UNITED STATES ENVIRONMENTAL PROTECTION AGENCY WASH IN GTO N , D .C

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OFFICE OF CHEMICAL SAFETY AND POLLUT ION PREVE N T ION 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 Noll en, 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, app les, 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 h ighest 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 1 1 2 (pH 5) = 4.4 days; MRID 44457782), and aqueous photolysis (T 1 1 2 (pH 9) = 23.2 days, T 1 12 (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 hydro lizes 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 chemica l 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 (FAO) mobility classification, prohexadione calcium is moderately mobi le 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 i s 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 ofprohexadione calcium, despropionyl prohexadione, appeared to be unstab le 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 2estimates of environmental concentrations resulting from app licat ion ofpesticides to rice paddies
. When a pesticide is applied l to 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**~onside red**  in this modeL The aqueous concentration that results from such a**partiti ~**  

### is described as follows (US EPA, 2007):
(' - lila; ,

w - 0.00105 + O
. OOOI3Kd and. if appropriate !
wher e : Kd = O.OIKO<'
Cw= water concentration (Jtg/L] lllai ' =mass applied per tmit area [kg/ba]**~<(!** = water
- s ediment partitioning co efficient [Likg] Koc = organic carbon partitioning co effici ent [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).



### a e . nputs an T bl 1 I 

### ea 

### s rQm t e 1er ce 

### o e. d P kEEC f 

### hT" IRi Mdl Input Parameter Single Application Annual Application (2 aoolications) Appli cation 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 

### Groundwater 

## I Based on the proposed ra tes for watercress and strawberries, the previously registered use, turf has the highest annual application rate (19.5 lbs a.i ./ acre) and thus re sults in the most conservative groundwater E DWCs. 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 app lications at 0. 75 lb a.i./acre fo r a maximum yearly application rate of 19.5 lb s a.i/acre on turf) to determine drinking water exposure in gr ound 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).

3Table 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 2.93 E- 07 2
.2 1E

-07 21957.11 GA Coastal 1.41E-09 Incomplete in 100 years Incomplete in 100 years NC Coastal 2.83E-11 I ncomplete in 100 years Incomplete in 100 years W I 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 U SGS National Water Qua lity Assessment (NWQA) program. Uncertainties The Tier 1 rice model is based on a stat ic shallow waterbody1 and does not account for degradation. While water cress is grown in a similar depth o f water, the water is standing and degradation would likely occur in the water cress use s ite. Therefore, it is likely that the tier 1 rice model greatly over-estimates the E DWCs. 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 ofthe new prohexadione calcium \.tses is water cress , which grows in standing water. There is no standard scenario for wa ter 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 ch ronic) based on Sci-Grow. This EDWC i s 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 U SEPA. 2007
. Tier I Rice Model vl.O for E stimating Pesticide Concentrations in Rice Paddies. Environmental Fate and Effects Division , Office of Pesticide Programs, United States Environmental Protection Agency. U SEPA
. 2010. Prohexadione Calcium Drinking Water Assessment in Suppor t ofNew Use Registration Action for Turf and Ornamentals. Data Package Barcode: D367843. Office of Pesticide Programs, Uni ted States Environmental Protection Agency
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