EPA 815-R-O1-O1O
__,,„,„,_„ November, 2OO1
:W«»KFW'!'r''i"'nii«HI,>--i',iiiti: 'UN .,„..,'«» I u,i» « i. i; f ~ ii;,.!-
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Preliminary Regulatory Determination Support Document for Metribuzin
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Disclaimers
This document is designed to provide supporting information regarding the regulatory determination
for metribuzin as part of the Contaminant Candidate List (CCL) evaluation process. This document is not
a regulation, and it does not substitute for the Safe Drinking Water Act (SDWA) or the Environmental
Protection Agency's (EPA's) regulations. Thus,it cannot impose legally-binding requirements on EPA,
States, or the regulated community, and may not apply to a particular situation based upon the
circumstances. Mention of trade names or commercial products does not constitute endorsement or
recommendation for use.
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Preliminary Regulatory Determination Support Document for Metribuzin
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Acknowledgments
This document was prepared in support of the EPA's Office of Ground Water and Drinking Water
regulatory determination for metribuzin as part of the Contaminant Candidate List (CCL) evaluation
process. Dan Olson and Karen With served as EPA's team leaders for the CCL regulatory determination
process and James Taft as Standards and Risk Management Division Chief. Tara Cameron and Karen
Wirth served as Work Assignment Managers. The CCL Work Group provided technical guidance
throughout. In particular, Karen Wirth, Dan Olson, and Joyce Donohue provided scientific and editorial
guidance. External expert reviewers and many stakeholders provided valuable advice to improve the
CCL Program and this document The Cadmus Group, Inc., served as the primary contractor providing
support for Ihis work. The major contributions of Matt Collins, Emily Brott, and Ashton Koo are
gratefully acknowledged. George Hallberg served as Cadmus'Project Manager.
ill
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Preliminary Regulatory Determination Support Document for Metribuzin
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USEPA, Office of Water Report: EPA815-R-01-010, November, 2001
CANDIDATE CONTAMINANT LIST
PRELIMINARYREGULATORY DETERMINATION
SUPPORT jpOqUMENT FO?L METRIBUZIN
Executive Summary
Metribuzin is a 1998 Contaminant Candidate List (CCL) preliminary regulatory determination
priority contaminant Metribuzin is one of Hie contaminants being considered by the U.S. Environmental
Protection Agency (EPA) for a regulatory determination. The available data on occurrence, exposure,
and other risk considerations suggest that regulating metribuzin may not present a meaningful opportunity
to reduce health risk. EPA presents preliminary CCL regulatory determinations and further analysis in
the Federal Register Notice, v
To make the preliminary regulatory determination for metribuzin, EPA used approaches guided by
the National Drinking Water Advisory Council's (NDWAC) Work group on CCL and Six-Year Review.
The Safe Drinking Water Act (SDWA) requirements for National Primary Drinking Water Regulation
(NPDWR) promulgation guided protocol development. The SDWA Section 1412(bXl)(A) specifies that
die determination to regulate a contaminant must be based on a rinding that each of the following criteria
are met (i) "the contaminant may have adverse effects on the health of persons"; (ii) "the contaminant is
known to occur or mere is substantial likelihood that the contaminant will occur in public water systems
with a frequency and at levels of public health concern"; and (iii) "in the sole judgement of the
Administrator, regulation of such contaminant presents a meaningful opportunity for health risk reduction
for persons served by public water systems." Available data were evaluated to address each of the three
statutory criterion. •"•
Metribuzin, a synthetic organic compound (S0C), is a selective triazinone herbicide used mostly to
discourage growth of broadleaf weeds and annual grasses among vegetable crops and turf grass.
Metribuzin accomplishes this by inhibiting photosynthesis. It is commonly applied to soybeans, potatoes,
alfalfe, sugarcane, barley, and tomatoes. Use patterns for metribuzin show that use is concentrated in the
soybean producing regions in the Midwest States (equivalent to the corn belt) and along the Mississippi
River Valley production region.
Metribuzin was monitored from 1993 to 1999 under the SDWA Unregulated Contaminant
Monitoring (UCM) program. In addition, EPA has recommended guidelines for exposure to metribuzin
in drinking water through a health advisory of 200 ug/L. The sale, use, and distribution of metribuzin is
controlled under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) and metribuzin is also
a Toxic Release Inventory (TRT) chemical requiring public reporting of environmental releases from
certain industrial sectors. Releases of metribuzin to the environment were reported in the TRI from.three
States and one territory.
Metribuzin has been detected in ambient surface and ground waters as noted by the United States
Geological Survey's (USGS) National Water Quality Assessment (NAWQA) program. Detection
frequencies and concentrations are low, especially in ground water. Even so, metribuzin was 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 is less than 15% of all
samples for all land-use settings. For ground water, the annual mean detection frequency is less than 4%
of all samples across land-uses. Maximum concentrations are below 1 fig/L for all surface and ground
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Preliminary Regulatory Determination Support Document for Metribuzin
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water sites, well below the Health Reference Level (HRL) of 91 ug/L, a preliminary reference level used
for this analysis. Midwestern ambient surface and ground water concentrations and detection frequencies
are also low.
Metribuzin has also been detected in public water system (PWS) samples collected under SDWA.
Occurrence estimates from a cross-section of States with UCM data are very low with only 0.003% of
samples showing detections. For the cross-section samples with detections, both the median and the 99th
percentile concentrations are 0.10 ug/L. Systems with detections constitute approximately 0.007% of
cross-section systems. Estimates of the national 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 ) may be served by PWSs with metribuziu detections. No PWSs reported detections greater
than half the HRL, Using more conservative estimates of occurrence from all States reporting SDWA
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 greater than the
minimum reporting level (MRL), while no PWSs are affected by concentrations above one half the HRL
or above the HRL.
Because the heaviest use of metribuzin is across the nation's corn-soybean production area, additional
data from the Midwest corn belt were also evaluated to supplement the cross-section data. 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 HRL in any sample, with the
highest concentration at 20 ug/L.
Exposure to metribuzin occurs primarily in occupational settings, particularly in the agriculture
industry where it is used as an herbicide. 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. Chronic studies of metribuzin, for instance, have reported effects on body
weight increases, liver enzyme activities, histopathological changes, and mortality.
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 tlirough
drinking water are low. Thus metribuzin does not appear to occur with a frequency, or at levels, of public
health concern.
VI
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Table of Contents
Disclaimers i
Acknowledgments iii
Table of Contents vii
List of Tables ix
List of Figures . xi
1.0 INTRODUCTION I'. 1 1
1.1 Purpose and Scope 1
1.2 Statutory Framework/Background .' .. 1
1.3 Statutory History of Metribuzin 2
1.4 Regulatory Determination Process 2
1.5 Determination Outcome 4
2.0 CONTAMINANT DEFINITION . 4
2.1 Physical and Chemical Properties 4
2.2 Environmental Fate/Behavior , 5
3.0 OCCURRENCE AND EXPOSURE .... 6
3.1 Use and Environmental Release . 6
3.1.1 Production and Use 6
3.1.2 Environmental Release 8
3.2 Ambient Occurrence 9
3.2.1 Data Sources and Methods 9
3.2.2 Results 10
3.2.2.1 NAWQA National Synthesis 10
3.2.2.2 Water Quality Investigations from the Corn Belt 12
3.3 Drinking Water Occurrence 13
3.3.1 Data Sources, Data Quality, and Analytical Approach 14
3.3.1.1 UCM.Rounds 1 and2 14
3.3.1.2 Developing a Nationally Representative Perspective — 14
3.3.1.2.1 Cross-Section Development 15
3.3.1.2.2 Cross-Section Evaluation 16
3.3.1.3 Data Management and Analysis 17
3.3;1.4 Occurrence Analysis 18
3.3.1.5 Additional Drinking Water Data from the Corn Belt .. 19
3.3.2 Results J 20
3.3.2.1 Occurrence Estimates 20
3.3.2.2 Occurrence in the Corn Belt 21
3.3.2.3 Regional Patterns ." 24
3.4 Conclusion 27
4.0 HEALTH EFFECTS 27
4.1 Hazard Characterization and Mode of Action Implications 27
4.2 Dose-Response Characterization and Implications in Risk Assessment 28
vu
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4.3 Relative Source Contribution 29
4.4 Sensitive Populations 29
4.5 Exposure and Risk Information 29
4.6 Conclusion 30
5.0 TECHNOLOGY ASSESSMENT .... 30
5.1 Analytical Methods , 30
5.2 Treatment Technology 31
6.0 SUMMARY AND CONCLUSIONS - DETERMINATION OUTCOME 32
References 35
Appendix A: Abbreviations and Acronyms 41
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List of Tables
Table 2-1: Physical and chemical properties 5
Table 3-1: Metribuzin use, 1990-1999 8
Table 3-2: Environmental releases (in pounds) for metribuzin in the United States, 1995-1998
9
Table 3-3: Metribuzin detections and concentrations in streams and ground water 11
.i
Table 3-4: Metribuzin detections in shallow ground water from various land-use settings 12
Table 3-5: Metribuzin occurrence in Midwest surface and ground water 13
Table 3-6: Summary occurrence statistics for metribuzin 22
Table 3-7: SDWA compliance monitoring data from the States of Illinois, Indiana, and Ohio 23
Table 3-8: Metribuzin occurrence in Midwest drinking water 24
Table 5-1: Analytical methods for metribuzin 31
IX
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List of Figures
Figure 3-1: Estimated annual agricultural use for metribuzin (1992) 7
Figure 3-2: Geographic distribution of cross-section States for Round 2 (SDWIS/FED) 16
Figure 3-3: States with PWSs with detections of metribuzin for all States with data in SDWIS/FED
(Round 2) , 25
Figure 3-4: 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) 26
XI
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Preliminary Regulatory Determination Support Documentfor Metribuzin
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1.0 INTRODUCTION
1.1 Purpose and Scope
This document presents scientific data and summaries of technical information prepared for, and used
in, the Environmental Protection Agency's (EPA) regulatory determination for metribuzin. Information
regarding metribuzin's physical and chemical properties, environmental fate, occurrence and exposure,
and health effects is included. Analytical methods and treatment technologies are also discussed.
Furthermore, the regulatory determination process is described to provide the rationale for the decision.
1.2 Statutory Framework/Background
The Safe Drinking Water Act (SDWA), as amended in 1996, requires the United States
Environmental Protection Agency (EPA) to publish a list of contaminants (referred to as the Contaminant
.Candidate List, or CCL) to assist in priority-setting efforts. The contaminants included on the CCL were
not subject to any current or proposed National Primary Drinking Water Regulations (NPDWR), were
known or anticipated to occur in public water systems, and were known or suspected to adversely affect
public health. These contaminants therefore may require regulation under SDWA. The first Drinking
Water CCL was published on March 2,1998 (USEPA, 1998d; 63 FR 10273), and a new CCL must be
published every five years thereafter.
The 1998 CCL contains 60 contaminants, including 50 chemicals or chemical groups, and 10
microbiological contaminants or microbial groups. The SDWA also requires the Agency to select 5 or
more contaminants from the current CCL and determine whether or not to regulate these contaminants
with an NPDWR. Regulatory determinations for at least 5 contaminants must be completed 3% years
after each new CCL.
Language in SDWA Section 1412(b)(l)(A) specifies that the determination to regulate a contaminant
must be based on a finding mat each of the following criteria are met:
Statutory Finding i: ...the contaminant may have adverse effects on the health of persons;
Statutory Finding ii: the contaminant is known to occur or there is substantial likelihood that
the contaminant will occur in public water systems with a frequency and at levels of public
health concern; and
Statutory Finding Hi: in the sole judgement of the Administrator, regulation of such
contaminant presents a meaningful opportunity for health risk reduction for persons served
by public water systems.
The geographic distribution of the contaminant is another factor evaluated to determine whether it
occurs at the national, regional or local level. This consideration is important because the Agency is
charged with developing national regulations and it may not be appropriate to develop NPDWRs for
regional or local contamination problems.
EPA must determine if regulating this CCL contaminant will present a meaningful opportunity to
reduce health risk based on contaminant occurrence, exposure, and other risk considerations. The Office
of Ground Water and Drinking Water (OGWDW) is charged with gathering and analyzing the
occurrence, exposure, and risk information necessary to support this regulatory decision. The OGWDW
must evaluate when and where this contaminant occurs, and what would be the exposure and risk to
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public health. EPA most evaluate the impact of potential regulations as well as determine the appropriate
measure(s) for protecting public health.
For each of the regulatory determinations, EPA must first publish in the Federal Register the draft
determinations for public comment EPA will respond to the public comments received, and will then
finalize regulatory determinations. If the Agency finds that regulations are warranted, the regulations
must then be formally proposed within twenty-four months, and promulgated eighteen months later. EPA
has determined that there is sufficient information to support a regulatory determination for metribuzin.
1.3 Statutory History of Metribuzin
Metribuzin has been monitored under the SDWA Unregulated Contaminant Monitoring (UCM)
program since 1993 (USEPA, 1992; 57 FR 31776). Monitoring ceased for small public water systems
(PWSs) under a direct final rule published January 8,1999 (USEPA, 1999a; 64 FR 1494), and ended for
large PWSs with promulgation of the new Unregulated Contaminant Monitoring Regulation (UCMR)
issued September 17,1999 (USEPA, 1999b; 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 continuing monitoring under the new UCMR. list.
EPA previously recommended guidelines for exposure to metribuzin in drinking water through a
health advisory (USEPA, 1988). As part of the CCL process, health effects data have been reviewed.
These are summarized in Section 4.0 of this document
Metribuzin is regulated or monitored by other federal programs as well. As a pesticide, its sale, uses
and distribution is controlled under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA).
JFJJPKA was amended in 1996 under the Food Quality Protection Act (FQPA). FIFRA requires
registration of all pesticides with EPA, and certain labeling, application, and use restrictions. Moreover,
pesticide manufacturing plants must be registered, and the manufacturer must provide EPA with scientific
data regarding the product's efficacy and demonstrating that it does not pose an unreasonable risk to
people or the environment (USEPA, 1998c). Metribuzin was first registered in the U.S. in 1973, and a
Registration Standard was issued for it by EPA in 1985 (USEPA, 1998b). The registration standard
classified metribuzin as "restricted use" because of questions regarding its potential to leach to ground
water and chronic toxicity. Data submitted by the manufacturer later resolved those questions and the'
restricted use classification was discontinued (EXTOXNET, 1998). Data Call-ins (DCIs) were issued hi
1991 and 1995, requiring additional scientific data on ecological effects, product chemistry,
environmental fate, and ground water impacts (USEPA, 1998b). Metribuzin was reregistered in 1998 and
is classified as a general use pesticide (USEPA, 1998a).
Metribuzin is also a Toxic Release Inventory (TRI) chemical. The TRI was established by the
Emergency Planning and Community Right-to-Know Act (EPCRA). EPCRA requires certain industrial
sectors to publicly report the environmental release or transfer of chemicals included in this inventory.
1.4 Regulatory Determination Process
In developing a process for the regulatory determinations, EPA sought input from experts and
stakeholders. EPA asked the National Research Council (NRC) for assistance in developing a
scientifically sound approach for deciding whether or not to regulate contaminants on the current and
future CCLs. The NRC's Committee on Drinking Water Contaminants recommended mat EPA: (1)
gather and analyze health effects, exposure, treatment, and analytical methods data for each contaminant;
(2) conduct a preliminary risk assessment for each contaminant based on the available data; and (3) issue
a decision document for each contaminant describing the outcome of the preliminary risk assessment
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The NRC noted that in using this decision framework, EPA should keep in mind the importance of
involving all interested parties.
One of the formal means by which EPA works with its stakeholders is through the National Drinking
Water Advisory Council (NDWAC). The NDWAC comprises members of the general public, State and
local agencies, and private groups concerned with safe drinking water, and advises the EPA Administrator
on key aspects of the Agency's drinking'water program. The NDWAC provided specific
recommendations to EPA on a protocol to assist the Agency in making regulatory determinations for
current and future CCL contaminants. Separate but similar protocols were developed for chemical and •
microbial contaminants. These protocols are intended to provide a consistent approach to evaluating
contaminants for regulatory determination, and to be a tool that will organize information in a manner that
will communicate the rationale for each determination to stakeholders. The possible outcomes of the
regulatory determination process are: a decision to regulate, a decision not to regulate, or a decision that
some other action is needed (e.g., issuance of guidance).
The NDWAC protocol uses the three statutory requirements of SDWA Section 1412(b)(l)(A)(i)-(iii)
(specified hi section 1.2) as the foundation for guiding EPA in making regulatory determination
decisions. For each statutory requirement, evaluation criteria were developed and are summarized below.
To address whether a contaminant may have adverse effects on the health of persons (statutory
requirement (i)), the NDWAC recommended that EPA characterize the health risk and estimate a health
reference level for evaluating the occurrence data for each contaminant.
Regarding whether a contaminant is known to occur, or whether there is substantial likelihood that
the contaminant will occur, in public water systems with a frequency, and at levels, of public health
concern (statutory requirement (ii)), the NDWAC recommended that EPA consider: (1) the actual and
estimated national percent of public water systems (PWSs) reporting detections above half the health
reference level; (2) the actual and estimated national percent of PWSs with detections above the health
reference level; and (3) the geographic distribution of the contaminant.
To address whether regulation of a contaminant presents a meaningful opportunity for health risk
reduction for persons served by public water systems (statutory requirement (in)) the NDWAC
recommended that EPA consider estimating the national population exposed above half the health
reference level and the national population exposed above the health reference level.
The approach EPA used to make preliminary regulatory determinations followed the general format
recommended by the NRC and the NDWAC to satisfy the three SDWA requirements under section
1412(b)(l)(A)(i)-(iii). The process was independent of many of the more detailed and comprehensive
risk management factors that will influence the ultimate regulatory decision making process. Thus, a
decision to regulate is the beginning of the Agency regulatory development process, not the end.
Specifically, EPA characterized the human health effects that may result from exposure to a
contaminant found in drinking water. Based on this characterization, the Agency estimated a health
reference level (HRL) for each contaminant
For each contaminant EPA estimated the number of PWSs with detections >ViHRL and >HRL, the
population served at these benchmark values, and the geographic distribution, using a large number of
occurrence data (approximately seven million analytical points) that broadly reflect national coverage.
Round 1 and Round 2 UCM data, evaluated for quality, completeness, bias, and representativeness, were
the primary data used to develop national occurrence estimates. Use and environmental release
information, additional drinking water data sets (e.g., State drinking water data sets, EPA National
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Preliminary Regulatory Determination Support Document for Metribuzin
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Pesticide Survey, and Environmental Working Group data reviews), and ambient water quality data (e.g.,
NAWQA, State and regional studies, and the EPA Pesticides in Ground Water Database) were also
consulted.
The findings from these evaluations were used to determine if there was adequate information to
evaluate the three SDWA statutory requirements and to make a preliminary determination of whether to
regulate a contaminant
1.5 Determination Outcome
After reviewing the best available public health and occurrence information, EPA has made a
preliminary determination not to regulate metribuzin with an NPDWR. This preliminary determination is
based on the finding that metribuzin is not known to occur, nor is it likely to occur, in public water
systems with a frequency, or at levels, of public health concern. All preliminary CCL regulations
determinations are presented in the CCL Federal Register Notice. The following sections summarize the
data used by the Agency to reach this preliminary decision.
2.0 CONTAMINANT DEFINITION
Metribuzrn, a synthetic organic compound (SOC), is a white crystalline solid with a moderately sharp
sulfurous odor (EXTOXNET, 1998; USEPA, 1998a). It is a selective triazanone herbicide used primarily
to discourage growth of broadleaf weeds and annual grasses among vegetable crops and turf grass.
Metribuzin accomplishes this by inhibiting photosynthesis (EXTOXNET, 1998;.USEPA, 1998a).
Common uses include application to soybeans, potatoes, alfalfa, sugarcane, barley, and tomatoes (Larson
etal., 1999; USEPA, 1998a).
2.1 Physical and Chemical Properties
Table 2-1 lists summary information regarding metribuzin's physical and chemical properties. Also
included are its CAS Registry Number and molecular formula.
4
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Table 2-1; Physical and chemical properties
CAS number
Molecular Formula
-21087-64-9
Boiling Point
Melting Point
Molecular Weight
LogK,,.
Water Solubility
Vapor Pressure
Henry's Law
Constantf
approx.126 °C
214.28 g/mol
1.61
1.70*
l,200ppmat200C
ilO-5mmHgat25°C
1.43
ctfer USEPA, 1998a; * USDA, 1999
fnote: this quantity is expressed in a dimensionlessform,
2.2 Environmental Fate/Behavior .
When metribuzin is released to the environment, it does not volatilize from either water or land
surfaces. This property, along with its high solubility in water and low soil adsorption potential, make it
available to runoff to surface waters and likely to leach to ground water (USEPA, 1998b). EPA considers
it among a group of pesticides most likely to contaminate ground water (EXTOXNET, 1998). Once in
the saturated zone, it is expected to persist because its primary degradation routes are through soil
microbial degradation and photolytic degradation on soil surfaces. Moreover, it is not subject to
hydrolysis with a hydrolysis half-life of 9-28 weeks (USEPA, 1998b; EXTOXNET,1998).
Metribuzin has a low soil adsorption potential, and is consequently easily leached, except where soils
have a high clay and/or organic matter content. Under these conditions, the half life of metribuzin can be
extended to several months. Other soil properties that promote adsorption of metribuzin, and therefore
increase the persistence of the compound in soil, are low soil moisture, low temperatures, and acidic
conditions (EXTOXNET, 1998). While photodegradation from soil surfaces is a primary degradation
route (half life: 2.5 days), its importance is diminished because probably only the top 1 millimeter of soil
is exposed to direct sunlight This is reflected in terrestrial field-dissipation half lives of 15-149 days. Its
aerobic soil metabolism half-life is estimated to be between 40-106 days (USEPA, 1998a).
In shallow surface waters with good light penetration, degradation by aqueous photolysis may be
rapid (half life: 4.3 hours). However, if the surface water is turbid, metribuzin will be more likely to
persist since light penetration will be minimal and metribuzin is stable to hydrolysis (USEPA, 1998a;
EXTOXNET,1998).
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3.0 OCCURRENCE AND EXPOSURE
This section examines the occurrence of metribuzin in drinking water. While no complete national
database exists of unregulated or regulated contaminants in drinking water froto public water systems
(PWSs) collected under the Safe Drinking Water Act (SDWA), this report aggregates and analyzes
existing State data that have been screened for quality, completeness, and representativeness. Populations
served by PWSs exposed to metribuzin are estimated, and the occurrence data are examined for regional
or other special trends. To augment the incomplete national drinking water data and aid in the evaluation
of occurrence, information on the use and environmental release, as well as ambient occurrence of
metribuzin, is also reviewed.
3.1 Use and Environmental Release
3.1.1 Production and Use
Recent national estimates of agricultural use for metribuzin are available. Using its own proprietary
data, data from the United States Department of Agriculture (USDA) and the National Center for Food
and Agricultural Policy (NCFAP), the USEPA (1998a) estimates 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) estimates approximately 2.7 million pounds
of active ingredient used for the year 1992, with roughly 8.4 million acres treated (USGS, 1999a). 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 U.S.
for 1992. A breakdown of use by crop is also included. Again, the map was compiled using State-level
data sets on pesticide use rates available from the National Center for Food and Agricultural Policy
(NCFAP) and county-level data on harvested crop acreage from the Census of Agriculture (CA). As
such, 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 (USEPA, 1998a).
. Metribuzin use patterns have been documented by the USDA as well. USDA 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, whiter wheat, spring and durum wheat, and fall potatoes).
Farm operators are surveyed for crop practice information on a field-by-field basis (USDA, 1997; USDA,
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 par square mie
of county par year
D No Estimated Use
El =>2.648
Total
^OP* PcundsAppled
soybeans
potatoes
aifettahay
sugarcane: sugar&arod
wheat and grains
tuiiulcos
jtekJ and gpass^^nd
txjm
, flfinsytfTOif}
drypeas
1,707.148
377,255
216,321
189,811
82,793
48,913
29, 7M
26,825
14,377
8,574
PcnsGifl
NaBoreilUse
6113
13.95
7.SS
7.0B
a os
1.81
1.M
a 99
a 53
as
after USGS, 1998b
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Table 3-1; Mctribuzin 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
1,4^8
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
6^37
6,705
7,706
8,924
Data for the years 1990-1995, after VSDA, 1997
Data for the years 1996-1999, after USDA, 2000
'avcragejigure based on available data
3.1.2 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 m. 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 a TRI chemical (USEPA, 1996; USEPA, 2000d).
Under these conditions, facilities are required to report the pounds per year of metribiizin 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, and decrease throughout the period of record. A sharp
decrease is evident between the 1996 and 1997 reporting years, resulting in a decreasing trend for total
on- and off-site releases. Interestingly, over the period for which data is available (1995-1998), surface
water discharges generally increase. Again, the trend is exaggerated between the reporting years 1996
and 1997. Whether these abrupt shifts reflect actual jumps or drops in surface water discharges and air
emissions, respectively, 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; USEPA, 2000b).
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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
afterUSEPA. 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 manufecture and processing of 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. In addition, the TRI data is meant to reflect releases and should not be used to
estimate general exposure to a chemical (USEPA, 2000c; USEPA, 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 U.S. (Figure 3-1). Estimated
annual use appears to be modestly declining in the last decade (Table 2-1). Metribuzin is also a TRI
chemical. Industrial releases have been reported since 1995 in three States and one U.S. territory. On-
site releases to air constitute the majority of these reported releases, and decline throughout the period of
record.
3.2 Ambient Occurrence
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 U.S. are being produced through the USGS's National 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.)
3.2.1 Data Sources and Methods
The USGS instituted the NAWQA program in 1991 to examine water quality status and trends in the
U.S. NAWQA is designed and implemented in such a manner as 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 U.S. 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
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through intensive assessment over a ten-year period (Leahy and Thompson, 1994), The first round of
intensive monitoring (1991-96) targeted 20 study units.. This first group was more heavily 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., 2000; Larson et al., 1999; USGS, 1999b).
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 Minimum Reporting Level (MRL) for menibuzin
is 0.004 ug/L (Kolpin et al., 1998), substantially lower than most drinking water monitoring reporting
levels.
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, USEPA's 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.
3.2.2 Results
3.2.2.1 NAWQA National Synthesis
Detection frequencies and concentrations of metribuzin in ambient surface and ground water are low,
especially in ground water (Table 3-3). Most herbicides monitored in the first round of the NAWQA
program were detected in the greatest concentrations and frequencies in surface water as compared to
ground water. Surface waters show the highest maximum concentration of metribuzin at 0.5 ug/L, well
below the Health Reference Level (HRL) of 91 ug/L.
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 3-3). Larson and others (1999) found that for 50 stream sites monitored over a 1 year
period, one site had a detection frequency of greater than 50% of all samples (detections were reported for
metribuzin concentrations ^0.01 ug/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 surface water, detection in more than
1% of ground water samples at concentrations greater than or equal to 0.05 ug/L make 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 ug/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 to ground water (USGS, 1998c; USEPA, 1998b;
EXTOXNET, 1998). Also, the herbicide ranks among the top 200 agricultural pesticides in use (USGS,
1999b).
Herbicides often demonstrate detection frequencies in streams that correlate with patterns of use
(USGS, 1998c). Patterns of pesticide use often do not correlate with detection frequency in ground water,
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Preliminary'Regulatory Determination Support Document for Metribuzin
concUti^^^ conditions (depth and type of aquifer, soil
- . ,
, demonstrate a statistically significant correlation between detection
frequency andjntensity of use (Kolpin et al, 1998). Metribuzin detection frequence M^er °
n^TSi^^Tf ^ Tr*" comP^ with shallow groSd water in urban iSs
SS i 5'*2f • ?*f 6lJ E result of ^^buzin's primary use as an agricultural pesticide (USEPA
1998a) Metribuztn is detected most frequently in shallow ground water from land-use Stegorii
Table 3-3: Metribuzin detections and concentrations in streams and ground water
Detection frequency
(% samples ;> MKL*)
streams
urban
integrator
agricultural
all sites
ground water
shallow urban
shallow
agricultural
major aquifers
all sites
after USGS, 1998c
% > 0.004 fifl/T.
.-
6.73%
14.29%
13.70%
13.82%
1.66%
3.46%
0.75%
1.95%
% * 0.01 ug/T.
5.50%
9.39%
8.20%
9.94%
0.33%
2.81%
0.32%
1.36%
Concentrations
(all samples; ug/L)
95*
median percentile
nd** 0.011
nd 0.020
nd 0.016
nd 0.026
nd nd
nd nd
nd nd
nd nd
maximum
0.100
0.130
0.330
0.530
0.043
0.300
0.045
0.300
* MM (Mmiman Reporting Level) for msiribuzin in water studies.-0.004 vzO.
"not detected in concentration greater than MKL
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Table 3-4: Metribuzin detections in shallow ground water from various land-use settings
November, 2001
Land-use settings*
Detection frequency
Detection frequency
All
Com and soybeans > 20%
Com and alfalfa > 20%
Com > 50%
Peanuts>50%
Wheat and small grains> 50%
Wheat and small grains and alfalfa > 20%
Alfalfa > 50%
Pasture > 90%
Orchards or vineyards > 50%
Urban
tfterKoWn et at.. 1998 "
3.1%
6.6%
2.1%
0.0%
1.6%
93%
6.2%
0.0%
0.0%
0.0%
1.8%
nr**
s 10%
0-2%
0-2%
<5%
< 10%
*5%
0-2%
0-2%
0-2%
0-2%
Devaluated as crop-groups occupying a percent of the total land
**not reported
33.2.2 Water Quality Investigations from the Corn Belt
h tCr ?Mli!y mvesti8ations ««1 °fcer State and national studies are summarized
below to provide ambient data in States where metribuzin use is high (see Figure 3-1)
water concentrations and detection frequencies were low during theyL 1
n rou
0'^^
Maximum ^ncentrations of metribuzin in surface waters of the Mississippi Ri
tabutanes peaking at less than O.lug^, were considerably lower than
' ^ Percent °f Samples ^ detections ™
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Table 3-5: Metribuzin occurrence in Midwest surface and ground water
surface water
ground water
VMRL
max. cone.
ug/L
sites % samples sites % samples
USGS
Midwest Near-Surface Aquifers (1991)1
Midwest Near-Surface Aquifers (1992-94/
Mss. River and Major Tributaries >(199tf
Miss. River and Major Tributaries (1991-92)4
Midwest Reservoirs (1992?
Pesticides in Ground Water Database (I971-91)6
1.3%
nr
1.0%
1.4%
54% nr
100% 40%
12% 6.5%
4.3%
nr
0.57
0.22
0.08
0.03
nr
25.1
'Kolpinetal.,1994
'Kojpinetal.,1996
' Periera and Hostettler, 1993
4 GoolsbyandBattaglin, 1993
sGoolsbyetaL,1993
' Barbash andResek, 1996; data are national results including some Midwestern States
-The Health Reference Level (HRL) used for metribuzin is 91 ug/L. This is a draft value for working review onfy.
-Minimum Reporting Levels (MRL) vary by study.
- nr ** "not reported"
33 Drinking Water Occurrence
The 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 NPDWR, but they are contaminants
that were formally listed and 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 to monitor 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 (USEPA,
1991; 56 FR3526) for required monitoring mat beganin 1993 (USEPA, 1992; 57 FR31776).
Metribuzin has been monitored under the SDWA Unregulated Contaminant Monitoring (UCM)
program since 1993 (USEPA, 1992; 57 FR 31776). Monitoring ceased for small public water systems
(PWSs) under a direct final rule published January 8,1999 (USEPA, 1999a; 64 FR 1494), and ended for
large PWSs with promulgation of the new Unregulated Contaminant Monitoring Regulation (UCMR)
issued September 17,1999 (USEPA, 1999b; 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
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3.3.1 Data Sources, Data Quality, and Analytical Approach
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 contaminant occurrence analyses findings
presented in this report are based on a national cross-section of aggregated state data (i.e., a representative
subset of available state data) derived from the SDWIS/FED database.
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. Note, however, that the SDWIS/FED 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 USEPA, 2001a, 2001c).
33.1.1 UCM Rounds 1 and 2
The 1987 UCM contaminants include 34 volatile organic compounds (VOCs; USEPA, 1987; 52 FR
25690). 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 EPA. These data from Round 1 were collected by 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) (USEPA, 1992;
57 FR 31776). Monitoring for the UCM (1993) contaminants began coincident with the Phase WV
regulated contaminants in 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 several years, requested that the States
submit these historic data to EPA and they are 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, taut 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 USEPA (2001a) and USEPA (2001c).
3.3.1.2 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-
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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 comprised of the Round 2 state contaminant occurrence databases was
established using the approach developed for the EPA report A Review of Contaminant Occurrence in
Public Water Systems (USEPA, 1999d). This approach was developed to support occurrence analyses for
EPA's Chemical Monitoring Reform (CMR) evaluation, and 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
from the SDWIS/FED (Round 2) database was developed. The details of the approach are presented in
other documents (USEPA, 2001a, 2001b); readers are referred to these for more specific information.
33.1.2.1 Cross-Section Development
As a first step in developing the cross-section, the State data contained in the SDWIS/FED database
(that contains 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 the
data was recorded with incorrect units. Data sets only including detections are obviously biased, over-
representing high-occurrence settings. Other problems included substantially incomplete data sets
without all PWSs reporting (USEPA, 2001a Sections n and ffl).
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 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 U.S. are represented, again balancing the varied conditions that affect transport and
fate of contaminants, as well as conditions that affect naturally occurring contaminants (USEPA, 2001c
Sections IH.A. and ffl.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 (USEPA, 2001c Section IELA..). 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 equally from all quartiles for
both pollution potential factors to ensure representation, for example, from: States with high agrochemical
pollution potential rankings and high manufacturing pollution potential rankings; States with high
agrochemical pollution potential rankings and low manufacturing pollution potential rankings; States with
low agrochemical pollution potential rankings and high manufacturing pollution potential rankings; and
States with low agrochemical pollution potential rankings and low manufacturing pollution potential
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rankings (USEPA, 2001c-Section DOLE.). 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). At the same time, States within the specific quartiles were considered
collectively across all quartiles in an attempt to provide geographic coverage across all regions of the U.S.
The data quality screening, pollution potential rankings, and geographic coverage analysis established
a national cross-section of 20 Round 2 (SPWIS/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 3-2).
Figure 3-2: Geographic distribution of cross-section States for Round 2 (SBWIS/FED)
Round 2 (SDWIS/FED) Cross-Section
States
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
33.1.2.2 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. 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 States with biased data sets eliminated from the 24 State cross-section for
data quality reasons, and a cross-section composed of all 40 Round 1 States (USEPA, 2001c Section
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
apparently too small to provide balance both geographically and with pollution potential, a finding that
concurs with past work (USEPA, 1999c). The CMR analysis suggested that a minimum of 6-7 States was
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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 (USEPA, 1999c). 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 (USEPA,
2001c Section nLB.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. Including greater data
from more States improves the national representation and the confidence hi the results, as long as the
States are balanced related to pollution potential and spatial coverage. The 20-State cross-section
provides the best, nationally representative cross-section for the Round 2 data.
33.1.3 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-system.? 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. Obviously, 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 of 20 SDWIS/FED
Round 2 cross-section States with usable data for lOCs and VOCs. "Special" samples, or "investigation"
samples (investigating a contaminant problem that would bias results) and samples of unknown type,
were not used in the analyses. Various quality control and review checks were made of the results,
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 problematic data were insignificant
relative to the total number of observations, they were dropped from the analysis (for further details see
Cadmus, 2000).
As indicated above, Massachusetts is included in the 20-State, Round 2 national cross-section.
Massachusetts' SOC data were problematic. Massachusetts reported Round 2 sample results for SOCs
from only 56 PWSs, while reporting VOG 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
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33.1.4 Occurrence Analysis
To evaluate national contaminant occurrence, a two-stage analytical approach has been developed.
The first stage of analysis provides a straightforward, conservative, non-parametric evaluation of
occurrence of the CCL regulatory determination priority contaminants as described above. These Stage 1
descriptive statistics are summarized here. Based in part on the findings of the Stage 1 Analysis, EPA
•will determine whether more rigorous parametric 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, 2000,2001).
The summary descriptive statistics presented in Table 3-6 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 detection^) of the contaminant
(simple detection, > MRL) at any time during the monitoring period; or a detections) greater than half
the HRL; or a detections), greater than the HRL.
Metribuzin is not considered to be a linear carcinogen by the oral route of exposure. Accordingly, the
MCLG is derived using a Reference Dose (RfD) approach. The value used as the Health Reference Level
(HRL) for this occurrence evaluation is derived from the RfD using the following equation:
HRL=RfD x Body Weight x Relative Source Contribution
Drinking Water Intake
The body weight used in the calculation is an average adult body weight (70 Kg) and the value for daily
water intake is 2 L. In the calculation of the HRL, the relative source contribution is 20%. A different
relative source factor might be used to calculate the MCLG if a determination is made to regulate
metribuzin.
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 99* 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 MRL (denoted by "<" in Table 3-6). For the same reason, summary statistics such as the 95th
percentile 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 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 * 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
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Preliminary Regulatory Determination Sttpport Document for Metribuzin
November, 2001
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 3-6).
In Table 3-6, 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 (USEPA, 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 3-6). To generate the estimate of national occurrence based on the cross-section
occurrence findings, the national number of PWSs (of population served by PWSs) is simply multiplied
by the percentage value for the particular cross-section occurrence statistic (e.g., the national estimate for
the total number of PWSs with detections (5) is the product of the total national number of PWSs
(65,030) and the percentage of PWSs with detections (0.007%)).
Included in Table 3-6'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 to a national estimate. Using the biased States' data should provide conservative
estimates, likely overestimates, of national occurrence for metribuzin.
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 me 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 hi the Federal
Register Notice. The presentation in the Federal Register Notice of only the actual results of the cross-
section analysis maintains a straight-forward description, 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.
33.1.5 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 USEPA, 1999c). These state data sets were available from an
independent review of contaminant monitoring in drinking water (USEPA, 1999c). Finally, the Ohio
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Preliminary Regulatory Determination Support Document for Metribuzin
November, 2001
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 farther
perspective on corn belt occurrence of metribuzin. The Iowa State-Wide Rural Well-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 waterj
including data from Midwestern slates, for the years 1988-1990 (USEPA, 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 Hie respective reports.
3.3.2 Results
33.2.1 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 3-6). 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)
No PWSs reported detections at levels above Vz HRL or above the HRL. Detection frequencies are higher
for ground water systems when compared to surface water systems, as surface water systems reported
zero detections. Concentrations are also low: for samples with detections the median and 99th percentile
concentrations are 0.10 jig/L. These figures are identical because for metribuzin, Washington was the
only state that reported a detection (0.10 ug/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 3-6). Approximately 0.28% of PWSs (estimated at 182 PWSs nationally) experienced
detections above the MRL, while no PWSs experienced detections greater than V2 HRL or HRL These
figures indicate that about 1.61% of the population is affected by concentrations above me MRL
(approximately 3.4 million people nationally), and 0% of the population is afifected by concentrations
above K HRL or HRL. The proportion of surface water PWSs with detections was greater than ground
water systems. The median and 99th percentile concentrations of detections are 1 ue/L and 3 ue/L
respectively. .s '
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 3-6).
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33.2.2 Occurrence in the Corn Belt
SDWA compliance monitoring data from the com 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 99* percentile concentration of detections among me three States was for
Illinois at 0.7 u.g/L (Table^3-7). Illinois also had the highest maximum concentration at 20 pg/L, still well
below the HRL (after USEPA, 1999c). SDWA compliance monitoring from Iowa for the years 1988-
1995 show similar results, although the data are not presented in Table 3-7 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
The 99* percentile concentration of all samples was a non-detect (HaUberg et al., 1996). *
Metribuzin detection frequencies are generally much greater in surface water when compared to
ground water (Tables 3-8 and 3-9). 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 with no metribuzin detections in surface water (Table 3-7).
Table 3-8 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 frbin Ohio surface water PWSs shows 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 3-8,
3.7 ug/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 hi 29 cities. Metribuzin was
found hi 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 s:50 ft deep (Table 3-8). ^Comparisons between
raw and finished water in Iowa show detection frequencies of metribuzin in surface water increased from
the raw to finished State (Table 3-8; 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).
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Preliminary Regulatory Determination Support Document for Metribuzin
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Table 3-6: Summary occurrence statistics for metribuzin
Freonencv Factors
Total Number of Samples
Percent of Samples with Detections
99* Pereentilc Concentration Call samples)
Health Reference Level
Minimum Reporting Level (MRL)
99 Pereentilc Concentration of Detections
Median Concentration of Detections
Total Number of PWSa
Number of OWPWSs
Numberof SWPWSs y.
Total Population
Population of GWPWSs
Ponulation of SWPWSs
% PWSs with detections (> MRL)
Range
GW PWSs with detections
SW PWSs with detections
% PWSs > 1/2 Health Reference Level (HRL)
Range
GW PWSs > 1/2 Health Reference Level
SW PWSs > 1/2 Health Reference Level
% PWSs > Health Reference Level
Range
GW PWSs > Health Reference Level
SW PWSs > Health Reference Level
54 PWS Population Served with detections
Range
GW PWS Population with detections
SWPWS Population with detections
% PWS Population Served > 1/2 Health Reference Level
Range
GW PWS Population > 1/2 Health Reference Level
SW PWS Population > 1/2 Health Reference Level
% PWS Population Served > Health Reference Level
Range
GW PWS Population > Health Reference Level
SW PWS Ponulation > Health Reference Level
20 State
Cross-Section1
- (Round 2)
34,507
0.003%
<(Non-detect)
9ltigfL
Variable4
0.10 UK/L
O.lOfig/L
13,512
11,833
1,679
50,633,068
14,886,153
35.746.91 S
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)
91 ug/L
Variable4
3.0 ugflL
LOugflL
15,333
13,311
2,022
62,397,416
16,255,818
46.141.508
0.28%
0-14.29%
0.14%
1.24%
0.00%
0-0.00%
f> OA%
l/.wU/O
0.00%
0.00%
, 0-0.00%
onn%
u.vu/o
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%
n nno/
U.l/U/o
National System &
Population Nnmbers3
_
_
- .
65,030
59,440
5,590
213,008,182
85,681,696
5
N/A
5
0
0
N/A
0
0
N/A
1,000
N/A
1,000
0
0
N/A
0
0
0
N/A
182
N/A
83
69
o
N/A
0
0
o
N/A
0
3,420,000
N/A
208,000
2,656,000
o
N/A
0
0
o
N/A
0
UCM(I993)ROUnd2
™>*aunat.
tea reporting data (right) using the Baseline Handbook system and
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Preliminary Regulatory Determination Support Document for Metribuzin
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Table 3-7: SDWA compliance monitoring data from the States of Illinois, Indiana, and Ohio
Freauencv Factors
Total Number of Samples
Percent of Samples with Detections
99 Percentile Concentration fall samples)
Health Reference Level
Minimum Reporting Level (MRL)
99 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 > 1/2 Health Reference Level (HRL)
GW PWSs > 1/2 Health Reference Level
SW PWSs > 1/2 Health Reference Level
% PWSs > Health Reference Level
GW PWSs > Health Reference Level
SW PWSs > Health Reference Level
Tllinnfc1
14,818
0.2%
91 ug/L
Variable4
0.2 ue/L
0.2 ug/L
0.2 ug/L
392
345
47
0.26%
0.29%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
Ohin3
4,039
0.0%
'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, % Health Reference Level, or Health Reference Level, respectively
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Preliminary Regulatory Determination Support Document for Metribuzin
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Table 3-8: Metribuzin occurrence in Midwest drinkin
Ground Water Surveys
National Pesticide Survey (1988-90)1
Iowa State-Wide Rural Well-Water
Survey2
•wells < 50 ft deep
•wells .* 50ft deep .*.
Special Surface Water Studies
ra\v water
Iowa (1986)3
Illinois (1985-87)3
finished -water
Ohio (1993- )4
Iowa ( 1986)3
% sites
2MRL
nd
3.0%
1.4%
nr
nr
79.9%
nr
5 water
% samples
2>MRL
nd
nr
nr
7.0%
15.0%
22.3%
12.0%
maximum
concentration
(ftSfi-)
nd
0.43
0.72
0.89
3.70
1.8
0.45
'Kross ad., 1990
'cited in ffallberg, 1989
*USEPA,1999c
-MKLsvary by study. , .
-nd" results below the respective reporting level
-nr ""not reported"
33.23 Regional Patterns
Occurrence results are displayed graphically by State in Figures 3-3 and 3-4 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 3-3). Another 21 States did not detect metribuzin. The
remaining 3 States detected metribuzin in drinking water and are located on Ihe east and west coasts of
the United States (Figure 3-3). In contrast to the summary statistical data presented in the previous
section, this simple spatial analysis includes the biased Massachusetts data.
The simple spatial analysis presented in Figures 3-3 and 3-4 does not suggest any special regional
patterns. Further, use and environmental release information, (Section 3.1) and ambient water quality
data (Section 3.2), indicate that menibuzin 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; USEPA, 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 greater than the HRL. :
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Preliminary Regulatory Determination Support Document for Metribuzin
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Figure 3-3: States with PWSs with detections of metribuzin for all States with data in SDWIS/FED
(Round 2)
AH States
Metribuzin Detections in Round 2
B States not in Round 2
No data for Metribuzin
States with No Detections (No PWSs > MRL)
States with Detections (Any PWSs > MRL)
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Preliminary Regulatory Determination Support Document for Metribuzin
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Figure 3-4: Round 2 cross-section States with PWSs with detections of metribuziH (any PWSs with
results greater than the Minimum Reporting Level [MRLJ; above) and concentrations greater than
the Health Reference Level (HRL; below)
• Statecfltinadiaaa b on malkrwllh H.29% PWSs > MXL
MetribnziB Occurrence 1» Round 2
States not in Cross-Section
No dm for Melrifanzin
0.00% PWSs > MRL
0.01-1.00%PWS»MRL
>1J)0%PWS»>MRI,»
MetribnziB Occnrrence ia Ronnd 2
States not in Cross-Section
No data for Mettibuzin
0.00% PWSs > HRL
0.01 - 1.00% PWSs > HRL
>1.00%PWS»>HRL
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Preliminary Regulatory Determination Sttpport Document for Metribuzin
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3.4 Conclusion
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
melribuzin 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 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 man the MRL. For the Round 2
cross-section samples with detections, both the median and the 99th percentile concentrations are 0.10
ug/L. Systems wifh detections constitute approximately 0.007% of Round 2 cross-section systems.
National estimates for tire 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 greater than the MRL, and no PWSs reported detections greater than Vz
HRL or HRL. Using more conservative estimates of occurrence from all States reporting SDWA Round
2 monitoring data, including States with biased date, 0.28% of the nation's PWSs (approximately 182
systems and 3.4 million people served) are affected by metribuzin concentrations greater than the MRL,
while no PWSs are affected by concentrations greater than Vt. HRLor 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 com 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 ug/L.
4.0 HEALTH EFFECTS
A description of health effects and dose-response information associated with exposure to metribuzin
is summarized below. For more detail, please refer to the Health Effects Support Document for
Metribuzin (USEPA, 2001b).
4.1 Hazard Characterization and Mode of Action Implications
There are no epidemiological studies that have assessed adverse human health effects caused by
exposure to metribuzin. Exposure to metribuzin may occur primarily in an occupational setting,
particularly hi the agriculture industry where it is used as an herbicide. However, high 50% lethal dose
values resulting from acute toxicity animal studies have indicated that metribuzin may potentially have
low toxicity levels (Kimmerle et al., 1969; Morgan, 1982).
/•'-•'
Subchronic studies in animals suggest that metribuzin may cause adverse effects on body and organ
weight, and hematological parameters. Wistar rats, exposed to metribuzin through their diet at 1500 ppm
for 3-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 hi Beagle dogs did not affect body
weight gain or food consumption; only clinical parameters such as liver enzyme (SCOT and SGPT) levels
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Preliminary Regulatory Determination Support Document for Metribuzin
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were affected (Chaisson and Cueto, 1970). Metribuzin causes slight dermal irritation in rabbits, but has
not been found to cause eye irritation (Kimmerle et al., 1969).
Chronic studies of metribuzin on rats also report effects on body weight gain, mortality, and liver
enzyme and histopathological changes. While 2-year feeding studies conducted on rats (0,25,35,100 or
300 parts/million [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 2-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). The latter 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. In Beagle dogs, chronic exposure to 1,500 ppm caused a
significant increase in the mortality rate and liver dysfunction as evidenced by increased activity of the
liver enzymes SGOT, SGPT and OCT (Loser and Mirea, 1974). Thyroid weight also increased.
Histopathologic findings included liver and kidney damage at the highest dose, 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 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). A two-generation study in rats reported that both first
and second generations consumed less food and gained less body weight (Porter et al., 1988). Autopsy
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 class D, not classifiable as to human carcinogenicity because of
inadequate data in humans or 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).
4.2 Dose-Response Characterization and Implications in Risk Assessment
The EPA's reference dose (RfD) is an estimate of a daily exposure to the human population
(including sensitive subgroups) that is likely to be without appreciable risk of deleterious effects over a
lifetime. The principal study utilized for RfD derivation was the 2-year chronic study in rats conducted
by Christenson and Wahle (1993), where 344 Fisher 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) of metribuzin for 104 weeks. At 30 ppm,
both sexes exhibited increased absolute and relative thyroid weights, and statistically significant increases
in blood levels of thyroxine (T4) and decreases in blood levels of triiodothyronine (T3). In addition,
females exhibited decreased lung weight Since the health effects exhibited by both sexes were
considered to be biologically insignificant, 30ppm was 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. The HRL was derived from the RfD as discussed in
section 3.3.1.4. .
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43 Relative Source Contribution
Relative source contribution analysis compares 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 jig/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 * 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 0.010
mg/kg-day.
As part of the Food and Drug Administration's (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 grams, milk products, fruits or vegetables. In addition, no detections were
found in 218 domestic and 298 imported fish and shellfish samples. Thus, 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. Metribuzin is a solid at ambient
temperatures and has a low vapor pressure. Thus, partitioning of metribuzin into air is highly unlikely.
While the average daily intake for the general population is anticipated to be close to zero, inhalation of
metribuzin may be a potentially significant occupational exposure. The occupational subgroup may
include workers involved hi 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 * lO^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:dairy 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 most individuals, the majority of metribuzin exposure will be from water. For the purpose of
estimating the HRL from the RfD, a conservative default value of 20% was used for the relative source
contribution.
4.4 Sensitive Populations
No populations sensitive to metribuzin have been identified.
4.5 Exposure and Risk Information
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 about 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 ha 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 ug/L, this value is both the median and 99th percentile concentration. 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.
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Preliminary Regulatory Determination Sttpport Document for Metribuzin
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4.6 Conclusion
In conclusion, while there is evidence from animal studies that metribuzin may cause adverse health
effects at high doses, low doses do not appear to be very toxic. There are no available studies, either
epidemiological studies or case-studies of accidentally exposed agricultural workers, that assess adverse
health effects in humans from metribuzin exposure. Its occurrence in public water systems and the
number of people potentially exposed through drinking water is generally low. Thus it is unlikely that
metribuzin will occur in drinking water at frequencies lhat are of public health concern or that regulation
represents a meaningful opportunity for health risk reduction in persons served by public water systems.
All preliminary CCL regulatory determinations and future analysis will be presented hi the Federal
Register Notice.
5.0 TECHNOLOGY ASSESSMENT
If a determination is made to regulate a contaminant, SDWA requires development of proposed
regulations within 2 years of making the decision. It is critical to have suitable monitoring methods and
treatment technologies to support regulation development according to the schedules defined ha the
SDWA.
5.1 Analytical Methods
The availability of analytical methods does not influence EPA's determination of whether or not a
CCL contaminant should be regulated. However, before EPA actually regulates a contamhiant and
establishes a Maximum Contaminant Level (MCL), there must be an analytical method suitable for
routine monitoring. Therefore, EPA needs to have approved methods available for any CCL regulatory
determination contamhiant before it is regulated with an NPDWR. These methods must be suitable for
compliance monitoring and should be cost effective, rapid, and easy to use.
Metribuzin is an unregulated contamhiant for which monitoring was required under the Unregulated
Contamhiant Monitoring Program (USEPA, 1987; 52 FR 25690). Monitoring for metribuzin was
initiated through rulemaking hi 1991 (USEPA, 1991; 56 FR 3526), and began hi 1993. It already has
well-documented analytical methods developed specifically for low-level drinking water analyses (see
Table 5-1).
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Preliminary Regulatory Determination Support Document for Metribuzht
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Table 5-1: Anal
Method .
EPA 507
EPA 508.1
EPA 525.2
EPA 55 1.1
ytical methods for metribnzin
Type
gas chromatography (GC)/
Nitrogen/Phosphorous
detector
GC/ electron capture
detectors (BCD)
GC/ quadrupole mass
spectrometry
GC/ Ion trap mass
spectrometry
GC/ECP
Method Detection
Limit ftig/L)
0.029
0.009
0.062
0.09
0.005
5.2 Treatment Technology
Treatment technologies also do not influence the determination of whether or not a contaminant
should be regulated. But before a contaminant can be regulated with an NPDWR, treatment technologies
must be readily available. EPA's Office of Research and Development (ORD) has researched treatment
technologies for all of the organic compounds listed as regulatory determination priorities on the CCL,
including metribuzin. The two appropriate technologies reviewed were granular activated carbon (GAC)
and air stripping.
Granular activated carbon treatment removes contaminants via the physical and chemical process of
sorption; by which the contaminants attach to the carbon surface as water passes through the carbon bed.
Activated carbon has a large sorption capacity for many water impurities including synthetic organic
contaminants, taste and odor causing compounds, and some species of mercury. Adsorption capacity is
typically represented by the Freundlich isotherm constants, with higher Freundlich K values indicating
greater sorption potential.
Air stripping involves the continuous contact of air with the water being treated, allowing volatile .
dissolved contaminants to transfer from the source water to the air. After contact, the "contaminated air"
is swept from the system, taking the contaminant out of contact with the treated water. The driving force
for the water-to-air transfer of the volatile contaminants is the contaminant's concentration gradient
between the water and air. The Henry's Law constant is a commonly used indicator of the tendency of a
contaminant to partition from water to air. A larger Henry's constant indicates a greater equilibrium of
the contaminant hi the air: Thus, contaminants having larger Henry's constant are more easily removed
by air stripping.
Predictive computer modeling and specific chemical characteristics were used to determine the
isotherm constants needed to evaluate the two treatment technologies. The rule of thumb used for SDWA
compounds, learned through the development of cost-and-technology documents to support other
drinking water regulations, is that GAC is considered to be cost-effective if the contaminant has a
Freundlich (K) value above 200 (Speth and Adams, 1993). For air stripping, a compound with a Henry's
constant above dibromochloropropane (0.005) or ethylene dibromide (0.037) is considered strippable at a
reasonable cost.
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Preliminary Regulatory Determination Support Document for, Metribuzin
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Metribuzin has a predicted Freundlich (K) value of 25,200 and a predicted Henry's Law constant of
7.2x10"*. Therefore, only GAC is an applicable treatment technology for metribuzin. Its low
volatilization potential makes air stripping impractical. '
6.0 SUMMARY AND CONCLUSIONS - DETERMINATION OUTCOME
Three statutory criteria are used to guide the preliminary determination of whether regulation of a
CCL contaminant is warranted: 1) the contaminant may adversely affect the health of persons; 2) the
contaminant is known or is likely to occur in public water systems with a frequency, and at levels, of
public health concern; and 3) regulation of the contaminant presents a meaningful opportunity for health
risk reduction for persons served by public water systems. As required by SDWA, a decision to regulate
a contaminant commits the EPA to propose a Maximum Contaminant Level Goal (MCLG) and
promulgate a National Primary Drinking Water Regulation (NPDWR) for the contaminant. A decision
not to regulate a contaminant is considered a final Agency action and is subject to judicial review. The
Agency can choose to publish a Health Advisory (a non-regulatory action) or other guidance for any
contaminant on the CCL that does not meet the criteria for regulation.
Exposure to metribuzin occurs primarily in occupational settings, particularly in the agriculture
industry where it is used as an herbicide. Although there are no studies assessing adverse effects of
metribuzin on human health, animal studies indicate that metribuzin has the potential to cause adverse
health effects at high doses.- Chronic studies of metribuzin, for instance, have reported effects on body
weight increases, mortality, liver enzyme activities, and histopathological changes. The RfD of 0.013
mg/kg-day was derived from a study reporting the adverse health effects of metribuzin in rats. Currently,
metribuzin is classified as a class D carcinogen, due to inadequate carcinogenicity data in humans and
animals.
While metribuzin has been detected in ambient surface and ground water, detection frequencies and
concentrations from PWS samples collected under the SDWA are low. Contaminant releases to the
environment have been reported in the TRI from only three States and one territory. Round 2 cross-
section occurrence estimates are very low, with only 0.003% of all samples showing detections.
Significantly, the values for the 99th percentile (0.10 ug/L) and median concentrations (0.10 ug/L) of all
samples are less than the HRL. When all the Round 2 data are considered, a national extrapolation of the
data indicates that 1.6%, or approximately 3.4 million people nationally, are exposed to any
concentration of metribuzin.
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 show
very low occurrence of metribuzin. Special, targeted surface water studies from Ohio have the highest
detection frequency of metribuzin.
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 mmcimnm daily intake for a
contact intensive worker would be 5.3 * 10"3 mg/kg-day, which is below the RfD 0.013 mg/kg-day. There
is no evidence to suggest that children, or any other population subgroup, would be more sensitive than
others when exposed to metribuzin. In addition, EPA has applied an uncertainty factor in deriving the
HRL that adequately protects sensitive subgroups of the population.
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Preliminary Regulatory Determination Support Document for Metribuzin
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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. Thus metribuzin may not occur in drinking water with a frequency, or at levels,
of public health concern. All preliminary CCL regulatory determinations and further analysis will be
presented in the Federal Register Notice.
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results from the National Water-Quality Assessment Program. US Geological Survey
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Geological Survey Open-File Report 94-70. 4 pp. Available on the Internet at:
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Loser, E. and D. Mirea*. 1974. Bay 94337: Chronic toxicity studies on dogs (Two-year feeding
experiment). Report No. 4887; Report No. 41814. Unpublished report submitted to the EPA by
Mobay Chemical Corp., Kansas City, MO. MRID 00061260.
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Preliminary Regulatory Determination Support Document for Metribuzin
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Loser, E. and U. Mohr*. 1974. Bay 94337: Chronic toxicity studies on rats (Two-year feeding
experiment). Report No. 4888; Report No. 41816. Unpublished study submitted to the EPA by
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No. 41818. Unpublished study submitted to the EPA by Mobay Chemical Corp., Kansas City, MO.
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on rats (Three-month feeding experiment). Unpublished study submitted to the EPA by Mobay
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Programs. EPA report 540-9-80-005. 120pp.
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tributaries by herbicides: Environ. Sci. Technol. 27(8): 15642-1552 (as cited in Larson etal., 1997).
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Porter, W.P., S.M. Green, N.L. Debbink and I. Carlson. 1993. Groundwater pesticides: interactive
effects of low concentrations of carbamates, aldicarb and methomyl and the triazine metribuzin on
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Preliminary Regulatory Determination Support Document for Metribuzin
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USEPA. 1988. Health advisories for 50 pesticides (Including...metribuzin...). Washington, DG: Office of
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inorganic Chemicals; Monitoring for Unregulated Contaminants; National Primary Drinking Water
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Chemicals and Inorganic Chemicals; National Primary Drinking Water Regulations Implementation.
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* . " - '
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modified March 23,2000. Link to site at: http://www.epa.gov/tri/chemicd.htm
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V ' ' • ' ...
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Available on the Internet at: www.epa.gov/triexplorer/yearsum.htm Last modified May 5,2000.
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38'
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Internet at: http://www.epa.gov/tri/general.htm Last modified February 28,2000.
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National Water Quality Assessment Program (NAWQA). Provisional data-subject to revision.
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Circular 1225. 82pp.
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"Confidential Business Information submitted to the Office of Pesticide Programs.
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Appendix A: Abbreviations and Acronyms
ARMS - Agricultural Resources Management Study
CA - Census of Agriculture
CAS - Chemical Abstract Service
CCL - Contaminant Candidate List
CMR - Chemical Monitoring Reform
CPS -CroppingPractices Survey <
CWS - conununity water system
DBCP - dibromochloropropane
DCI - data call-in
BCD - electron capture detectors
EPA -Environmental Protection Agency
EPCRA - Emergency Planning and Community Right-to-Know Act
EXTOXNET - Extension Toxicology Network, Pesticide Management Education Program
FDA - Food and Drag Administration
FIFRA - Federal Insecticide, Fungicide, and Rodenticide Act
FQPA - Food Quality Protection Act
FR - federal register
GAC - granular activated carbon (treatment technology for organic compounds)
GC - gas chromatography (a laboratory method)
g/mol - grams per mole
GW • -groundwater
HRL - Health Reference Level
IOC - inorganic compound
KO,. - organic carbon partition coefficient
K,,,, - octanol-water partitioning coefficient
L - -liter
MCL - maximum contaminant level
mg -milligram
mg/kg-day - milligram per kilogram per day
mmHg - millimeter mercury
MRL - minimum reporting level
NAWQA - National Water Quality Assessment Program
NCFAP - National Center for Food and Agricultural Policy
NCOD - National Drinking Water Contaminant Occurrence Database
NDWAC - National Drinking Water Advisory Council
NIRS - National Inorganic and Radionuclide Survey
nm - nanometer
NOAEL - no-observed adverse effect level
NPDWR -National Primary Drinking Water Regulation
NFS - National Pesticide Survey
NTNCWS - non-transient non-community water system
OGWDW -Office of Ground Water and Drinking Water
ORD -Office of Research and Development
PGWD -Pesticides in Ground Water Database
ppm -part per million
PWS - public water system
RCRA - Resource Conservation and Recovery Act
RfD - reference dose
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SARA. Title m
SDWA
SDWIS/FED
SOC
SW
TPQ
TRI
UCM
UCMR
URCIS
USDA
USEPA
USGS
VOC
HE
>MCL
>MRL
• Superfund Amendments and Reauthorization Act
- Safe Drinking Water Act
• Federal Safe Drinking Water Information System
• synthetic organic compound
• surface water
• threshold planning quantity
- Toxic Release Inventory
• Unregulated Contaminant Monitoring
- Unregulated Contaminant Monitoring Regulation/Rule
- Unregulated Contaminant Monitoring Information System
- United States Department of Agriculture
• United States Environmental Protection Agency
- United States Geological Survey
• volatile organic compound
-micrograms
• percentage of systems with exceedances
- percentage of systems with detections
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