### Nguyen, Khue From: 

### Grable , Melissa Sent: To : 

### Thursday , March 31 , 2016 5:50PM Nguyen , Khue Subject: FW : 

### thidiazuron--quick sit-down with EFED

From: 

### Eckel, Wi l l iam 

### Sent: Thursday, March 31 , 2016 9 :55AM To: 

### Grable, Mel issa <Grable
. Melissa@epa
.gov> 

### Subject: RE : thidiazuron --quick sit-down with EFED Melissa: OK, this is from written notes. It is a simple equilibrium calculation of the water concentration on the field, given the application rate and partitioning coefficient, Kf. The median desorption coefficient (Kf,d) for thidiazuron is 11.8 L/kg. The maximum application rate is 0.211b/A. In the top acre-inch of soil, assuming a soil density of 1.6 g/cm3, the application rate is equivalent to 0.58 mg a.i./kg-soil. Using Kf =soil (mg/kg) 

## I water (mg/L), and solving for the water concentration, we get 0.05 mg/L (SO 

### ppb), well below the solubility limit of 31 mg/L. Hope this helps. Let me know if you have any questions. Bill From: 

### Grable , Melissa 

### Sent: Wednesday, March 

### 30, 2016 6 :04 PM To: 

### Eckel , William < Eckei.William@epa.gov> 

### Cc: Nguyen, Khue < Nguyen.Khue@epa.gov>; Motilall, Christina <Motilaii.Christina@epa.gov>; Wait, Monica <Wait.Monica@epa.gov>; Carey, Stephen <Carey.Stephen@epa.gov> 

### Subject: RE : thidiazuron--quick sit-down with EFED Super! That would be great if you could dig it out for us. We will use this in our document to address this comment. Thanks so much! Melissa From: 

### Eckel, William 

### Sent: Wednesday, March 

### 30, 2016 6 :00PM To: 

### Grable, Mel issa <Grable.Melissa@epa.gov> 

### Cc: Nguyen, Khue <Nguyen.Khue@epa.gov>; Motilall, Christina <Motilaii.Christina@epa.gov>; Wait, Monica <Wait.Monica@epa.gov>; Carey, Stephen <Carey.Stephen@epa.gov> 

### Subject: Re : thidiazuron -- quick sit-down with EFED Melissa: 1

## Nguyen,Khue From: Sent: To: Cc: Subject: Follow Up Flag: Flag Status: Khue : Ecke l , W illiam Tuesday , December 15 , 2015 12 :52 PM Nguyen , Khue ; Wa it , Monica ; Carey , Stephen Motil all , Christina ; Grable , Mel issa RE : thidiazuron--quick sit-down with EFED Follow up Completed

Per our discussions, EFED is providing additional characterization of the potential chronic risk to birds from drinking water alone (SIP model), that was indicated by our risk assessment for thidiazuron (barcode 

### 424977, June 

### 12, 2015)
. The SIP model is intended as a conservative screen (more conservative than a Tier 1 risk assessment) to determine if the drinking water route of exposure , by itself, might be of concern . SIP assumes that the chemical is present in the source water at its solubility limit 

### (31 mg/L for thidiazuron), that 

### 100% of drinking water needs are obtained from that source, and that the most sensitive bird, weighing 

### 20 grams, is exposed. It is highly unlikely that the chemical would be present at its solubility l imit. Relaxing this assumption to half the so l ubility limit 

### (15 mg/L) , SIP indicates no concern for bird chronic effects
. Because even this exposure is very unlikely, EFED does not have any concern for bird chronic effects from drinking water alone, given the current toxicity data (NOAEC of 

### 298 mg a. i./kg-diet). Bill Eckel From: Nguyen, K hue Sent: Wednesday, December 

### 09, 2015 4 : 14 PM To: Wait, Monica <Wait
. Monica@epa
.gov>; Eckel, William <Eckei.William@epa
.gov> ; Carey, Stephen <Carey
.Stephen@epa
.gov> 

### Cc: Motilall, Christina <Motilaii.Christina@epa.gov>; Grable, Melissa <Grable.Melissa@epa.gov> Subject: thidiazuron--quick sit-down with EFED Hi all , Can we schedule a quick sit-down with EFED sometime next week? We want to better understand the possible chronic drinking water risk for birds from use of thidiazuron. For example, how at risk are birds? How does the SIP model work? Thanks! Khue Nguyen Chemical Review Manager Risk Management and Implementation Branch 1 Pesticide Re -evaluation Division Office of Pesticide Programs, EPA 

### 703-347
-0248 Nguye n. khue@epa
.gov

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