UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

OFFICE OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION 

PC Code: 112600 

DP Barcode: D419983 

Date: January 30, 2015 

MEMORANDUM 

Subject: 

To: 

From: 

Tier II Drinking Water Assessment for the Proposed Prohexadione Calcium Use 

on Watercress and Strawberries 

William Wassell, HED Primary Reviewer 

Anwar Dunbar 

James S. Miller 

Risk Assessment Branch 1 

Health Effects Division (7509P) 

Laura Nollen, Risk Manager Reviewer 

Barbara Madden, Minor Use Team Leader 

Marion Johnson, Branch Chief 

Risk Integration Minor Use, and Emergency Response Branch 

Registration Division (7505P) 

~ \/3o/l~ ( ' .- Edom Seifu, Envirorup. ental Scienc~rai~e :..+,")r s-....... \ \.\ 3()\ 1 

') 

Stephen Wente, Ph.D., Biologist . ~ :: ) - _.I 

Sujatha Sankula, Ph.D., Branch Chief. . ;.::.:-;==- ~\3d'<? 

Environmental Risk Branch 1 

Environmental Fate and Effects Division (7507P) 

This drinking water assessment was completed for the proposed section 3 new use of 

prohexadione calcium on watercress and strawberries. 

Prohexadione calcium is a plant growth regulator (PGR) in the end-use product Apogee PGR 

(BASF; EPA Reg. No. 7969-188). Apogee PGR has been previously assessed for use on sweet 

cherry, turf, ornamentals, apples, grass, peanuts, and pome fruit. It reduces vegetative growth and 

produces a larger stem diameter in watercress and controls vegetative growth in strawberries. 

The highest annual application rate for the proposed uses is 0.275 lb a.i./acre, resulting from two 

applications of 0.138 lb a.i./acre (7 -day interval) on watercress. Prohexadione calcium can be 

applied to both watercress and strawberries through ground or aerial application and chemigation 

for watercress. The highest annual application rate for all registered and proposed uses of 

prohexadione calcium remains 19.5lbs a.i./acre (26 applications of0.75lbs a.i./acre; 14 day 

interval) for use on turf. 
Environmental Fate and Transport 

Envirorunental fate data indicate that prohexadione calcium is not expected to persist in the 

natural envirorunent. Laboratory and field half-life values range from 1.4 to 65 days. Major 

routes of degradation of prohexadione calcium are soil microbial degradation under aerobic soil 

conditions (T 112 = 1.4 days; MRID 44457785), rapid hydrolysis under acidic conditions (T 112 (pH 5) 

= 4.4 days; MRID 44457782), and aqueous photolysis (T 112 (pH 9) = 23.2 days, T 112 (pH 5) = 9.9 

days; MRID 44457783). 

Following application, prohexadione calcium can reach surface water via spray drift and runoff. 

Although prohexadione calcium is expected to be moderately mobile in most soils, it is not 

expected to pose a threat to ground water because it readily degrades microbially in soil under 

aerobic conditions. Some envirorunental conditions, however, such as excessive precipitation 

occurring immediately after application or during preferential flow conditions in structural soil, 

could result in rapid runoff or leaching to ground water. 

Prohexadione calcium hydrolizes with a half-life of 65 days in pH 7 sterile aqueous buffer 

solutions, photodegrades with half-life of 32 days in soil, and degrades with a half-life of 117 

days in flooded loamy sand soil incubated anaerobically in darkness. The chemical dissipates 

with half-lives ranging from one to 20 days in various terrestrial field dissipation studies 

conducted on bare ground. Therefore, the terrestrial field dissipation half-lives are in agreement 

with prohexadione calcium's laboratory half-lives. AccordiJ;J.g to the Food and Agriculture 

Organization (F AO) mobility classification, prohexadione calcium is moderately mobile in most 

soils. It is highly soluble in water and not likely to bioaccumulate with a Kow of 1.25 x 1 o-3 

(MRID 44457714). In addition, it is not expected to volatilize from water and soil surfaces. 

Prohexadione calcium degrades to despropionyl, which degrades further into tricarballylic acid 

and citric acid, both of which are naturally occurring substances. Further, these two acids are 

subsequently mineralized to C02. The major degradate of prohexadione calcium, despropionyl 

prohexadione, appeared to be unstable in a freezer storage stability study. 

Drinking Water Exposure Modeling 

Surface Water 

The previous highest surface water Estimated Drinking Water Concentrations (EDWCs), 

determined using the PRZM/EXAMS model, were 52.4 J.tg/L (acute) and 6.19 J.tg/L (chronic) 

based on turf use (USEPA 2010, D367843). Watercress, which is planted in flooded paddies, is 

drained before prohexadione calcium application. Following the application and a brief drying 

period, the paddies are flooded again with water, similar to rice. Because ofthe characteristics of 

the system and the unique characteristics of accumulation, tije Tier I Rice model was used to 

calculate an aquatic EDWC value based on exposure of runoff water from flooded watercress 

fields. 

The Tier 1 Rice model was used to generate Estimated Envirorunental Concentrations (EECs). 

The Tier I Rice Model v 1.0 relies on an equilibrium partitioQing concept to provide conservative 

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estimates of environmental concentrations resulting from application of pesticides to rice 

paddies. When a pesticide is appliedlto a rice paddy (or in this case a watercress paddy), the ·. 

model assumes that it will instantaneously partition between a water phase and a sediment pqase. 

Neither the degradation of the pesticide, nor the mass transfer from the aqueous phase to the 

sediment is ~onsidered in this modeL The aqueous concentration that results from such a 

partiti~ is described as follows (USEP A, 2007): 

(' - lila; , 

w - 0.00105 + O.OOOI3Kd 

and. if appropriate! 

where: 

Kd = O.OIKO<' 

Cw = water concentration (Jtg/L] 

lllai' =mass applied per tmit area [kg/ba] 

~<(! = water-sediment partitioning coefficient [Likg] 

Koc = organic carbon partitioning coefficient [Likg] 

The Tier I Rice Model produced a ghly conservative peak EDWC of 170.1 J.Lg a.i./A (acute and 

chronic) for prohexadione calcium in the aqueous concentration in a watercress system following 

two applications of0.138lbs a.i./A with an application interval of7 days (Table 1). 

T a e bl 1 . I nputs an d P ea kEEC s f rQm t e hT" 1er IRi ce Mdl o e. 

Input Parameter Single Application Annual 

Application (2 

aoolications) 

Application Rate lbs a.i./ A 0.138 0.275 

Water-Sediment Partitioning Coefficient (Likg) 5.85 5.85 

Peak EEC (Jtg a.i.IL) 85.37 170.1 

Groundwate r I 

Based on the proposed rates for watercress and strawberries, the previously registered use, turf 

has the highest annual application rate (19.5 lbs a.i./acre) and thus results in the most 

conservative groundwater EDWCs. 

For grOtmd water, the PRZM-GW (J SEPA, 2012a) and SCI-GROW models were employed to I 

model the highest application rate of all prohexadione calcium uses (26 applications at 0. 75 lb 

a.i./acre for a maximum yearly application rate of 19.5 lbs a.i/acre on turf) to determine drinking 

water exposure in ground water. The recommended EDWCs for ground water are 0.137 J.Lg/L 

based on SCI-GROW. The PRZM-QW EDWCs were many orders of magnitude lower (Table 

2). 

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Table 2. PRZM-GW Groundwater EDWCs for Prohexadione Calcium. 

Scenario Acute (Peak) ug/L Post Breakthrough Average Breakthrough 

Average (ug/L) Time (days) 

DelMarVa 1.3E-08 Incomplete in 1 00 years Incomplete in 1 00 years 

FL Citrus 2.21E-06 1.81E-06 25640.35 

FL Potato 2.93E-07 2.2 1E-07 21957.11 

GA Coastal 1.41E-09 Incomplete in 1 00 years Incomplete in 100 years 

NC Coastal 2.83E-11 Incomplete in 100 years Incomplete in 100 years 

WI Sands 4.45E-12 Incomplete in 1 00 years Incomplete in 100 years 

Monitoring Data 

No surface water or ground water monitoring data were loca ed for prohexadione calcium. The 

chemical was not surveyed in the surface and ground water under the USGS National Water 

Quality Assessment (NWQA) program. 

Uncertainties 

The Tier 1 rice model is based on a static shallow waterbody1 and does not account for 

degradation. While water cress is grown in a similar depth of water, the water is standing and 

degradation would likely occur in the water cress use site. Therefore, it is likely that the tier 1 

rice model greatly over-estimates the EDWCs. 

Conclusions 

The previous highest surface water EDWCs were 52.4 Jlg/L (acute) and 6.19 Jlg/L (chronic) 

based on the turf use. One of the new prohexadione calcium \.tses is water cress, which grows in 

standing water. There is no standard scenario for water cress. Using the tier 1 Rice Model to 

generate highly conservative EDWCs for water cress produced EDWCs of 170 Jlg/L (acute and 

chronic). 

For groundwater, the EDWC is 0.137 Jlg/L (acute and chronic) based on Sci-Grow. This 

EDWC is for turf use, which is the use with the highest annual application rate (19.5 lbs a.i./ A) 

for any prohexadione calcium use. The PRZM-GW EDWCs are found to be many orders of 

magnitude lower. 

References 

USEPA. 2007. Tier I Rice Model vl.O for Estimating Pesticide Concentrations in Rice Paddies. 

Environmental Fate and Effects Division, Office of Pesticide Programs, United States 

Environmental Protection Agency. 

USEPA. 2010. Prohexadione Calcium Drinking Water Assessment in Support ofNew Use 

Registration Action for Turf and Ornamentals. Data Package Barcode: D367843. Office of 

Pesticide Programs, United States Environmental Protection Agency. 

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