UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

MEMORANDUM 

SUBJECT: 

FROM: 

THRU: 

TO: 

September 18, 2012 

OFFICE OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION 

PC Code: 038904 

DP Barcode: 404321 

De.c-tsroN: 4b4Z4~ 

Drinking Water Exposure Assessment for Proposed New Use of 

Endothall Dipotassium on Apple Trees for Thinning of Blossoms 

James Lin, Environmental Engineer t-=- ~ - Environmental Risk Branch II 

En-vironmental Fate and Eff~cts Di:fJ/~07P~ . 

Bnan Anderson, Branch Chief // ( ~, '{ (/} 

R. David Jones, Senior Agronomist /( l)vvJ/ /t--;7'~ Environmental Risk Branch II (.__ 

Environmental Fate and Effects Division (7507P) 

Kathryn Montague, Risk Manager 

Grant Rowland, Risk Manager Reviewer 

Registration Division (7505P) 

This memo presents the drinking water exposure assessment of ThinRite blossom thinner 

(5.37% of endothall dipotassium) use on apples. The proposed annual maximum 

application rate is 12 pints (0.846 lbs ai or 0.60 acid equivalent). This is a terrestrial food 

use. Considering the possible exposure of drinking water, the greatest potential will come 

from the direct application to drinking water reservoirs. In a previous drinking water 

assessment, EFED has concluded the use of endothall-treated irrigation water represents 

the most conservative estimates for drinking water assessment (DP 356316, dated 

September 9, 2009, see attachment). Therefore, the drinking water exposure estimates 

(shown below) based on the direct water applications should continue to be used for the 

human health risk assessment. 

Media Max Concentration Source (ppb, J.lg/L) 

Surface water (acute) 100 Maximum Contaminant Level (MCL) 

Surface water (chronic) 31 Calculated Annual Average 

1 

1111111111 

2087954 
Attachment 

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 1 0% of the reservoir treated at one time as stated on the label, so that 1 0 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 

2 
ppm (100 11g/L), EFED assumed 100 J.lg/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 

J,lg!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 Hydrothol191 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 1110 ofthe 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 11g/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 1. The estimated maximum 1-in-1 0-year exposures from terrestrial uses were: 

7.1 11g /L for the 1 in 10 year annual peak concentration (acute) 

2.5 11g /L for the 1 in 10 year annual mean concentration (non-cancer chronic) and 

2.4 11g /L for the 36 year annual mean concentration (cancer chronic). 

Maximum ground water concentrations were less than 0.1 !lg /L. 

The greatest potential exposures from endothall use, however, come from direct application to 

drinking water reservoirs. Assuming 1 00% of the reservoir is treated at the maximum rate, the 

maximum peak (single-day) exposure could be as high as 5000 !lg/1 (5 ppm), based on the 

maximum application rate to the reservoir. However, Endothall has a Maximum Contaminant 

Level (MCL) of 100 11g/L (0.1 ppm); therefore, EPA used the MCL as the maximum exposure 

1 "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. 

3 
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 f..lg/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% ofthe cPAD for infants <1 yr 

old and 45% ofthe cPAD for children 1-22. 

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 assessment3. 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 

Molecular Weight, g/Mol 

Vapor pressure (Torr at 24.3 °C) 

Solubility (mg/L) 

Kd (mUg) 

Aerobic Soil Metabolic Half-life 

INPUT 

VALUE 

186.2 

100,000 

2.4 

43.5 

COMMENTS 

2005 RED 

2005 RED 

2005 RED 

Average of values in MRID 41616404 

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

2 "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. 

3 "Environmental Fate and Ecological Risk Assessment of Endothall- Revised". 

Memorandum from Embry et al, EFED, to Mika Hunter, SRRD, April 22, 2005. 

4 
MODEL INPUT VARIABLE 

(days) 

Aerobic Aquatic Metabolic Half-life 

(days) 

Anaerobic Aquatic Metabolism halflife (days) 

INPUT 

VALUE 

30 

27 

COMMENTS 

MRID 44949401 

3X I 0 day half-life based on extractable residues in 

MRID 42618901 

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 11g/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 endothalllevels 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-1 0-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 11g /L peak and 10 11g /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 11g/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 f.lg/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 

5 
average exposures will likely be well below the MCL. 

For chronic exposures, EFED used the following estimates: 

(1) an assumed 100 f..LgiL (0.1 ppm) concentration during the 70-day treatment window 

(2) first-order degradation, based on a 30-day aquatic metabolism half-life (3X the single 

measured value provided by the registrant), using the 

Ct = Coe-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 J.lg /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 halflife (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 

varymg · d egra d" at10n rates an d reservOir ·o ow-t h h rougl rates. 

Exposure Scenario Annual average 

Endothall Flow-through Effective dissipation (chronic) 

degradation half life (m3

/hr) I equiv rate (da-1

) concentration 

( da) I rate ( da-1

) turnover rate (da-1

) (J.lg /L) 

Static reservoir (no water flow-through) 

30 da I 0.0231 0 0.0231 31 (0.031 ppm) 

10 da I 0.0693 0 0.0693 23 (0.023 ppm) 

FL reservoir (FL su_g_arcane scenario) 

30 da I 0.0231 121 mjlhr I 0.0201 0.0432 25 (0.025 ppm) 

10 da I 0.0693 121 mjlhr I 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: 

6 
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 

r 1vestoc k d b d water pon s ase on varymg · d d egra ation · ra t d es an fl ow-th roug1 h t ra es. 

Exposure Scenario Annual average concentration, 

mg/L (ppm) 

Endothall Pond turnover Effective 0.3 ppm 3ppm Sppm 

degradation rate (% daily dissipation rate applied applied applied 

half life ( da) turnover) (da-1

) 

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 

7 
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. 

8