  SEQ CHAPTER \h \r 1 

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C.  20460

OFFICE OF

PREVENTION, PESTICIDES, AND

TOXIC SUBSTANCES

PC Code: 038901, 038903, 038904, 038905 

DP Barcode: 356316

	

September 9, 2009

MEMORANDUM

SUBJECT:	Drinking Water Assessment for the IR-4 Tolerance Petition for
the Use of Endothall-treated Irrigation Water on a Variety of Crops

TO:		Sidney Jackson, Risk Manager

		Barbara Madden, Risk Manager

Registration Division (7505P)

FROM:	Nelson Thurman, Senior Environmental Scientist

		Environmental Risk Branch 2

		Environmental Fate and Effects Division (7507P)

THROUGH:	Tom Bailey, Branch Chief

		Environmental Risk Branch II

		Environmental Fate and Effects Division (7507P)

Summary

A drinking water exposure assessment has been conducted to support the
human health risk assessment for the proposed use of endothall-treated
irrigation water on a variety of crops.  

The maximum potential exposure of endothall in drinking water sources is
expected to result from the direct application of endothall to drinking
water reservoirs to control aquatic weeds.  EFED assumed that the entire
reservoir would be treated at the maximum rates, with no more than 10%
of the reservoir treated at one time as stated on the label, so that 10
treatments were applied 7 days apart to get the entire reservoir.  Since
the label specified that the community water system (CWS) could not
supply treated drinking water unless the residues were below 0.1 ppm
(100 µg/L), EFED assumed 100 µg/L (0.1 ppm) as the acute (peak)
exposure and the constant exposure during the treatment period and then
modeled residue decline by degradation after the final treatment.  This
resulted in a chronic (annual average) concentration of 31 µg/L (0.031
ppm) for endothall.  This represents the likely high-end chronic
exposure from endothall from the use most likely to generate the highest
exposures (treatment of a reservoir).  

Nature of Action

Endothall is an herbicide with both terrestrial and aquatic uses. 
Interregional Research Project No. 4 (IR-4) submitted field trial data
to support the use of endothall as an algaecide and aquatic herbicide in
irrigation and drainage canals.  The irrigation water would subsequently
be applied to a variety of vegetables, berries, grains, nuts, grapes,
and feed crops.  With no geographic limitations, potentially all
irrigated crops could be subject to endothall residues from treated
irrigation waters.  Endothall may be applied to irrigation canals at a
maximum single rate of 5 ppm (as endothall acid), with a maximum of 30
ppm during the growing season, with a minimum 7-day application interval
(based on both the Hydrothol 191 and Aquathol® K labels). Based on the
maximum seasonal rate, EFED assumed a maximum exposure from 6
applications at 5 ppm with 7-day intervals between applications or 10
applications at 3 ppm for the 3 ppm maximum on one label.

Endothall may also be applied to lakes, ponds, and impoundments,
including drinking water reservoirs, at rates of up to 3 ppm (Hydrothol)
to 5 ppm (Aquathol).  For submerged aquatics in lakes or ponds, no more
than 1/10 of the impounded water may be treated at one time.  The labels
state that consumption of water by the public is allowed only when the
concentration of endothall in water is less than the Maximum Contaminant
Level (MCL) of 100 µg/L (0.1 ppm).

Previous Drinking Water Exposure Assessment

In May, 2004, EFED conducted a drinking water assessment for endothall
uses on terrestrial crops in the western US (potatoes, cotton, clover,
alfalfa for seed, hops) and direct aquatic applications.  The estimated
maximum 1-in-10-year exposures from terrestrial uses were:

7.1 µg /L for the 1 in 10 year annual peak concentration (acute)  

2.5 µg /L for the 1 in 10 year annual mean concentration (non-cancer
chronic) and  

2.4 µg /L for the 36 year annual mean concentration (cancer chronic).  


Maximum ground water concentrations were less than 0.1 µg /L.

The greatest potential exposures from endothall use, however, come from
direct application to drinking water reservoirs.  Assuming 100% of the
reservoir is treated at the maximum rate, the maximum peak (single-day)
exposure could be as high as 5000 µg/l (5 ppm), based on the maximum
application rate to the reservoir. However, Endothall has a Maximum
Contaminant Level (MCL) of 100 µg/L (0.1 ppm); therefore, EPA used the
MCL as the maximum exposure level expected from direct application of
endothall to reservoirs.  Although the MCL is likely to overestimate
average (i.e., chronic) residues of endothall in drinking water, EPA
believes it provides a reasonable high-end estimate of potential acute
drinking water concentrations from the aquatic uses of endothall. For a
screening approach, EFED used the MCL of 100 µg/L (0.1 ppm) for both
the acute and chronic exposures in the dietary risk assessments.  The
2005 Revised Human Health Risk Assessment for the RED indicated that
chronic dietary exposure assuming an average 100 ppb concentration for
endothall accounted for 99% of the cPAD for infants <1 yr old and 45% of
the cPAD for children 1-2. 

Summary of Environmental Fate Properties of Endothall

The environmental fate and transport properties of endothall are
described in the 2005 EFED environmental fate and ecological risk
assessment.  This section briefly summarizes the RED characterization.
Endothall exists in three forms in the environment: endothall
dipotassium, endothall potassium cation, and endothall acid.  These
chemical forms are highly soluble and mobile in the environment. 
Exposure is expressed in terms of endothall acid equivalents. 

Endothall degrades by biotic processes such as aerobic metabolism. 
Single laboratory studies measured first-order degradation half-lives of
14.5 days for aerobic soil metabolism, 10 days for aerobic aquatic
metabolism, and 9 days for anaerobic soil metabolism.  Terrestrial
dissipation studies measured dissipation half-lives from the soil
surface of 13 to 19 days.  Dissipation/disappearance half-lives in
aquatic dissipation studies ranged from 4 to 30 days (median 8.5 days)
in lab studies, and 0.5 to 20 days (median 4 days) in ponds and lakes. 
As noted in Table 1, EFED multiplied the single half-life measurement by
3 to reflect the uncertainty in the range of potential half-life values
in the field. The resultant 3X value for aquatic metabolism is within
the range of dissipation half-lives reported for the aquatic dissipation
studies while the single value is closer to the median dissipation
half-lives from the studies.  

Table 1.  Endothall Inputs Used for Drinking Water Exposure Modeling.

MODEL INPUT VARIABLE	

INPUT VALUE	

COMMENTS



Molecular Weight, g/Mol	

186.2	

2005 RED



Vapor pressure (Torr at 24.3 oC)	

 ADVANCE \d4 2.2e-10	

2005 RED



Solubility (mg/L)	

100,000	

2005 RED



Kd (ml/g)	

2.4	

Average of values in MRID 41616404



Aerobic Soil Metabolic Half-life (days)	

43.5

	

3X 14.5 day half-life based on extractable residues in MRID 44949401



Aerobic Aquatic Metabolic Half-life (days)	

30	

3X 10 day half-life based on extractable residues in MRID 42618901



Anaerobic Aquatic Metabolism half-life (days)	

27	

3X 9 day half-life based on extractable residues in MRID 42903901



Drinking Water Exposure Assessment for the Proposed Use

For chronic drinking water exposures, the 100 µg/L (0.1 ppm) MCL is the
presumable maximum threshold for surface water sources of drinking
water.  That threshold assumes that the CWS downstream of the
agricultural areas that are releasing treated irrigation canal water
into the larger water bodies are aware of the applications so they can
ensure that endothall levels stay below that threshold in the water they
release to the public.  In the event that the CWS is not aware of what's
going on upstream, it is possible that short-term exposures from
endothall may exceed the MCL because of the high rates used in treating
the waters.  For flowing water bodies, the endothall residues would
quickly dissipate downstream so that elevated exposures would only be
expected for short periods after endothall-treated irrigation water is
released to the water body. 

 

Application of endothall-treated irrigation water to crops would result
in lower exposure concentrations released to surface water sources of
drinking water due to (a) filtration of irrigation water through the
crops and soil and (b) increased time for degradation from the time of
treatment.  Estimated 1-in-10-year peak and average annual
concentrations from irrigation applications of endothall are expected to
be in the same range as those estimated for endothall applications to
terrestrial crops in the 2005 RED (up to 39 µg /L peak and 10 µg /L
annual). 

The maximum potential drinking water exposures are expected to result
from the direct application of endothall as an aquatic herbicide in a
drinking water reservoir.  In a maximum potential exposure scenario, the
entire reservoir is treated at maximum rates (3 to 5 ppm, depending on
the label), with no more than 10% of the reservoir treated at one time. 
This would result in 10 treatments of 3 ppm applied 7 days apart to get
the entire reservoir (assuming a 30 ppm maximum seasonal rate).  The
labels state that consumption of water by the public is allowed only
when the concentration of endothall in water is less than the MCL of 100
µg/L (0.1 ppm).  At the maximum rates, even with a 600-foot setback
from the drinking water intake, endothall residues may exceed the MCL
for one or a few days after application. However, based on the label
specifications, the water cannot be released for public consumption
until the concentration is less than the MCL. Thus, the maximum peak
concentration of endothall residues would be 100 µg/L (0.1 ppm).

Given the degradation/dissipation rates of endothall, the concentrations
will likely decline fairly quickly once treatments have ended, even in
the most static of reservoirs.  So longer-term average exposures will
likely be well below the MCL. 

For chronic exposures, EFED used the following estimates:

an assumed 100 µg/L (0.1 ppm) concentration during the 70-day treatment
window

first-order degradation, based on a 30-day aquatic metabolism half-life
(3X the single measured value provided by the registrant), using the 

			Ct = C0e-kt   

where Co = initial concentration, k =  first-order degradation rate
(hr-1), and  t = time.

The calculated daily concentrations were averaged over the year to
generate a chronic (annual average) concentration of 31 µg /L (0.031
ppm) for endothall.  This would represent the likely high-end chronic
exposure from endothall from the use that is most likely to generate the
highest exposures (treatment of a drinking water reservoir).  

To characterize the range in potential exposures, EFED varied the
degradation/ dissipation half-life (30 days vs 10 days) and included an
estimate of reservoir flow-through rates, similar to the approach in the
2004 EFED drinking water assessment.  These are summarized in Table 2. 
The FL reservoir had the greatest flow-through rates (see the 2004 EFED
drinking water assessment for details).

Table 2: Range in estimated annual average concentrations of endothall
residues based on varying degradation rates and reservoir flow-through
rates.

Exposure Scenario	Annual average (chronic) concentration 

(µg /L)

Endothall degradation half life (da) / rate (da-1)	Flow-through (m3/hr)
/ equiv turnover rate (da-1)	Effective dissipation rate (da-1)

	Static reservoir (no water flow-through)

30 da / 0.0231	0	0.0231	31 (0.031 ppm)

10 da / 0.0693	0	0.0693	23 (0.023 ppm)

FL reservoir (FL sugarcane scenario)

30 da / 0.0231	121 m3/hr / 0.0201	0.0432	25 (0.025 ppm)

10 da / 0.0693	121 m3/hr / 0.0201	0.0894	22 (0.022 ppm)



Livestock Drinking Water Exposure Assessment

To evaluate potential endothall residues in milk/dairy products, HED
asked EFED for an estimate of potential endothall residues in
livestock/animal drinking water.  The highest exposures would come from
a static pond with no water flow-through.  Endothall would soon be
diluted out of any water body with flow in it (canals, etc) and, even
with multiple applications, the higher concentrations would soon be
moving away from the source.

The labels have certain waiting periods before treated water could be
used for animal consumption depending on the application rate:

Applied 0.3 ppm – 7 days after application

Applied 3.0 ppm – 14 days after application

Applied 5.0 ppm – 25 days after application

The estimated concentrations use the simple first-order degradation
model, assuming either static (no flow) or varying water turnover rates
in the farm water body.  Endothall concentrations degraded with a
first-order model starting immediately after application. No water was
consumed during the specified waiting periods; annual average
concentrations reflect daily concentrations beginning after the waiting
period.  The uncertainty in the estimates is bound by the range in
potential degradation rates of endothall and the potential water
turnover rates for the water body.  At the high end of the half-life
range in water (30 days), average annual water exposures in static water
bodies are 0.26 and 0.35 ppm, for the 3 and 5 ppm treatments
respectively; at the low-end (10 days) of the half-life range, average
water exposures are below 0.1 ppm.  

Table 3: Range in estimated annual average concentrations of endothall
residues in livestock water ponds based on varying degradation rates and
flow-through rates.

Exposure Scenario	Annual average concentration,

mg /L (ppm)

Endothall degradation half life (da)	Pond turnover rate (% daily
turnover)	Effective dissipation rate (da-1)	0.3 ppm applied	3 ppm
applied	5 ppm applied

Single application

30 da 	0%	0.0231	0.03	0.26	0.35

	1%	0.0331	0.02	0.16	0.19

	5%	0.0731	0.007	0.04	0.03

10 da	0%	0.0693	0.007	0.04	0.04

	1%	0.0793	0.006	0.03	0.02

	5%	0.1193	0.003	0.01	0.006

3 applications spread throughout the year (1, 123, 245 days)

30 da	0%	0.0231	0.09	0.82	0.78

	1%	0.0331	0.06	0.51	0.44

	5%	0.0731	0.02	0.13	0.07

10 da	0%	0.0693	0.02	0.15	0.08

	1%	0.0793	0.02	0.11	0.06

	5%	0.1193	0.009	0.04	0.01

6 applications at 7-day treatment intervals w/ no consumption during
treatment

30 da	0%	0.0231	0.12	1.06	1.42

	10%	0.1231	0.002	0.01	0.004

10 da	0%	0.0693	0.01	0.08	0.06

	10%	0.1693	0.001	0.002	0.001



In most instances, some water turnover/exchange (replenishment with rain
and/or runoff) occurs in a farm pond over time.  Ranges of 1 to 5%
turnover on a daily basis is not uncommon.  At a 5% water exchange rate
for a pond, the year-long average concentrations for the 3 rate/waiting
period combinations were below 0.2 ppm.  Table 3 also provides average
annual concentration estimates for a variety of multiple application
scenarios.  Each of the scenarios assume that the waiting period is
enforced between treatments.

 “Drinking Water Assessment for Endothall for both Terrestrial and
Aquatic Uses”.  Memorandum from James Breithaupt, EFED, to Susan Lewis
and Pat Dobak, SRRD, May 5, 2004.

 “Endothall: Revised Human Health Risk Assessment. HED Chapter of the
Reregistration Eligibility Decision Document (RED)”. Memorandum from
Zendzian et al, HED, to Mika Hunter, SRRD, 09/30/05.

 “Environmental Fate and Ecological Risk Assessment of Endothall -
Revised”. Memorandum from Embry et al, EFED, to Mika Hunter, SRRD,
April 22, 2005.

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