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Guidance for Making Temperature Adjustments to Metabolism Inputs to EXAMS and PE5

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Guidance for Making Temperature Adjustments to Metabolism Inputs to EXAMS and PE5 (4 pp, 946K, About PDF)

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Memorandum

October 18, 2010

SUBJECT: Guidance for Making Temperature Adjustments to Metabolism Inputs to EXAMS and PE5

FROM: Donald Brady, Director /s/, Environmental Fate and Effects Division (7507P), Office of Pesticide Programs

TO: Environmental Fate and Effects Division (7507P), Office of Pesticide Programs

This guidance is effective October 11, 2010. The Environmental Fate and Effects Division (EFED) routinely evaluates aerobic and anaerobic aquatic metabolism studies where laboratory-derived measurements were conducted at temperatures other than 25°C. When studies are conducted at temperatures other than the EXAMS-assumed default of 25°C and then used as an input to EXAMS or PE5, misrepresentation of the metabolic rates may result. The Water Quality Tech Team has developed the attached advisory which contains instructions to fate scientists and modelers on how to adjust the study data so that it may be properly used in EXAMS.

Any assessment that begins on or after October 11, 2010 that includes aerobic and anaerobic metabolism studies conducted at temperatures other than 25°C should employ the instructions and methods in the attached advisory1 prior to estimating concentrations using EXAMS or PE5.

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Memorandum

DATE: September 21, 2010

SUBJECT: WQTT Advisory Note Number 9: Temperature Adjustments for Aquatic Metabolism Inputs to EXAMS and PE5

FROM: Dirk F. Young, Ph.D., Model Coordinator

TO: Water Quality Technical Team, Environmental Fate and Effects Division (7507C), Office of Pesticide Programs

THROUGH: Marietta Echeverria, Acting WQTT Management Representative; Jim Hetrick, Ph.D. WQTT Science Advisor; Nelson Thurman, WQTT Science Advisor; Reuben Baris, WQTT Co-Chair; Charles Peck, WQTT Co-Chair

Purpose

The purpose of this guidance is to advise PE5 and EXAMS users on adjusting the aquatic aerobic and anaerobic aquatic metabolism inputs when laboratory-derived measurements were conducted at other than 25°C.

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Background

Registrants supply estimates of aqueous aerobic and anaerobic metabolism rates that are derived from laboratory tests that are typically conducted at 20 to 30°C. When studies are conducted at temperatures other than the EXAMS-assumed default of 25°C and then used as an input to EXAMS or PE5, misrepresentation of the degradation rate may result. This is because temperature impacts degradation rates, and EXAMS automatically adjusts the input degradation rate according to the EXAMS-determined ambient temperature (as determined from the meteorological file). This guidance does not apply to aerobic and anerobic soil 2, hydrolysis, or photolysis degradation rates. The adjustment that EXAMS uses to modify the aerobic and anaerobic aquatic degradation rates is as follows:

Equation 1

µEXAMS = µmeasured[Q10((T-Tref)/10)]

where

µEXAMS = degradation rate coefficient used internally by EXAMS, [day-1]

µmeasured = laboratory measured aerobic metabolism rate, [day-1]

Q10 = factor by which degradation increases for a 10°C temperature rise.

T = temperature of modeled water body [°C]

Tref = temperature of laboratory study, assumed 25 by EXAMS [°C].

In a standard EPA assessment, the default Q10 is equal to 2, so this temperature modification doubles the degradation rate for every 10°C rise in temperature. Also, in a standard EPA assessment, the default reference temperature is 25°C- that is, EXAMS assumes by default that the input metabolism rate was derived from a laboratory experiment conducted at 25°C. For example, if the actual rate were conducted at 20°C, then EXAMS would assume by default that the aquatic degradation rate was conducted at 25°C, and the effective EXAMS degradation rate would be 70% of it actual value. Therefore, metabolism studies conducted at temperatures other than 25°C should be corrected prior to entering them into EXAMS or PE5.


2 Anaerobic soil metabolism should be used in aquatic modeling in the absence of acceptable data for anaerobic aquatic metabolism when hydrolysis is shown to be insignificant (USEPA, 2009). Therefore, temperature adjustments for aquatic metabolism studies would apply to anaerobic soil studies when used in absence of anaerobic aquatic metabolism data.

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Guidance

When aerobic or anaerobic aquatic metabolism rates are derived from studies conducted at other than 25°, they should be adjusted before entering them into EXAMS or PE5. The adjustment should be as follows:

Equation 2

µinput = [2((25-Texp)/10)measured

µinput = input value for metabolism rate, [day-1]

µmeasured = laboratory measured aerobic metabolism rate, [day-1]

Texp = temperature of laboratory study [°C].

In addition, when multiple studies are received and a 90th percentile confidence limit is calculated on the half lives (as according to Input Parameter Guidance), then this adjustment should be made on the individual studies prior to that calculation being made.

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Example

As an example of an application, consider a case in which 4 studies were submitted:

  1. with a half life of 100 days conducted at 17 °C,

  2. one with a half life of 100 days conducted at 20°C,

  3. one with a half life of 100 days conducted at 25°C, and

  4. one with a half life of 100 days conducted at 27°C.

Half lives are inversely proportional to rates, so using equation (2), the adjusted half lives are:

µinput 1 = [100 days / (2((25-17)/10))] = 57 days

µinput 2 = [100 days / (2((25-20)/10))] = 71 days

µinput 3 = [100 days / (2((25-25)/10))] = 100 days

µinput 4 = [100 days / (2((25-27)/10))] = 114 days

Using the EFED input parameter guidance and calculating the upper 90th percentile confidence of the 4 values gives an input half life of 107 days (mean = 85.5 days, standard deviation = 26 days).

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Reference

USEPA 2009. Guidance for Selecting Input Parameters in Modeling the Environmental Fate and Transport of Pesticides. U.S. Environmental Protection Agency, Office of Prevention, Pesticides and Toxic Substances, Office of Pesticide Programs, Environmental Fate and Effects Division, Oct. 22, 2009.

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