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).
On this page:
- Calculating the upper bound estimate of exposure
- Adjusting acute toxicity values
- Adjusting chronic toxicity values
- Determining whether drinking water exposure is of potential concern
- Pesticide-specific Mineau scaling factors
SIP employs the following conservative assumptions to derive upper bound exposure estimates:
The chemical concentration in drinking water is at the solubility limit in water (at 25°C).
The assessed animals obtain 100% of their daily water needs through drinking water.
The daily water need is equivalent to the daily water flux rate as calculated by Nagy and Peterson (1988).
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).
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).
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
Fluxwater = (1.180 * BW0.874) / 1000
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
Fluxwater = (0.708 * BW0.795) / 1000
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.
Dose = (Fluxwater * Solubility) / BW
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,
- AT = adjusted toxicity value (mg/kg-bw);
- LD50 = endpoint reported by toxicity study (mg/kg-bw);
- TW = body weight of tested animal
(350 g rat, 1580 g mallard duck, 178 g Northern bobwhite quail or weight defined by the model user for an alternative species);
- AW = body weight of assessed animal (g);
- x = Mineau scaling factor.
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.
Adjusted Toxicity Value
AT = (LD50) ((AW / TW)(x-1))
Adjusted Toxicity Value
AT = (LD50) ((TW / AW)0.25)
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.
Adjusting chronic toxicity values
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
FI = 0.0582 * BW0.651
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):
Chronic Toxicity Value
Dose Equiv. Toxicity = (NOAEC * FI) / BW
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.
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).
Adjusted Toxicity Value
AT = (NOAEL) ((TW / AW)0.25)
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.
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.
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.
|Pesticide||Mineau Scaling Factor|
|Sodium fluoroacetate (Compound 1080)||1.3180|
Note: not all of the pesticides listed in Table 4 are currently registered in the U.S.