only 22 percent of the area had sheetflow distances of

more than 10 feet. Thus, buffers adjacent to water may

be limited in their ability to trap pesticides unless land

can be shaped to encourage sheetflow or spreader devices are incorporated into the design.

intercept both sheet and concentrated flow from fields

and also can intercept pesticides close to the source.

Wider grass strips encourage more sheetflow and infiltration as runoff enters the edges of waterways.

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

because they are located within fields and are on the

contour, thus maximizing sheetflow across the buffer.

Herbaceous wind barriers and cross wind trap strips

located close to treated fields trap wind eroded particles

containing adsorbed pesticides. Typically, the ratio of

runoff source areas to buffers is smaller for this type of

buffer than most edge-of-field buffers, which also

increases trapping efficiency. Grassed waterways

Numbering system of stream orders

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 thirdorder stream, and so on. Most experts conclude that

streamside 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, intermittent as well as firstand second-order perennial streams require more vegetative buffer protection. Little “new” water enters thirdand fourth-order streams over banks. Conservation buffers along these larger streams provide 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 can be identified based on cropping patterns. This information can be used to prioritize placement of conservation buffers.
11

Fields

Level spreader

Diversion

berm

Level spreader

Forested Riparian Buffer

Gravel Erosion

fabric

Spreader ditch

Gravel

Design of level spreader used for dispersing agricultural runoff through

a forested riparian buffer

Concentrated flow through a grass buffer

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

by leveling when possible. Land can also be shaped to

encourage broad, shallow sheet flow. New techniques

have been developed to disperse concentrated flow. For

example, level spreaders are constructed to laterally

disperse runoff uniformly across a slope. 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 wellvegetated. Small berms, or “water bars” 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 sheetflow.

Detailed cross-section of level spreader trench for dispersing runoff

along the contour

How wide is wide enough?

Appropriate widths for conservation buffers are debatable. Widths are defined here as flow length across the

buffer. Buffer per unit area is affected by runoff flow rate

and depth as well as by conditions within the buffer, such

as soil type and antecedent moisture, that affect water

infiltration. Amount of runoff is affected by source area

size and properties as well as rainfall intensity and quantity.

Selecting an appropriate buffer size often involves 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- and 15-foot strips of orchardgrass trapped 84 and 70

percent 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- and 15-

foot strips of fescue trapped 75 and 52 percent 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 effective, depending on site-specific conditions. A

buffer width of at least 50 feet was necessary to protect

wetlands and streams under most conditions.
12

Flow

Contributing area

Filter strip length

Filter strip width

Width and Length Used in Conservation Buffers

The width is measured in the direction of flow. Since

conservation buffers are placed along the contour or

perpendicular to the prevailing wind direction as feasible as possible, their direction at the narrow point is

called width. This is analogous to the width of a cultivated area in stripcropping or width of a contour strip.

The flow of water moves parallel with the width. (The

same is the case of other conservation practices, such as

cross wind trap strips, which have movement of the

wind across the width.) The length of a conservation

buffer is the longitudinal distance across the landscape

that the strip occupies perpendicular to the direction of

flow. Other terms, such as flow length, may be used to

depict the direction of flow (see insert figure).

The slope of the buffer and soil in the buffer area impact

the overall buffer performance. Steeper slopes in the

conservation buffer strips increase flow velocity and

shorten the time the contaminant material carried in the

runoff water, both particulate and soluble, has to interact with the vegetation and soil in the buffer area. The

soil is an important parameter in judging buffer effectiveness. Hydrologic soil groups (A, B, C, and D) are

indicative of the infiltration and runoff potential of the

soil. Soil groups A and B have higher infiltration potential; therefore, less runoff than groups C and D. Buffer

strips located on hydrologic soil groups C and D are less

effective in treating run-on than buffer areas on A and B

soils. The soil drainage class also determines the extent

of soil moisture conditions and water storage available

in a soil.

With most conservation buffers, the greater flow length

(width) of buffer area provides the greater entrapment

and removal of contaminants. However, an optimal

length or area is soon reached where further distance

does not result in proportionally greater efficiency. A

buffer strip that achieves 100 percent removal of contaminants or completely reduces the water discharge to

zero would be difficult and impractical to design and

maintain. Most practical designs are based on contaminant removals of at least 50 to 60 percent (up to 80

percent for sediment) and at the same time allow some

discharge at the end of the buffer.

Type and density of vegetation also influence buffer

effectiveness. A large number of vegetative stems (usually greater than 50 per square foot for most grasses) is

required to retard water and wind flows and provide

enough surface area for attachment of contaminate

material passing through the buffer strip. Stems should

stand upright during runoff and wind events.

The width of a conservation buffer strip depends on a

number of factors. The purpose of the buffer strip must

be defined. Buffers to entrap and deposit sediment are

not required to be as wide (only at least 20 feet) as buffers

used to remove soluble compounds, such as nitrate

nitrogen or pesticides (as wide as 100 feet or longer). It

takes more surface area and longer flow paths to adsorb

and infiltrate soluble material than to entrap solid

material. Climate conditions and storm events

anticipated during the expected runoff events influence

the effectiveness of the buffer to retard flow and remove

pollutants. At times, conditions such as frozen and snow

covered soil, saturated soil, and crusted soil surfaces,

severely reduce the function and effectiveness of buffer

strips.
13

Adequate buffer widths 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 area 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 to 35, less than 60:1

in regions with USLE R factor values 35 to 175, and less

than 50:1 in regions with USLE R factor values of more

than 175.” Consult the local NRCS Field Office Technical Guide for filter strip standards because these criteria

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. For example, finetextured soils generally have lower 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.

Narrow buffers have sometimes trapped pesticides effectively. The specific pesticide studies cited in this

publication found that buffers as narrow as 1.6 feet

could be effective in trapping significant quantities of

pesticides. Increasing buffer width did not always significantly improve pesticide trapping. 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

loss of metribuzin was reduced by at least 73 percent, and

runoff loss of metolachlor was reduced at least 67 percent

by all buffer widths.

While site characteristics, such as large source areas or

slow permeability soils, may dictate larger buffers for

high pesticide trapping efficiency, relatively small

buffers should provide significant water quality benefits.

Typical buffer widths of about 50 feet can be effective in

reducing pesticide runoff by at least 50 percent if sheet

flow is maintained, depending on a number of factors as

described previously. Wider buffers may provide

greater protection than narrow buffers in many settings,

but where space or cost considerations limit buffer

widths, a narrow buffer is better than no buffer at all.

For more information

Section IV of the NRCS Field Office Technical Guide in

each state contains Conservation Practice Standards

developed for that state. These state standards are based

on national standards in the NRCS National Handbook of

Conservation Practices. National standards establish

minimum requirements for state standards, which are

specifically tailored to each state's local conditions.

Conservation Practice Standards include a practice

definition, purposes of the practice, conditions where

the practice applies, criteria for applying the practice,

special considerations in applying the practice, practice

plans and specifications, and practice operation and

maintenance requirements. All applicable conservation

buffer practices have standards in the local Field Office

Technical Guide which include required buffer widths.

Information on selecting and sizing buffer practices for

the conservation buffer initiative is available at:

http://www.ftw.nrcs.usda.gov/tpham/buffer/akey.htm

Species selection

Conservation buffers can be planted to perennial

grasses, legumes and forbs, woody plants, or a

combination of the three. When available and able to

perform the desired functions of specific buffer types,

native plant species are preferred. Some annually

harvested crops, such as small grains or legume-grass

forages, can serve the purpose of buffers, either when

planted adjacent to watercourses or in stripcropping

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 warmseason 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.
14

Ground water

Subsurface flow

Surface runoff

ZONE 2

Managed Forest

ZONE 3

Runoff Control

ZONE 1

Undisturbed Forest

Hyporheic Zone

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 coolseason grass filter strips.

Ideally, buffer grasses should

produce dense vegetation

with stiff, upright stems near

ground level. Species that

form sods rather than clumps

provide more uniform coverage. Because increased infiltration and percolation of

water into buffers is an important pesticide removal mechanism, species with deeper rooting patterns may also be

more effective. Upright growth and stiff stems can slow

runoff velocity and increase sediment deposition and infiltration. Weak-stemmed species may be pushed over by

runoff, mat on the soil surface, and decrease infiltration.

Because buffers 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

that 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 percent of

sediment coarser than 125 mm (fine sands and coarser).

Such hedges can be used in contour strips. Over time,

trapped sediment forms natural terraces. Short hedges

can also 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 northern

and southern climates and is widely used for conservation buffer and grass hedge applications. Warm-season

grasses, such as switchgrass and big bluestem, are also

tolerant to triazine herbicides that may be present in field

runoff.

Conservation buffer grass species and varieties should

be adapted to local conditions. Check local information

sources, such as NRCS and Extension, before making

selections. These sources can also recommend seeding

rates and procedures. Some cost sharing programs may

also have specific seeding requirements.

Woody species—Trees and shrubs can also trap sediment, nutrients, and pesticides, as well as provide wildlife

habitat and streambank protection. The deep roots of trees

also help to intercept subsurface water-flow containing

nitrate and introduce organic matter deep into the soil,

facilitating denitrification and acting as a carbon source for

pesticide-degrading micro-organisms.

Trees and shrubs are often used in combination with a

grass buffer located adjacent to crop fields. Schultz, et al.

(1995) describe a three-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 three-zone

riparian buffer, where zone 1 is permanent woody vegetation immediately adjacent to the streambank, zone 2

is managed forest occupying a strip upslope from zone 1,

and zone 3 is an herbaceous filter strip upslope from zone

2. (See figure above.)

Selection of appropriate shrubs and trees for three-zone

riparian buffers depends highly on the following factors:

• climate

• site conditions, including soil type and depth to water

table

• intended uses (possible harvest)

• species of wildlife desired

• tolerance of the vegetation species to the pesticides

contained in the runoff

A three-zone riparian forest buffer
15

For more information contact the local Soil and

Water Conservation District office. Additional

information:

• Banks and Buffers, A Guide to Selecting Native

Plants for Streambanks and Shorelines. Available

on CD-ROM from Tennessee Valley Authority.

Call 423-751-7338.

• Plants for Conservation Buffers. USDA, NRCS.

Economic considerations

When planning installation of a buffer, consider the cost

of installation and maintenance and the economic consequences of taking land out of crop production. Proper

design and installation can increase the expected life

span of a buffer and reduce maintenance costs. Payments from Federal, State, and private sources can help

to offset reductions in income and cover installation

costs. Some states allow a real estate tax exemption for

the area of a farm planted to a permanent conservation

buffer. Some income may be generated from buffers if

haying or tree harvest is allowed, helping to offset loss of

income from the traditional crop.

Cost-sharing programs often have specific design criteria that vary with locality. Local input is essential to

ensure that buffers are properly designed and installed

so that they qualify for payments. Installation costs vary,

depending on species and design. A typical riparian

forest buffer with mixed hardwood seedlings and a

switchgrass strip can cost about $400 per acre to install

(USDA-NRCS, 1999). Cost-share funds contributing 50 to

75 percent or more of the cost of practice installation are

widely available.

Maintenance

Sediment removal—Conservation buffers must be

intensively managed to maintain pesticide-trapping efficiency. Sediment trapped by buffers changes land shape

and may cause runoff to flow parallel to buffers, rather

than across them. Similarly, sediment trapped in the

center of grassed waterways may cause runoff to flow

along the edge of waterways, eroding gullies and increasing concentrated flow. Sediment must be removed

periodically from these areas, and vegetation reestablished when necessary. It is critical that sediment loads

flowing across buffers be limited as much as possible by

soil conservation practices applied to source fields. The

draft NRCS Conservation Practice Standard for Filter

Strips requires that average sheet and rill erosion above

the filter strip be less than 10 tons per acre per year.

Mowing—Buffers may require mowing for weed control or aesthetic reasons. Mowing can both positively

and negatively affect pesticide trapping efficiency. It

can encourage some grass species to tiller and produce

denser vegetation at the soil surface. Mowing too short,

especially with stiff-stemmed species, may reduce the

flow retardance of the vegetation and injure the grass.

Actively growing vegetation is more biologically active,

taking up and degrading pesticides, and supplying carbon for microbial degradation.

Harvest of grass or trees—One function of conservation buffers is to trap nutrients, such as nitrogen and

phosphorus. Periodic harvest of buffer vegetation removes trapped nutrients from the system, preventing

eventual release to the soil and potential movement to

water.

Impact of trapped herbicides—Herbicides trapped

by buffers are degraded in the soil by microbial and

chemical processes. Some herbicide may be taken up by

buffer plants either by roots or through foliage and

metabolized. However, excessive loads of certain
16

herbicides can injure buffer vegetation. Some preemergence herbicides have little impact on established plants.

However, a few, such as the triazines, can injure grasses

if present in high enough concentrations. Most buffer

studies have reported either no injury to buffer grasses,

or only slight injury. In Iowa, bromegrass in buffers grew

most vigorously nearest source areas, apparently because of nutrients trapped by the buffer (Arora, et al.,

1996). Atrazine, cyanazine, and metolachlor trapped by

the buffers did not harm the grass. Grass seedlings are

most sensitive to herbicides in runoff. Thus, the greatest

chance for harmful impact of herbicides in runoff would

occur during buffer establishment. Warm-season grasses,

such as switchgrass, big bluestem, and bermudagrass,

tolerate triazine herbicides well and would not be injured by runoff.

Avoid overspray—While buffers usually tolerate herbicide concentrations in runoff, direct application or

drift of some herbicides can harm grasses and woody

plants. Nonselective herbicides used as burndown treatments in no-till production systems and on some herbicide-tolerant crop varieties can be especially damaging

to buffer vegetation. Care should be taken to turn off

spray booms and use control drip nozzles when driving

over buffers, or if booms extend over buffers when

turning. Squaring up cropland areas by varying buffer

widths along irregular streams or field borders makes

application of herbicides easier, with fewer “point rows”

and chances for overspray over buffers.

Turning on buffers—While buffers at the edge of fields

make convenient turning areas, driving heavy equipment on buffers can cause damage, compacting soil and

reducing water infiltration, and causing ruts when soil is

wet. Ruts may then encourage concentrated flow that

bypasses filtering ability of the buffer. Avoid driving on

buffers as much as possible, especially under wet soil

conditions.

Livestock grazing—Grazing reduces buffer efficiency

by compacting soil and reducing grass heights. Woody

species may also be injured. Livestock can cause significant streambank degradation and directly contaminate

water. Some livestock producers would like to graze

livestock in fields adjacent to buffers for limited times

without having to fence buffers, for example, to allow

gleaning of waste grain following harvest. Plan a grazing

system to allow quick, intensive foraging under good

soil moisture conditions. Remove livestock when soils

are wet to reduce potential damage to buffers.

Weed control—Buffers may harbor weeds requiring

control. Vigorous grass growth prevents growth of many

annual weeds, but some perennial weeds may require

either mowing or spot treatment with herbicides. Noxious weeds must be controlled in the buffer.

Insect concerns—Buffers may harbor insect pests that

move into crop fields. In some cases, such grassy areas

are sprayed with insecticides to prevent damage to

adjacent crops. If necessary, such treatments should be

selected considering potential risks to adjacent aquatic

ecosystems. Buffers also can be a safe harbor for beneficial insects. Populations of these insects can build up

within buffer areas and stay outside the cropland area

treated with insecticides. Tailor vegetation and buffer

maintenance to promote beneficial insect populations

within buffer areas.

Regional considerations in buffer

design and maintenance

Appropriate buffer design depends on many local factors

including climate, soils, hydrology, and farming

practices. Buffer species need to be adapted to the region

and appropriate for other functions, such as wildlife

habitat. Climate can greatly affect buffer function. For

example, winter rains on frozen soil in northern areas

produce runoff that cannot be processed by buffers.

Rainfed agriculture produces different runoff patterns

than agriculture dominated by irrigation. For these

reasons, planners must get local input to benefit from

local experience and research. In addition, specific

buffer design specifications are often required to qualify

for cost-sharing programs.

Local NRCS and soil and water conservation district

offices can provide design specifications for your area.

Extension is also a source of expertise and information.

Many State Extension Services have developed publications on buffer design and maintenance.
17

Example of Extension publications:

• Buffer Strip Design, Establishment, and Maintenance.

Publication Pm-1626b, Iowa State University Extension.

• Landowner’s Guide to Managing Streams in the Eastern United States. Publication 420-141, Virginia Cooperative Extension.

• Vegetative Filter Strips: Application, Installation, and

Maintenance. Publication AEX-467-94, Ohio State

University Extension.

Impact of Buffers on

Leaching of Pesticides and

Nitrate

Because buffers increase water infiltration, concern has

been expressed that leaching of pesticides and nitrate

might be increased, possibly to shallow ground water.

When examining this possibility, consider the properties of pollutants normally present in field runoff. Because nitrate is water soluble and rarely adsorbed to soil

particles, it quickly moves into the soil with rainfall. In

most settings surface runoff contains little nitrate. As

described previously, nitrate is carried to surface water

primarily by subsurface flow. Similarly, weakly adsorbed

pesticides (which would have the greatest leaching risk)

often are not detected at significant concentrations in

runoff, as they quickly move into the soil. Pesticides

detected in runoff are primarily strongly adsorbed compounds attached to suspended sediment and moderately adsorbed compounds both adsorbed to sediment

and dissolved in water.

Strongly adsorbed pesticides have very low leaching

potential because of adsorption to soil. Moderately

adsorbed pesticides can sometimes leach below the

root zone in small concentrations. However, quantities

leached are normally as much as 1,000 times smaller

than quantities carried off fields in surface runoff. Parts

per million concentrations of some products can be

detected in runoff at the field edge, while concentrations detected in shallow ground water are often only a

few parts per billion, if detected at all. Because many

buffers are located near streams, pesticides or nitrate

leaching into buffers would most likely be carried by

subsurface flow to streams. Movement of runoff through

the root zone soil of buffers before discharge to streams

by subsurface flow is much better than allowing surface

runoff to directly enter streams. Pesticides can be

adsorbed and degraded and nitrate taken up by plants or

denitrified within buffers.

Because of relatively low concentrations of pesticide

trapped in buffers, leaching risk from buffers should be

much less than that from source fields. For example, in

an Iowa study, atrazine concentrations in a source corn

field were 4,800 ppb in the surface 2 cm of soil after the

first runoff event of the season (Fawcett, et al., 1995).

Atrazine concentrations in the buffer strip were 750 ppb.

Using BMPs to reduce pesticide runoff from source

fields not only reduces pesticide loads ultimately reaching surface water, but also reduces loads trapped by

buffers. Conservation buffers have been shown to degrade pesticides and to attenuate pesticide concentrations in subsurface water-flow. In Iowa (Schultz, et al.,

1997), atrazine concentrations in soil water 2 feet below

a corn field were 13 ppb. Atrazine concentrations beneath an adjacent grass and woody vegetation buffer

were only 0.2 ppb. In a Georgia study (Lowrance, et al.,

1997), no atrazine was detected in shallow ground water

beneath a 3-zone buffer for the first 2 years of the study.

In the third year, a large rain event soon after herbicide

application resulted in atrazine detection in monitoring

wells 6.6 feet deep. A concentration of 6 ppb was detected at the field edge. At the downslope edge of a 26-

foot-wide grass strip adjacent to the field, atrazine concentrations declined to 2 ppb. At the downslope edge of

the tree strip at the stream edge, atrazine was detected

at only 0.2 ppb.

Considering the relatively small load of pesticide intercepted by buffers compared to that applied to crop

fields, and the adsorption and degradation of pesticides

by soil and vegetation in buffers, increased leaching of

pesticides does not appear to be a significant risk from

conservation buffers.
18

Integrating Buffers with

BMPs

Conservation buffers can trap and degrade part of the

pesticides that run off fields either adsorbed to sediment or dissolved in water. However, buffers seldom

trap all pesticides in runoff. Buffers have been described

as “the last line of defense” or as acting to “polish” runoff

after it has been treated by other practices. Other BMPs

are needed in a systems approach to adequately protect

water quality. Many practices can be used to reduce

offsite movement of pesticides, and can be selected and

integrated into cropping systems where appropriate

and effective. Although not an exhaustive list, some

common pesticide BMPs and descriptions are listed

below.

Integrated Pest Management (IPM)—IPM systems

utilize pesticides in concert with nonchemical pest management techniques. Pest populations are determined

and pesticides or other techniques are used only when

pest populations exceed economic thresholds. The lowest effective pesticide rate is used, and pesticide products are selected to target specific pests and protect

nontarget organisms.

Pesticide selection—Pesticides applied at low rates

reduce amounts available to run-off. Products that are

strongly adsorbed are less likely to move off fields

dissolved in runoff.

Pesticide application timing—Risk of pesticide runoff is greatest when heavy rains closely follow pesticide

application. Avoid applying pesticides if heavy rain is

imminent. Sometimes long-term weather records can

indicate application times when heavy rains are less

likely. Post-emergence applications result in less runoff

than soil applications, as the crop and weeds behave

similar to a buffer, increasing infiltration and pesticide

adsorption by soil and foliage.

Banded application—Application of herbicides in

bands over crop rows, combined with cultivation to control weeds between rows, reduces total amounts of chemical applied compared to broadcast applications.

Soil incorporation—Some herbicides and insecticides

are effective when mechanically incorporated into the soil

(in the case of herbicides) or placed in crop furrows (in the

case of insecticides). Placing some of the applied chemical

below the soil surface protects it from surface runoff.

Because tillage performed to incorporate pesticides buries

surface crop residue, it may increase erosion risk.

Conservation tillage—Surface crop residue reduces erosion and often increases water infiltration, reducing pesticide runoff (Fawcett, et al., 1994). No-till, especially after

having been practiced for several years, has sometimes

dramatically reduced pesticide runoff, although pesticides

intercepted by surface crop residue may be subject to

runoff if heavy rains follow application.

Nutrient management—Supplying the amount, selecting the form, and determining the timing and placement of

crop nutrients provide adequate soil fertility for food, fiber,

and forage production while at the same time protect

against any detrimental environmental risk.

Contour planting—Contour rows reduce erosion, slow

runoff, and increase infiltration. Orientation of rows adjacent to buffers may need to be adjusted to direct runoff as

sheetflow across buffers.

Stripcropping—When strips of densely planted crops,

such as forages or small grains, are alternated with strips

of row crops, the densely planted crop acts as a buffer.

When the strips are planted on the contour, runoff and

erosion are reduced and more runoff water enters the soil.

Crop rotation—Rotation of crops can disrupt life cycles

of insects, diseases, and weeds, and reduce the necessity

for pesticide treatments. Pesticides can be rotated as the

crop is rotated, thus reducing the amount of any one

pesticide used on that field.

Terraces/detention ponds—These structures shorten

slope length, trap sediment, and increase water infiltration,

reducing pesticide runoff.

Irrigation timing—Irrigation after application of soilapplied herbicides moves the chemical into the soil profile,

improving weed control and protecting the chemical from

later rainfall-runoff events. Use of polyacrylamide (PAM),

which reduces irrigation-induced erosion, also improves

water infiltration, thereby reducing pesticide runoff.
19

Irrigation water management—Improved management of the rate and amount of irrigation water can

reduce deep percolation, tailwater, and erosion losses.

Compaction reduction—Correcting compaction problems encourages water infiltration and reduces runoff.

Subsurface drainage—A high water table can result in

excessive runoff and pesticide loss. Improving drainage

increases water infiltration and reduces pesticide runoff. As subsurface drainage carries nitrate to streams,

treatment of tile effluent in a constructed wetland or

buffer area may be desirable.

Summary

Conservation buffers are an effective tool to reduce

pesticides losses to water when used in conjunction

with other BMPs. Pesticide trapping is most efficient

when sheetflow, rather than concentrated flow, occurs

across buffers. Sheetflow can be encouraged by proper

buffer design, including such innovations as level spreaders, water bars, and stiff-grass hedges. As sediment is

trapped, waterflow patterns are changed. Thus, buffer

maintenance is critical. Sediment must be periodically

removed and buffers reshaped to maintain effectiveness. Other soil conservation practices must be used in

conjunction with buffers to prolong the effective buffer

life.

Conservation buffers provide many other benefits, including trapping sediment and nutrients, providing wildlife habitat, protecting streambanks, and increasing farming safety. By varying buffer width along irregular streams

or field borders, the cropped areas can be “squared up,”

reducing sprayer overlaps and making fields more compatible with Global Positioning Systems controls used

in precision farming. Many buffer types can be selected

to match site conditions and desired benefits. Appropriate buffer plant species should be selected to match

local conditions. Research into buffer effectiveness in

pesticide trapping is a relatively new field. As research

continues, buffer designs and maintenance procedures

will undoubtedly be refined to maximize their effectiveness.

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