Methodology

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The methods used for estimating risk are based closely on USFS risk assessment methodology, in which three estimates are calculated for the exposure (dose) received by a particular receptor as a result of various herbicide use scenarios. Each dose estimate is based on a set of best-case, most-probable, or worst-case assumptions based on exposure parameters appropriate to that scenario. The dose estimates are then compared to Toxicity Reference Values (TRVs) to assess risk if the scenario were to occur. See Appendix C in the Herbicide BMPs manual for the TRVs used in the risk assessment.

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Guidance Documents for USFS Risk Assessment Methods

Exposure estimates were calculated using the risk assessment spreadsheets created by Syracuse Environmental Research Associates (SERA) for the USFS and the Bureau of Land Management (BLM), published between 2007 and 2014. A full description is available in the report Preparation of Environmental Documentation and Risk Assessments. Detailed risk assessments for each of the herbicides discussed here are also downloadable from the USFS site. An explanation of the methods used to estimate risks to wildlife is also available in Chapter 2 of the
2010 Marin Municipal Wastewater District (MMWD) Herbicide Risk Assessment. Some parameter values and methods used for the risk estimates differ from previous risk assessments. Each of these changes is discussed below.

Modifications to USFS Risk Estimation Methods

Several modifications to USFS/SERA default values were made for this report:

TRVs: Toxicity Reference Values (TRVs) based on LD50 or LC50 values were transformed to “No Effect” levels by incorporating an additional uncertainty factor of 20, the methodology used by US EPA to adjust TRVs for assessment of effects to endangered species. This transformation ensures that all TRVs are based on “No Effect” levels, and allows direct comparison of herbicides. This change has been incorporated into the more recent USFS herbicide risk assessments, and PRI updated the older risk assessments to include this change.

Percent of diet contaminated: In more recent versions of the USFS/SERA herbicide risk assessments, the percentage of an animal’s diet assumed to be contaminated was modified to 10% (best-case), 30% (most-probable) or 100% (worst-case). PRI applied the same change to herbicides not yet adopted by USFS, to ensure an “apples to apples” comparison between herbicides. Residue rates assumed for herbicides on food (fruit, vegetation and prey) were based on the most up-to-date values from USFS/SERA (WorksheetMaker 6.0). The caloric error factor, which was introduced in recent versions of USFS/SERA worksheets, was not utilized here.

Herbicide Residue Rates: USFS changed the residue rates used in the latest version of their risk calculation spreadsheets for estimating exposures from consumption of contaminated fruit, insects and vegetation. This change lowers the best-case predicted dose for wildlife from consumption of contaminated food. In the new versions of the spreadsheets, a new lower residue rate was introduced that is equivalent to the following:

Best-case residue rate = Most-probable rate x (Most-probable rate ÷ Worst-case rate)

For example, for consumption of short grass, the values changed from 85, 85, and 240 mg of pesticide ingested/kg of body weight per lb of pesticide applied/acre to 30, 85, and 240 mg/kg per lb/acre.

These values were incorporated into the calculations for all of the herbicides to ensure comparison of equivalent values.

Insect Contamination Rate: The USFS changed the mass of a honey bee from 93 mg to 116 mg and the surface area from 2.66 cm2 to 1.42 cm2 in the more recent herbicide reviews. The net effect is to reduce the estimated dose received by the honey bee. These values were incorporated into the calculations for all of the herbicides to ensure comparison of equivalent values.

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