United States
Environmental Protection
Agency
Health Effects Support
Document for Metribuzin
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Health Effects Support Document
for Metribuzin
U.S. Environmental Protection Agency
Office of Water (43 04T)
Health and Ecological Criteria Division
Washington, DC 20460
www.epa.gov/safewater/
EPA 822-R-03-004
February 2003
Printed on Recycled Paper
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FOREWORD
The Safe Drinking Water Act (SDWA), as amended in 1996, requires the Administrator
of the Environmental Protection Agency to establish a list of contaminants to aid the agency in
regulatory priority setting for the drinking water program. In addition, SDWA requires EPA to
make regulatory determinations for no fewer than five contaminants by August 2001. The
criteria used to determine whether or not to regulate a chemical on the CCL are the following:
The contaminant may have an adverse effect on the health of persons.
The contaminant is known to occur or there is a substantial likelihood that the
contaminant will occur in public water systems with a frequency and at levels of public
health concern.
In the sole judgment of the administrator, regulation of such contaminant presents a
meaningful opportunity for health risk reduction for persons served by public water
systems.
The Agency's finding for the criteria are used in making a determination to regulate a
contaminant. The Agency may determine that there is no need for regulation when a
contaminant fails to meet one of the criteria. The decision not to regulate is considered a final
agency action and is subject to judicial review.
This document provides the health effects basis for the regulatory determination for
metribuzin. In arriving at the regulatory determination, data on toxicokinetics, human exposure,
acute and chronic toxicity to animals and humans, epidemiology, and mechanisms of toxicity
were evaluated. In order to avoid wasteful duplication of effort, information from the following
risk assessments by the EPA and other government agencies were used in development of this
document.
U.S. EPA. 1998a. U.S. Environmental Protection Agency. Registration Eligibility
Decision (RED): Metribuzin. Office of Prevention, Pesticides, and Toxic Substances.
February 1998.
U.S. EPA. 1998b. U.S. Environmental Protection Agency. R.E.D. Facts: Metribuzin.
Office of Prevention, Pesticides, and Toxic Substances. February 1998.
U.S. EPA. 1993. U.S. Environmental Protection Agency. Integrated Risk Information
System (IRIS): Metribuzin. Cincinnati, OH. December 1, 1993.
U.S. EPA. 1988. U.S. Environmental Protection Agency. Health advisories for 50
pesticides. Office of Drinking Water. August 1988.
Metribuzin February 2003 iii
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Information from the published risk assessments was supplemented with information
from recent studies of metribuzin identified by literature searches conducted in 1999 and 2000
and the primary references for key studies.
Generally a Reference Dose (RfD) is provided as the assessment of long-term toxic
effects other than carcinogenicity. RfD determination assumes that thresholds exist for certain
toxic effects such as cellular necrosis. It is expressed in terms of milligrams per kilogram per
day (mg/kg-day). In general, the RfD is an estimate (with uncertainty spanning perhaps an order
of magnitude) of a daily exposure to the human population (including sensitive subgroups) that
is likely to be without an appreciable risk of deleterious effects during a lifetime.
The carcinogenicity assessment for metribuzin includes a formal hazard identification.
Hazard identification is a weight-of-evidence judgment of the likelihood that the agent is a
human carcinogen via the oral route and the conditions under which the carcinogenic effects may
be expressed.
Guidelines that were used in the development of this assessment may include the
following: the Guidelines for Carcinogenic Risk Assessment (U.S. EPA,1986a), Guidelines for
the Health Risk Assessment of Chemical Mixtures (U.S. EPA, 1986b), Guidelines for
Mutagenicity Risk Assessment (U.S. EPA, 1986c), Guidelines for Developmental Toxicity Risk
Assessment (U.S. EPA, 1991), Proposed Guidelines for Carcinogen Risk Assessment (1996a),
Guidelines for Reproductive Toxicity Risk Assessment (U.S. EPA, 1996b), and Guidelines for
Neurotoxicity Risk Assessment (U.S. EPA, 1998c); Recommendations for and Documentation of
Biological Values for Use in Risk Assessment (U.S. EPA, 1988); and Health Effects Testing
Guidelines (OPPTS series 870, 1996 drafts; U.S. EPA 40 CFRPart 798, 1997); Peer Review
and Peer Involvement at the U.S. Environmental Protection Agency (U.S. EPA, 1994a); Use of
the Benchmark Dose Approach in Health Risk Assessment (U.S. EPA, 1995); Science Policy
Council Handbook: Peer Review (U.S. EPA, 1998d, 2000f); Memorandum from EPA
Administrator, Carol Browner, dated March 21, 1995, Policy for Risk Characterization; Science
Policy Council Handbook: Risk Characterization (U.S. EPA, 2000g).
The chapter on occurrence and exposure to metribuzin through potable water was
developed by the Office of Ground Water and Drinking Water. It is based primarily on
unregulated contaminant monitoring (UCM) data collected under SDWA. The UCM data are
supplemented with ambient water data as well as information on production, use, and discharge.
Metribuzin February 2003 iv
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ACKNOWLEDGMENTS
This document was prepared under the U.S. EPA contract No. 68-C-01-002, Work
Assignment No. B17 with Sciences International, Inc., Alexandria, Virginia. The Lead U.S.
EPA Scientist is Octavia Conerly, Health and Ecological Criteria Division, Office of Science and
Technology, Office of Water.
Metribuzin February 2003
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Table of Contents
FOREWORD iii
ACKNOWLEDGMENTS v
LIST OF TABLES ix
LIST OF FIGURES x
1.0 EXECUTIVE SUMMARY 1-1
2.0 IDENTITY: PHYSICAL AND CHEMICAL PROPERTIES 2-1
3.0 USES AND ENVIRONMENTAL FATE 3-1
3.1 Production and Use 3-1
3.2 Environmental Release 3-3
3.3 Environmental Fate 3-4
4.0 EXPOSURE FROM DRINKING WATER 4-1
4.1 Occurrence and Monitoring Data of Ambient Water 4-1
4.1.1 Data Sources and Methods 4-1
4.1.2 Results 4-2
4.2 Occurrence and Monitoring Data in Drinking Water 4-5
4.2.1 Data Sources, Data Quality, and Analytical Methods 4-6
4.2.2 Data Management and Analysis 4-10
4.2.3 Results 4-14
4.3 Conclusions 4-21
5.0 EXPOSURE FROM MEDIA OTHER THAN WATER 5-1
5.1 Exposure from Food 5-1
5.1.1 Exposures of the General Population 5-1
5.1.2 Exposures of Subpopulations 5-3
5.2 Exposure from Air 5-3
5.2.1 Exposures of the General Population 5-3
5.2.2 Exposures of Subpopulations 5-3
5.3 Exposure from Soil 5-4
5.3.1 Exposures of the General Population 5-4
5.3.2 Exposures of Subpopulations 5-4
5.4 Other Residential Exposures 5-5
5.5 Summary 5-5
6.0 TOXICOKINETICS 6-1
6.1 Absorption 6-1
6.2 Distribution 6-1
Metribuzin February 2003 vi
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7.
7.
6.3 Metabolism 6-1
6.4 Excretion 6-1
7.0 HAZARD IDENTIFICATION 7-1
7.1 Human Effects 7-1
7.1.1 Short-Term Studies 7-1
7.1.2 Long-Term and Epidemiological Studies 7-1
7.2 Animal Studies 7-1
7.2.1 Acute Toxicity 7-1
7.2.2 Short-Term Studies 7-1
7.2.3 Subchronic Studies 7-1
7.2.4 Neurotoxicity 7-3
7.2.5 Developmental/Reproductive Toxicity 7-3
7.2.6 Chronic Toxicity 7-5
12.1 Carcinogenicity 7-6
7.3 Other Key Data 7-7
7.3.1 Mutagenicity/Genotoxicity 7-7
7.3.2 Immunotoxicity 7-8
7.3.3 Hormonal Disruption 7-8
7.3.4 Physiological or Mechanistic Studies 7-8
7.3.5 Structure-Activity Relationship 7-9
7.4 Hazard Characterization 7-9
7.4.1 Synthesis and Evaluation of Major Non-Cancer Effects 7-9
7.4.2 Synthesis and Evaluation of Carcinogenic Effects 7-10
7.4.3 Mode of Action and Implications in Cancer Assessment 7-10
7.4.4 Weight of Evidence Evaluation for Carcinogenicity 7-11
7.4.5 Sensitive Populations 7-11
8.0 DOSE-RESPONSE ASSESSMENT 8-1
8.1 Dose-Response for Non-Cancer Effects 8-1
8.1.1 RfD Determination 8-1
8.1.2 RfC Determination 8-1
8.2 Dose-Response for Cancer Effects 8-2
9.0 REGULATORY DETERMINATION AND CHARACTERIZATION OF RISK
FROM DRINKING WATER 9-1
9.1 Regulatory Determination for Chemicals on the CCL 9-1
9.1.1 Criteria for Regulatory Determination 9-1
9.1.2 National Drinking Water Advisory Council Recommendations 9-2
9.2 Health Effects 9-2
9.2.1 Health Criterion Conclusion 9-3
9.2.2 Hazard Characterization and Mode of Action Implications 9-3
9.2.3 Dose-Response Characterization and Implications in
Risk Assessment 9-4
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9.3 Occurrence in Public Water Systems 9-4
9.3.1 Occurrence Criterion Conclusion 9-5
9.3.2 Monitoring Data 9-5
9.3.3 Use and Fate Data 9-6
9.4 Risk Reduction 9-7
9.4.1 Risk Criterion Conclusion 9-7
9.4.2 Exposed Population Estimates 9-7
9.4.3 Relative Source Contribution 9-8
9.4.4 Sensitive Populations 9-8
9.5 Regulatory Determination Summary 9-9
10.0 REFERENCES 10-1
APPENDIX A: Abbreviations and Acronyms A-l
APPENDIX B: Round 2 Metribuzin Occurrence B-l
Metribuzin February 2003 viii
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LIST OF TABLES
Table 3-1. Metribuzin Use, 1990-1999 3-3
Table 3-2. Environmental Releases (in pounds) for Metribuzin in the United States,
1995-1998 3-4
Table 4-1. Metribuzin Detections and Concentrations in Streams and Ground Water. . . . 4-3
Table 4-2. Metribuzin Detections in Shallow Ground Water from Various
Land-Use Settings 4-4
Table 4-3. Metribuzin Occurrence in Midwest Surface and Ground Water 4-5
Table 4-4. Summary Occurrence Statistics for Metribuzin 4-12
Table 4-5. SDWA Compliance Monitoring Data from the States of Illinois, Indiana,
and Ohio 4-16
Table 4-6. Metribuzin Occurrence in Midwest Drinking Water 4-18
Table 5-1. Exposures of the General Population to Metribuzin in Media
Other Than Water 5-6
Table 5-2. Exposures of Subpopulations to Metribuzin in Media Other Than Water 5-6
Table 7-1. Acute Toxic Effects of Metribuzin 7-2
Metribuzin February 2003
IX
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LIST OF FIGURES
Figure 3-1. Estimated Annual Agricultural Use for Metribuzin (1992) 3-2
Figure 4-1. Geographic Distribution of Cross-Section States for Round 2
(SDWIS/FED) 4-9
Figure 4-2. States with PWSs with Detections of Metribuzin for All States with
Data in SDWIS/FED (Round 2) 4-19
Figure 4-3. Round 2 cross-section states with PWSs with detections of metribuzin (any
PWSs with results greater than the Minimum Reporting Level [MRL]; above)
and concentrations greater than the Health Reference Level (FtRL; below). . 4-20
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1.0 EXECUTIVE SUMMARY
The U.S. Environmental Protection Agency (EPA) has prepared this Health Effects
Support Document for Metribuzin to support a determination regarding whether to regulate
metribuzin with a National Primary Drinking Water Regulation (NPDWR). The available data
on occurrence, exposure, and other risk considerations suggest that, because metribuzin does not
occur in public water systems at frequencies and levels of public health concern, regulating
metribuzin will not present a meaningful opportunity to reduce health risk. EPA will present a
determination and further analysis in the Federal Register Notice covering the CCL proposals.
Metribuzin (Chemical Abstracts Services Registry Number 21087-64-9) is a synthetic
organic compound used as a selective triazinone herbicide. It is a white crystalline solid, is
soluble in water up to 1,200 ppm (1.2 g/L), and has a sulfurous odor. Metribuzin is released into
the environment primarily during agricultural spraying operations and is moderately absorbed on
soils with high clay or organic content. It may be released into surface and ground waters during
runoff events in agricultural regions. Metribuzin is listed as a Toxic Release Inventory (TRI)
chemical, with air emissions constituting the majority of on-site releases.
Human exposure to metribuzin occurs through inhalation and ingestion, usually in
agricultural settings. Although it is applied to food crops to discourage the growth of broadleaf
weeds and grasses, metribuzin has not been detected in any food samples tested. Occupational
exposure to metribuzin includes agricultural workers, sprayers, and handlers. General
population exposures are thought to be minimal. There are no reports of accidental human
exposures to metribuzin.
There is little information on the adverse health effects of metribuzin exposure to
humans. Hazard characterization has therefore been accomplished in animal toxicity studies.
Acute studies in animals indicate that metribuzin exhibits a low order of toxicity, as indicated by
high LD50 values. Acute exposure studies also indicate that metribuzin does not possess ocular
or dermal irritation properties. Subchronic studies suggest that metribuzin could cause adverse
effects in body weight gain, organ weight, and hematological parameters. Specifically, studies
in Wistar rats indicate that liver and thyroid weights were increased and body weight gain was
decreased. In rats, chronic effects may include changes in body weight gain, liver enzyme
activities and histopathological changes. In addition, increases in corneal neovascularization and
discolored zones in the liver, and enlarged adrenal and thyroid glands, have been observed in
rats. At high doses, chronic metribuzin exposure has been observed to cause significant
increases in mortality, liver dysfunction, and thyroid weight in Beagle dogs. Developmental
studies in rats and rabbits indicate that effects to the fetus only occur subsequent to maternal
toxicity. Similarly, in reproductive studies, both parents and pups experienced decreased body
weight and exaggerated liver cell growth.
Drinking water monitoring of metribuzin is conducted under the Unregulated
Contaminant Monitoring (UCM) program. Metribuzin was not among the contaminants
monitored in Round 1 of the UCM program; metribuzin monitoring began in Round 2. A cross-
section analysis of 20 states participating in Round 2 of the UCM program indicate that the
Metribuzin February 2003 1-1
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frequency of detection of metribuzin in public water systems (PWSs) is low. The 20-state cross-
section analysis indicates that 0.007% of PWSs detected metribuzin at levels above the
Minimum Reporting Level (MRL). The percentage of the population served by PWSs reporting
metribuzin detections is 0.0003%. National extrapolation of this data indicates that 5 PWSs
nationally would contain detectable levels of metribuzin, and that 1000 people would be
exposed. However, no drinking water concentrations of metribuzin in the cross-section analysis
were greater than the Health Reference Level (HRL) or half the HRL. Using more conservative
estimates of occurrence from all states reporting Round 2 monitoring data, including states with
biased data, 0.28% of the nation's PWSs (approximately 182 systems and 3.4 million people
served) are affected by metribuzin concentrations > MRL, while no PWSs are affected by
concentrations > /^ HRL or > HRL.
In accordance with current cancer guidelines (U.S. EPA, 1986a), metribuzin is classified
as a Class D carcinogen due to inadequate carcinogenicity data in humans and animals. Chronic
exposure studies in rats and mice were negative for the induction of tumors by metribuzin.
Based on a 2-year feeding study in rats, the oral Reference Dose (RfD) was determined to be
0.013 mg/kg-day.
Metribuzin February 2003 1 -2
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2.0 IDENTITY: PHYSICAL AND CHEMICAL PROPERTIES
Metribuzin is a white crystalline solid with a melting point of 126 ฐC. Pure metribuzin is
soluble in water up to 1,200 ppm (1.2 g/L). It is also soluble in dimethylformamide at 1,780,
cyclohexanone at 1,000, chloroform at 850, acetone at 820, ethylacetate at 470, methanol at 450,
dichloromethane at 333, benzene at 220, n-butanol at 150, ethanol at 190, toluene at 120, xylene
at 90 and n-hexane at 2 g/kg at 20 ฐC. Metribuzin has a slight sulfurous odor. It is reported to
have a vapor pressure of between 5 and 10 mm Hg at 20ฐC and a density of 1.28 between 4 and
20ฐC (U.S. EPA, 1998a; HSDB, 2000).
Common Name:
Chemical Name:
Chemical Family:
CAS Registry Number:
Molecular Weight:
Empirical Formula:
Metribuzin
4-amino-6-( 1,1 -dimethylethyl)-3 -(methylthio)-1,2,4-triazin-5(4H)-
one
Triazinone
21087-64-9
214.28
C8H14N4OS
Metribuzin Structural Formula:
Metribuzin has several trade names and synonyms. These names and synonyms are
listed below in alphabetical order (RTECS, 2000; HSDB, 2000; U.S. EPA, 1998a).
4-Amino-6-tert-butyl-4,5-dihydro-3-methylthio-l,2,4-triazin-5-one
4-Amino-6-tert-butyl-3-(methylthio)-as-triazin-S(4H)-one
4-Amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one
4-Amino-6-(l,l-dimethylethyl)-3-(methylthio)-l,2,4-triazin-5(4H)-one
4-Amino-6-tert-butyl-3-(methylthio)- l,2,4-triazin-5-one
Bay 61597, Bayer 94337, Bay DIC 1468, Bayer 6159H, Bayer 6443H
DIC 1468, NTN 70
Lexone, Lexone DF, Lexone 4L
Metribuzine, Preview
Sencor, Sencor 4, Sencoral, Sencor DF, Sencorer, Sencorex , Sengoral
Zenkor
Metribuzin February 2003
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3.0 USES AND ENVIRONMENTAL FATE
3.1 Production and Use
Metribuzin is a synthetic organic compound (SOC). It is a selective triazinone herbicide
used primarily to discourage growth of broadleaf weeds and annual grasses among vegetable
crops and turf grass. Metribuzin accomplishes this by inhibiting electron transport in
photosynthesis (EXTOXNET, 1998; U.S. EPA, 1998a). Common uses include application to
soybeans, potatoes, alfalfa, sugarcane, barley, and tomatoes (Larson et al., 1999; U.S. EPA,
1998a).
Recent national estimates of agricultural use for metribuzin are available. Using its own
proprietary data, data from the United States Department of Agriculture (USD A) and the
National Center for Food and Agricultural Policy (NCFAP), the U.S. EPA (1998a) estimated
U.S. average annual use for the years 1990-94 at approximately 2.8 million pounds of active
ingredient (a.i.) with approximately 8.5 million acres treated. The United States Geological
Survey (USGS) estimated approximately 2.7 million pounds of active ingredient used for the
year 1992, with roughly 8.4 million acres treated (USGS, 2000a). These estimates were derived
using state-level data sets on pesticide use rates available from NCFAP combined with
county-level data on harvested crop acreage from the Census of Agriculture (CA) (Thelin and
Gianessi, 2000).
Figure 3-1 shows the geographic distribution of estimated average annual metribuzin use
in the United States for 1992. A breakdown of use by crop is also included. Non-agricultural
uses are not reflected here and any sharp spatial differences in use within a county are not well
represented (USGS, 1998a). Existing data suggest that non-agricultural use of metribuzin is
minimal (U.S. EPA, 1998a).
Metribuzin use patterns have been documented by the USD A as well. USD A Cropping
Practices Surveys (CPS) for field crops (1964-1995) merged with the Farm Costs and Returns
Survey (FCRS) in 1996 to form the Agricultural Resources Management Study (ARMS). As
was the case with the CPSs, the ARMS is conducted in major producing states and provides
information on metribuzin use on particular field crops (corn, soybeans, cotton, winter wheat,
spring and durum wheat, and fall potatoes). Farm operators are surveyed for crop practice
information on a field-by-field basis (USD A, 1997; USD A, 2000). Table 3-1 shows the amount
of metribuzin used annually and the number of acres treated. Metribuzin use appears to be
modestly declining over the ten-year period.
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Figure 3-1. Estimated Annual Agricultural Use for Metribuzin (1992).
METRIBUZIN
ESTIMATED ANNUAL AGRICULTURAL USE
Average use of
Active Ingredient
Pounds per square mile
of county per year
D No Estimated Use
D < 0.028
D 0.028-0.145
D 0.146-0.730
D 0.731 - 2.547
>= 2.548
Total
^P8 Pounds Applied
soybeans
potatoes
alfalfa tey
sugar cans: sugar & seed
wheat and grains
tomatoes
ItokJ and grass seed
com
asparagus
dypeas
1, 707, 148
377,256
216,321
189,811
82,793
48,913
29,704
26,825
14,377
8,574
Percent
National Use
63.13
13.95
7.96
7.02
3.06
1.81
1.10
0.99
0.53
0.32
USGS, 1998b
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Table 3-1. Metribuzin Use, 1990-1999.
Year
1999
1998
1997
1996
1995
1994
1993
1992
1991
1990
Pounds of Active
Ingredient (x 1000)
1,214
1,261
2,207
1,785
1,498
1,773
2,003
1,975
2,537
2,959
Acres Treated
(x 1000)
4,542*
6,432
8,646
6,547
5,892
5,811
6,437
6,705
7,706
8,924
Data for the years 1990-1995, USD A, 1997
Data for the years 1996-1999, USDA, 2000
* aver age figure based on available data
3.2 Environmental Release
Metribuzin is also listed as a toxic release inventory (TRI) chemical. In 1986, the
Emergency Planning and Community Right-to-Know Act (EPCRA) established the Toxic
Release Inventory (TRI) of hazardous chemicals. Created under the Superfund Amendments and
Reauthorization Act (SARA) of 1986, EPCRA is also sometimes known as SARA Title III. The
EPCRA mandates that larger facilities publicly report when TRI chemicals are released into the
environment. This public reporting is required for facilities with more than 10 full-time
employees that annually manufacture or produce more than 25,000 pounds, or use more than
10,000 pounds, of TRI chemical (U.S. EPA, 1996; U.S. EPA, 2000d).
Under these conditions, facilities are required to report the pounds per year of metribuzin
released into the environment both on- and off-site. The on-site quantity is subdivided into air
emissions, surface water discharges, underground injections, and releases to land (see Table 3-2).
For metribuzin, air emissions constitute most of the on-site releases; these emissions decrease
throughout the period of record. A sharp decrease is evident between the 1996 and 1997
reporting years. In contrast, over the period for which data are available (1995-1998), surface
water discharges generally increase. Again, the trend is exaggerated between the reporting
Metribuzin February 2003
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Table 3-2. Environmental Releases (in pounds) for Metribuzin in the United States,
1995-1998.
Year
1998
1997
1996
1995
On-Site Releases
Air
Emissions
339
359
1,012
1,936
Surface
Water
Discharges
26
24
5
9
Underground
Injection
0
0
0
0
Releases
to Land
0
0
0
0
Off-Site
Releases
255
0
0
0
Total On- &
Off-site
Releases
620
383
1,017
1,945
U.S. EPA, 2000b
years 1996 and 1997. Whether these abrupt shifts reflect actual increases in surface water
discharges and decreases in air emissions is unclear. Interpretation is confounded by the
relatively short period of record. These TRI data for metribuzin were reported from three states
and one territory (IA, MO, NB, Puerto Rico; U.S. EPA, 2000b).
Although the TRI data can be useful in giving a general idea of release trends, it is far
from exhaustive and has significant limitations. For example, only industries that meet TRI
criteria (at least 10 full-time employees, and manufacture and process quantities exceeding
25,000 Ibs/yr, or use of more than 10,000 Ibs/yr) are required to report releases. These reporting
criteria do not account for releases from smaller industries. Threshold manufacturing and
processing quantities also changed from 1988-1990 (dropping from 75,000 Ibs/yr in 1988 to
50,000 Ibs/yr in 1989 to its current 25,000 Ibs/yr in 1990) creating possibly misleading data
trends. Also, the TRI data is meant to reflect releases and should not be used to estimate general
exposure to a chemical (U.S. EPA, 2000c; U.S. EPA, 2000a).
In summary, metribuzin is used as an herbicide on crops and has limited non-agricultural
use. Applications are primarily targeted to soybeans, potatoes, alfalfa, and sugar cane, and the
geographic distribution of use largely reflects the distribution of these crops across the United
States (Figure 3-1). Estimated annual use appears to be modestly declining in the last decade
(Table 3-1). Metribuzin is also a TRI chemical. Industrial releases have been reported since
1995 in three states and one U.S. territory.
3.3 Environmental Fate
Metribuzin is released into the environment primarily during agricultural spraying
operations. It is moderately adsorbed on soils with high clay or organic content, as reflected by
the organic carbon partition coefficient (Koc = 95) (HSDB, 2000). Adsorption decreases with
increasing soil pH since metribuzin is adsorbed via a hydrogen-bonding mechanism. Although
little leaching occurs in soils with a high organic content, metribuzin is readily leached in sandy
soils. The soil half-life ranges from 14-60 days.
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Based on its low vapor pressure, metribuzin should exist in the vapor and particulate
phases at ambient temperature (HSDB, 2000). In the vapor phase, metribuzin is degraded by
reaction with photochemically formed hydroxyl radicals with a half-life of approximately 11
hours (HSDB, 2000). In the particulate phase, metribuzin is removed from the atmosphere by
dry deposition. In addition, metribuzin has been detected in rainwater, indicating that it can be
removed from air by wet deposition (HSDB, 2000).
The primary fate process for metribuzin in soil is microbial degradation (HSDB, 2000).
The rate of degradation is increased by the activity of soil microorganisms, higher temperatures,
and aerobic conditions. Metribuzin is degraded to carbon dioxide in soil. Metabolites observed
in plants, such as the 3,5-diketo and deaminated diketo metribuzin, have been found in soil
(HSDB, 2000). Loss from soil surfaces by photodecomposition and volatilization are not
expected (HSDB, 2000).
In the aquatic environment, volatilization from water and bioconcentration in fish are not
anticipated to be relevant (HSDB, 2000). No data are available for the biodegradation of
metribuzin in water.
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4.0 EXPOSURE FROM DRINKING WATER
4.1 Occurrence and Monitoring Data of Ambient Water
To understand the presence of a chemical in the environment, an examination of ambient
occurrence is useful. In a drinking water context, ambient water is source water existing in
surface waters and aquifers before treatment. The most comprehensive and nationally consistent
data describing ambient water quality in the United States are being produced through the United
States Geological Survey's (USGS) National Ambient Water Quality Assessment (NAWQA)
program. NAWQA, however, is a relatively young program and complete national data are not
yet available from their entire array of sites across the nation.
4.1.1 Data Sources and Methods
The USGS instituted the NAWQA program in 1991 to examine water quality status and
trends in the United States. NAWQA is designed and implemented in such a manner to allow
consistency and comparison between representative study basins located around the country,
facilitating interpretation of natural and anthropogenic factors affecting water quality (Leahy and
Thompson, 1994).
The NAWQA program consists of 59 significant watersheds and aquifers referred to as
"study units." The study units represent approximately two-thirds of the overall water usage in
the United States and a similar proportion of the population served by public water systems.
Approximately one half of the nation's land area is represented (Leahy and Thompson, 1994).
To facilitate management and make the program cost-effective, approximately one-third
of the study units at a time engage in intensive assessment for a period of 3 to 5 years. This is
followed by a period of less intensive research and monitoring that lasts between 5 and 7 years.
This way all 59 study units rotate through intensive assessment over a ten-year period (Leahy
and Thompson, 1994). The first round of intensive monitoring (1991-96) targeted 20 watersheds
which were slanted toward agricultural basins. A national synthesis of results from these study
units focusing on pesticides and nutrients has been compiled and analyzed (Kolpin et al., 1998;
Larson etal., 1999; USGS, 1999).
Metribuzin is an analyte for both surface and ground water NAWQA studies. Two of the
first round study units, the Central Nebraska Basins and the White River Basin in Indiana, are
located in the corn belt where metribuzin is heavily used (see Figure 3-1). The Method
Detection Limit (MDL) for metribuzin is 0.004 |ig/L (Kolpin et al., 1998), substantively lower
than most drinking water monitoring reporting levels. Additional information on analytical
methods used in the NAWQA study units, including method detection limits, are described by
Gilliom and others (in press).
Data are also available for metribuzin occurrence in ground water and surface water for
key corn belt states. The majority of these data are the result of USGS regional water quality
investigations with a focus on near-surface aquifers and surface waters. Additionally, EPA's
Metribuzin February 2003 4-1
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Pesticides in Ground Water Database (PGWD) provides a large data set on pesticide occurrence
in ground water that spans a period of 20 years and contains data from 68,824 sites. It is a
compilation of numerous national, regional, state, and local studies and therefore the data are a
mix of the results of a variety of study designs, sampling techniques, and reporting limits.
However, the size and temporal scope of the data set make it a valuable resource. Details
regarding sampling and analytical methods for the USGS studies and the PGWD report are
described in the respective reports.
4.1.2 Results
NAWQA National Synthesis
Detection frequencies and concentrations of metribuzin in ambient surface and ground
water are low, especially in ground water (Table 4-1). Most herbicides monitored in the first
round of the NAWQA program were detected in the greatest concentrations and frequencies in
surface water compared to ground water. Surface waters show the highest maximum
concentration of metribuzin at 0.5 |ig/L, well below the Health Reference Level (HRL) of 91
Hg/L. The Health Reference Level is a preliminary estimated health effect level used for the
present analysis.
Frequencies and concentrations of metribuzin in streams in agricultural settings are
greater than those in urban settings, with integrator sites (a combination of agricultural and
urban) having the highest occurrence (Table 4-1). Larson and others (1999) found that for 50
stream sites monitored over a 1-year period, one site had a detection frequency of > 50% of all
samples (detections were reported for metribuzin concentrations > 0.01 |ig/L). Ninety percent of
sites, however, had detection frequencies of less than 20% of all samples. The annual mean
frequency of metribuzin detection was less than 15% in all land-use settings at all concentrations
(calculated as the average of the 12 monthly detection frequencies from each site; Larson et al.,
1999).
While occurrence in ground water is considerably lower than in surface water, detection
in > 1% of ground water samples at concentrations > 0.05 |ig/L makes metribuzin one of the 21
most commonly detected pesticides in the first round of intensive NAWQA monitoring (the 21
are detected at concentrations > 0.05 jig/L in more than 10% of stream samples or more than 1%
of ground water samples). Metribuzin exceeded the ground water criteria partly because its high
water solubility and low soil adsorption potential allow it to leach into ground water (USGS,
2000b; U.S. EPA, 1998b; EXTOXNET, 1998). Also, the herbicide ranks among the top 200
agricultural pesticides in use (USGS, 1999).
Metribuzin February 2003 4-2
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Table 4-1. Metribuzin Detections and Concentrations in Streams and Ground Water.
streams
urban
integrator
agricultural
all sites
ground water
shallow urban
shallow
agricultural
major aquifers
all sites
Detection frequency
(% samples ;> MDL*)
% > 0.004 ug/L
6.73%
14.29%
13.70%
13.82%
1.66%
3.46%
0.75%
1.95%
%> 0.01 ug/L
5.50%
9.39%
8.20%
9.94%
0.33%
2.81%
0.32%
1.36%
Concentration percentiles
(all samples fig/L)
median
nd**
nd
nd
nd
nd
nd
nd
nd
95th
0.011
0.020
0.016
0.026
nd
nd
nd
nd
maximum
0.100
0.130
0.330
0.530
0.043
0.300
0.045
0.300
USGS, 2000b
* MDL (Method Detection Limit) for metribuzin in water studies: 0.004 ffg/L
* *not detected in concentration greater than MDL
Herbicides often demonstrate detection frequencies in streams that correlate with patterns
of use (USGS, 2000b). Patterns of pesticide use often do not correlate with detection frequency
in ground water, probably because of the variable effect of local hydrogeologic conditions (depth
and type of aquifer, soil conditions) on pesticides in ground water (USGS, 2000b). Metribuzin,
however, is one of six pesticides that, for shallow ground water, demonstrate a statistically
significant correlation between detection frequency and intensity of use (Kolpin et al., 1998).
Metribuzin detection frequencies are higher in shallow ground water in agricultural areas when
compared with shallow ground water in urban areas (Table 4-1). This is most likely a result of
metribuzin's primary use as an agricultural pesticide (U.S. EPA, 1998a). Metribuzin is detected
most frequently in shallow ground water from land-use categories containing wheat, wheat and
alfalfa, corn and soybeans, and corn and alfalfa as major crops or crop-groups (Table 4-2).
Metribuzin February 2003
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Table 4-2. Metribuzin Detections in Shallow Ground Water from Various Land-Use
Settings.
Land-use settings*
All
Corn and soybeans > 20%
Corn and alfalfa > 20%
Corn > 50%
Peanuts > 50%
Wheat and small grains > 50%
Wheat and small grains and alfalfa > 20%
Alfalfa > 50%
Pasture > 90%
Orchards or vineyards > 50%
Urban
Detection frequency
>0.004 ug/L
3.1%
6.6%
2.1%
0.0%
1.6%
9.3%
6.2%
0.0%
0.0%
0.0%
1.8%
Detection frequency
>0.010 ug/L
jy**
< 10%
0 - 2%
0 - 2%
<5%
< 10%
< 5%
0 - 2%
0 - 2%
0 - 2%
0 - 2%
after Kolpinet al, 1998
*evaluated as crop-groups occupying a percent of the total land
** not reported
Water Quality Investigations from the Corn Belt
USGS regional water quality investigations and other state and national studies are
summarized below to provide ambient data in states where metribuzin use is high (see Figure 3-
1). Midwest ground water concentrations and detection frequencies were low during the years
1991-1994 (Table 4-3). The highest detected ground water concentration, 25. 1 |ig/L, is found in
the national Pesticides in Ground Water Database, which draws only a portion of its data from
Midwestern states. This concentration is still well below the Health Reference Level (HRL) of
91
Maximum concentrations of metribuzin in surface waters of the Mississippi River and
major tributaries for all years, peaking at < 0. 1 |ig/L, were considerably lower than the HRL.
Although all 9 sampling sites in the Mississippi River and major tributaries had a least one
detection of metribuzin (100% of sites) from April 1991 to March 1992, the percentage of
samples with detections was 40%.
Metribuzin February 2003
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Table 4-3. Metribuzin Occurrence in Midwest Surface and Ground Water.
uses
Midwest Near-Surface Aquifers (1991)1
Midwest Near-Surface Aquifers (1992-94)2
Miss. River and Major Tributaries (199 1)3
Miss. River and Major Tributaries (1991-92)4
Midwest Reservoirs (1992)5
Pesticides in Ground Water Database (1971-91)6
Ground water
*MRL
% %
sites samples
1.3%
nr
4.3%
1.0%
1.4%
nr
Surface water
*MRL
%
sites
54%
100%
12%
%
samples
nr
40%
6.5%
Max.
cone.
Hg/L
0.57
0.22
0.08
0.03
nr
25.1
'Kolpin et al, 1994
2Kolpin et al, 1996
3Periera andHostettler, 1993 (cited in Larson et al., 1997)
* Goolsby andBattaglin, 1993 (cited inLarson et al, 1997)
5 Goolsby et al, 1993 (cited in Larson et al, 1997)
6 U.S. EPA, 1992 (cited in Barbash andResek, 1996); data are national results including some Midwestern states
- The Health Reference Level (HRL) used for metribuzin is 91 ffg/L. This is a draft value for working review only.
- Minimum Reporting Levels (MRL) vary by study.
-nd = results below the respective reporting level
-nr = "not reported"
4.2 Occurrence and Monitoring Data in Drinking Water
The Safe Drinking Water Act (SDWA), as amended in 1986, required Public Water
Systems (PWSs) to monitor for specified "unregulated" contaminants on a five-year cycle and to
report the monitoring results to the states. Unregulated contaminants do not have an established
or proposed National Primary Drinking Water Regulation (NPDWR), but they are contaminants
that were formally listed and were required for monitoring under federal regulations. The intent
was to gather scientific information on the occurrence of these contaminants to enable a decision
as to whether or not regulations were needed. All non-purchased community water systems
(CWSs) and non-purchased non-transient non-community water systems (NTNCWSs), with
greater than 150 service connections, were required to conduct this unregulated contaminant
monitoring. Smaller systems were not required to conduct this monitoring under federal
regulations, but were required to be available for monitoring if the state decided such monitoring
was necessary. Many states collected data from smaller systems. Additional contaminants were
added to the Unregulated Contaminant Monitoring (UCM) program in 1991 (56 FR 3526) for
required monitoring that began in 1993 (57 FR 31776).
Metribuzin February 2003
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Metribuzin has been monitored under the SDWA Unregulated Contaminant Monitoring
(UCM) program since 1993 (57 FR 31776). Monitoring ceased for small PWSs under a direct
final rule published January 8, 1999 (64 FR 1494), and ended for large PWSs with promulgation
of the new Unregulated Contaminant Monitoring Regulation (UCMR) issued September 17,
1999 (64 FR 50556) and effective January 1, 2001. At the time the UCMR lists were developed,
the Agency concluded there were adequate monitoring data for a regulatory determination. This
obviated the need for continued monitoring under the new UCMR list.
4.2.1 Data Sources, Data Quality, and Analytical Methods
Currently, there is no complete national record of unregulated or regulated contaminants
in drinking water from PWSs collected under SDWA. Many states have submitted unregulated
contaminant PWS monitoring data to EPA databases, but there are issues of data quality,
completeness, and representativeness. Nonetheless, a significant amount of state data are
available for UCM contaminants that can provide estimates of national occurrence.
The National Contaminant Occurrence Database (NCOD) is an interface to the actual
occurrence data stored in the Safe Drinking Water Information System/Federal version
(SDWIS/FED) and can be queried to provide a summary of the data in SDWIS/FED for a
particular contaminant. The drinking water occurrence data for metribuzin presented here were
derived from monitoring data available in the SDWIS/FED database.
The data in this report have been reviewed, edited, and filtered to meet various data
quality objectives for the purposes of this analysis. Hence, not all data from a particular source
were used, only data meeting the quality objectives described below were included. The sources
of these data, their quality and national aggregation, and the analytical methods used to estimate
a given contaminant's national occurrence (from these data) are discussed in this section (for
further details see U.S. EPA, 2001a,b).
UCM Rounds 1 and 2
The 1987 UCM contaminants include 34 volatile organic compounds (VOCs) (52 FR
25720). Metribuzin, a synthetic organic compound (SOC), was not among these contaminants.
The UCM (1987) contaminants were first monitored coincident with the Phase I regulated
contaminants, during the 1988-1992 period. This period is often referred to as "Round 1"
monitoring. The monitoring data collected by the PWSs were reported to the states (as primacy
agents), but there was no protocol in place to report these data to U. S EPA. These data from
Round 1 were collected by U.S EPA from many states over time and put into a database called
the Unregulated Contaminant Information System, or URCIS.
The 1993 UCM contaminants include 13 SOCs and 1 inorganic contaminant (IOC) (56
FR 3526). Monitoring for the UCM (1993) contaminants began coincident with the Phase II/V
regulated contaminants from 1993 through 1998. This is often referred to as "Round 2"
monitoring. The UCM (1987) contaminants were also included in the Round 2 monitoring. As
with other monitoring data, PWSs reported these results to the states. EPA, during the past
Metribuzin February 2003 4-6
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several years, requested that the states submit these historic data which is now stored in the
SDWIS/FED database.
Monitoring and data collection for metribuzin, a UCM (1993) contaminant, began in
Round 2. Therefore, the following discussion regarding data quality screening, data
management, and analytical methods focuses on SDWIS/FED. Discussion of the URCIS
database is included where relevant, but it is worth noting that the various quality screening, data
management, and analytical processes were nearly identical for the two databases. For further
details on the two monitoring periods as well as the databases, see U.S. EPA (2001a and 2001b).
Developing a Nationally Representative Perspective
The Round 2 data contain contaminant occurrence data from a total of 35 primacy
entities (including 34 states and data for some tribal systems). However, data from some states
are incomplete and biased. Furthermore, the national representativeness of the data is
problematic because the data were not collected in a systematic or random statistical framework.
These state data could be heavily skewed to low-occurrence or high-occurrence settings. Hence,
the state data were evaluated based on pollution-potential indicators and the spatial/hydrologic
diversity of the nation. This evaluation enabled the construction of a cross-section from the
available state data sets that provides a reasonable representation of national occurrence.
A national cross-section from these state Round 2 contaminant databases was established
using the approach developed for the EPA report A Review of Contaminant Occurrence in Public
Water Systems (U.S. EPA, 1999). This approach was developed to support occurrence analyses
for EPA's Chemical Monitoring Reform (CMR) evaluation. It was supported by peer reviewers
and stakeholders. The approach cannot provide a "statistically representative" sample because
the original monitoring data were not collected or reported in an appropriate fashion. However,
the resultant "national cross-section" of states should provide a clear indication of the central
tendency of the national data. The remainder of this section provides a summary description of
how the national cross-section for the SDWIS/FED (Round 2) database was developed. The
details of the approach are presented in other documents (U.S. EPA, 2001a,b); readers are
referred to these for more specific information.
Cross-Section Development
As a first step in developing the cross-section, the state data contained in the
SDWIS/FED database (containing the Round 2 monitoring results) were evaluated for
completeness and quality. Some state data in SDWIS/FED were unusable for a variety of
reasons. Some states reported only detections, or their data had incorrect units. Datasets only
including detections are obviously biased. Other problems included substantially incomplete
data sets without all PWSs reporting (U.S. EPA, 2001a Sections II and III).
The balance of the states remaining after the data quality screening were then examined
to establish a national cross-section. This step was based on evaluating the states' pollution
Metribuzin February 2003 4-7
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potential and geographic coverage in relation to all states. Pollution potential is considered to
ensure a selection of states that represent the range of likely contaminant occurrence, and a
balance with regard to likely high and low occurrence. Geographic consideration is included so
that the wide range of climatic and hydrogeologic conditions across the United States are
represented, again balancing the varied conditions that affect transport and fate of contaminants,
as well as conditions that affect naturally occurring contaminants (U.S. EPA, 2001b Sections
III.A. and III.B.).
The cross-section states were selected to represent a variety of pollution potential
conditions. Two primary pollution potential indicators were used. The first factor selected
indicates pollution potential from manufacturing/population density and serves as an indicator of
the potential for VOC contamination within a state. Agriculture was selected as the second
pollution potential indicator because the majority of SOCs of concern are pesticides (U.S. EPA,
200Ib Section III.A.). The 50 individual states were ranked from highest to lowest based on the
pollution potential indicator data. For example, the state with the highest ranking for pollution
potential from manufacturing received a ranking of 1 for this factor and the state with the lowest
value was ranked as number 50. States were ranked for their agricultural chemical use status in a
similar fashion.
The states' pollution potential rankings for each factor were subdivided into four
quartiles (from highest to lowest pollution potential). The cross-section states were chosen from
all quartiles for both pollution potential factors to ensure representation, for example, from:
states with high agrichemical pollution potential rankings and high manufacturing pollution
potential rankings; states with high agrichemical pollution potential rankings and low
manufacturing pollution potential rankings; states with low agrichemical pollution potential
rankings and high manufacturing pollution potential rankings; and states with low agrichemical
pollution potential rankings and low manufacturing pollution potential rankings (U.S. EPA,
2001b Section III.B.). In addition, some secondary pollution potential indicators were
considered to further ensure that the cross-section states included the spectrum of pollution
potential conditions (high to low). The cross-section was then reviewed for geographic coverage
throughout all sectors of the United States.
The data quality screening, pollution potential rankings, and geographic coverage
analysis established a national cross-section of 20 Round 2 (SDWIS/FED) states. The cross-
section states provide good representation of the nation's varied climatic and hydrogeologic
regimes and the breadth of pollution potential for the contaminant groups (Figure 4-1).
Metribuzin February 2003 4-8
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Figure 4-1. Geographic Distribution of Cross-Section States for Round 2 (SDWIS/FED).
Round 2 (SDWIS/FED)
Alaska
Arkansas
Colorado
Kentucky
Maine
Maryland
Massachusetts
Michigan
Minnesota
Missouri
New Hampshire
New Mexico
North Carolina
North Dakota
Ohio
Oklahoma
Oregon
Rhode Island
Texas
Washington
Cross-Section Evaluation
To evaluate and validate the method for creating the national cross-sections, the method
was used to create smaller state subsets from the 24-state, Round 1 (URCIS) cross-section and
aggregations. Again, states were chosen to achieve a balance from the quartiles describing
pollution potential and a balanced geographic distribution, to incrementally build subset cross-
sections of various sizes. For example, the Round 1 cross-section was tested with subsets of 4, 8
(the first 4 state subset plus 4 more states), and 13 (8 state subset plus 5) states. Two additional
cross-sections were included in the analysis for comparison: a cross-section composed of 16
biased states eliminated from the 24 state cross-section for data quality reasons and a cross-
section composed of all 40 Round 1 states (U.S. EPA, 2001b Section III.B.l).
These Round 1 incremental cross-sections were then used to evaluate occurrence for an
array of both high- and low-occurrence contaminants. The comparative results illustrate several
points. The results are quite stable and consistent for the 8-, 13-, and 24-state cross-sections.
They are much less so for the 4-state, 16-state (biased), and 40-state (all Round 1 states) cross-
sections. The 4-state cross-section is too small to provide balance both geographically and with
pollution potential, a finding that concurs with past work (U.S. EPA, 1999). The CMR analysis
suggested that a minimum of 6-7 states was needed to provide balance both geographically and
with pollution potential, and the CMR report used 8 states out of the available data for its
nationally representative cross-section (U.S. EPA, 1999). The 16-state and 40-state cross-
sections, both including biased states, provided occurrence results that were unstable and
inconsistent for a variety of reasons associated with their data quality problems (U.S. EPA,
2001b Section III.B.l).
The 8-, 13-, and 24-state cross-sections provide very comparable results, are consistent,
and are usable as national cross-sections to provide estimates of contaminant occurrence.
Metribuzin February 2003
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Including greater data from more states improves the national representation and the confidence
in the results, as long as the states are balanced in terms of pollution potential and spatial
coverage. The 20-state cross-section provides the best, nationally representative cross-section
for the Round 2 data.
4.2.2 Data Management and Analysis
The cross-section analyses focused on occurrence at the water system level; i.e., the
summary data presented discuss the percentage of public water systems with detections, not the
percentage of samples with detections. By normalizing the analytical data to the system level,
skewness inherent in the sample data is avoided. System level analysis was used since a PWS
with a known contaminant problem usually has to sample more frequently than a PWS that has
never detected the contaminant. The results of a simple computation of the percentage of
samples with detections (or other statistics) can be skewed by the more frequent sampling results
reported by the contaminated site. The system level of analysis is conservative. For example, a
system need only have a single sample with an analytical result greater than the Minimum
Reporting Limit (MRL), i.e., a detection, to be counted as a system with a result "greater than the
MRL."
Also, the data used in the analyses were limited to only those data with confirmed water
source and sampling type information. Only standard SDWA compliance samples were used;
"special" samples, or "investigation" samples (investigating a contaminant problem that would
bias results), or samples of unknown type were not used in the analyses. Various quality control
and review checks were performed, including follow-up questions to the states providing the
data. Many of the most intractable data quality problems encountered occurred with older data.
These problematic data were, in some cases, simply eliminated from the analysis. For example,
when the number of data with problems were insignificant relative to the total number of
observations they were dropped from the analysis (For further details see Cadmus, 2000).
As indicated in Figure 4-1, Massachusetts is included in the 20-state, Round 2 national
cross-section. Noteworthy for SOCs like metribuzin, however, Massachusetts' SOC data were
problematic. Massachusetts reported Round 2 sample results for SOCs from only 56 PWSs,
while reporting VOC results from over 400 different PWSs. Massachusetts SOC data also
contained an atypically high percentage of systems with analytical detections when compared to
all other states. Through communications with Massachusetts data management staff it was
learned that the state's SOC data were incomplete and that the SDWIS/FED record for
Massachusetts SOC data was also incomplete. For instance, the SDWIS/FED Round 2 data for
Massachusetts indicates 14.3% of systems reported detections of metribuzin. The cross-section
state with the next highest detection frequency reported only 0.2% of systems with detections.
In contrast, Massachusetts' data characteristics and quantities for lOCs and VOCs were
reasonable and comparable with other states' results. Therefore, Massachusetts was included in
the group of 20 SDWIS/FED Round 2 cross-section states with usable data for lOCs and VOCs,
but its metribuzin (SOC) data were omitted from Round 2 cross-section occurrence analyses and
summaries presented in this report.
Metribuzin February 2003 4-10
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Occurrence Analysis
To evaluate national contaminant occurrence, a two-stage analytical approach has been
developed. The first stage of analysis provides a straightforward, conservative, broad evaluation
of occurrence of the CCL preliminary regulatory determination priority contaminants as
described above. These descriptive statistics are summarized here. Based on the findings of the
Stage 1 Analysis, EPA will determine whether more intensive statistical evaluations, the Stage 2
Analysis, may be warranted to generate national probability estimates of contaminant occurrence
and exposure for priority contaminants. (For details on this two stage analytical approach see
Cadmus, 2000.)
The summary descriptive statistics presented in Table 4-4 for metribuzin are a result of
the Stage 1 analysis and include data from Round 2 (SDWIS/FED, 1993-1997) cross-section
states (minus Massachusetts). Included are the total number of samples, the percent of samples
with detections, the 99th percentile concentration of all samples, the 99th percentile concentration
of samples with detections, and the median concentration of samples with detections. The
percentages of PWSs and population served indicate the proportion of PWSs whose analytical
results showed a detect!on(s) of the contaminant (simple detection, > MRL) at any time during
the monitoring period; or a detect on(s) greater than one-half the Health Reference Level (HRL);
or a detection(s) greater than the Health Reference Level. The Health Reference Level, 91 |ig/L,
is a preliminary estimated health effect level used for this analysis.
The HRL was derived from the RfD (developed in Chapter 8 of this document) as a
preliminary estimated health effect level as follows:
HRL = RfD x Body Weight x Relative Source Contribution
Drinking Water Intake
HRL = 0.013mg/kg x 70kg x 20%
2L
HRL = 0.091 mg/L or 91 jig/L
The 99th percentile concentration is used here as a summary statistic to indicate the upper
bound of occurrence values because maximum values can be extreme values (outliers) that
sometimes result from sampling or reporting error. The 99th percentile concentration is presented
for both the samples with only detections and all of the samples because the value for the 99th
percentile concentration of all samples is below the Minimum Reporting Level (MRL) (denoted
by "<" in Table 4-4). For the same reason, summary statistics such as the 95th percentile
Metribuzin February 2003 4-11
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Table 4-4. Summary Occurrence Statistics for Metribuzin.
Frequency Factors
Total Number of Samples
Percent of Samples with Detections
99th Percentile Concentration (all samples)
Health Reference Level
Minimum Reporting Level (MRL)
99th Percentile Concentration of Detections
Median Concentration of Detections
Total Number of PWSs
Number of GW PWSs
Number of SW PWSs
Total Population
Populations of GW PWSs
Populations of SW PWSs
Occurrence by System
PWSs with detections (>MRL)
Range
GW PWSs with detections
SW PWSs with detections
PWSs > '/2 Health Reference Level (HRL)
Range
GW PWSs > H Health Reference Level
SW PWSs > H Health Reference Level
PWSs > Health Reference Level
Range
GW PWSs > Health Reference Level
SW PWSs > Health Reference Level
Occurrence by Population Served
PWS Populations Served with detections
Range
GW PWS Population with detections
SW PWS Populations with detections
PWS Population Served > V2 Health Reference
Level
Range
GW PWS Population > Vi Health Reference Level
SW PWS Population > Vi Health Reference Level
PWS Population Served > Health Reference Level
Range
GW PWS Population > Health Reference Level
SW PWS Population > Health Reference Level
20 State Cross-Section1
(Round 2)
34,507
0.003%
< (Non -detect)
91 (ig/L
Variable4
0.10|ig/L
0.10|ig/L
13,512
11,833
1,679
50,633,068
14,886,153
35,746,915
0.007%
0-0.17%
0.008%
0.00%
0.00%
0-0.00%
0.00%
0.00%
0.00%
0-0.00%
0.00%
0.00%
0.0003%
0-0.01%
0.00%
0.00%
0.00%
0-0.00%
0.00%
0.00%
0.00%
0-0.00%
0.00%
0.00%
All Reporting States2
(Round 2)
42,856
0.23%
< (Non -detect)
91p.g/L
Variable4
3.0 (ig/L
1.0 (ig/L
15,333
13,311
2,022
62,397,416
16,255,818
46,141,598
0.28%
0-14.29%
0.14%
1.24%
0.00%
0-0.00%
0.00%
0.00%
0.00%
0-0.00%
0.00%
0.00%
1.61%
0-14.92%
0.24%
2.09%
0.00%
0-0.00%
0.00%
0.00%
0.00%
0-0.00%
0.00%
0.00%
National System and
Population Numbers3
-
65,030
59,440
5,590
213,008,182
85,681,696
127,326,486
National Extrapolation5
5
N/A
5
0
0
N/A
0
0
0
N/A
0
0
182
N/A
83
69
0
N/A
0
0
0
N/A
0
0
1,000
N/A
1,000
0
0
N/A
0
0
0
N/A
0
0
3,429,000
N/A
206,000
2,661,000
0
N/A
0
0
0
N/A
0
0
' Summary Results based on data from 20-State Cross-Section (minus Massachusetts), from SDWIS/FED, UCM (1993) Round 2.
2 Summary Results based on data from all reporting states from SDWIS/FED, UCM (1993) Round 2.
3 Total PWS and population numbers are from EPA March 2000 Water Industry Baseline Handbook (U.S. EPA, 2000e).
4 See text for discussion.
5 National extrapolations are from the 20-State cross-section data (left) and all Round 2 states reporting data (right) using the Baseline
Handbook system and population numbers.
- PWS = Public Water Systems; GW = Ground Water; SW = Surface Water; MRL = Minimum Reporting Level (for laboratory analyses);
HRL = Health Reference Level, an estimated health effect level used for preliminary assessment for this review; N/A = Not Applicable.
- The Health Reference Level (HRL) used for metribuzin is 91 fJg/L. This is a draft value for working review only.
- Total Number of Samples = the total number of analytical records for metribuzin.
- 99th Percentile Concentration = the concentration value of the 99th percentile of either all analytical results or just the detections (in
Median Concentration of Detections = the median analytical value of all the detections (analytical results greater than the MRL) (in
- Total Number of PWSs = the total number of public water systems with records for metribuzin
- Total Population Served = the total population served by public water systems with records for metribuzin
- % PWS with detections, % PWS > '/2 Health Reference Level, % PWS > Health Reference Level = percent of the total number of public water
systems with at least one analytical result that exceeded the MRL, 'A Health Reference Level, Health Reference Level, respectively
- % PWS Population Served with detections, % PWS Population Served > ฅ2 Health Reference Level, % PWS Population Served > Health
Reference Level = percent of the total population served by P WSs with at least one analytical result exceeding the MRL, '/2 Health Reference
Level, or the Health Reference Level, respectively
Metribuzin February 2003
4-12
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concentration of all samples or the median (or mean) concentration of all samples are omitted
because these also are all "<" values. This is the case because only 0.003% of all samples
recorded detections of metribuzin in Round 2.
As a simplifying assumption, a value of one-half the MRL is often used as an estimate of
the concentration of a contaminant in samples/systems whose results are less than the MRL. For
a contaminant with relatively low occurrence, such as metribuzin in drinking water occurrence
databases, the median or mean value of occurrence using this assumption would be half the MRL
(0.5 x MRL). However, for these occurrence data this is not straightforward. For Round 2,
states have reported a wide range of values for the MRLs. This is in part related to state data
management differences as well as real differences in analytical methods, laboratories, and other
factors.
The situation can cause confusion when examining descriptive statistics for occurrence.
For example, most Round 2 states reported non-detections as zeros resulting in a modal MRL
value of zero. By definition the MRL cannot be zero. This is an artifact of state data
management systems. Because a simple meaningful summary statistic is not available to
describe the various reported MRLs, and to avoid confusion, MRLs are not reported in the
summary table (Table 4-4).
In Table 4-4, national occurrence is estimated by extrapolating the summary statistics for
the 20-state cross-section (minus Massachusetts) to national numbers for systems, and
population served by systems, from the Water Industry Baseline Handbook, Second Edition
(U.S. EPA, 2000e). From the handbook, the total number of community water systems (CWSs)
plus non-transient, non-community water systems (NTNCWSs) is 65,030, and the total
population served by CWSs plus NTNCWSs is 213,008,182 persons (see Table 4-4). To arrive
at the national occurrence estimate for the cross-section, the national estimate for PWSs (or
population served by PWSs) is simply multiplied by the percentage for the given summary
statistic [i.e., the national estimate for the total number of PWSs with detections (5) is the
product of the percentage of PWSs with detections (0.007%) and the national estimate for the
total number of PWSs (65,030)].
Included in Table 4-4, in addition to the results from the cross-section data, are results
and national extrapolations from all Round 2 reporting states. The data from the biased states
are included because of metribuzin's very low occurrence in drinking water samples in all states.
For contaminants with very low occurrence, such as metribuzin where very few states have
detections, any occurrence becomes more important, relatively. For such contaminants, the
cross-section process can easily miss a state with occurrence that becomes more important. This
is the case with metribuzin.
Extrapolating only from the cross-section states, metribuzin's very low occurrence
clearly underestimates national occurrence. For example, while data from biased states like
Massachusetts exaggerate occurrence because of incomplete reporting, the detections are real
and need to be accounted for because extrapolations from the cross-section states do not predict
enough detections in the biased states. Therefore, results from all reporting Round 2 states,
including the biased states, are also used here to extrapolate a national estimate. Using the
biased states' data should provide conservative estimates of national occurrence for metribuzin.
Metribuzin February 2003 4-13
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As exemplified by the cross-section extrapolations for metribuzin, national extrapolations
of these Stage 1 analytical results can be problematic, especially for contaminants with very low
occurrence, because the State data used for the cross-section are not a strict statistical sample.
For this reason, the nationally extrapolated estimates of occurrence based on Stage 1 results are
not presented in the CCL Federal Register Notice. The presentation in the Federal Register
Notice of only the actual results of the cross-section analysis maintains a straight-forward
presentation, and the integrity of the data, for stakeholder review. The nationally extrapolated
Stage 1 occurrence values are presented here, however, to provide additional perspective. A
more rigorous statistical modeling effort, the Stage 2 analysis, could be conducted on the cross-
section data (Cadmus, 2001). The Stage 2 results would be more statistically robust and more
suitable to national extrapolation. This approach would provide a probability estimate and
would also allow for better quantification of estimation error.
Additional Drinking Water Data from the Corn Belt
To augment the SDWA drinking water data analysis described above, and to provide
additional coverage of the corn belt states where metribuzin use is highest (Figure 3-1),
independent analyses of finished drinking water data from the states of Iowa, Illinois, Indiana,
and Ohio are reviewed below. The Iowa analysis examined SDWA compliance monitoring data
from surface and ground water PWSs for the years 1988-1995 (Hallberg et al., 1996). Illinois
and Indiana compliance monitoring data for surface and ground water PWSs were evaluated.
The data were mostly for the years from 1993 to 1997, though some earlier data were also
analyzed (after U.S. EPA, 1999). These state data sets were available from an independent
review of contaminant monitoring in drinking water (U.S. EPA, 1999). Finally, the Ohio Round
2 data analyzed with the 20-state cross-section are examined independently for comparison with
the other supplemental data sets from corn belt states.
Additional reviews of national and state drinking water monitoring results are included
for further perspective on corn belt occurrence of metribuzin. The Iowa State-Wide Rural Weil-
Water Survey was conducted in 1988-1989 to assess pesticide occurrence in rural private wells
(Kross et al., 1990). The National Pesticide Survey (NFS) provides extensive national
monitoring data for drinking water, including data from Midwestern states, for the years 1988-
1990 (U.S. EPA, 1990). Hallberg (1989) reviewed special contaminant occurrence studies of
raw surface water supplies in Illinois (1985-1987), and both raw and finished drinking water
from surface water in Iowa (1986). Data sources, data quality, and analytical methods for these
analyses are described in the respective reports.
4.2.3 Results
Occurrence Estimates
As noted, the extrapolation from cross-section states underestimates national metribuzin
occurrence, and the resulting percentages of PWSs with detections are very low (Table 4-4).
The cross-section shows approximately 0.007% of PWSs (about 5 PWSs nationally) experienced
detections of metribuzin above the MRL, affecting less than 0.0003% of the population served
(approximately 1,000 people nationally) (see also Figure 4-3). No PWSs reported detections at
levels > l/2 HRL or > HRL. Detection frequencies are higher for ground water systems when
Metribuzin February 2003 4-14
-------
compared to surface water systems, as surface water systems reported zero detections. For
samples with detections, the median and 99th percentile concentrations are 0.10 |ig/L. These
figures are identical because for metribuzin, Washington was the only state that reported a
detection (0.10 |ig/L) and thus this statistic is both the median and 99th percentile concentration.
Because metribuzin's low occurrence yields an underestimate from cross-section states,
all data are used, even the biased data, to present a conservative upper bound estimate.
Conservative estimates of metribuzin occurrence using all of the Round 2 reporting states still
show relatively low detection frequencies (Table 4-4). Approximately 0.28% of PWSs
(estimated at 182 PWSs nationally) experienced detections > MRL, while no PWSs experienced
detections > /^ HRL, and > HRL. These figures indicate that about 1.61% of the population is
affected by concentrations > MRL (approximately 3.4 million people nationally), and 0% of the
population is affected by concentrations > 1A HRL or > HRL. The proportion of surface water
PWSs with detections is greater than ground water systems. The median and 99th percentile
concentrations of detections are 1 |ig/L and 3 |ig/L, respectively.
The Round 2 reporting states and the Round 2 national cross-section show a
proportionate balance in PWS source waters compared to the national inventory. Nationally,
91% of PWSs use ground water (and 9% surface waters); Round 2 national cross-section states
show 88% use ground water (and 12% surface waters); Round 2 reporting states show 87% use
ground water (and 13% surface waters). The relative populations served are not as comparable.
Nationally, about 40% of the population is served by PWSs using ground water (and 60% by
surface water). For the Round 2 cross-section, 29% of the cross-section population is served by
ground water PWSs (and 71% by surface water). For all Round 2 reporting states, 26% of the
population is served by ground water PWSs (and 74% by surface water). The resultant national
extrapolations are not additive as a consequence of these disproportions (Table 4-4).
Occurrence in the Corn Belt
SDWA compliance monitoring data from the corn belt states of Illinois, Indiana, and
Ohio also show very low occurrence of metribuzin. The pesticide was not detected above the
Health Reference Level in any case, and the highest 99th percentile concentration of detections
among the three states was for Illinois at 0.7 |ig/L (Table 4-5). Illinois also had the highest
maximum concentration at 20 |ig/L, still well below the HRL (after U.S. EPA, 1999). SDWA
compliance monitoring from Iowa for the years 1988-1995 show similar results, although the
data are not presented in Table 4-5 because they were not compiled at the system level in the
same manner. Approximately 0.8% of samples analyzed for metribuzin in Iowa drinking water
had detections of the compound with a maximum concentration of 1.6 |ig/L. The 99th percentile
concentration of all samples was a non-detect (Hallberg et al., 1996).
Metribuzin detection frequencies are generally much greater in surface water when
compared to ground water (Tables 4-5 and 4-6). Two exceptions are the Iowa SDWA
compliance data, in which surface and ground water detection frequencies are essentially the
same (0.77% and 0.76%, respectively), and the Indiana SDWA compliance data which had no
metribuzin detections in surface water (Table 4-5).
Metribuzin February 2003 4-15
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Table 4-5. SDWA Compliance Monitoring Data from the States of Illinois, Indiana, and
Ohio.
Frequency Factors
Total Number of Samples
Percent of Samples with Detections
99th Percentile Concentration (all samples)
Health Reference Level
Minimum Reporting Level (MRL)
99th Percentile Concentration of Detections
Median Concentration of Detections
Minimum Concentration of Detections
Total Number of PWSs
Number of GWPWSs
Number of SW PWSs
Occurrence by System
% PWSs with detections (>MRL)
GW PWSs with detections
SW PWSs with detections
% PWSs > !/2 Health Reference Level
GW PWSs > !/2 Health Reference Level
SW PWSs > !/2 Health Reference Level
% PWSs > Health Reference Level
GW PWSs > Health Reference Level
SW PWSs > Health Reference Level
Illinois1
14,818
0.2%
<(ND)
91 Hg/L
Variable4
0.7 [ig/L
0.2|ig/L
0.1 [ig/L
1,139
1,030
109
0.97%
0.10%
9.17%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
Indiana2
1,033
0.1%
<(ND)
91 Hg/L
Variable4
0.2 jig/L
0.2 jig/L
0.2 jig/L
392
345
47
0.26%
0.29%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
Ohio3
4,039
0.0%
<(ND)
91^g/L
Variable4
Ojig/L
Ojig/L
Ojig/L
2,178
2,017
161
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
1 After an independent analysis of Illinois SDWA compliance monitoring data from 1993-1997 (U.S. EPA, 1999).
2 After an independent analysis of Indiana SDWA compliance monitoring data from 1993-1997 (U.S. EPA, 1999).
3 Summary results based on analysis of Ohio data from the SDWIS/FED UCM (1993), Round 2.
4 See text for discussion.
- PWS = Public Water Systems; GW = Ground Water; SW = Surface Water; MRL = Minimum Reporting Level (for laboratory analyses);
- HRL = Health Reference Level, an estimated health effect level used for preliminary assessment for this review
- The Health Reference Level (HRL) used for metribuzin is 91 fJg/L. This is a draft value for working review only.
- Total Number of Samples = the total number of analytical records for metribuzin
- 99h Percentile Concentration = the concentration value of the 99th percentile of either all analytical results or just the detections (in [ig/L)
- Median Concentration of Detections = the median analytical value of all the detections (analytical results greater than the MRL) (in ng/L)
- Total Number of PWSs = the total number of public water systems with records for metribuzin
- % PWS with detections, % PWS > 'A Health Reference Level, % PWS > Health Reference Level = percent of the total number of public water
systems with at least one analytical result that exceeded the MRL, '/z Health Reference Level, or Health Reference Level, respectively
Metribuzin February 2003
4-16
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Table 4-6 presents data from a number of national and state drinking water monitoring
studies with results in corn belt states. The National Pesticide Survey reports no detections for
metribuzin. Compliance monitoring from Ohio surface water PWSs show the highest detection
frequency of metribuzin by system (79.9%), but the data are from a targeted study of sensitive
surface waters so results may not be representative. The highest reported concentration of the
studies summarized in Table 4-6, 3.7 |ig/L, is well below the HRL. Environmental Working
Group reports were reviewed; however only, preliminary results were available from a special
study of finished tap water in 29 cities. Metribuzin was found in unspecified concentrations in
7% (2) of the 29 cities (Cohen et al., 1995).
The Iowa State-Wide Rural Well-Water Survey established a statistically significant
correlation between increasing well depth and decreasing pesticide contamination, as evidenced
by the lower detection frequency of metribuzin in drinking water wells >50 ft deep (Table 4-6).
Comparisons between raw and finished water in Iowa show detection frequencies of metribuzin
in surface water increased from the raw to finished state (Table 4-6; Hallberg, 1989). This is
probably a result of either analytical variance, imprecise matching between raw and finished
water samples, or pesticide adsorption to and subsequent release from, filtration/treatment
materials (Hallberg, 1989).
Regional Patterns
Occurrence results are displayed graphically by state in Figures 4-2 and 4-3 to assess
whether any distinct regional patterns of occurrence are present. Thirty-four states reported
Round 2 data but 10 of those states have no data for metribuzin (Figure 4-2). Another 21 states
did not detect metribuzin. The remaining 3 states detected metribuzin in drinking water and are
located on the east and west coasts of the United States (Figure 4-2). In contrast to the summary
statistical data presented in the previous section, this simple spatial analysis includes the biased
Massachusetts data.
Metribuzin February 2003 4-17
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Table 4-6. Metribuzin Occurrence in Midwest Drinking Water.
Ground Water Surveys
National Pesticide Survey (1988-90)1
Iowa State-Wide Rural Well-Water Survey 2
Wells < 50 ft deep
Wells > 50 ft deep
Special Surface Water Studies
Raw water
Iowa (1986)3
Illinois (1985-87)3
Finished water
Ohio (1993- )4
Iowa (1986)3
% sites
*MRL
nd
3.0%
1.4%
nr
nr
79.9%
nr
% samples
^MRL
nd
nr
nr
7.0%
15.0%
22.3%
12.0%
maximum
concentration
Oig/L)
nd
0.43
0.72
0.89
3.70
1.8
0.45
U.S. EPA, 1990; data are national results including some Midwestern states
2 Kr asset al, 1990
3 cited in Hallberg, 1989
4 U.S. EPA, 1999
- HRL = 91 fig/L
- MRLs vary by study.
- nd = results below the respective reporting level
-nr = "not reported"
The simple spatial analysis presented in Figures 4-2 and 4-3 does not suggest any special
regional patterns. Further, use and environmental release information (Chapter 3) and ambient
water quality data (Section 4.1) indicate that metribuzin has low detection even in non-drinking
water sources. According to TRI data, industrial releases have occurred since 1995 in only three
states and one U.S. territory (IA, MO, NB, Puerto Rico; U.S. EPA, 2000b). However, the use
patterns for metribuzin (Figure 3-1) do show that use is concentrated in soybean producing
regions (similar to the corn belt) in the Midwest states and along the Mississippi River Valley
production region. These states are missing from the Round 2 data, hence, a special review was
conducted to evaluate data from Iowa, Illinois, Indiana, and Ohio. Occurrence rates in these
states are much greater than other areas, but even in these states no PWSs had results > HRL.
Metribuzin February 2003
4-18
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Figure 4-2. States with PWSs with Detections of Metribuzin for All States with Data in
SDWIS/FED (Round 2).
All States
Metribuzin Detections in Round 2
1 States not in Round 2
] No data for Metribuzin
] States with No Detections (No PWSs > MRL)
| States with Detections (Any PWSs > MRL)
Metribuzin February 2003
4-19
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Figure 4-3. Round 2 cross-section states with PWSs with detections of metribuzin (any
PWSs with results greater than the Minimum Reporting Level [MRL];
above) and concentrations greater than the Health Reference Level (HRL;
below).
* State of Massachusetts is an mttlierwith 14.29% PWSs > MRL
Metribuzin Occurrence in Round 2
| | States not in Cross-Section
| | No data for Metribuzin
| | 0.00% PWSs > MRL
I ~| 0.01 - 1.00% PWSs > MRL
> 1.00% PWSs > MRL *
Metribuzin Occurrence in Round 2
| | States not in Cross-Section
gNo data for Metribuzin
0.00% PWSs > HRL
0.01 -1.00% PWSs > HRL
\~M > 1.00% PWSs > HRL
Metribuzin February 2003
4-20
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4.3 Conclusions
Detection frequencies and concentrations of metribuzin in ambient surface and ground
water are low, especially in ground water. Even so, it is one of the 21 most commonly detected
pesticides in ground water from the first round of NAWQA intensive data collection. The
annual mean frequency of metribuzin detection in surface water was less than 15% for all land-
use settings and concentrations. Midwestern ambient surface and ground water concentrations
and detection frequencies are also low. Releases of metribuzin to the environment were reported
in the TRI from only three states and one territory.
Metribuzin has been detected in PWS samples collected under the Safe Drinking Water
Act (SDWA). Cross-section occurrence estimates are very low with only 0.003% of all samples
showing detections. Significantly, the values for the 99th percentile and median concentrations
of all samples are less than the Minimum Reporting Level (MRL). For the Round 2 cross-
section samples with detections, both the median and the 99th percentile concentrations are 0.10
|ig/L. Systems with detections constitute approximately 0.007% of Round 2 cross-section
systems. National estimates for the population served by PWSs with detections using the cross-
section data are also low: approximately 1,000 people (about 0.0003% of the national PWS
population ) are served by PWSs with metribuzin detections > MRL, and no PWSs reported
detections > 1A HRL or > HRL. Using more conservative estimates of occurrence from all states
reporting SDWA Round 2 monitoring data, including states with biased data, 0.28% of the
nation's PWSs (approximately 182 systems and 3.4 million people served) are affected by
metribuzin concentrations > MRL, while no PWSs are affected by concentrations > /^ HRL or >
HRL.
The heaviest use of metribuzin is across the nation's corn-soybean production area.
These states are not well represented in the Round 2 database. Therefore, additional data from
the Midwest corn belt were also evaluated. Drinking water data from the corn belt states of
Iowa, Indiana, Illinois, and Ohio also show very low occurrence of metribuzin. Special targeted
surface water studies from Ohio have the highest detection frequency of metribuzin (79.9% of
systems). The pesticide was not detected above the Health Reference Level in any sample, with
the highest concentration at 20 |ig/L.
Metribuzin February 2003 4-21
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5.0 EXPOSURE FROM MEDIA OTHER THAN WATER
This section summarizes human population exposures to metribuzin from food, air, and
soil. The primary purpose is to estimate average daily intakes of metribuzin by members of the
general public. When exposure data on subpopulations were located, such as occupationally
exposed persons, these data were also summarized and included in this section.
5.1 Exposure from Food
5.1.1 Exposures of the General Population
Concentrations of Metribuzin in Food Items
Metribuzin is a triazine herbicide used to control small seeded grasses and broadleaf
weeds in crops such as soybeans, potatoes and sugarcane (Bouchard, 1982). Several studies
have evaluated its residues in food (as mentioned below). However, not all studies may be
representative of concentrations that the general population would typically be exposed to from
food items. In the animal studies summarized below, metribuzin concentrations administered in
feed were greater than those that would occur under typical feeding conditions. In the plant
studies summarized below, metribuzin residues in plant tissues and food products were measured
at the point of application. During the time lapse between food production and consumption by
the general public, metribuzin may further be metabolized and dissipated in the food product,
and also be removed through washing and food preparation. Thus, the metribuzin concentrations
reported in some studies are most likely higher than those that the general population would
encounter in their diets.
In 1999, approximately 9,438 domestically produced and imported food samples were
analyzed for 366 different pesticides as part of the Food and Drug Administration's (FDA)
Regulatory Monitoring Program. Metribuzin was not detected (detection limit not reported) in
any samples of grains, milk products, fruits or vegetables. Metribuzin was also not detected
(detection limit not reported) in any of the 218 domestic or 298 imported fish and shellfish
samples analyzed (US FDA, 1999).
One study examined the uptake and metabolism of metribuzin in soybeans. After a pre-
emergence soil application of 14C-metribuzin at 0.3 Ib ai/A (active ingredient/acre), soybean
plants and mature soybean seeds contained total radioactive residues (expressed as metribuzin
equivalents) of 12.1 ppm (mg/kg) and 0.48 ppm (mg/kg), respectively. The major metribuzin
metabolite in both soybean plants and mature seeds was the 6-(l,l-dimethylethyl)-3,5-(diketo)-
l,2,4-triazin-5-(2 H,4H)-dione (DADK) (U.S. EPA, 1998a).
Growth chamber experiments measured metribuzin absorption and distribution in
soybean plants during a 6-day period after emergence. Soybean plants grown in soils treated
with 0.28 kg/ha (kg/hectare) 14C-metribuzin contained metribuzin concentrations ranging from
8.86 to 9.99 |ig/g (mg/kg) metribuzin. Shoot concentrations were not as high as those in roots,
and little radioactivity was found in leaves of plants (Hargroder and Rogers, 1974).
Metribuzin February 2003 5-1
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Post-emergence treatment of wheat with 5-14C-metribuzin at 0.15 Ib ai/acre resulted in
0.2 ppm (ppm, expressed as metribuzin equivalents) total radioactive residues in wheat grains
after 33 days. About 9.3% of these residues consisted of metribuzin and its metabolites (U.S.
EPA, 1998a).
Field studies conducted in California, Delaware, Illinois, Michigan, New Jersey, Texas
and Washington evaluated metribuzin residues in carrots after post-emergence herbicide
treatment. Metribuzin and metribuzin metabolite residues in carrots were below the EPA
tolerance level of 0.3 ppm after multiple applications of up to four times the maximum allowable
rate (U.S. EPA, 1998a).
Cessna (1998) measured metribuzin residues in lentil crops in Canada. After post-
emergence application of 0.28 kg/ha, lentils contained residues of 1 mg/kg metribuzin. Residues
in lentils decreased five- and ten-fold after one and two weeks, respectively, and were not
detected after six weeks. At lentil maturity, metribuzin was below the detection limit of 0.02
mg/kg.
A ruminant (goat) metabolism study evaluated the distribution of metribuzin and its
metabolites in various tissues (U.S. EPA, 1998a). Goats were administered 410 ppm 5-14C-
metribuzin by diet for three consecutive days. This corresponds to approximately 59 times the
calculated dietary burden of metribuzin for ruminants. Total radioactive residues reported in
various tissues were 2.66 mg/kg in liver, 4.27 mg/kg in kidney, 0.97 mg/kg in fat, 0.44 mg/kg in
muscle, and 0.25-2.09 mg/kg in milk (U.S. EPA, 1998a). Because animals were administered
metribuzin in their diets at concentrations of up to 59 times their dietary burden, the resulting
tissue residues may tend to be up to 59 times higher than those that would occur under typical
feeding conditions.
In a poultry metabolism study, hens were given feed at a concentration of 400 ppm 5-14C-
metribuzin for three days. This corresponds to approximately 500 times the calculated dietary
burden of metribuzin for poultry. Radioactive residues of 33.6 mg/kg in liver, 36.3 mg/kg in
kidney, 1.6 mg/kg in muscle, and 0.2-1.0 mg/kg in eggs were reported (U.S. EPA, 1998a).
Because animals in this study were administered metribuzin in their diets at concentrations of up
to 500 times their dietary burden, the resulting tissue residues may tend to be up to 500 times
higher than those that would occur under typical feeding conditions.
Intake of Metribuzin from Food Items
From the studies mentioned above, the analysis of metribuzin in food items conducted by
the FDA Regulatory Monitoring Program appears to be most representative of general
population exposures to metribuzin in food items. Although additional studies reporting
metribuzin residue levels in food items were identified, these studies were conducted at dietary
concentrations that are higher than anticipated to occur under typical herbicide use conditions.
Metribuzin was not detected in 9,438 domestic and imported food items analyzed during FDA
Metribuzin February 2003 5-2
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pesticide regulatory monitoring (US FDA, 1999). Based on this, the typical average daily intake
of metribuzin from food for the general population is anticipated to be close to zero.
Both imported and domestic fish and shellfish samples analyzed for pesticides during
FDA regulatory monitoring did not contain metribuzin at detectable levels (US FDA, 1999).
Based on this, the typical average daily intake of metribuzin from fish and shellfish for the
general population is anticipated to be close to zero.
5.1.2 Exposures of Subpopulations
No evidence was located in the available literature indicating the existence of
subpopulations with dietary intakes of metribuzin different from those of the general population.
5.2 Exposure from Air
5.2.1 Exposures of the General Population
Concentrations of Metribuzin in Air
Information on ambient levels of metribuzin measured in air were not located in the
available literature.
Intake of Metribuzin from Air
Concentration data on ambient levels of metribuzin were unavailable to estimate average
intakes by the general population from air. However, based upon its physical properties,
metribuzin is not expected to be present in ambient air. Metribuzin is a solid at ambient
temperatures, and has a low vapor pressure (4.4 x 10"7 mm Hg at 20ฐC). Therefore, it is not
likely to readily partition into ambient air. Additionally, any partitioning of metribuzin into air
would most likely occur in areas where it is used. These areas are typically agricultural regions,
remote from the general population. Based upon this, ambient air concentrations of metribuzin
are most likely close to zero. Thus, the typical average daily intake for the general population is
anticipated to be close to zero.
5.2.2 Exposures of Subpopulations
Concentrations of Metribuzin in Air
Occupational exposures to metribuzin may occur as part of its regular use. Persons
involved in mixing, loading, applying or handling the various dry and liquid formulations of
metribuzin during its ground and aerial applications have the potential to be exposed.
Concentration data for metribuzin in air were not obtained from the available literature.
However, the EPA has estimated baseline inhalation exposures for workers involved in the
regular use of metribuzin as a herbicide ranging from 0.006 to 91.14 mg/day (U.S. EPA, 1998a).
The greatest inhalation exposure estimates are for persons involved in the handling of powdered
Metribuzin February 2003 5-3
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and dry bulk forms of metribuzin. The estimated daily inhalation exposure for a person mixing
wettable powder for application to sugarcane crops is 91.14 mg/day, and 31.3 mg/day for a
worker loading or mixing dry bulk fertilizer. The lowest daily inhalation exposures (0.006
mg/day) were seen in workers involved in the liquid applications of metribuzin to turf grass by
plane (U.S. EPA, 1998a).
Intake of Metribuzin from Air
The EPA estimated inhalation exposures for workers involved in the regular use of
metribuzin to range from 0.006 to 91.14 mg/day (U.S. EPA, 1998a). Dividing these estimates by
a body weight of 70 kg results in daily metribuzin intakes for adult workers ranging from 8.6 x
10'5 to 1.3 mg/kg-day.
5.3 Exposure from Soil
5.3.1 Exposures of the General Population
Concentrations of Metribuzin in Soil
Information was not located regarding metribuzin levels in residential soils. Metribuzin
is not labeled for residential use by homeowners or certified applicators. Thus, it is not
anticipated to be found in residential soils.
Intake of Metribuzin from Soil
Based on its approved uses, exposure to metribuzin in soil is not anticipated to be a
typical route of exposure for the general population. Because metribuzin is not labeled for
residential use by homeowners or certified applicators, the intake of metribuzin through soil by
most of the general population is probably close to zero.
5.3.2 Exposures of Subpopulations
Concentrations of Metribuzin in Soil
Several studies (Burgard et al., 1994; Brown et al., 1985; Dao, 1995; Gallaher and
Meuller, 1996) have reported metribuzin levels in agricultural soils, which may be a source of
occupational exposure for those involved in the handling or application of this chemical. Soil
concentrations were dependant upon application rate, and decreased over time following
application. At application rates ranging from 0.56 to 1.1 kg/ha, initial metribuzin
concentrations in soil ranged from 0.09 to 0.78 mg/kg. After 86-195 days, concentrations ranged
from 0.007 to 0.11 mg/kg.
Post-application exposures to metribuzin in soil may occur for persons entering areas that
have been treated with metribuzin. The general public may be exposed after the application of
metribuzin to turfgrass in public areas (e.g. parks, athletic fields, or golf courses) that have been
Metribuzin February 2003 5-4
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treated with metribuzin (U.S. EPA, 1998a). Specific information on post-application
concentrations in recreational areas or exposure estimates were not available in the obtained
literature.
Intakes of Metribuzin from Soil
At an application rate of 0.56 kg/ha, metribuzin concentrations in soil may be as high as
0.78 mg/kg upon first application, with levels decreasing over time (Gallaher and Mueller,
1996). At this concentration, and a daily soil intake of 480 mg/day for a contact intensive
worker (U.S. EPA, 1997), the maximum total daily intake of metribuzin for a 70 kg adult worker
would be 5.3 x 10"3 mg/kg-day.
5.4 Other Residential Exposures
Metribuzin may be transported from agricultural fields during runoff events. Sediment in
run-off water from winter wheat fields in eastern Washington contained metribuzin
concentrations ranging from below detection limits (200 |ig/kg sediment) to 3440 |ig/kg wet
weight (Brown et al., 1985). These samples were collected from three major runoff events
during 1979-1980 that produced more than 7.5 grams of sediment. Run-off samples generating
less than 7.5 grams of sediment were not analyzed for metribuzin in this study. Metribuzin
concentrations in 18 run-off water samples ranged from below detection limits (5 |ig/L) to 44
An additional study evaluated metribuzin concentrations in surface runoff samples
collected from midwestern streams during the first major runoff event after its application. The
90th percentile concentrations of metribuzin collected in 1989, 1994, and 1995 were 1.4, 1.2, and
0.5 ppb, respectively (U.S. EPA, 1998a).
5.5 Summary
Concentration and intake values for metribuzin in media other than water to the general
population and an occupationally exposed adult subpopulation are summarized in Tables 5-1 and
5-2. Based on the available information, the general population, on average, is not typically
expected to be exposed to metribuzin from food, air or soil. However, for persons who are
occupationally exposed to metribuzin, inhalation appears to be the main route of exposure to this
chemical.
Metribuzin February 2003 5-5
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Table 5-1. Exposures of the General Population to Metribuzin in Media Other Than
Water.
Parameter
Concentration in
medium
Estimated daily
intake (mg/kg-day)
Medium
Food
Adult
not detected
0.0*
Child
not detected
0.0*
Air
Adult
Child
NA
0.0*
0.0*
Soil
Adult
Child
NA
0.0*
0.0*
NA= Not Available in literature
* expected to be close to zero based upon physical properties and/or use of chemical (see
Sections 5.1.1, 5.2.1 and 5.3.1)
Table 5-2. Exposures of Subpopulations to Metribuzin in Media Other Than Water.
Parameter
Concentration in
medium
Estimated daily
intake (mg/kg-day)
Medium
Food
Adult Worker
NA
Air
Adult Worker
0.006 to 91. 14
mg/day
8.6x 10-5to 1.3 **
mg/kg-day
Soil
Adult Worker*
0.78 mg/kg
5.3 xlO'3***
mg/kg-day
NA= Not Available in literature.
= Unable to estimate based on available information
* Estimates are for a contact intensive worker, with direct soil contact.
** Based on U.S. EPA (1998a) estimates for inhalation exposures of workers handling or
applying metribuzin (see Section 5.2.2).
***High-end estimate based upon maximum soil concentration after application.
Metribuzin February 2003
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6.0 TOXICOKINETICS
6.1 Absorption
Based on urinary excretion data, 36-52% of orally administered metribuzin was absorbed
in Sprague-Dawley rats (U.S. EPA, 1993). In dogs, 52-60% of the administered oral dose was
absorbed (U.S. EPA, 1993). However, in a recent study, Mathew et al. (1998) reported poor
absorption (< 15%) of metribuzin in Sprague-Dawley rats fed extracts of soybean plants
containing 14C-metribuzin for 2 days (6,392-156,000 disintegrations per minute/g body weight).
6.2 Distribution
Mathew et al. (1998) reported on the distribution of 14C-metribuzin in Sprague-Dawley
male rats (n=4) after a 2-day feeding with a normal rat chow containing a methanol extract of
beans and shoots of soybean plants radiolabeled with metribuzin (6,392-156,000 dpm/g body
weight). There was no radioactivity detected in tissues such as heart, kidney, and liver,
suggesting that the metabolites of metribuzin, or the parent compound, are not accumulated.
6.3 Metabolism
Animal studies suggest that metribuzin undergoes deamination and deketonization during
its metabolism. The presence of metribuzin metabolites, diketo metribuzin and deaminated-
diketo metribuzin in urine was also reported (U.S. EPA, 1993). In a study conducted by Cain et
al. (1987), the authors reported the metabolism of metribuzin in rats to involve deamination,
dethioalkylation, hydroxylation of the t-butyl side chain and conjugation with glutathione.
6.4 Excretion
Studies in Wistar rats performed by Cain et al. (1987) used either a single low dose (5
mg/kg) of 14C-metribuzin (98.4-99.4% active ingredient; Specific Activity 20.8 mCi/nmol), or a
single high dose (500 mg/kg). No significant changes were observed in the rates or the routes of
14C-elimination between male and female rats in either the low-dose or high-dose administration
groups. In general, 27.3-43.4% of the radiolabel was excreted in the urine and from 55.8 to
71.5% of the radiolabel was excreted in feces after 96 hours.
About 90% of administered metribuzin in rats was excreted within 16 days in one study
or within 5 days by another. The half-life for elimination of radiolabeled metribuzin was
reported as 19.1-30.4 hours for male rats and 22.4-33.6 hours for female rats (U.S. EPA, 1993).
Moreover, in dogs, over 90% of the oral dose was excreted between 72 and 120 hours, with
about 52-60% excreted in the urine as metabolites or conjugates and about 30% excreted in the
feces predominantly as unchanged metribuzin (U.S. EPA, 1993).
Mathew et al. (1998) reported that approximately 85% of the radioactivity was
eliminated in the feces of rats 4 days after a 2-day feeding with extracts of soybean plants
Metribuzin February 2003 6-1
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containing radiolabeled metribuzin. About 1-8% of the radioactivity was eliminated in the
urine.
Metribuzin February 2003 6-2
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7.0 HAZARD IDENTIFICATION
7.1 Human Effects
7.1.1 Short-Term Studies
There are no short-term studies available which report the effect of metribuzin on human
health.
7.1.2 Long-Term and Epidemiological Studies
There are no long-term epidemiological studies available which have examined the
relationship between exposure to metribuzin and human health effects.
7.2 Animal Studies
7.2.1 Acute Toxicity
Animal studies have demonstrated that metribuzin exposure induces low acute toxicity.
The doses of metribuzin that cause acute toxic effects are summarized in Table 7-1 (U.S. EPA,
1998a). Kimmerle et al. (1969) found that metribuzin was not an eye irritant in a primary eye
irritation test in rabbits. In a primary dermal irritation study also conducted by Kimmerle et al.
(1969), metribuzin exposure produced very slight irritation of rabbit skin. However, metribuzin
exposure has not been shown to produce sensitization effects in guinea pigs (ACGIH, 1986).
7.2.2 Short-Term Studies
There are no short-term animal studies available which have examined the relationship
between metribuzin exposure and adverse health effects.
7.2.3 Subchronic Studies
Flucke and Hartmann (1989) evaluated systemic and dermal toxicity in New Zealand
rabbits (HC:NZW strain) that were dermally exposed to metribuzin (DIG 1468, technical 94%)
at 0, 40, 200, or 1,000 mg/kg-day (6 hours/day; 5 days/week) for 3 weeks. Neither dermal
irritation nor mortality were observed in the study. However, high-dose males and females did
demonstrate a dose-related increase in cholesterol. Triiodothyronine (T3) was decreased in all
males, but this decrease was statistically significant only at the high dose. The authors reported
statistically significant increases in liver enzymes such as N-demethylase and cytochrome P450
activities in high-dose males.
Chaisson and Cueto (1970) studied toxic effects in Beagle dogs (4 animals/sex/group)
orally fed metribuzin in the diet at 0, 50, 150 or 500 ppm for 90 days. No differences in body
Metribuzin February 2003 7-1
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Table 7-1. Acute Toxic Effects of Metribuzin
Species
Rats
Rats
Rat
Rat
Mouse
Cat
Guinea Pig
Guinea Pig
Rat and
Rabbit
Rabbit
Rat
Rat
Mouse
Rat
Rat
Mouse
Active
Ingredient
Not Specified
Technical
(% not
specified)
Not Specified
Not Specified
Not Specified
Not Specified
Not Specified
Not Specified
Not Specified
Technical
(% not
specified)
Not Specified
Not Specified
Not Specified
Not Specified
92.6%
Not Specified
Route of
Exposure
Oral
Oral
Oral
Oral
Oral
Oral
Oral
Oral
Dermal
Dermal
Percutaneous
Dermal
Inhalation
Inhalation
Inhalation
Intraperitoneal
Results
LD50: l,100mg/kg
LD50:
Males l,090mg/kg
Females l,206mg/kg
LD50:
Males 2,379 mg/kg
Females 2,794 mg/kg
LD50:
Males 2,300 mg/kg
Females 2,200 mg/kg
LD50: 698- 711 mg/kg
LD50: > 500 mg/kg
LD50:
Males 245 mg/kg
Females 274 mg/kg
LD50: 250 mg/kg
LD50: > 2,000 mg/kg
LD50: > 20,000 mg/kg
LD50: > 20,000 mg/kg
LD50: > 5,000 mg/kg
LC50: >860mg/m3
LC50: > 20,000 mg/m3
LC50: > 648 mg/m3
LD50: 210 mg/kg
Reference
Morgan, 1982
Crawford and
Anderson, 1974
Mobay Chemical,
1978a
Kimmerleetal.,1969
Hartley and Kidd, 1987
Hartley and Kidd, 1987
Crawford and
Anderson, 1974
Hartley and Kidd, 1987
ACGIH, 1986
Crawford and
Anderson, 1972
Hartley and Kidd, 1987
Mobay Chemical,
1978a
ACGIH, 1986
Mobay Chemical,
1978a
Shiotsuka, 1986
PCBPBS, 1984
Metribuzin February 2003
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weight gain or food consumption were observed at any of the doses tested. However, in both
male and female animals, dose-related increases in liver weight, and liverbody weight and
liverbrain weight ratios, were reported. Blood chemistry analysis did not reveal any differences
between the control and treated groups, except a small decrease in SGOT (serum glutamate-
oxaloacetate transaminase) and SGPT (serum glutamate-pyruvate transaminase) activities in the
high-dose male group at the end of the study. These findings implicate the liver as a possible
target organ. However the dose levels were not verifiable.
Loser et al. (1969) reported toxic effects in Wistar rats (5 animals/sex/group) fed
metribuzin at 0, 50, 150, 500 or 1,500 ppm for three months. The authors observed no
statistically significant changes in food consumption; however, there was a significant reduction
in body weight gain and an increase in liver and thyroid weights observed in the high-dose
(1,500 ppm) group. Pathology in the lung and liver was unremarkable in either the control or
treatment groups.
In studies conducted by Lindberg and Richter (1970), Beagle dogs (four/sex/dose) which
were administered oral doses of 50, 150 or 500 ppm (about 1.25, 3.75 or 12.5 mg/kg-day, based
on dietary assumptions of Lehman, 1959) of technical metribuzin for 90 days showed no
significant differences in body weights, food consumption, behavior, mortality, hematological
findings, urinalysis, gross pathology or histopathology.
In a study reported by ACGIH (1986), no effects were observed during a 3-week period
of daily dermal application of 1,000 mg metribuzin/kg. A 3-week inhalation study conducted in
rats (ACGIH, 1986) (aerosol exposure six hours daily, 5x/week) at an air concentration of 31
mg/m3 was without observable effects.
In a 21-day inhalation toxicity study, Thyssen (1981) administered metribuzin (DIG
1468, 93.1-98.2% active ingredient) at doses ranging from 0-720 mg/m3 daily for 6 hours.
Increased N-demethylase, O-demethylase and cytochrome P450 activities along with increased
liver and thyroid weights were noted in the high dose (720 mg/m3) group.
7.2.4 Neurotoxicity
There are no studies available which correlate exposure to metribuzin with neurotoxic
effects.
7.2.5 Developmental/Reproductive Toxicity
In a developmental toxicity (teratology) study, metribuzin (92.6% active ingredient) was
administered to pregnant Charles River rats (Crl:CD BR) in doses of 0, 25, 70 or 200 mg/kg-day
by gavage on gestation days 6-18. Maternal toxic effects such as a reduction in body weight
gain during the entire gestation period, and a decrease in food consumption, were observed at all
doses. The high-dose (200 mg/kg-day) group showed a statistically significant increase in
thyroid weight. A decrease in thyroxine (T4) levels was reported in both the 70 and 200 mg/kg-
day dose groups (Kowaski et al., 1986). In another developmental toxicity study, Machemer
Metribuzin February 2003 7-3
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(1972) reported a reduction in maternal weight gain only in the high-dose rat group (FB 30
Strain) fed metribuzin at 0, 5, 15, 50 and 100 mg/kg-day during gestation period days 6-15. No
evidence of fetal toxicity was reported in the rats administered metribuzin at doses of 100
mg/kg-day or below.
In a 3-generation reproduction study, Loser and Siegmund (1974) administered technical
metribuzin in the feed at dose levels of 0, 35, 100 or 300 ppm (about 0, 1.75, 5 or 15 mg/kg-day,
based on dietary assumptions of Lehman, 1959) to FB30 (Elberfeld breed) rats during mating,
gestation and lactation. Following treatment, fertility, lactation performance, and pup
development were evaluated. No treatment-related effects were reported at any dose tested.
In a developmental toxicity (teratology) study conducted by Clemens and Hartnagel
(1989), American Dutch rabbits (17 females/dose group) were dosed with 0,10, 30 or 85 mg/kg-
day of metribuzin (92.7% active ingredient) by gavage on gestation days 6-18. Maternal
toxicity was noted at doses of 30 mg/kg-day and above, based on a reduction in maternal body
weight gain on gestation days 18-28, and a decrease in food consumption on gestation days
7-19 at the high-dose level. A no-observable-adverse-effects-level (NOAEL) for maternal
toxicity was determined to be 10 mg/kg-day and a maternal lowest-observable-adverse-effects-
limit (LOAEL) of 30 mg/kg-day.
In another developmental toxicity (teratology) study, New Zealand white rabbits were
administered 0, 15, 45, or 135 mg/kg-day of metribuzin by gavage on gestation days 6-18.
Maternal systemic toxicity was noted at 45 mg/kg-day as reduced body weight gain, and reduced
food and water intake. Additionally, at 135 mg/kg-day there was an increased incidence of
abortions and decreased body weights. There were no significant differences between control
and treatment groups reported for the mean number of corpus lutea, implantation sites, early or
late resorptions, and live or dead fetuses (Unger and Shellenberger, 1981).
In a two-generation reproduction study conducted by Porter et al. (1988), Cr:CD BR rats
were exposed orally via diet, with 0, 30, 150 or 750 ppm of metribuzin (Sencorฎ technical
92.6% active ingredient). Compared to the control animals, the high-dose adult males and
females of both the F0 and Fx generations consumed less food and gained less body weight.
Necropsy findings on both the F0 and Fx generations were not affected by the metribuzin
treatment. There were no treatment related effects observed in the pathology of the reproductive
organs or the pituitary tissues. However, a dose-related increase in the hypertrophy of the
hepatocytes of the centrilobular and mid zonal regions was noted in the high-dose (750 ppm)
males and mid- and high-dose (150 and 750 ppm) females. No other biologically relevant
observations were noted. Therefore, for reproductive toxicity and systemic effects a NOAEL of
30 ppm and LOAEL of 150 ppm were established.
7.2.6 Chronic Toxicity
In a 2-year feeding study conducted by Loser and Mohr (1974), 40 Wistar rats/sex/group
received 25, 35, 100 and 300 ppm of metribuzin (99.5% pure) in their diets; 80 rats/sex served as
controls. These doses corresponded to 0, 1.3, 1.9, 5.3 and 14.4 mg metribuzin/kg/day,
Metribuzin February 2003 7-4
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respectively, in the males and 0, 1.7, 2.3, 6.5 and 20.4 mg metribuzin/kg/day, respectively, in
females. Metribuzin exposure resulted in no significant difference in either food consumption or
mortality rate when compared to the control groups. The body weights of the rats in the 25, 35
and 100 ppm dose groups (both sexes) at the end of 2-years did not differ significantly from their
respective controls. However, body weight gains in the male high-dose groups were
significantly decreased during weeks 70-80 and 90-100; high-dose female body weights were
significantly decreased from weeks 20-100.
Hayes (1981) investigated the systemic effects of metribuzin in a 2-year feeding study in
outbred CD-I mice. In this study, metribuzin technical (92.9% pure) dissolved in corn oil and
added to a commercial diet was administered to the mice (50/sex/group) at levels of 0, 200, 800
or 3,200 ppm (about 30, 120 or 480 mg/kg-day, based on the dietary assumptions of Lehman,
1959) for 104 weeks. The body weights of the treated males (all dose groups) and females (all
dose groups with the exception of 800 ppm group) did not differ significantly from those of the
control group. Metribuzin treatment induced inconsistent results in the hematological
parameters. Survival rates were not altered by metribuzin exposure.
In a 2-year feeding study conducted by Loser and Mirea (1974), four Beagle
dogs/sex/group were administered 0, 25, 100, or 1,500 ppm (0, 0.8, 3.4 or 55.7 mg/kg-day for
males; 0, 0.8, 3.6, or 55.3 mg/kg-day for females) of metribuzin (Bay 94 337 technical 99.5%) in
the diet. Mortality rates were observed in the high-dose (1,500 ppm) group at 75% in both males
and females. The clinical tests performed after twelve months of metribuzin exposure suggested
the presence of liver dysfunction in the dogs. Elevated activities of liver enzymes such as
SGOT, SGPT, OCT (ornithine-carbamyl transferase) and alkaline phosphatase along with an
increase in BSP (bromsulphthalein) retention were reported in the males. Increased SGPT, OCT
and serum protein levels were observed in high-dose females. There were no major changes in
kidney function. An increase in thyroid weight was observed in the high-dose groups of both
sexes.
Christenson and Wahle (1993) conducted a 2-year feeding study, in which Fischer 344
rats received 0, 30, 300 or 900 ppm (0, 1.3, 13.8, 42.2 mg/kg-day in males; 0, 1.6, 17.7, 53.6
mg/kg-day females) metribuzin (93.0% active ingredient) for 104 weeks. There were no major
changes in the food consumption or the mortality rates observed subsequent to metribuzin
exposure. A decrease in body weight gain was noticed in high-dose males (900 ppm) and mid-
and high-dose females (300 and 900 ppm). Increases in brain:body weight, heart:body weight,
kidney:body weight, and liverbody weight ratios were observed, in addition to increases in
thyroid weights and thyroid:body weight ratios, in the high-dose groups for both sexes. In
general, thyroxine (T4) levels increased in all dose levels, while triiodothyronine (T3) levels
decreased at all dose levels, but no other systemic effects were observed. A significant increase
in corneal neovascularization was observed in male rats receiving 300 and 900 ppm metribuzin.
An incidence of macroscopic changes in the 300 and 900 ppm metribuzin treated male rats
included a discolored zone in the liver, an enlarged adrenal and thyroid gland, ocular opacity, an
enlarged abdomen and epididymal mass. Ovarian cysts were detected in the 300 and 900 ppm
metribuzin treated females.
Metribuzin February 2003 7-5
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7.2.7 Carcinogenicity
Hayes (1981) conducted studies in which technical metribuzin was administered in the
diet to albino CD-I mice (50/sex/dose) at 200, 800 or 3,200 ppm (approximately 30, 120 or 480
mg/kg-day) for 24 months. Although some increase in the number of tumor-bearing animals was
observed in low- and mid-dose animals, significant increases in the incidence of specific tumor
types were not observed at any dose level. The authors concluded that, under the conditions of
the test, there was no increase in the incidence of tumors in mice.
Subsequently, statistical analysis of this data was performed by the EPA's Office of
Pesticide Programs using the Chi square test (U.S. EPA, 1993). Reevaluations resulted in a
statistically significant (p=0.037 and p=0.045, respectively) decrease in malignant and total
tumor-bearing male mice in the high-dose group. The number of tumor-bearing female mice
appeared to increase in the low-dose group (not statistically significant, p=0.071) and did
significantly increase in the middle-dose group (p=0.45 for malignant tumors and p=0.0499 for
benign tumors). The tumor incidence in high-dose females was comparable to that of the female
control group. The overall conclusion drawn was that, under the test conditions in the Hayes
(1981) study, metribuzin exposure did result in an increase of tumor incidence in mice.
In a 2-year feeding study by Loser and Mohr (1974), 40 Wistar rats/sex/group received
25, 35, 100 and 300 ppm metribuzin (99.5% pure) in their diets; while 80 rats/sex served as a
control group. These doses corresponded to 0, 1.3, 1.9, 5.3 and 14.4 mg metribuzin/kg/day,
respectively, in the males and 0, 1.7, 2.3, 6.5 and 20.4 mg metribuzin/kg/day, respectively, in
females. Loser and Mohr (1974) reported through initial evaluation that there were statistically
significant increases in the incidence of liver bile duct adenomas and pituitary gland adenomas in
the female high-dose groups by pair-wise comparison. The incidence of bile duct adenoma in
the females was 13/71, 4/10, 5/10, 1/10 and 19/35 in the control, 25, 35, 100 and 300 ppm
groups, respectively, while, the incidence in males was 19/66, 10/10, 8/10, 5/10 and 9/29,
respectively. The incidence of pituitary gland adenomas in the female control and high-dose
groups was 27/71 and 21/35 respectively, while in males the incidences were 10/62 and 6/29,
respectively. It was determined that the incidence of tumors in male rats was not significantly
different in any of the tissues examined.
The EPA's Office of Pesticide Programs reevaluated the original histopathological
findings reported by Loser and Mohr (1974). In this reevaluation, all of the female liver bile duct
adenomas were reclassified as bile duct proliferation. The pituitary glands from all animals were
also histopathologically reevaluated. It was determined that the incidences of pituitary adenoma
in the female groups were 16/71 (23%), 6/34 (18%), 9/31 (29%), 11/33 (33%) and 14/35 (40%)
in the control, 25-, 35-, 100- and 300-ppm groups, respectively (U.S. EPA, 1993).
In another 2-year feeding cancer study conducted by Christenson and Wahle (1993),
Fischer 344 rats received 0, 30, 300 or 900 ppm (0, 1.3, 13.8, 42.2 mg/kg-day in males and 0,
1.6, 17.7, 53.6 mg/kg-day females) of metribuzin (93.0% active ingredient) for 104 weeks. The
most significant change observed was thyroid follicular hyperplasia in male rats at the highest
dose. There were no significant changes in the neoplastic lesions of other tissues (kidney,
Metribuzin February 2003 7-6
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pituitary) observed in both the high-dose males and females. Christenson and Wahle (1993)
concluded that there was no evidence of carcinogenicity in any of the tissues examined.
7.3 Other Key Data
7.3.1 Mutagenicity/Genotoxicity
Metribuzin was determined to be nonmutagenic when tested in unspecified strains of
Salmonella typhimurium and Escherichia coll (Mobay Chemical Corp., 1977, 1978b).
Metribuzin exposure did not induce a reverse mutation in the D7 strain of Saccharomyces
cerevisiae either in the presence or absence of metabolic activation (Mobay Chemical Corp.,
1987). Metribuzin was determined to be negative when tested for dominant lethal effects in
male and female mice (unspecified strain) treated with doses of 300 mg metribuzin/kg (Mobay
Chemical Corp., 1974a, 1975, 1976). It was determined that doses of 100 mg metribuzin/kg did
not induce chromosomal aberrations in Chinese hamster spermatogonia (Mobay Chemical Corp.,
1974b). Metribuzin exposure did not cause a significant increase in the unscheduled DNA
synthesis when added to test cultures of rat primary hepatocytes (Mobay Chemical Corp., 1986a)
and was determined to be negative in the CHO/HGPRT mutation assay (Mobay Chemical Corp.,
1986b). However, S-9 activated (but not nonactivated) metribuzin was determined to be
clastogenic in CHO cells (Mobay Chemical Corp., 1990).
In vitro tests suggest that metribuzin (Sencor) exposure can result in adduct formation.
Using a 32P-postlabeling method, Shah et al. (1997) reported that adducts were formed when
metribuzin and its S9-metabolites (1 mM) were reacted with calf thymus DNA for 3.5 hours.
The adducts were analyzed using either nuclease PI or butanol enrichment method.
Benzo(a)pyrene was used as the positive control. Compared to the adducts formed in control
DNA (5.6 adducts per 109 nucleotides), metabolites of metribuzin produced 48.0 total adducts
per 109 nucleotides, as analyzed by nuclease PI enrichment method. Analysis of the adducts
produced by metribuzin, utilizing the butanol enrichment method, yielded three unique adducts.
The major adduct was produced at 281.5 total adducts per 109 nucleotides. Adduct formation by
metribuzin is less than the adduct formation from benzo(a)pyrene. Benzo(a)pyrene produced
1893 and 1707 adducts per 109 nucleotides as analyzed by the nuclease PI and the butanol
enrichment methods, respectively, under similar conditions. Utilizing nuclease PI and the
butanol enrichment methods, Shah et al. reported that, unlike benzo(a)pyrene, the type of
adducts formed by metribuzin metabolites were not similar. However, metribuzin (Sencor) did
test positive for DNA adduct formation, when using 32P-postlabeling with nuclease PI
enrichment.
Metribuzin exposure produced a negative response in the SOS Chromotest (DNA
damage) conducted in Escherichia coli with or without metabolic activation (Xu and Schurr,
1990). In a recent study, Venkat et al. (1995) reported mild genotoxic effects of metribuzin
using a similar SOS chromotest. The test compounds were tested in 10% dimethylsulfoxide
(DMSO) or in micellar solution in sodium taurocholate in order to simulate conditions that are
present in the gastrointestinal tract. The activity of the p-galactosidase was 199 and 636 per
[imole depending on whether metribuzin was dissolved in DMSO or sodium taurocholate
Metribuzin February 2003 7-7
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solution, respectively. The genotoxic activity of metribuzin was 26-43 times less than that of
the positive control, 4-nitroquinoline oxide (4-NQO). The activity of the p-galactosidase for the
positive control (4-NQO) was 8,557 and 16,734 per [imole in DMSO and sodium taurocholate
solutions, respectively.
7.3.2 Immunotoxicity
There are no studies available which examine the relationship between metribuzin
exposure and immunotoxic effects.
7.3.3 Hormonal Disruption
In vivo metribuzin exposure has been shown to affect the endocrine system. For
example, Porter et al. (1993) measured thyroxine and somatotropin levels in rats after exposure
to metribuzin. Metribuzin was administered orally in drinking water to Sprague-Dawley rats
(n=6; 125-150g) for 6 or 16 weeks. It was observed that the rats treated with metribuzin had
hyperthyroidism. Metribuzin exposure (0-10,000 ppm) caused a significant (p < 0.0005)
increase in the plasma thyroxine levels after 7, 13 or 16 weeks of exposure in both male and
female rats. It was determined that somatotropin levels were not affected by metribuzin
exposure.
Christenson and Wahle, (1993) reported a statistically significant increase in thyroxine
(T4) and decrease in triiodothyronine (T3) levels at all dose levels in Fischer 344 rats receiving
0, 30, 300 or 900 ppm metribuzin (93.0% active ingredient) for 104 weeks.
Flucke and Hartmann (1989) also observed a decrease in triiodothyronine (T3) in male
New Zealand rabbits exposed dermally to metribuzin (DIG 1468, technical 94%) at 0, 40, 200, or
1,000 mg/kg-day (6 hours/day; 5 days/week) for 3 weeks.
Kowaski et al. (1986) administered metribuzin (92.6% active ingredient) to pregnant
Charles River rats in doses of 0, 25, 70 or 200 mg/kg-day by gavage on gestation days 6-18.
The high-dose (200 mg/kg-day) group exhibited a statistically significant increase in thyroid
weight. A decrease in T4 levels was observed in both the 70 and 200 mg/kg-day dose groups.
7.3.4 Physiological or Mechanistic Studies
The major toxic effects that result from metribuzin exposure are changes in body weight
gain, survival rate, and liver and thyroid function. These effects are mainly systemic and the
mode of action has not been investigated. In addition, metribuzin exposure has also been shown
to affect hormonal levels such as triiodothyronine, thyroxine, and somatotropin.
7.3.5 Structure-Activity Relationship
There are no studies available which examine the structure-activity relationship of
metribuzin.
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7.4 Hazard Characterization
7.4.1 Synthesis and Evaluation of Major Non-Cancer Effects
While the studies relating to the metribuzin exposure on human health effects are lacking,
the hazard characterization is performed from the available animal studies. Studies conducted in
animals suggest that metribuzin exposure causes low acute toxicity as evidenced by the reported
high LD50 values (Kimmerle et al., 1969; Morgan, 1982; Hartley and Kidd, 1987). Also, acute
exposure studies suggest that metribuzin, at the doses tested, does not result in eye or dermal
irritations (Kimmerle et al., 1969). Subchronic studies suggest that metribuzin could cause
adverse effects in body weight gain, organ weight, and hematological parameters. For example,
a significant reduction in body weight gain and an increase in liver and thyroid weights were
reported in Wistar rats exposed to metribuzin at 1,500 ppm (Loser et al., 1969). Three weeks of
dermal exposure to metribuzin (1,000 mg/kg) in rabbits also resulted in an increase in liver
enzymes such as N-demethylase and cytochrome P450 (Flucke and Hartmann, 1989). These
effects are not pronounced when the studies were conducted at lower doses in dogs. Three-
month metribuzin exposure to Beagle dogs did not affect body weight gain or food consumption,
but altered the clinical parameters such as SGOT and SGPT levels (Chaisson and Cueto, 1970).
Chronic effects of metribuzin exposure may include changes in body weight gain,
mortality, liver enzyme activities and histopathological changes. Two-year feeding studies were
performed on rats (Loser and Mohr, 1974; Christenson and Wahle, 1993), mice (Hayes, 1981)
and Beagle dogs (Loser and Mirea, 1974). In general, there were no significant differences in
body weight gain, food consumption or mortality after two years of exposure to metribuzin to
rats (Loser and Mohr, 1974) and mice (Hayes, 1981). However, Christenson and Wahle (1993)
observed a decrease in body weight gain in rats after metribuzin treatment. The differences in
body weight gain observed in rats could possibly be attributed to the higher dose (900 ppm)
administered by Christenson and Wahle (1993) as compared to a maximum dose of 100 ppm
given by Loser and Mohr (1974).
Major histopathological changes reported by one study after chronic feeding of
metribuzin include a significant increase in corneal neovascularization, the incidence of a
discolored zone in the liver, an enlarged abdomen, enlarged adrenal and thyroid glands, ocular
opacity, and enlarged epididymal mass in male rats and the presence of ovarian cysts in female
rats (Christenson and Wahle 1993).
Chronic exposure to metribuzin (1,500 ppm) could cause a significant increase in the
mortality rate in Beagle dogs. Liver dysfunction was also observed as evidenced by elevation in
the activities of liver enzymes such as SGOT, SGPT and OCT. In addition, an increase in thyroid
weight was observed in the high dose group of both males and females (Loser and Mirea, 1974).
However, inconsistent hematological results were observed in mice following chronic exposure
to metribuzin (Hayes, 1981).
There are a few studies available on metribuzin treatment and developmental and
reproductive effects. These studies were performed using rats (Kowaski et al., 1986; Machemer,
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1972) and rabbits (Unger and Shellenberger, 1981; Clemens and Hartnagel, 1989). In general,
the maternal toxic effects are accompanied by little toxic effect to the fetus. These maternal
toxic effects are characterized by a reduction in body weight gain and food consumption. In a
two-generation reproduction study, Porter et al. (1988) reported that F0 and Fx generations
consumed less food and gained less body weight. Necropsy findings in both the F0 and Ft
generations were not affected by metribuzin exposure. Also, no treatment-related effects were
reported in a 3-generation reproduction study in rats (Loser and Siegmund, 1974).
There are no animal studies available which have examined neurotoxic or immunotoxic
effects of metribuzin. However, metribuzin exposure could produce some endocrine effects in
vivo. For example, evidence suggests that metribuzin could elevate plasma thyroxine (T4) levels
in rats (Porter et al., 1993; Christenson and Wahle, 1993) and decrease triiodothyronine (T3)
levels in rats (Christenson and Wahle, 1993) and rabbits (Flucke and Hartmann, 1989).
A few inhalation studies are available on metribuzin exposure and the effects are
comparable to the existing oral exposure studies. An increase in thyroid and liver weights as
well as liver enzyme activities such as N-demethylase, O-demethylase and cytochrome P450 was
reported in Wistar rats exposed to metribuzin at 720 mg/m3 (Thyssen, 1981).
7.4.2 Synthesis and Evaluation of Carcinogenic Effects
There are no human studies available which have examined the relationship between
exposure to metribuzin and cancer. Metribuzin exposure did not increase the incidence of
tumors in a lifetime dietary study using CD-I mice when compared to both concurrent and
historic controls (Hayes, 1981). In a 2-year feeding study utilizing Wistar rats, there were no
significant differences in neoplastic findings between the test and control groups (Loser and
Mohr, 1974; Christenson and Wahle, 1993). Short-term studies in bacteria and mammalian
systems suggest that metribuzin is not mutagenic. Recent in vitro studies suggest, however, that
metribuzin can induce adduct formation (Shah et al., 1997).
7.4.3 Mode of Action and Implications in Cancer Assessment
The available evidence from animal studies suggest that there is little data to support
metribuzin-induced carcinogenicity and therefore no studies are reported which examine the
mode of action of metribuzin for cancer effects.
7.4.4 Weight of Evidence Evaluation for Carcinogenicity
There are no studies identified that examine the carcinogenic effects of metribuzin on
humans. There are three lifetime studies which have been reported, one in mice and two in rats,
that examine the relationship between metribuzin exposure and tumor incidence. Evidence from
these animal studies is inadequate and therefore, metribuzin is classified as a class D carcinogen,
applying the criteria described in the EPA's guidelines for the assessment of carcinogenic risk
(U.S. EPA, 1986).
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7.4.5 Sensitive Populations
There are no human studies available which examine the toxic effects of metribuzin and
its effect on sensitive populations.
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8.0 DOSE-RESPONSE ASSESSMENT
8.1 Dose-Response for Non-Cancer Effects
8.1.1 RfD Determination
Choice of Principal Study
The oral Reference Dose (RfD) is based on the assumption that thresholds exist for
certain toxic effects. The RfD is expressed in units of mg/kg-day. In general, the RfD is an
estimate (with uncertainty spanning perhaps an order of magnitude) of a daily exposure to the
human population (including sensitive subgroups) that is likely to be without an appreciable risk
of deleterious effects during a lifetime.
The principal study utilized for RfD derivation, as recommended by the OPP/HED RfD
Committee, was the chronic study in rats conducted by Christenson and Wahle (1993) in rats
described in section 7.2.6. A 2-year feeding study was conducted in which Fischer 344 rats
received 0, 30, 300 or 900 ppm (0, 1.3, 13.8, 42.2 mg/kg-day in males; 0, 1.6, 17.7, 53.6 mg/kg-
day females) metribuzin (93.0% active ingredient) for 104 weeks. At 30 ppm (1.3 mg/kg-day for
males and 1.6 mg/kg-day for females), both sexes exhibited increased absolute and relative
thyroid weights, statistically significant increases in blood levels of thyroxine (T4), and
statistically significant decreases in blood levels of triiodothyronine (T3). Females also
exhibited decreased lung weight. However, these effects were considered to be of marginal
biological significance. Therefore, the RfD Committee determined that the 30 ppm dose (1.3
mg/kg-day in males) should be considered the NOAEL.
RfD Derivation
RfD = 1.3 mg/kg-dav = 0.013 mg/kg-day
100
Based on a chronic exposure study, an uncertainty factor of 100 was used to account for
inter-species extrapolation (10) and intra-species variability (10).
8.1.2 RfC Determination
There is insufficient data available from which to derive the RfC at this time.
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8.2 Dose-Response for Cancer Effects
In a study by Hayes (1981), metribuzin was orally administered via the diet to mice
(50/sex/dose) at dose levels of 200, 800 or 3,200 ppm (30, 120 or 480 mg/kg-day) for 24 months.
Following treatment, the incidence of tumor formation was analyzed in a variety of tissues.
Neoplasms of various tissues and organs were similar in type, location, time of occurrence, and
incidence in control and treated animals. The mice study is supported by the tumor incidence
data observed in 2-year feeding cancer studies in rats (Loser and Mohr, 1974; Christenson and
Wahle, 1993).
Applying the criteria described in EPA's guidelines for assessment of carcinogenic risk
(U.S. EPA, 1986), metribuzin should be classified in Group D: not classifiable as to human
carcinogenicity. This category is used for substances with inadequate animal evidence of
carcinogenicity.
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9.0 REGULATORY DETERMINATION AND CHARACTERIZATION OF RISK
FROM DRINKING WATER
9.1 Regulatory Determination for Chemicals on the CCL
The Safe Drinking Water Act (SOWA), as amended in 1996, required the U.S.
Environmental Protection Agency (EPA) to establish a list of contaminants to aid the agency in
regulatory priority setting for the drinking water program. EPA published a draft of the first
Contaminant Candidate List (CCL) on October 6, 1997 (62 FR 52193, U.S. EPA, 1997a). After
review of and response to comments, the final CCL was published on March 2, 1998 (63FR
10273, U.S. EPA, 1998). The CCL grouped contaminants into three major categories as follows:
Regulatory Determination Priorities - Chemicals or microbes with adequate data to
support a regulatory determination.
Research Priorities - Chemicals or microbes requiring research for health effects,
analytical methods, and/or treatment technologies.
Occurrence Priorities - Chemicals or microbes requiring additional data on occurrence in
drinking water.
The March 2, 1998 CCL included one microbe and 19 chemicals in the regulatory
determination priority category. More detailed assessments of the completeness of the health,
treatment, occurrence and analytical method data led to a subsequent reduction of the regulatory
determination priority chemicals to a list of 12 (one microbe and 11 chemicals) which was
distributed to stakeholders in November 1999.
SDWA requires EPA to make regulatory determinations for no fewer than five
contaminants in the regulatory determination priority category by August, 2001. In cases where
the Agency determines that a regulation is necessary, the Agency has two years to propose an
NPDWR and one and a half years to finalize the rule. The Agency is given the freedom to also
determine that there is no need for a regulation if a chemical on the CCL fails to meet the
statutory criteria established by SDWA and described in section 9.1.1.
9.1.1 Criteria for Regulatory Determination
These are the three criteria used to determine whether or not to regulate a chemical on the
CCL:
The contaminant may have an adverse effect on the health of persons
The contaminant is known to occur or there is a substantial likelihood that the
contaminant will occur in public water systems with a frequency and at levels of public
health concern
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In the sole judgment of the administrator, regulation of such contaminant presents a
meaningful opportunity for health risk reduction for persons served by public water
systems.
The findings for all three criteria are used in making a determination to regulate a
contaminant. As required by SDWA, a decision to regulate commits the EPA to publication of a
Maximum Contaminant Level Goal (MCLG) and promulgation of a National Primary Drinking
Water Regulation (NPDWR) for that contaminant. The Agency may determine that there is no
need for a regulation when a contaminant fails to meet one of the criteria. A decision not to
regulate is considered a final Agency action and is subject to judicial review. The Agency can
choose to publish a Health Advisory (a nonregulatory action) or other guidance for any
contaminant on the CCL, independent of the regulatory determination.
9.1.2 National Drinking Water Advisory Council Recommendations
In March 2000, the U.S EPA convened a Working Group under the National Drinking
Water Advisory Council (NDWAC) to help develop an approach for making regulatory
determinations. The Working Group developed a protocol for analyzing and presenting the
available scientific data and recommended methods to identify and document the rationale
supporting a regulatory determination decision. The NDWAC Working Group report was
presented to and accepted by the entire NDWAC in July 2000.
Because of the intrinsic difference between microbial and chemical contaminants, the
Working Group developed separate but similar protocols for microorganisms and chemicals.
The approach for chemicals was based on an assessment of the impact of acute, chronic, and
lifetime exposures, as well as a risk assessment that includes evaluation of occurrence, fate, and
dose-response. The NDWAC Protocol for chemicals is a semi-quantitative tool for addressing
each of the three CCL criteria. The NDWAC requested that the Agency use good judgement in
balancing the many factors that need to be considered in making a regulatory determination. The
U.S. EPA modified the semi-quantitative NDWAC suggestions for evaluating chemicals against
the regulatory determination criteria and applied them in decision making. The quantitative and
qualitative factors for metribuzin that were considered for each of the three criteria are presented
in the sections that follow.
9.2 Health Effects
The first criterion asks if the contaminant may have an adverse effect on the health of
persons. Because all chemicals have adverse effects at some level of exposure, the challenge is
to define the dose at which adverse health effects are likely to occur, and to estimate a dose at
which adverse health effects are either not likely to occur (threshold toxicant), or have a low
probability for occurrence (non-threshold toxicant). The key elements that must be considered in
evaluating the first criterion are the mode of action, the critical effect(s), the dose-response for
critical effect(s), the RfD for threshold effects, and the slope factor for non-threshold effects.
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A full description of the health effects associated with exposure to metribuzin is
presented in Chapter 7 of this document and summarized below in Section 9.2.2. Chapter 8 and
Section 9.2.3 present dose-response information.
9.2.1 Health Criterion Conclusion
Although there are no studies reporting the adverse effects of metribuzin on human
health, animal studies indicate that metribuzin has the potential to cause adverse health effects at
high doses. Exposure to metribuzin may occur primarily in an occupational setting, particularly
in the agriculture industry where it is used as an herbicide. The RfD of 0.013 mg/kg-day was
derived from a study reporting the adverse health effects of metribuzin in rats.
9.2.2 Hazard Characterization and Mode of Action Implications
There are no epidemiology studies that have assessed adverse human health effects
caused by exposure to metribuzin. Acute toxicity animal studies indicate that metribuzin induces
low toxicity as evidenced by the relatively high LD50 values (Kimmerle et al., 1969; Morgan,
1982; Hartley and Kidd, 1987). In addition, metribuzin has not been found to cause eye
irritation in rabbits, and causes only slight dermal irritation in rabbits (Kimmerle et al., 1969).
Subchronic studies in animals suggest that metribuzin may cause adverse effects on body
weight gain, organ weight and hematological parameters. Wistar rats exposed to metribuzin in
the diet at 1500 ppm for three months exhibited a significant reduction in body weight gain, and
increased liver and thyroid weights (Loser et al., 1969). However a 3-month dietary exposure in
Beagle dogs did not affect body weight gain or food consumption, and only altered clinical
parameters such as SGOT and SGPT levels (Chaisson and Cueto, 1970).
Chronic studies of metribuzin report effects on body weight gain, mortality, liver enzyme
activities and histopathological changes. Two-year feeding studies conducted in rats (0, 25, 35,
100 or 300 ppm) and mice (0, 200, 800 or 3200 ppm) indicated no significant differences in body
weight gain, food consumption, or mortality (Loser and Mohr, 1974; Hayes, 1981). Another
two-year feeding study in rats using a higher dose (900 ppm) of metribuzin did report a decrease
in body weight gain (Christenson and Wahle, 1993). This study also reported histopathological
changes such as significant increases in corneal neovascularization, discolored zones in the liver,
an enlarged abdomen, enlarged adrenal and thyroid glands, ocular opacity, an enlarged
epididymal mass in males, and the presence of ovarian cysts in female rats (Christenson and
Wahle, 1993). In Beagle dogs, chronic exposure to the highest dose of 1,500 ppm caused a
significant increase in the mortality rate and liver dysfunction as evidenced by increases in the
activity of liver enzymes such as SGOT, SGPT and OCT (Loser and Mirea, 1974). Thyroid
weight was also increased in the highest dose group. Histopathologic findings included liver and
kidney damage at the highest dose. The liver and kidney effects, decreased body weight gain,
and mortality at the highest dose are considered the critical effects of metribuzin exposure.
There are few studies that have assessed the developmental and reproductive effects of
metribuzin exposure. In general, maternal toxicity effects observed in rats and rabbits include
Metribuzin February 2003 9-3
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reduced body weight gain and food consumption, and are accompanied by slight toxicity to the
fetus (Kowaski et al., 1986; Machemer, 1972; Unger and Shellenberger, 1981; Clemens and
Hartnagel, 1989). A two-generation study in rats reported that both F0 and Fx generations
consumed less food and gained less body weight (Porter et al., 1988). Necropsy findings in both
generations were not affected by exposure to metribuzin. Another 3-generation reproduction
study in rats found no treatment-related effects (Loser and Siegmund, 1974).
No animal studies have addressed the neurologic or immunotoxic effects of metribuzin.
There is evidence of endocrine effects induced by metribuzin, including elevated plasma
thyroxine levels in rats and decreased triiodothyronine levels in rats and rabbits (Porter et al.
1993; Christenson and Wahle, 1993; Flucke and Hartmann, 1989).
The EPA has classified metribuzin as a class D carcinogen due to inadequate
carcinogenicity data in humans and animals. A lifetime dietary study in CD-I mice and 2-year
feeding studies in Wistar rats were negative for the induction of tumors compared to control
incidences (Hayes, 1981; Loser and Mohr, 1974; Christenson and Wahle, 1993).
9.2.3 Dose-Response Characterization and Implications in Risk Assessment
The principal study utilized for RfD derivation, as recommended by the OPP/FIED RfD
Committee, was the chronic study in rats conducted by Christenson and Wahle (1993) in rats
described in section 7.2.6. A 2-year feeding study was conducted in which Fischer 344 rats
received 0, 30, 300 or 900 ppm (0, 1.3, 13.8, 42.2 mg/kg-day in males; 0, 1.6, 17.7, 53.6 mg/kg-
day females) metribuzin (93.0% active ingredient) for 104 weeks. At 30 ppm (1.3 mg/kg-day for
males and 1.6 mg/kg-day for females), both sexes exhibited increased absolute and relative
thyroid weights, statistically significant increases in blood levels of thyroxine (T4), and
statistically significant decreases in blood levels of triiodothyronine (T3). Females also
exhibited decreased lung weight. However, these effects were considered to be of marginal
biological significance. Therefore, the RfD Committee determined that the 30 ppm dose (1.3
mg/kg-day in males) should be considered the NOAEL. The RfD of 0.013 mg/kg-day was
derived by dividing the NOAEL by an uncertainty factor of 100, which was used to account for
inter- and intra-species variability.
9.3 Occurrence in Public Water Systems
The second criterion asks if the contaminant is known to occur or if there is a substantial
likelihood that the contaminant will occur in public water systems with a frequency and at levels
of public health concern. In order to address this question, the following information was
considered:
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Monitoring data from public water systems
Ambient water concentrations and releases to the environment
Environmental Fate
Data on the occurrence of metribuzin in public drinking water systems were the most
important determinants in evaluating the second criterion. EPA looked at the total number of
systems that reported detections of metribuzin, as well as those that reported concentrations of
metribuzin above an estimated drinking water health reference level (HRL). For noncarcinogens
the estimated HRL risk level was calculated from the RfD assuming that 20% of the total
exposure would come from drinking water. For carcinogens, the HRL was the 10"6 risk level.
The HRLs are benchmark values that were used in evaluating the occurrence data while the risk
assessments for the contaminants were being developed.
The available monitoring data, including indications of whether or not the contamination
is a national or a regional problem, are included in Chapter 4 of this document and are
summarized below. Additional information on production, use, and environmental fate are
found in Chapters 2 and 3.
9.3.1 Occurrence Criterion Conclusion
The available data on metribuzin production and use indicate a modestly declining trend.
Although detection of metribuzin is found in both surface and ground waters of urban and
agricultural regions, concentrations are extremely low and well below the HRL or half the HRL.
In regards to drinking water, metribuzin detection frequencies and concentrations are extremely
low to undetectable, with the exception of detections in Pennsylvania, Indiana, Illinois and
Washington. These data indicate that although metribuzin is found in ambient waters, little to no
metribuzin is detected in drinking water systems.
9.3.2 Monitoring Data
Drinking Water
A national cross-section of 20 states reported metribuzin detection data in the
SDWIS/FED database. This cross-section provides a good representation of the nation's varied
climatic and hydrogeologic regions, and the breadth of pollution potential. In addition,
occurrence data is presented from all the states participating under the Unregulated Contaminant
Monitoring (UCM) program begun in 1991. Metribuzin was not included in this program until
Round 2, which began in 1993.
In the cross-section of 20 states, approximately 0.007% of Public Water Systems (PWS)
reported detections of metribuzin above the minimum reporting level (MRL), affecting about
0.0003% of the population. Only the state of Washington reported a metribuzin detection above
the MRL, at a level of 0.10 |ig/L, which is far below the Health Reference Level (HRL) of 91
l-ig/L. A national extrapolation of this data indicates that approximately 5 PWSs would
Metribuzin February 2003 9-5
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experience detections of metribuzin above the MRL, and that approximately 1,000 people would
be affected.
When all the states participating in Round 2 of the UCM program were considered,
0.28% of PWSs experienced detections above the MRL. This indicates that approximately
1.61% of the population, or 3.4 million people nationally, is affected by concentrations of
metribuzin above the MRL. No PWSs experienced detections > 1A HRL or > HRL. Therefore,
0% of the population is affected by metribuzin concentrations > 1A HRL or > HRL. The median
and 99th percentile concentrations of detections are 1.0 |ig/L and 3 |ig/L, respectively.
Ambient Water
The USGS began the National Ambient Water Quality Assessment (NAWQA) program
in 1991 to monitor water quality status and trends in the U.S. This program consists of 59
watersheds and aquifers referred to as "study units" and represents approximately two-thirds of
the overall water usage and a similar proportion of the population served by public water
systems. The Method Detection Limit (MDL) for metribuzin is 0.004 |ig/L.
Detection frequencies and concentrations of metribuzin in ambient surface and ground
waters are low. Surface waters exhibited the highest maximum concentration of metribuzin at
0.530 |ag/L, with a reported frequency of 13.82% of samples with concentrations greater than the
MDL. Although the occurrence in ground water is lower than in surface water, detection in
1.95% of ground water samples at a maximum concentration of 0.300 |ig/L make metribuzin one
of the 21 most commonly detected pesticides in intensive NAWQA monitoring. In both surface
and ground waters, metribuzin was more frequently detected in agricultural regions as compared
to urban areas.
9.3.3 Use and Fate Data
Metribuzin, a synthetic organic compound, is a selective triazinone herbicide used to
discourage growth of broadleaf weeds and annual grasses among vegetable crops and turf grass.
Using data from the USDA and NCFAP, the EPA estimates that the average annual use for the
years 1990-94 at 2.8 million pounds of active ingredient with 8.5 million acres treated. The
USGS estimates that 2.7 million pounds of metribuzin treating 8.4 million acres were used in
1992. The non-agricultural use of metribuzin is minimal. Table 3-1 of Chapter 3 indicates a
modest decline of metribuzin use from 1990-1999.
Data from the Toxic Release Inventory (TRI) indicate a general decline in environmental
releases of metribuzin between 1995 and 1998 (Table 3-2). Air emissions are reported to have
declined although surface water discharges have increased.
Metribuzin is a solid at ambient temperatures and has a low vapor pressure. Therefore, it
is unlikely to readily partition to air. Since metribuzin is not labeled for residential use, it is not
anticipated to be found in residential soils. The Organic Carbon Partition Coefficient (Koc) is 95,
and indicates that metribuzin is highly mobile in soil. It is also moderately adsorbed on soils
with high clay or organic content and leaches more readily from sandy soils. In soil,
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biodegradation is the primary fate process (HSDB, 2000). In the aquatic environment,
volatilization from water and bioconcentration in fish are not anticipated to be relevant (HSDB,
2000). No data are available for the biodegradation of metribuzin in water.
9.4 Risk Reduction
The third criterion asks if, in the sole judgement of the Administrator, regulation presents
a meaningful opportunity for health risk reduction for persons served by public water systems.
In evaluating this criterion, EPA looked at the total exposed population, as well as the population
exposed above the estimated HRL. Estimates of the populations exposed and the levels to which
they were exposed were derived from the monitoring results. These estimates are included in
Chapter 4 of this document and summarized in Section 9.4.2 below.
In order to evaluate risk from exposure through drinking water, EPA considered the net
environmental exposure in comparison to the exposure through drinking water. For example, if
exposure to a contaminant occurs primarily through ambient air, regulation of emissions to air
provides a more meaningful opportunity for EPA to reduce risk than regulation of the
contaminant in drinking water. In making the regulatory determination, the available
information on exposure through drinking water (Chapter 4) and information on exposure
through other media (Chapter 5) were used to estimate the fraction that drinking water
contributes to the total exposure. The EPA also evaluated effects on potentially sensitive
populations, including fetuses, infants and children. The sensitive population considerations are
included in Section 9.4.4.
9.4.1 Risk Criterion Conclusion
Based on the data from the cross-section analysis of 20 states, metribuzin exposure from
drinking water would be very low with only 1,000 people exposed nationally. When all the
Round 2 data are considered, including data from the state of Pennsylvania, approximately 3.4
million people nationally are exposed to any concentration of metribuzin. Aside from the
potential of occupational exposure, no other source of exposure would lead to significant doses
of metribuzin. These observations indicate that regulation of metribuzin in drinking water would
have little impact on human risk reduction.
9.4.2 Exposed Population Estimates
As described in 9.3.1, a cross-section survey of 20 states reported that 0.007% of Public
Water Systems had detections of metribuzin above the minimum reporting level (MRL),
affecting less than 0.0003% of the population. A national extrapolation of this data indicates that
approximately 1,000 people would be exposed to metribuzin through the drinking water. Of the
20 states in this cross-section survey, only the state of Washington reported a detection of
metribuzin. Since Washington is the only state to report a metribuzin detection at 0.10 |ig/L, this
value is both the median and 99th percentile concentrations.
Metribuzin February 2003 9-7
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However, when all of the participating states in Round 2 of the UCM program were
considered, 0.28% of PWSs reported detections above the MRL. National extrapolation of this
data indicates that approximately 1.6% of the population, or 3.4 million people, are exposed to
concentrations above the MRL.
9.4.3 Relative Source Contribution
Relative source contribution analysis compared the magnitude of exposure to metribuzin
expected via drinking water and the magnitude of exposure from other media, such as food, air
and soil. The intake of metribuzin from drinking water can be calculated from the median
concentrations described above for both the cross-section study and the study of all the Round 2
states. Using the median metribuzin level from the 20 state cross-section study of 0.10 |ig/L, an
average daily intake of 2 L/day for an adult, and an average weight of 70 kg for an adult, the
corresponding dose would be 2.8 x 10"3 mg/kg-day for adults. For children, assuming an intake
of 1 L/day and an average weight of 10 kg, the dose would be 1.0 x 10"2 mg/kg-day.
As part of the FDA's Regulatory Monitoring Program, 9,438 domestic and imported food
samples were analyzed for pesticides, including metribuzin. Metribuzin was not detected in any
samples of grains, milk products, fruits or vegetables. In addition, no detections were found in
218 domestic and 298 imported fish and shellfish samples. The daily intake of metribuzin from
food is anticipated to be close to zero.
No data are available for the ambient levels of metribuzin in air. However, metribuzin is
a solid at ambient temperatures and has a low vapor pressure; partitioning of metribuzin into air
is highly unlikely. Therefore, the average daily intake for the general population is anticipated to
be close to zero. However, inhalation of metribuzin may be potentially significant for
occupational exposure. The occupational subgroup may include workers involved in the mixing,
loading, handling and application of metribuzin. The EPA has estimated that inhalation
exposures of this subgroup range from 0.006 to 91.14 mg/day. Calculations of doses based on
this range of exposure and 70 kg body weight are 8.6 x 10"5 to 1.3 mg/kg-day.
Metribuzin is not labeled for residential use and so it is not anticipated to be found in
residential soils. General population exposures are anticipated to be close to zero. In
agricultural regions where metribuzin is applied, metribuzin may be found in soils in
concentrations as high as 0.78 mg/kg. Based on an average body weight of 70 kg and a daily soil
intake of 480 mg/day, the maximum daily intake for a contact intensive worker would be 5.3 x
10"3 mg/kg-day, which is below the RfD.
For estimating the HRL from the RfD, the default value of 20% was used for the relative
source contribution assuming that the total exposure to metribuzin is not from drinking water.
9.4.4 Sensitive Populations
No sensitive populations to metribuzin have been identified.
Metribuzin February 2003 9-8
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9.5 Regulatory Determination Summary
Although there is evidence from animal studies that metribuzin may cause adverse health
effects at high doses, its occurrence in public water systems and the numbers of people
potentially exposed through drinking water are low. In addition, there are no available studies,
either epidemiological or case-studies of accidentally exposed agricultural workers, assessing
adverse health effects in humans due to metribuzin. Overall, metribuzin is not anticipated to
cause adverse health effects in humans at the concentrations detected in public water systems
and is unlikely to expose a large number of people outside of an occupational setting. For these
reasons, EPA may not propose to regulate metribuzin with NPDWR. All final determinations
and future analysis will be presented in the Federal Register Notice covering CCL proposals.
Metribuzin February 2003 9-9
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APPENDIX A: Abbreviations and Acronyms
AA
ACGffl
a.i.
ANPRM
APHA
ARMS
ATSDR
CA
CAS
CASRN
CCL
CEC
CERCLA
CMR
CPS
CWS
DBCP
DCIs
DBA
DWEL
BCD
EDB
EDL
Eh
EHS
EPA
EPCRA
ESA
Atomic Absorption
American Conference of Governmental Industrial Hygienists
active ingredient
Advanced Notice of Proposed Rule-Making
American Public Health Association
Agricultural Resources Management Study
Agency for Toxic Substances and Disease Registry
Census of Agriculture
Chemical Abstract Service
Chemical Abstract Service Registry Number
Contaminant Candidate List
cation exchange capacity
Comprehensive Environmental Response, Compensation &
Liability Act
Chemical Monitoring Reform
Cropping Practices Survey
Community Water System
dibromochloropropane
Data Call-Ins
deethyl-atrazine
Drinking Water Equivalent Level
Electron Capture Detectors
ethylene dibromide
Estimated Detection Limit
oxidation-reduction potential
Extremely Hazardous Substance
Environmental Protection Agency
Emergency Planning and Community Right-to-Know Act
ethanesulfonic acid
FCRS
FDA
FIFRA
FQPA
GAC
GC
GW
GWP
Farm Costs and Returns Survey
Food and Drug Administration
Federal Insecticide, Fungicide, and Rodenticide Act
Food Quality Protection Act
granular activated carbon (treatment technology for organic
compounds)
gas chromatography (a laboratory method)
ground water
ground water - purchased
Metribuzin February 2003
A-l
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GUDI
GUP
HA
HAL
HRL
IDL
IGWM
IOC
IRIS
MCL
MDL
MMT
MRL
MS
NAWQA
NCFAP
NCOD
NDWAC
NERL
NIOSH
NPD
NPDES
NPDWR
NFS
NRMRL
NTIS
NTNCWS
NTTAA
OA
OCT
OGWDW
OMB
ORD
OSHA
PAH
PB
PBMS
PCE
PEL
Ground Water Under the Direct Influence (of surface water)
Ground Water Under Direct Influence - Purchased
Health Advisory
Health Advisory Level
Health Reference Level
Instrument Detection Level
Iowa Ground Water Monitoring Program
inorganic compound
Integrated Risk Information System
Maximum Contaminant Level
Method Detection Limit
methylcyclopentadienyl manganese tricarbonyl
Minimum Reporting Level
mass spectrometry (a laboratory method)
National Water Quality Assessment Program
National Center for Food and Agricultural Policy
National Drinking Water Contaminant Occurrence Database
National Drinking Water Advisory Council
National Environmental Research Laboratory
National Institute for Occupational Safety and Health
nitrogen/phosphorus detector
National Pollution Discharge Elimination System
National Primary Drinking Water Regulation
National Pesticide Survey
National Risk Management Research Laboratory
National Technical Information Service
Non-Transient Non-Community Water System
National Technology Transfer and Advancement Act
oxanilic acid
ornithine-carbamyl transferase
Office of Ground Water and Drinking Water
Office of Management and Budget
Office of Research and Development
Occupational Safety and Health Administration
polycyclic aromatic hydrocarbon
particle beam
performance-based measurement system
tetrachloroethylene
permissible exposure limit
Metribuzin February 2003
A-2
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PGWD
ppm
PWS
PWSF
PWSID
QA
QC
RCRA
RFA
RFF
RO
RPD
RSD
SARA Title III
SBREFA
SD
SDWA
SDWIS
SDWIS FED
SGOT
SGPT
SM
SMCL
SMF
SOC
SPE
SRF
STORE!
SW
SWP
TBD
TCE
IDS
THM
TNCWS
TPQ
TRI
UCM
UCMR
Pesticides in Ground Water Database
part per million
Public Water System
Public Water System Facility
Public Water System Identifier
quality assurance
quality control
Resource Conservation and Recovery Act
Regulatory Flexibility Act
Resources for the Future
reverse osmosis
relative percent difference
relative standard deviation
Superfund Amendments and Reauthorization Act
Small Business Regulatory Enforcement Fairness Act
standard deviation
Safe Drinking Water Act
Safe Drinking Water Information System
the Federal Safe Drinking Water Information System
serum glutamate-oxaloacetate transaminase
serum glutamate-pyruvate transaminase
standard methods
Secondary Maximum Contaminant Level
Standard Compliance Monitoring Framework
synthetic organic compound
solid phase extraction (a laboratory method)
State Revolving Fund
Storage and Retrieval System
surface water
surface water - purchased
to be determined
tri chl oroethy 1 ene
total dissolved solids
trihalomethane
Transient Non-Community Water System
Threshold Planning Quantity
Toxic Release Inventory
Unregulated Contaminant Monitoring
Unregulated Contaminant Monitoring Regulation/Rule
Metribuzin February 2003
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UMRA - Unfunded Mandates Reform Act of 1995
URCIS - Unregulated Contaminant Monitoring Information System
USDA - United States Department of Agriculture
U.S. EPA - United States Environmental Protection Agency
USGS - United States Geological Survey
VOC - volatile organic compound
|ig/L - micrograms per liter
mg/L - milligrams per liter
>MCL - percentage of systems with exceedances
>MRL - percentage of systems with detections
Metribuzin February 2003 A-4
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APPENDIX B: Round 2 Metribuzin Occurrence
Metribuzin Occurrence in Public Water Systems in Round 2, UCM (1993) results
State
Tribes (06)
AK
AL
AR
AZ
CA
CO
CT
IN
KY
LA
MA
MD
ME
MI
MN
MO
MS
NC
ND
NH
NJ
NM
OH
OK
OR
PA
RI
Total
Unique
PWS
1
20
536
750
69
418
56
684
2,650
1,264
638
623
296
557
715
2,178
107
1,135
358
15
#GW
PWS
1
17
431
538
35
204
29
627
2,570
1,234
437
567
258
524
686
2,017
82
984
231
6
#SW
PWS
0
3
105
212
34
214
27
57
80
30
101
56
38
33
29
161
25
151
127
9
% PWS
>MRL
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
14.29%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
9.50%
0.00%
%GW
PWS
>MRL
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
13.79%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
5.63%
0.00%
%sw
PWS
>MRL
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
14.81%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
16.54%
0.00%
% PWS
>HRL
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
%GW
PWS>
HRL
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
%sw
PWS>
HRL
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
99%
Value
Glg/L)
<0.00
<0.00
<0.00
<0.00
<0.00
< 10.00
2.00
<0.30
<0.00
<0.00
<0.60
<0.00
<0.02
<0.00
<0.60
<2.00
<0.00
<0.00
3.00
<0.53
Metribuzin February 2003
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Appendix B (continued)
State
SC
SD
TN
TX
VT
WA
WI
Total
20 States
19 States
Total
Unique
PWS
940
7
426
390
600
15,333
13,568
13,512
#GW
PWS
842
2
121
338
530
13,311
11,862
11,833
#SW
PWS
98
5
305
52
70
2,022
1,706
1,697
% PWS
>MRL
0.00%
0.00%
0.00%
0.00%
0.17%
0.28%
0.07%
0.01%
%GW
PWS
>MRL
0.00%
0.00%
0.00%
0.00%
0.19%
0.14%
0.04%
0.01%
%SW
PWS
>MRL
0.00%
0.00%
0.00%
0.00%
0.00%
1.24%
0.23%
0.00%
% PWS
>HRL
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
%GW
PWS>
HRL
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
%sw
PWS>
HRL
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
99%
Value
Gig/L)
<0.00
<0.00
<0.20
<0.00
<0.00
<2.00
<2.00
<2.00
1. Massachusetts data not included in "19 States" summary for metribuzin.
PWS = Public Water System; GW = Ground Water (PWS Source Water Type); SW = Surface Water (PWS Source Water Type); MRL = Minimum Reporting Limit
(for laboratory analyses)
The Health Reference Level (HRL) is the estimated health effect level as provided by EPA for preliminary assessment for this work assignment.
"% > HRL" indicates the proportion of systems with any analytical results exceeding the concentration value of the HRL.
The Health Reference Level (HRL) used for Metribuzin is 91 |ig/L. This is a draft value for working review only.
The highlighted States are part of the SDWI/FED 20 State Cross-Section.
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