UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON D.C., 20460 

OFFICE OF 

CHEMICAL SAFETY AND 

POLLUTION PREVENTION 

MEMORANDUM 

SUBJECT: 

FROM: 

THRU: 

TO: 

PC Code: 116002, 116004 

DP Barcode: 419489, 427507 

Response to Registrant's Rebuttal on the Drinking Water Numbers Used by 

HED in Support of the Removal of Grazing Restrictions for Lactating Dairy 

Animals on Triclopyr TEA (PC Code 116002) and BEE (PC Code 116004) 

Larry Liu, Ph.D, Chemist -f; / ' / 

Environmental Risk Branch 5 LA/' 

Environmental Fate and Effects Division (7507P) I 

· Ob/ 0 'f /IS--

Mah Shamim, Ph.D. Branch Chief JYtlti ~ I 

Environmental Risk Branch 5 I/ ff~ 

Environmental Fate and Effects Division (7507P) 

Mindy Ondish, Risk Manager Reviewer 

Reuben Baris. Risk Manager, RM-25 

Herbicide Branch 

Registration Division (7505P) 

Jenn ifer Tyler, Chemist 

RAB I 

Health Effects Division (7509P) 

EFED has reviewed comments received from the registrant (Dow AgroSciences LLC) regarding 

triclopyr dri nking water EECs used in the Acute and Chronic Aggregate Dietary (Food and 

Drinking Water) Exposure and Risk Assessment for the Proposed Livestock Tolerance Changes 

on three labels [triclopyr TEA (EPA. Reg. No. 62719-37), and triclopyr BEE (EPA Reg. No. 

62719-552 and 62719-63 7)]. Since the proposed removal of a grazing restriction for lactating 

dairy animals does not change the drinking water assessment, EFED is referring to the last 

assessment completed on 6/ 17/2002 (see anachment). In this drinking water assessment, EFED 

recommended values of 1,049 ppb and 390 ppb for acute and chronic drinking water EECs. In 

response, the registrant suggested that a value of 400 ppb should be used for acute and chronic 

drinking water EECs because the labels include the setback distance from the application site to 

a functional potable water intake and the intake must be turned off until the triclopyr 

concentration in the intake water is less than 400 ppb. EFED concurs with the registrant that 400 

ppb should be used for acute and chronic drinking water EEC. 
MEMORANDUM 

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

OFFICE OF 

PREVENTION. PESTICIDES ANO 

TOXIC SUBSTANCES 

PC Code: 116002 

DPBarcode: D 283 "f I 5 

SUBJECT: Revised Tier 1 Estimates for Drinking Water Concentrations Resulting from 

Triclopyr Use for Aquatic Weed Control 

DATE: 17 June 2002 

FROM: Michele Mahoney, Agronomist fl<. ~~l/18/oz 

Ian Kennedy, Hydrologist -r- tS-~- __ 1...,, / 

Environmental Risk Branch II ·- ..- ~ -~ ig"f ..;,... 2 • .:1c:n.. 

Environmental Fate and Effects Division (7507C) 

TO: Jim Tompkins 

THRU: 

Registration Division (7505C) 

Dana Vogel 

Health Effects Division (7509C) 1 

Tom Bailey, Branch Chief rr-----o7Jl7 

Environmental Risk Branch II . ../ 

Environmental Fate and Effects Division (7507C) 

Summary: EFED has calculated suggested setback distances for triclopyr applications near 

drinking water intakes and made predictions for peak and annual average drinking water 

concentrations resulting from direct application of triclopyr TEA to surface water at such 

setbacks. For the setback distances discussed below the estimated peak (acute) drinking water 

concentration ranges up to I 049 ppb and the estimated annual average drinking water 

concentration ranges up to 390 ppb. 

Discussion: The estimated environmental concentrations (EEC's) were based on treatment of an 

area of a lake at the maximum application rate of2.5 ppm, at setback distances which are 

dependent on the size of the area treated. EFED calculated setbacks to be used in order to keep 

the annual average concentration below 400 ppb and then used these setbacks in calculating 

annual average and peak EEC's. 
Estimates were made using the one-dimensional convection-dispersion equation 

The dimensionless parameters in this equation are C = c/c0, 't = vtll , Z = xll, P = vl/D, μ = 

ln(2)Llvt112• Where c is the concentration, c0 the initial concentration, l the length of the water 

body, x is distance along the water body, v is the water flow rate, D is a dispersion coefficient and 

t lll. is the half-life of the chemical. The dimensionless parameter Pis known as the Peclet number 

and can be used to tell how much a pulse will spread as it flows the length of the system. 

The solution to this equation, assuming an initial constant concentration between two points is 

(Toride et al., 1995) 

- = L (- 1)' e-μ r l - -erfi ' - -e 4r 

C ' ·[ l I {(Z-Z.)-r) f,!r pz_ P[(Z-Z,>+rf 

C0 i=O 2 .J 4 r I P 1r 

+i(1+ P(Z + Z;)+ Pr)e" erfr( (Z~ ')f] 

where the Z; are the start and end of the application zone, converted to dimensionless parameters 

in the same manner as Z, above. The function erfc(x) is the complementary error function and is 

defined as 

The yearly average concentration at different distances downstream was then calculated and used 

to detennine the setback distance needed to keep the annual average concentration below 400 

ppb. 

The equation uses only one space dimension, so only the length of the application area was 

considered. This length was calculated from the application area by assuming the treated area had 

proportions identical to those of the Index Reservoir (640m x 82m) and using the long side of the 

resulting rectangle as the length of the application (and because we considered only one 

dimension, the width of the lake). Thus for a 4 acre (16000 m2

) application area, we would 

assume a long, skinny lake 45m wide, with an application the full width and 360m long. These 

lengths are listed in Table I below, and shown graphically in Figure I. Note that this does not 

assume the entire lake is treated, and applications to other areas of the lake at different times 

would add to the EEC's presented here. The index reservoir. which is a flooded stream channel, 

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should represent the type of water body geometry likely to transport a direct 

application most efficiently to a water intake. Such a water body is also more 

likely to have lower dispersion, as assumed here. than a water body with more 

complex geometry. 

Parameters used for this calculation were D = 0.0005 m2/s and and a half-life of 

triclopyr of 142 days. Flow velocities ranged between 2.28 m/day and 6.28 

m/day, depending on the size of the application area, and were chosen to 

maximize the annual average concentrations. The flow velocities used in 

previous direct application EEC calculations ranged between 1.75 m/day and 

I 2.2 m/day, so these flow rates fall squarely into that range. 

These flow rates are somewhat lower than those used in previous modeling done 

for the registrant, but were chosen as conservative values. Higher water 

velocities would move the chemical more quickly and result in lower annual 

average concentrations. Lower water velocities will allow more time for the 

chemical to degrade before reaching the setback point. 

The dispersion value of 0.0005m2/s is fa irly small, lower than that used in 

modeling submitted by the registrant, and was chosen because small dispersion 

values result in more conservative EEC's. This value is on the low end of 

dispersion values for lakes (Chapra, 1997). Decreasing the dispersion to zero 

would result in plug flow, where the chemical moves in a single slug the size of 

the application area without spreading. Such a scenario wou ld result in 

somewhat higher annual average and peak concentrations. For the listed 

application areas and setbacks, annual average concentrations would remain 

below 400 ppb. Peak concentrations would increase to around 1150 ppb for all 

four cases. 

Note that for treatment areas of 8- I 6 and ~ 16 acres, the setback distances listed 

in the table are greater than those on the proposed label. The proposed label 

does not specify a maximum area that can be treated - the largest area is simply 

Figure 1. 

relative sizes of 

application 

areas 

listed as I 6 acres. Here the" ~ 16 acre" category was assumed to be 32 acres, and the treated 

length and setback was calculated using this area. The sizes and shapes of the treated areas are 

shown in Figure 1 . Setbacks are the distance downstream from the downstream end of each of 

these areas, and are not shown in the figure. Treated areas greater than 32 acres were not 

considered in this assessment, but setback distances would need to increase proportionally for 

larger application areas. 

EFED believes these calculations presented here represent a more protective estimate of EEC's. 

Flow rates were chosen to maximize annual average concentrations for each of the four cases 

considered. The dispersion value was used was small, which also results in somewhat higher 

EEC's. Greater dispersion and any change in flow rate would lead to lower predicted annual 

3 
average EEC's. Finally, the convection dispersion equation was solved in only one space 

dimension, so only longitudinal mixing of triclopyr was considered. This is equivalent to an 

application over the entire width of the lake . 

Table 1. Input Sizes, Setbacks and EECs 

Area Treated Setback Distance flow rate Concentration (ppb) 

acres m2 length (m) feet meters m/day annual avg peak 

sA 16187 360 1000 300 2.28 382 821 

4-8 32375 500 1400 425 3. 13 384 894 

8-16 64750 710 2000 600 4.44 389 983 

216 129500 lOOO 2600 792 6.28 390 1049 

References 

Chapra, S. C. (1997). Surface Water Quality Modeling. McGraw Hill, New York, p. 149 

Toride, N., F. J. Leij, and M. Th. van Genuchten, 1995. The CXTFITCodefor Estimating 

Transport Parameters from laboratory or Field Tracer Experiments. USDA-ARS, U.S. Salinity 

Laboratory Research Report No. 13 7 

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