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User's Guide Screening Imbibition Program (SIP) Version 1.0

(August 19, 2010)

Contact: This spreadsheet was developed by the Terrestrial Exposure Technical Team (TETT) of the Environmental Fate and Effects Division (EFED). For more information or assistance, please contact the TETT co-chairs.

Description: The purpose of this model is to provide an upper bound estimate of exposure of birds and mammals to pesticides through drinking water alone. This model is intended for use in problem formulation to determine whether or not drinking water exposure alone is a potential pathway of concern. This does not aggregate drinking water exposure with other exposure routes (i.e., diet, inhalation, dermal).

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Assumptions

SIP employs the following conservative assumptions to derive upper bound exposure estimates:

  1. The chemical concentration in drinking water is at the solubility limit in water (at 25°C).

  2. The assessed animals obtain 100% of their daily water needs through drinking water.

  3. The daily water need is equivalent to the daily water flux rate as calculated by Nagy and Peterson (1988).

  4. The body weight of the assessed bird is equivalent to the smallest generic bird modeled in T-REX (i.e., 20 g). This assumption results in the highest ratio of exposure to toxicity for the 3 assessed avian body weights of T-REX (i.e., 20, 100, 1000 g).

  5. The body weight of the assessed mammal is equivalent to the largest generic mammal modeled in T-REX (i.e., 1000 g). This results in the highest ratio of exposure to toxicity for the 3 assessed mammalian body weights of T-REX (i.e., 15, 35, 1000 g).

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Calculating the upper bound estimate of exposure

The daily water intake rates (Fluxwater; units in L) for birds are calculated using allometric equations, which are based on body weight (BW) in grams (Nagy and Peterson 1988). These equations calculate the total daily water needs of an animal from all sources, including food and drinking water.

For birds, the daily water intake rate is calculated using the equation below. This equation is representative of passerine birds, which represent the majority of bird species visiting agricultural areas and which have higher daily water flux requirements than other birds. As a result, the equations represent the most conservative estimate of pesticide concentrations in water. The resulting daily water intake rate for the 20 g bird is 0.0162 L.

Daily Water Intake Rate
Birds

For mammals, the daily water intake rate is calculated using the equation below. This equation is representative of eutherian herbivore mammals, which have higher daily water flux requirements compared to other mammals that visit agricultural areas. The only equation that would generate higher estimates of daily water flux corresponds to marsupial carnivores, which are not considered to be representative of the majority of mammals that visit agricultural areas. The resulting daily water intake rate for a 1000 g mammal is 0.172 L.

Daily Water Intake Rate
Mammals

The model calculates the upper bound estimate of exposure in drinking water (dose-based; units in mg/kg-bw) by multiplying the daily water intake rate (L) by the chemical solubility (mg/L) and then dividing by the body weight (in kg) of the assessed animal (See equation below). In cases where water characteristics (e.g., pH) influence the solubility of a chemical in water, the user should select the highest available water solubility for use in SIP.

Upper Bound
Exposure Estimate

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Adjusting acute toxicity values

LD50 values for mammals and birds are adjusted using the same approach employed by T-REX (USEPA 2008). These equations are provided below. In these equations,

Chemical specific values for x may be located in the worksheet titled "Mineau scaling factors". If no chemical specific data are available, the default value of 1.15 should be used for this parameter.

Birds

Acute
Adjusted Toxicity Value
Birds

Mammals

Acute
Adjusted Toxicity Value
Mammals

Note that if no acute toxicity data are available for mammals or birds exposed to the assessed chemical, the user should enter 0 as the toxicity value. In that case, any test species may be selected.

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Adjusting chronic toxicity values

Birds

Chronic avian toxicity studies produce endpoints based on concentration in food, not dose. The endpoint is a No Observed Adverse Effects Concentration (NOAEC) that is assumed to be relevant to all birds, regardless of body weight. In order to convert a reported avian NOAEC (mg/kg-diet) value to a dose equivalent toxicity value for the assessed animal, the daily food (dry) intake of the test bird is considered. The daily food intake rate (FI; units in kg-food) of the test bird is calculated using the equation below, where BW = body weight in kg (USEPA 1993). This equation corresponds to daily food intake rate for all birds, which generates a lower food intake rate compared to passerines. This equation is more conservative because it results in a lower dose-equivalent toxicity value.

Daily Food Intake Rate
Birds

The FI value (kg-diet) is multiplied by the reported NOAEC (mg/kg-diet) and then divided by the test animal's body weight to derive the dose-equivalent chronic toxicity value (mg/kg-bw):

Dose Equivalent
Chronic Toxicity Value
Birds

The user enters the lowest available NOAEC for the mallard duck, for the bobwhite quail, and for any other test species. The model calculates the dose equivalent toxicity values for all of the modeled values (Cells F20-24 and results worksheet) and then selects the lowest dose.

Mammals

SIP relies upon the No Observed Adverse Effects Level (NOAEL; mg/kg-bw) from a chronic mammalian study. If only a NOAEC value (in mg/kg-diet) is available, the model user should divide the NOAEC by 20 to determine the equivalent chronic daily dose. This approach is consistent with that of T-REX, which relies upon the standard FDA lab rat conversion (USEPA 2008). Mammalian NOAEL values are adjusted using the same approach employed by T-REX (USEPA 2008). The equation for mammals is provided below (variables are defined above).

Chronic
Adjusted Toxicity Value
Mammals

Note that if no chronic toxicity data are available for mammals or birds exposed to the assessed chemical, the user should enter 0 as the toxicity value. In this case, any test species may be selected.

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Determining whether drinking water exposure is of potential concern

For acute exposures, if the ratio of the upper bound dose to the adjusted LD50 value is < 0.1, the risk assessor can conclude that pesticide exposure to mammals or birds through drinking water by itself is not an exposure route of concern. If the ratio of the upper bound dose to the adjusted LD50 value is ≥ 0.1, the risk assessor can conclude that pesticide exposure to mammals or birds through drinking water by itself is an exposure route of concern.

For chronic exposures, if the ratio of the upper bound dose to the adjusted chronic toxicity value is < 1, the risk assessor can conclude that pesticide exposure to mammals or birds through drinking water by itself is not an exposure route of concern. If the ratio of the upper bound dose to the adjusted chronic toxicity value is ≥ 1, the risk assessor can conclude that pesticide exposure to mammals or birds through drinking water by itself is an exposure route of concern.

If no data are available for a specific endpoint, the risk assessor cannot preclude risk to the taxa.

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References

Mineau, P., Collins, B.T. and A. Baril. 1996. On the use of scaling factors to improve interspecies extrapolation of acute toxicity in birds. Regulatory Toxicology and Pharmacology, 24: 24-29.

Nagy, K.A. and C.C. Peterson. 1988. Scaling of Water Flux Rate in Animals. University of California Press, Berkeley.

USEPA. 1993. Wildlife Exposure Factors Handbook. United States Environmental Protection Agency. Office of Research and Development. EPA/600/R-93/187.

USEPA. 2008. User's Guide: T-REX Version 1.4.1 (Terrestrial Residue Exposure model). United States Environmental Protection Agency. Environmental Fate and Effects Division.
http://www.epa.gov/oppefed1/models/terrestrial/trex/t_rex_user_guide.htm

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Table 4
Pesticide-specific Mineau scaling factors
(from Mineau et al. 1996)
PesticideMineau Scaling Factor
3-chloro-p-toluidine0.9724
4-Aminopyridine0.9970
Aldicarb1.4021
Alphachloralose1.2780
Bufencarb1.1161
Brodifacoum0.7589
Carbaryl1.5518
Carbofuran0.8891
Chlorfenvinfos1.2561
Chlorpyrifos1.1573
Coumaphos1.3424
Demeton1.2018
Diazinon0.6284
Dicrotophos1.1180
Dieldrin1.2447
EPN1.2432
Fenitrothion1.0401
Fensulfothion1.2909
Fenthion1.2081
Methiocarb1.4079
Methomyl1.0778
Metomidate1.1044
Mevinphos0.8371
Mexacarbate0.8135
Monocrotophos0.8938
Nicotine sulfate1.5370
Parathion1.1761
Phencyclidine HCL1.1142
Phosphamidon1.1508
Pirimicarb1.1320
Propoxur (carbamate)1.2942
Sodium fluoroacetate (Compound 1080)1.3180
Starlicide0.7828
Strychnine1.1509
Temephos1.2116
Trichlorfon1.3153

Note: not all of the pesticides listed in Table 4 are currently registered in the U.S.

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