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 lliam 

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, Melissa <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: 

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Nguyen,Khue 

From: 

Sent: 

To: 

Cc: 

Subject: 

Follow Up Flag: 

Flag Status: 

Khue: 

Eckel, William 

Tuesday, December 15, 2015 12:52 PM 

Nguyen, Khue; Wait, Monica; Carey, Stephen 

Motilall, Christina; Grable, Melissa 

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 limit. Relaxing this assumption to half the 

solubility 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, Khue 

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 

703-34 7-0248 

Nguyen. khue@epa .gov 

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