BEST MANAGEMENT PRACTICES FOR SILVICULTURAL

CHEMICALS AND THE SCIENCE BEHIND THEM

JERRY L. MICHAEL

USDA-FS, Southern Research Station, 520 Devall Drive, Auburn, AL 36849–5418, U.S.A.

(for correspondence, e-mail: michajl@auburn.edu)

Abstract. Silvicultural chemicals include fertilizers and pesticides applied for forest management.

All states east of the Rockies have at least some form of silvicultural chemical best management

practices (SCBMPs) and it is widely accepted that SCBMPs effect some protection of water quality.

All SCBMPs recommend handling and application precautions and a minimum width streamside

management zone (SMZ) on each side of streams for protection of water quality and aquatic ecosystems. Typically these zones increase in width as stream width increases. In areas where cold-water

fish are present, additional widths are designated for their protection. Minimum SMZ widths range

from 8 to 91 meters on each side of the stream bank for intermittent and perennial streams. Most

SMZ recommendations do not cover ephemeral drainages, ditches or canals. SMZs are generally

described as being site specific with width dependent on slope, soil type and other conditions as

well as the stream’s designation as perennial or intermittent. The science behind SCBMPs is often

elusive. Spray drift can be controlled through proper selection and use of application technology

while considering site specific conditions. SMZs greatly reduce the amount of herbicides reaching

streams. Available toxicological data and research on aquatic ecosystem impacts from herbicide use

suggest that additional protection from silvicultural chemicals may not be necessary. However, there

is evidence to suggest that protection of ephemeral and intermittent channels can further reduce entry

of silvicultural chemicals into streams and also reduce sedimentation. This may be the most fruitful

area of research we can pursue in light of the knowledge gaps listed.

Keywords: BMP, ephemeral streams, fertilizers, herbicides, intermittent streams, perennial streams,

silvicultural chemicals, SMZ, forestry

1. Introduction

Forest vegetation management activities significantly alter portions of the forest

ecosystem. Activities that have the greatest impacts are harvest, planting site

preparation, and management of competing vegetation. These activities occur principally at the end of one rotation and at the beginning of a new rotation and can

result in movement of nutrients, pesticides and large amounts of sediment into

streams. Depending on the crop species and the product for which it is grown, a rotation may last 20 to 80 years or more so most of the introduction of pollutants into

streams occurs at the end of one rotation and beginning of another. Vegetation control and fertilization may occur 2–3 times during a rotation. Depending on the tool

used the attendant ecosystem changes can be significant on the temporal and local

 The U.S. Government’s right to retain a non-exclusive, royalty free licence in and to any

copyright is acknowledged.

Water, Air, and Soil Pollution: Focus 4: 95–117, 2004.

© 2004 Kluwer Academic Publishers. Printed in the Netherlands.
96 J. L. MICHAEL

scale. Along with changes in the landscape comes changes in wildlife habitat and

potentially the associated aquatic ecosystem. Most timber producing countries in

the world have instituted forest regulations in an effort to respond to concerns over

potential changes in site productivity, water quality, and to promote sustainability.

Authorizing regulations in the U.S.A. include the Federal Insecticide, Fungicide,

and Rodenticide Act (FIFRA); the National Environmental Policy Act 1969 and as

amended; the Endangered Species Act 1973; and the Clean Water Act of 1977 and

as amended.

There are three inter-related components of water quality protection from silvicultural chemicals in the U.S.A.: 1) Use of Best Management Practices (BMPs) to

protect against water contamination from direct application to water, drift, movement through the soil profile to groundwater, and overland flow; 2) Toxicity which

is a function of frequency and duration of exposure and the dose received at each

exposure; 3) Risk-based assessment to determine what constitutes an unreasonable adverse effect on the environment. The latter is mandated by Congress under

FIFRA § 3(c)(5), (8). Regulatory authority is vested in the individual states and

takes the form of Best Management Practices (BMPs). These BMPs may be voluntary or mandatory, depending on the state, and are developed in each state to help

landowners and forestry practitioners protect water quality.

Water quality includes the physical, chemical and biological attributes of water, but the terminology and measures that have been devised and codified for its

protection are diverse among countries, provinces, and states. Herein the terms

streamside management zone, riparian management zone, special management

zone, streamside reserve, will all be represented by the abbreviation SMZ. An SMZ

is a designated area that consists of the stream and an adjacent area of varying

width where management practices that might impact water quality are modified

or restricted. The standard convention for expressing SMZ width is to indicate the

width on one side of the stream measuring from the streambank. For ephemeral

or intermittent streams with nearly nondefined banks, it is usually expressed from

the middle of the channel. SMZs may include many components depending on

geography, topography, and purpose of the regulation. An SMZ may include such

components as buffers, filter strips, primary zones, secondary zones, stringers, etc.

SMZ width recommended in BMPs may range from no recommendation in

some instances for intermittent and ephemeral streams to more than 90 m for perennial streams. The issues of sedimentation, wildlife habitat and rare and endangered

plant species are very important, but these issues are outside the scope of this paper

which is concerned only with water quality. The purpose of this paper is to review

the silvicultural chemical best management practices (SCBMPs) established by

states east of the Rocky Mountains (excluding ND, SD, KA, OK, MA, and RI).
BMPS FOR SILVICULTURAL CHEMICALS 97

TABLE I

Percent of states east of the Rocky Mountains in U.S.A. (ND, SD, KA, OK, MA,

and RI are not included) recommending minimum SMZ widths indicated (see

Table II for specific information and references)

Minimum width Stream designation

in ft (m) Perennial Intermittent Ephemeral Ditches,

canals, etc.

NSRa 0 12.9 80.6 93.5

Protect b−20 (6.1) 0 6.5 16.1 6.5

21–35 (6.4–10.7) 38.7 45.2 3.2 0

36–50 (11–15.2) 48.4 25.8 0 0

> 50 (15.2) 12.9 9.7 0 0

a No specific SMZ width recommendation given.

b No specific width indicated, but ‘protection’ recommended. Some indicate use

of vegetative filter strip, stringer (a narrow strip of trees left for stabilization of

soil), or treatment as intermittent if ephemeral transitions into intermittent.

2. States’ Recommendations

Silvicultural chemical BMPs apply to fertilizers and pesticides used on forest sites

for management of crop trees. In this context, pesticides and fertilizers are applied

as needed to improve growth and yield and for protection of the crop tree from a

host of pests, including competing vegetation. The use of herbicides in silvicultural activities may occur two or three times in the period of a rotation, typically

20–80 years depending on the crop tree and the product for which it is grown

(lumber, pulp, etc.). BMPs recommended by the various states for protection of

water quality include several levels of specificity based primarily on stream classification, hillside slope, soils, and presence of anadromous fish (Tables I and II).

Most states combine considerations of water quality as affected by use of fertilizers and pesticides into the SCBMP and the most common aspect of that BMP

is the recommendation concerning SMZ width. Other recommendations found in

SCBMPs are largely founded on common sense and the law explicit on manufacturer’s product labels for pesticides. For specific details for individual states, please

consult Table II for references to those states.

Silvicultural chemical BMPs are comprised of general guidelines, voluntary in

some states and mandatory in others. The diversity of land ownerships, values

states’ citizens place on forest land, variety of landowner owner goals and objectives coupled with some federal and state regulations governing water quality

insure a variety of general guidelines. While the states’ guidelines vary slightly in

wording there are some constants that allow for a summary of the states’ BMPs.

Most states have in common the recommendation of specific SMZ widths for the

protection of water quality. The remaining general guidelines fall into several cat-
98 J. L. MICHAEL

TABLE II

Minimum streamside management zone width recommendations by stream classification for individual states east of the Rocky Mountain states in the U.S.A. and the variables used to customize

SMZ width for specific conditions. Some states have no specific recommendations (NSR) in their

manuals while others say protection may be needed. Recommendations for control of spray drift

into SMZs are also included

State Variables a Stream classification Drift Reference

Ephemeral Intermittent Perennial control

ft (m) ft (m) ft (m)

Alabama NSR NSR 35 (10.7) 35 (10.7) Follow label AFC 1999

Arkansas Sl NSR 35 (10.7) 35 (10.7) Follow label ARFC 2002

Connecticut Sl NSR 25 (7.6) 25 (7.6) NSR CRCD 1998

Delaware E, Sl Protect, 50 (15.2) 50 (15.2) Follow label, DDAFS 2002

NSR prevent drift

Florida E, Sl, SW NSR Stringer 35 (10.7) Keep out FDOF 2000

of SMZ

Georgia Sl, Tr 20 (6.1) 20 (6.1) 40 (12.2) Avoid drift GFC 1999

Illinois Sl NSR 25 (7.6) 50 (15.2) Follow label, IDFR 2000

prevent drift

Indiana Sl, SW No spray, 25 (7.6) 50 (15.2) Follow label IDF 1998

NSR

Iowa SW NSR NSR 50 (15.2) Follow label, IDNR not

prevent drift dated

Kentucky Sl NSR 25 (7.6) 25 (7.6) Follow label KDF 2001

Louisana SW Maintain 35 (10.7) 50 (15.2) Follow label LDAF 2001

vegetated

filter strip,

NSR

Maine NSR NSR 75 (22.9) 75 (22.9) Minimize MDEP 2000

drift MBPC not

dated

Maryland NSR NSR 35 (10.7) 35 (10.7) NSR Tjaden and

Weber 1998

Michigan Sl NSR 100 (30.5) 100 (30.5) Follow label, MDEQ not

prevent drift dated

Hausler and

Peterson 2001

Minnesota FT, MS, Sl, NSR NSR 50 (15.2) Minimize MFRC 1999

SW, Tr drift

Mississippi Sl NSR 30 (9.1) 30 (9.1) NSR MFC 2000

Missouri Sl NSR 25 (7.6) 25 (7.6) NSR MDC 2000

Nebraska SW NSR NSR 50 Follow label NFS 1998
BMPS FOR SILVICULTURAL CHEMICALS 99

TABLE II

(Continued)

State Variables a Stream classification Drift Reference

Ephemeral Intermittent Perennial control

ft (m) ft (m) ft (m)

New Hampshire Sl NSR 50 (15.2) 50 (15.2) NSR DRED 2001

New Jersey Sl, E NSR 25 25 Maintain NJBFM 1995

50 ft buffer

for aerial

application

follow label,

avoid drift

New York Sl NSR 50 50 NSR NYS 2000

North Carolina Sl, Tr NSR 50 (15.2) 50 (15.2) Follow label NCDFR 1989

NCDFR 2003

Ohio Sl NSR 25 (7.6) 25 (7.6) NSR ODNR 1992

Pennsylvania Sl NSR NSR 25 NSR PSU 1999

PDEP 1999

South Carolina Sl, Tr Protect, 40 (12.2) 40 (12.2) Follow label, SCFC

NSR avoid drift not dated

Tennessee Sl Protect, 25 (7.6) 25 (7.6) Follow label, TDADF 1993

NSR avoid drift UTAES 1995

TDADF 2000

Texas Sl NSR 50 (15.2) 50 (15.2) Follow label, TFS 2000

avoid drift

Vermont Sl NSR 50 (15.2) 50 (15.2) NSR DPFR 1987

Virginia Sl, Tr NSR 50 (15.2) 50 (15.2) Do not VDF 2002

apply when

wind speed

> 5 mph

West Virginia NSR 25 (7.6) 100 (30.5) 100 (30.5) NSR WVDF 2001

Wisconsin NAV NSR 35 (10.7) 100 (30.5) Follow label, WDNR 2002

prevent drift,

use drift

control agent

a Variables for which states have given in their BMPs specific recommendations for increasing the minimum

recommended SMZ width: E, erodibility; FT, forest type; MS, management system (even-age vs. uneven-age);

NAV, navigability; Sl, slope; SW, stream width; Tr, designation as trout or non-trout stream. Stringer, narrow strip

of trees left for stabilization of soil, no width specified.
100 J. L. MICHAEL

egories: following all product label instructions; disposal of excess chemical and

containers; how, when, and where to apply or not apply; maintenance and service

of application equipment; prevention of direct application to surface water, and

prevention of contamination by drift; and what to do in case of spills. It is not

possible to include each SCBMP general guideline for each state, but they can

be summarized to some extent by sacrificing some details. The summary (with

the states recommending the general guideline in parentheses, see Table II for

individual state BMP manual references) below is generalized in this way and the

reader is directed to seek specific information for any state of interest:

• Follow silvicultural chemical label instructions (AR, DE, GA, IL, IA, LA, MI,

MN, NC, NJ, SC, TN, TX, VA, WI).

• Properly dispose of excess chemicals and empty containers (AR, FL, GA, IA,

LA, MI, NC, SC, VA).

• Apply at appropriate rates (AR, IL, IA, KY, LA, NC, SC).

• Do not apply fertilizer in SMZs (AL, FL, GA, IN, KY, MS, NC).

• Do not handle, mix, or store pesticides or fertilizers in SMZs (GA, IL, IA,

LA, MN, NC, NJ, SC, TX, VA, WI).

• Do not allow silvicultural chemicals to leak from equipment (AR, IL, IA, LA,

WI).

• Do not service application equipment near streams (AR, GA, MI, MS, NC,

TX).

• Minimize use of streams, lakes, ponds or rivers as sources of water for mixing

of silvicultural chemicals (AR, LA, TX, WI).

• Do not apply immediately before precipitation or after if there is still runoff

(AR).

• Clean up and report all chemical spills immediately (GA, IL, IA, LA, MN,

NC, SC, TX, WI).

• Do not use broadcast herbicide applications where excessive runoff will be a

problem (IN, IA, LA).

• Do not broadcast apply pesticides in SMZs (FL, GA, IL, IN, LA, MS, MS,

MO, NC, SC, VA).

• Consider weather conditions, equipment and chemical fomulations to avoid

drift into streams (AR, DE, FL, GA, IL, IN, IA, LA, MI, MN, NC, NJ, SC,

TN, TX, VA, WI).

• Shut off chemical application during turns over open water (LA, TX).

• Do not apply directly to water unless registered for aquatic use (AL, GA, LA,

MN, MS, NC, SC, TN, TX, VA).

The categories above are not mutually exclusive. The first category is follow

label instructions, but most labels have instructions covering the next two, disposal

of containers and following recommended application rates. The categories above

generalize the kinds of recommendations made by the various states in their efforts

to protect water quality.
BMPS FOR SILVICULTURAL CHEMICALS 101

The key to maintaining water quality is protection of streams from direct application of pesticides and fertilizers, principally through the appropriate and judicious

application of SMZs to different classes of streams. Streams may be classified in

many ways, but the simplest classification is in terms of the amount and seasonality of water passing through them. Perennial streams flow most of the year and

include a well defined channel. Intermittent streams also contain a well defined

channel and flow through the year, but flow is more seasonal and usually present

in the wet portions of the year. Ephemeral streams may have poorly- or un-defined

channels and frequently contain decaying and fresh organic matter during portions

of the year, but usually exhibit surface scouring following precipitation events.

Flow in these portions of streams occurs only in response to rainfall or snowmelt.

Ephemeral drains are frequently connected directly to intermittent and perennial

channels such that any sediment, fertilizer or herbicide moving in them flows

directly into intermittent and perennial streams. Ditches and canals are artificial

channels that also contribute to streamflow. They are usually deeply incised into

the topography sharing the common feature of a defined channel with perennial

streams. Ditches and canals may be ephemeral, intermittent or perennial and are

frequently connected to intermittent or perennial streams.

SMZs are universally applied to perennial streams and other bodies of water. SMZs are frequently applied to intermittent streams, but ephemeral streams,

ditches and canals are often left unprotected by SMZs. It is widely recognized

that the maintenance of SMZs protect water quality to varying degrees, and it is

common to recommend greater widths for more protection. In the United States of

America east of the Rocky Mountains, for example, a survey of SMZ widths for

streams of various types shows 80.6% of the states do not recommend protection

of ephemeral drains and 93.5% do not recommend protection of drains, canals and

ditches (Table I).

SMZ width in some states is based on stream width (Table II). Florida’s SMZ

width recommendations for perennial streams located on soils rated as low erodibility and on hillside slope of 2% or less are 35 ft for streams < 20 ft (6.1 m) in

width, 75 ft (22.9 m) for streams 20–40 ft (6.1–12.2 m) in width, and 200 ft (61 m)

for streams > 40 ft in width (FDOF, 2000). Minnesota recommends SMZ widths of

50 ft for streams < 3 ft (0.9 m) in width and 100 ft (30.5 m) for streams > 10 ft (3.0

m) in width (MFRC, 1999) while Louisana recommends 50 ft width for streams

< 20 ft wide and 100 ft for streams >20 ft in width (LDAF, 2001). It is clear that

some states’ SMZ recommendations are based on the assumption that wider SMZs

are more protective of water quality than narrower. However, the ephemeral stream

portion of any river system may be quite large. For example 55% of the total stream

network in the Chattooga River System in the Southern Appalachians is composed

of ephemeral streams while 28% are perennial and 17% are intermittent (Hansen,

2001). Clearly, more than half of that system is ephemeral in nature. Thus, most of

the stream network in the Chattooga River system (and other similar river systems)

is not protected under current SCBMP SMZ width recommendations. Considering
102 J. L. MICHAEL

the variable source-area watershed concept, in which source of runoff expands in

area through an increase in flow from intermittent and ephemeral channels, length

rather than width may be the most important factor for protection against chemical

pollutants and sediment transport. In fact, some states (GA, LA, SC, TN, WV)

recognize that if an ephemeral channel transitions into an intermittent stream there

is the possibility of pollutants moving in that continuum to the intermittent and

subsequently to the perennial streams. These states recommend protection of such

ephemeral streams either as intermittent, through the use of specified SMZ widths,

or by the use of special precautions such as maintenance of filter strips or protection

of litter (Table II).

3. Scientific Basis for Silvicultural Chemical BMPs

Almost all states recommend a minimum SMZ width and many stipulate increases

in width based on such factors and combinations of factors as stream width, slope

of adjacent land, soil erodibility, and whether the affected stream is populated with

cold or warm water fish (Table II). A few states make additional SMZ width recommendations based on whether the stream is navigable, even intermittent streams

may be considered navigable if a canoe can be maneuvered during wet seasons

(WDNR, 2002). Minnesota recommends chemical characteristics (water solubility

greater than 30 mg L−1, Koc less than 300–500, and half-life greater than 21 days)

which should be avoided in order to reduce leaching potential to groundwater and

runoff to surface water (MFRC 1999). These characteristics, should they become

mandatory, would preclude the use of herbicides past and presently registered by

EPA (Table III). The basis for BMP recommendations is sometimes well founded

and at others appears to be more political than scientific. The scientific basis is built

on a consideration of toxicity and of potential for contamination from drift, leaching through the soil profile to groundwater, and storm runoff. A few field studies

have investigated the actual impacts on aquatic ecosystems and have extrapolated

potential adverse impacts on water quality based on observed levels of contamination and toxicological data. The following discussion will focus primarily on

herbicides.

3.1. TOXICITY

There are several terms useful in any discussion of toxicity. The term LD50 is in

popular use and denotes the dose, which kills half the test organisms. While toxicity

testing is not generally conducted on humans, several surrogate mammalian species

are used for tests. However, LD50 is not a very useful concept since it means that

half of the animals die and the other half are probably very sick. A more acceptable

and useful term is the no observable effect level (NOEL, or concentration, NOEC).

The NOEL/NOEC is the highest exposure at which no effect on the test organism is
BMPS FOR SILVICULTURAL CHEMICALS 103

TABLE III

Properties of some silvicultural chemicals. Data for properties are from

Wauchope et al., 1992

.

Pesticide Water solubility Koc Soil half-life

(mg L−1) (days)

Clopyralid 300000 6 40

2,4-D ester 100 100 10

Glyphosate 900000 24000 47

Hexazinone 33000 54 90

Imazapyr 500000 100 90

Metsulfuron 9500 35 30

Picloram 200000 16 90

Sulfometuron 70 78 20

Triclopyr ester 23 780 46

observed. The oral reference dose (RfD) is the total amount of a pesticide that can

be consumed daily over a lifetime without adverse health effects. Calculated from

the lowest animal NOEL, EPA applies an additional safety factor of 100 (10 for

interspecific and 10 for intraspecific variation) to 1000 depending on their estimate

of the reliability of available data. Another measure of relative safety to mammals

is the health advisory level (HAL), the maximum concentration allowable for a

herbicide in drinking water, which if consumed daily over a lifetime would not be

expected to cause adverse health effects. HALs can be approximated by scaling

10% of the RfD up to 70 kg to represent a 70 kg adult (Table IV). Most herbicides

registered for forestry use in the U.S. are classified in Cancer Groups D or E, either

not classifiable as to human carcinogenicity (D) or evidence of noncarcinogenicity

(E) and the risk of developing cancer from exposure to these chemicals is currently

assessed as less than 1 in 100 000. The HALs, RfDs, and NOELs from Table IV can

be used to evaluate potential toxicity from herbicide values observed in water. It

should be remembered that every chemical, natural or synthetic, including water is

toxic. A recent incident in Springville, Utah points out all too vividly how toxicity

of even the most innocuous compounds may lead to death when sufficient exposure

occurs (ABCNews, 2002). In this incident, a four-year-old child died after drinking

large quantities of water. The mode of action in this case is believed to be the

lowering of sodium concentration in the child’s blood. Decreased blood sodium

levels triggered brain swelling and caused the child’s death. The amount and route

of exposure determine whether organisms will be adversely affected by toxicity.

Even the most toxic compounds can be relatively innocuous when exposures are

sufficiently low.
104 J. L. MICHAEL

TABLE IV

Toxicity of some herbicides. From U.S. Environmental Protection Agency sources (http://www.

epa.gov/waterscience/drinking/standards/dwstandards.pdf) and Office of Drinking Water. Cancer

Groups D, not classifiable as to human carcinogenicity; E, evidence of noncarcinogenicity

Herbicide Cancer HAL a Oral RfD Mammalian Aquatic Aquatic

group (mg/L) (mg/kg/day) NOEL NOEC NOEC

(mg/kg/day) (mg/L) references

2,4-D (acid) D 0.07 0.01 1–37 2–110 USDA, 1984

Glyphosate D 0.7 0.1 10–500 25–50 USEPA, 1993

Hexazinone D 0.4 0.05 10–250 10– > 80 USEPA, 1994

Imazapyr E [17] 2.5 300–10000 0.024– > 100 SERA, 1999

Metsulfuron D [1.75] 0.25 25–700 > 150 PIP 1993

Sulfometuron D [0.14] 0.02 50 > 1.2 SERA, 1998

Triclopyr D [.35] 0.05 2.5–240 NA USEPA, 1998

a Brackets enclose an approximated HAL = (0.1∗RfD)∗(70).

NA, not available for triclopyr acid; triclopyr triethylamine salt (Garlon 3A) has NOECs ranging from

80.7 to 104 mg/L for aquatic organisms and is categorized as practically nontoxic to slightly toxic;

aquatic organism NOECs not available for triclopyr butoxyethyl ester (Garlon 4E) which is rated as

moderately to highly toxic for these organisms.

3.2. DRIFT DURING APPLICATION

Spray drift is the movement of spray droplets or vapors from the target area to

areas where application was not intended and can contaminate streams. Spray drift

occurs when droplets formed by the application nozzles move with a trajectory

different than straight down. Spray nozzles are designed to maximize the amount of

spray that actually hits its target and save on application costs; however, under the

right conditions very small droplets may move a mile or more off site. Herbicide

granules or dried particles of herbicides may move from the target area in high

winds but are not considered important sources of herbicide drift. Low levels of

drift may pose no environmental hazard.

Spray drift is one of the components of stream contamination for which we

have a solid scientific background and understanding. We can come very close,

with simple mathmatical calculations, to determining how far a droplet of known

mass will travel under specific conditions of speed and height of application and

therefore how wide an SMZ will be required to minimize contamination of streams.

There are some very sophisticated models like AgDRIFT, FSCBG, and AGDISP

(FEMVTF, 2002), which give an even better estimate of required SMZ width

(Teske and Ice, 2002; Payne et al., 1988). These models take into consideration

spray nozzle drop distribution patterns which describe the mass of any spray pattern

that occurs in drops of various particle sizes, wind speed and direction, height of
BMPS FOR SILVICULTURAL CHEMICALS 105

application and SMZ porosity to estimate the amount of contamination that may

occur from specific applications. This information, when used in conjunction with

toxicological data for aquatic organisms, allows one to calculate the SMZ width

needed to afford a specified level of protection to organisms in streams and to

define the conditions under which spraying can be allowed.

Payne and others (1988, 1990) have used the modeling approach to calculate

SMZ width. Payne et al., (1988) determined that 65.6 ft (20 m) was sufficient

to protect streams and limit mortality of Aedes aegypti (yellow fever mosquito)

to less than 10% from an application of permethrin at the rate of 0.035 kg ha−1.

The species they were most interested in protecting was trout, a species relatively

tolerant of permethrin. Aedes aegypti was selected as the indicator species in this

study because it was considered the most sensitive species in the stream. In a study

on glyphosate, Payne et al., (1990) set the toxicological endpoint conservatively for

trout protection from glyphosate at 1 mg L−1 even though the 96-hr LC50 was 52–

55 mg L−1 and determined the required SMZ width was 82 ft (25 m). Using EPA’s

level-of-concern approach, 5 mg L−1 may have been a more reasonable end-point

and it would have resulted in a much narrower SMZ. Hildebrand et al. (1980) also

demonstrated in field tests that peak glyphosate stream concentrations of 249–480

mg L−1 had no effect on rainbow trout mortality and resulted in 90% survival of

Daphnia magna. This value is greater than EPA’s value of 25 mg L−1 for trout

NOEC. It is clear that the toxicological endpoint used to determine acceptable

water concentrations greatly affects width of SMZ. The important point here is that

one must select the species or organisms to be protected, establish concentrations

that will effect that protection, and then calculate the required SMZ width. For very

precise calculations of SMZ width for adequate drift protection of stream ecosystems, good toxicological data and knowledge of the composition of the aquatic

communities within the stream are necessary.

3.3. LEACHING THROUGH THE SOIL PROFILE

Herbicide movement through soil, except in the case of macropore transport, is

relatively slow and most forest herbicides have not been detected at great depths in

the soil profile. Glyphosate is almost immobile in soil and is not thought to move

through the soil below 15 cm (EPA, 1993). Triclopyr has been found to move to

depths of 10–30 cm (Lee et al., 1985, Stephenson et al., 1990). Hexazinone, perhaps the most studied non-phenoxy forestry herbicide, has been detected at depths

down to 75 cm, but more commonly is not detected below about 50 cm (Roy et

al., 1989, Feng and Navratil 1990, Allender 1991, Michael et al., 1999). Imazapyr

is rarely detected at 50 cm, but is known to move below 30 cm in several soil

types (Rahman et al., 1993). Similarly, the sulfonyl ureas (sulfometuron and metsulfuron) are known to move to 50 cm, but are not reported to move significantly

below 70 cm (Walker and Welch, 1989, Lym and Swenson, 1991). In the process
106 J. L. MICHAEL

of movement through the soil profile, herbicides are acted upon by a variety of

degradative and dilution processes.

Biological and chemical degradation play a large role in reduction of herbicide movement through soil profiles with the end result being destruction of the

herbicide molecule. The residence time of silvicultural chemicals in a given layer

of soil (e.g., 0–15 cm depth) is described in terms of half-life or the time required for dissipation of half of the beginning concentration (Michael and Neary,

1993). Half-lives of some silvicultural chemicals currently in use are presented

in Table III. Advective dispersion also contributes to the chemical and biological

degradation by slowing the movement of herbicide through the soil profile. This

form of dispersion is like that observed in chromatographic systems in which the

solute front is partially retarded by interaction with the chromatographic medium,

soil in this case. The retardation of movement both dilutes the front (resulting in

reduced concentrations observable at greater soil depths) and retains the herbicide

in the root zone for a longer period of time during which degradation can occur.

Soils with more organic matter and clay provide for greater advective dispersion of

nonpolar pesticides. This process and the attendant lack of significant movement

through the soil profile preclude significant groundwater contamination by forestry

herbicides (Michael, 2000).

The greater the capacity for water (and therefore soluble herbicides) movement

through the soil profile, the lower is the potential for overland flow. Overland flow

occuring shortly after herbicide application potentially contains very high concentrations of herbicide which it carries to streams. Because of its short residence time

prior to entry into streams, overland flow results in higher levels of stream contamination than that observed from herbicide which has leached through the soil

profile and entered streams through baseflow (Michael and Neary, 1993; Michael

et al., 1999; Michael 2003).

3.4. STREAM CONTAMINATION DURING APPLICATION AND STORM RUNOFF

The results of several studies, which monitored the environmental fate of forestry

herbicides used in the southern United States, have been reported (Michael and

Neary, 1993). Another report (Michael, 2000) summarized surface and groundwater contamination data from reports for North America published between 1974 and

2000. It is clear from these reports that the maximum concentrations of herbicide

observed in streams is related to the method of application. Broadcast applications

generate the highest concentration which is often observed on the day of application. Broadcast applications permit less control over herbicide placement and

frequently result in application of herbicide into ephemeral and intermittent stream

channels. If application to ephemeral or intermittent channels occurs during a time

when they contain water, movement into perennial channels is inevitable, even

when the perennial stream is protected with an SMZ. Under these conditions, aerial

broadcast applications may result in 0.4 to 0.6 mg L−1 of herbicide in streams on
BMPS FOR SILVICULTURAL CHEMICALS 107

the day of application while concentrations resulting from overland flow during the

first storm typically do not exceed 0.2 to 0.4 mg L−1, even when the application rate

is 3 to 4 times the prescribed rate. Periods between storms, principally composed

of baseflow, are characterized by herbicide concentrations near or below analytical

detection limits (Michael and Neary, 1993, Michael et al., 1999; Michael, 2003).

Each subsequent storm results in stream contamination at decreasing levels until

the 4th or 5th storm at which time herbicides are typically not detected in streams

draining treated areas. The peak concentrations observed in streams, whether on

the day of application or during storm events, are short-lived and last from a few

minutes to a few hours. During storm runoff, concentrations are greatest around

peak discharge for the storm event and seldom last longer than 30 minutes, but

even these highest observed concentrations seldom exceed drinking water quality

standards.

Single stem injection treatments (herbicide is injected directly into each plant

to be killed) and spot applications (placement of herbicide directly onto the soil

surface) result in observed stream contamination levels up to 0.04 mg L−1. These

methods of application permit the maximum amount of control over where the

herbicides are applied and it is very easy to keep them out of stream channels

during application.

Herbicide may also move through the SMZ via overland flow and into streams.

The concern over movement of pollutants (herbicides, nutrients, sediment, etc.)

from hillside slopes through the SMZ and into streams has resulted in many states’

use of hillside slope as a basis for increasing the SMZ width beyond established

minimum standards. The formula takes the general form:

SMZ width = minimum base width + X feet per Y change in % slope

where the minimum base width varies by state and stream type (Table II). The

amount of change (X) and the change in percent slope categories (Y) also vary

by state. Some states recommend a nearly linear change (e.g., 10 ft for each 5%

of slope up to 40% in Kentucky). Other states recommend SMZ width increases

based on broad categories of slope (e.g., for < 7%, 35 ft; 7–20%, 50 ft; and > 20%,

80 ft [24.4 m] in Arkansas) or narrow slope categories (e.g., 40 additional feet

for each increase of 10% in slope for perennial streams in Illinois). Additional

increases are based on soil erodibility factors and stream width. For example there

are 18 SMZ width recommendations (ranging from 35 ft to 300 ft [91.4 m]) based

on 3 soil erodibility classes and 6 slope classes for perennial streams of 0–20 feet

in width in Florida. There are also 18 additional recommendations for perennial

streams of 20–40 feet width, 18 for perennial streams > 40 feet and similarly there

are an additional 18 recommendations for intermittent streams. While herbicides

are mobile in soil, little lateral movement has been observed through the soil profile

following infiltration.

Downslope movement of herbicides occurs principally in the form of overland

flow or macropore flow (Michael et al., 1989, Michael et al., 1990, Michael and
108 J. L. MICHAEL

Neary, 1991; Michael et al., 1999; Bastardie et al., 2002, Buttle and McDonald,

2002). Overland flow is a function of antecedent soil moisture conditions, precipitation rate, infiltration rates, and drainage capacity, but occurs to some extent

any time the instantaneous precipitation rate exceeds the infiltration rate. When

antecedent soil moisture is low, and infiltration rates are high, little downslope

movement occurs until infiltration rates are exceeded. However, when antecedent

soil moisture is high or the soil is saturated, infiltration rates may be exceeded

quickly resulting in overland flow. Overland flow will always occur on poorly

drained soils more quickly than well drained soils under the same conditions of

slope, precipitation intensity and duration.

Macropore flow can also contribute to downslope movement of herbicides.

When macropores such as those created by burrowing animals (moles, earthworms,

etc.) and those left where roots have died and decayed are intact and prevalent on

a site, flow through these channels may be very high and may move considerable

amounts of herbicide and other water soluble compounds very deep in the soil

(Bastardie et al., 2002). Movement through macropores may result in transport

of herbicides to impermeable layers (bedrock, etc.) where they move along those

impermeable layers to streams (Williams et al., 2002; Buttle and McDonald, 2002).

3.5. STREAM IMPACTS

Stream impacts can be projected from toxicological data compared with herbicide

concentrations observed in operational treatment monitoring studies. Such projections are not straightforward because toxicological data are usually the result

of highly controlled laboratory studies of static exposures that typically last from

48 to 96 hours instead of the short duration (a few minutes or hours) and pulsed

concentration exposures that occur in aquatic ecosystems. In addition, indirect toxicity and cumulative effects are not measured in toxicological studies conducted in

most screening laboratories. Field-based environmental fate studies provide a more

complete picture of adverse impacts from herbicide use because they include considerations of direct and indirect toxicity, cumulative impacts, and considerations

of trophic level functioning.

There have been several environmental fate studies conducted in North America, but three provide information on the environmental consequences of the three

herbicides most commonly used in forest management. The studies reported were

conducted with the commercially formulated product containing the active ingredients described below. They are the Carnation Creek Glyphosate Study, the Coosa

County Hexazinone Study and the Florida Wetlands Imazapyr study.

3.5.1. Carnation creek glyphosate study

The study, conducted from 1984–1986 on a portion of the Carnation Creek Experimental Watershed Project, which covers 10 km2 in British Columbia, Canada,

aerially applied 2 kg ai glyphosate ha−1 as the commercially available product
BMPS FOR SILVICULTURAL CHEMICALS 109

Roundup (Reynolds et al., 1989). It assessed the short-term impacts of glyphosate

on stream water, vegetation, soils, and stream biota. Long-term indirect impacts

on water quality, erosion processes, and stream biota were also assessed. Many

scientists were involved in the study (Reynolds, 1989), which began with the aerial

spraying in September 1984 of 41.7 ha of the watershed. Application observed a

10m buffer strip on each side of the streams except that two tributaries and their

associated wetlands were intentionally sprayed as a part of the study. Maximum observed glyphosate concentration in water occurred 2 hours post-application (0.162

mg L−1) and decreased to near the detection limit of 0.0001 mg L−1 within 96-

hours of treatment. Stream sediment glyphosate concentration peaked at 0.0068 mg

kg−1 24 days after treatment. Concentrations of glyphosate in leaf tissue peaked at

0.261 mg kg−1 in red alder and 0.448 mg kg−1 in salmonberry on day of treatment decreasing to less than 5% of the peak within 15 days. Caged coho salmon

fingerlings suffered 2.6% mortality in the tributary that was intentionally sprayed,

but mortality was 8.8% in the unsprayed control tributary. The researchers found

nothing to suggest the survival of coho salmon or cutthroat trout was affected

by glyphosate (Holtby and Baillie, 1989). In laboratory tests, salmonid LC50 for

formulated Roundup was determined to be 15 to 55 mg L−1 (LC50 was 22 mg L−1

of the total Roundup formulation, 11 mg L−1 of the glyphosate isopropyl amine

salt for Coho Salmon fingerlings; Chapman, 1989). There were no statistically

significant impacts on periphyton or on aquatic invertebrates, but some qualitative

changes in drift patterns may have occurred (Reynolds, 1989).

3.5.2. Coosa county hexazinone study

This study used by the USEPA for hexazinone reregistration was conducted by

the U.S. Forest Service in Coosa County, Alabama, U.S.A. (Michael et al., 1999).

The study conducted from 1990–1992 utilized a 290 ha portion of the Hatchet

Creek drainage and included 3 watersheds, two treated and one untreated control.

The study assessed the direct and indirect impacts of hexazinone applied as the

commercially available products Velpar L (one watershed) and Velpar ULW (one

watershed) on stream water, vegetation, litter, soil, and stream biota. The study

began with the aerial application in April 1990 of 6.72 kg ha−1 (3 times normal

rate for this site) of hexazinone active ingredient to two watersheds.. The applicator

observed a 10m buffer strip on each side of the streams except that the ephemeral

portions of each channel were left unprotected. Maximum observed hexazinone

concentration in water occurred during application (0.473 mg L−1) and decreased

to near detection limit of 0.001 mg L−1 within 24-hours. No damage was observed

to vegetation in the SMZ, therefore the hexazinone contamination observed during

application was the result of application to the ephemeral channel. All subsequent

inputs (peak with first storm was 0.230 mg L−1) were of continuously decreasing

concentrations and driven by precipitation events. Stream sediment hexazinone

concentration peaked at 1.7 mg kg−1. Concentrations of hexazinone were highest

in leaf tissue on the day of treatment with the liquid formulation of hexazinone
110 J. L. MICHAEL

(Velpar L) for Vaccinium sp (526 mg kg−1), Cornus sp (702 mg kg−1), fern (384 mg

kg−1), and grass species (626 mg kg−1). Dissipation from leaf tissue was rapid and

nearly complete in 180 days. Native fish were not affected by hexazinone in this

study. Benthic macroinvertebrate diversity, population numbers, and trophic level

function were unaffected by hexazinone treatment. The six principal metabolites

of hexazinone were also monitored, but were not of significant concentrations in

any samples analyzed (Michael et al., 1999).

3.5.3. Florida wetlands imazapyr study

Conducted jointly by the U.S. Forest Service and the University of Florida, Center for Wetlands with support from the National Pesticide Impact Assessment

Program, and the National Council for Air and Stream Improvement, this study

investigated the impacts of imazapyr applied as the commercially available product

Arsenal Applicator’s Concentrate at extremely high rates on benthic macroinvertebrates (Fowlkes et al., 2003). The study site, located 40 km north of Gainesville,

Florida on the National Council for Air and Stream Improvement’s Wetlands Study

Site (owned by Plum Creek Timber Company) consisted of a 30 ha block of pine

flatwoods with intermittent pond cypress domes. The microcosm experiment to

determine toxicity of imazapyr to benthic macroinvertebrates, especially Chironomus sp was conducted in a seasonally inundated, logged pond cypress dome.

Microcosms were treated with 0, 0.56, 5.6, and 56 kg imazapyr active ingredient

ha−1, equivalent to 0, 1, 10, and 100 times the normal prescription rate for this

location. Imazapyr treatment had no statistically significant impact on taxa richness

or total abundance for all taxa, dipterans, and chironomids, even at the highest

treatment rate, which resulted in a water concentration of 18.7 mg L−1.

The results of these very large studies have consistently shown no adverse

effects attributable to the respective herbicides used in forest management, even

when experimental conditions utilized treatments at rates up to 100 times those

normally used in forestry.

4. SMZ Length vs. Width

Michael and Neary (1993) have reported stream contamination from herbicide

applications utilizing very conservative application technology. Both single-stem

injection and soil spot treatments applied with 32.8 ft (10 m) SMZs resulted in

stream contamination at levels up to approximately 0.04 mg L−1. While this is a

very small amount, it may represent a level of contamination that cannot be eliminated by current methods of stream protection. It has been shown that herbicide

contamination during operational and experimental applications when ephemeral

and intermittent streams are not protected by SMZs resulted in stream contamination levels of 0.4 to 0.6 mg L−1 on the day of application, even in the absence of

precipitation (Michael and Neary 1993, Michael et al., 1999). These contributions
BMPS FOR SILVICULTURAL CHEMICALS 111

to perennial stream contamination are probably due to direct application to stream

water in the unprotected ephemeral or intermittent channel. Therefore increasing

the width of the SMZ subtending the perennial channel will do little to further

decrease stream contamination. However, in light of the findings by Hansen (2001)

that 55% of the river system may be composed of ephemeral channels, it is reasonable to assume that increasing the protection of streams to include the ephemeral

channels would decrease the amount of stream contamination from aerial applications. Contamination of streams at levels up to 0.04 mg L−1 may be unavoidable,

but this is a ripe area for research. It should also be pointed out that soil type does

play a role and there may be differences to be observed in other physiographic

provinces, i.e., mountains and coastal plain.

5. Conclusions

BMPs provide some guidelines for protection of water quality and some of these

guidelines are well founded in science and practicality. The most important features are the guidelines that govern handling of silvicultural chemicals and SMZ

width recommendations. Increasing SMZ width beyond 10 m on each side of the

channel may not provide much additional protection against water contamination

from the use of silvicultural chemicals. It is clear that protection of the intermittent

and ephemeral channels may provide much additional protection for water quality,

especially in reference to erosion and sedimentation but these are outside the scope

of this discussion and have been covered in other papers at this symposium.

The degree of stream contamination observed during use of silvicultural chemicals compares favorably with current toxicological data and indicates that most

contamination results in drinking water quality well within EPA’s HAL standards.

Field studies that have been conducted with glyphosate, hexazinone, and imazapyr

indicate that little or no adverse aquatic ecosystem impacts are to be expected.

Hexazinone has been widely studied, but fewer environmental fate and impact

studies have been conducted on glyphosate, imazapyr, sulfometuron, metsulfuron,

and both the butoxyethyl ester (Garlon 4E) and triethylamine salt (Garlon 3A) of

triclopyr. Studies to corroborate the few that have been conducted on glyphosate,

imazapyr, sulfometuron, metsulfuron, and both the butoxyethyl ester (Garlon 4E)

and triethylamine salt (Garlon 3A) of triclopyr are needed.

It should also be noted that additional regulation concerning allowable stream

contamination levels may be set by local governments and other agencies that are

not bound by the same regulations limiting EPA authority under FIFRA to considerations of ‘unreasonable adverse effects’. In the absence of the unreasonable

adverse effects clause, zero-tolerance in reference to stream contamination with

silvicultural chemicals may become wide spread. Learning everything possible

now concerning how we can limit silvicultural chemical incursion into streams and

its biological implications may become very important to the future availability
112 J. L. MICHAEL

of silvicultural chemicals. Ephemeral and intermittent channel protection will be a

very important step in this process.

6. Knowledge Gaps

Currently, it appears that use of silvicultural chemicals according to manufacturers’

label directions, and following states’ BMPs is sufficient to protect streams from

significant impacts. Even when moderate impacts occur, they are localized and

temporal in scale. It should be noted, however, that there is a lot we do not know.

Secondly it should be noted that some areas of the U.S. are moving in the direction

of zero tolerance. Zero tolerance is defined largely by the skill and capability of the

chemist whose goal is to continue to improve. Thirty years ago, chemical analysis

to the parts per million (ppm) was possible, but for many compounds this really

stretched technology. Today it is possible to analyse at the ppb level routinely and

in many cases it is relatively easy to go a thousand times lower to the parts per

trillion (ppt) level. Within a few years, we may be routinely analysing at the ppt

level. Analytical capability is improving at a much greater rate than our ability to

preclude stream contamination. It is important, therefore, to improve our knowledge base concerning the impacts of silvicultural chemicals on water quality with

all its ramifications. It is also important to improve our ability to minimize stream

pollution to the greatest extent possible so that forest management for production

of consumer needs can continue regardless of the social agenda imposed on that

management. I offer the following as some of our data gaps and research needed:

1. Soluble contaminants (pesticides and nutrients) are carried in both sheet flow

and in concentrated flow during overland flow conditions. Sheet flow is not

usually so deep and energy is decreased by surface roughness factors while

concentrated flow is less affected by surface roughness factors. In order to

mitigate against stream contamination, it will be necessary to know how to

decrease the energy of concentrated flow or how to avoid it.

2. Macropore flow contributes to the contamination of streams and it occurs very

rapidly. Assessment of macropore capacity in soils near to streams and how

to intervene in this flow path is not known.

3. Sorption and uptake of nutrients and pesticides in SMZs may reduce the

amount moving to streams. Surface roughness, species composition and density, and soil structure are components of this process that must be understood

in order to provide information on SMZ design to maximize sorption and

uptake.

4. Minimum amount and structure of ephemeral channel protection required to

optimize water quality protection is not known. Extension of even minimal

protection to ephemeral and intermittent channels while decreasing SMZ

width for higher order channels may afford greater water quality protection

and reduce production losses due to the land base tied up in SMZs. This may
BMPS FOR SILVICULTURAL CHEMICALS 113

be the most productive area of research for knowledge gaps in silvicultural

BMPs.

5. A key factor in design of SMZs is the knowledge of threshold contamination

levels below which no unacceptable adverse impacts will occur. While there

has been much research on the environmental impacts of different pesticides,

little is known about the potential synergism or antagonism that may occur for

tank mixes as they enter streams. Synergistic activity may result in adverse

impacts above the sum of potential impacts for individual chemicals. In the

same way antagonism may also occur. The impacts of tank mixes on water

quality is not known.

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