Alternative to EPA’s Highly Conservative Total Toxic Residue Approach for Ecological and Drinking Water Risk Assessments
March 21, 2017
A pesticide active ingredient and its degradates of concern are collectively referred to by the United States Environmental Protection Agency (EPA) as residues of concern (ROC) or total toxic residues (TTR). In determining ROCs for pesticide ecological and drinking water exposure and risk assessments, EPA requires identification of a pesticide’s degradates of concern (generally defined as individual degradates accounting for greater than 10% of total residues from laboratory studies) and including these degradates in risk assessments. Identifying a pesticide’s degradates of concern can be complicated and costly as it often requires conducting a number of additional studies to reduce uncertainties. When data are unavailable to confirm whether the concern for the degradates is justified, EPA frequently conducts the risk assessments simply by including all major degradates as residues of concern and defaulting to the highly conservative TTR approach.

Total Toxic Residue Approach


The TTR approach is one of the most conservative methods that EPA uses in risk assessments. With this approach, exposure modeling is performed for a pesticide and its degradates presumed or experimentally shown to be of similar toxicity. Degradation half-lives used to estimate exposure following this approach are calculated by regressing the total residues (rather than individual compounds) across the study intervals. In addition, the most conservative adsorption value (Kd or Koc; a measure of soil mobility) among the pesticide and its degradates is used, which often is the smallest Kd or Koc value (representing a condition of maximum soil mobility). Exposure estimates are then compared to the most sensitive toxicity endpoint of the parent or degradates. These conservative assumptions for exposure modeling often lead to overestimates of exposure and result in risk exceedances, which can cause the rejection of new product registrations or the cancellation of pesticides undergoing reregistration.

Alternative Approach: Residue Summation Method

An alternative to the TTR approach is to apply the EPA’s residue summation (RS) method, an exposure modeling approach for a pesticide and its major degradates in which degradation rates of individual compounds are determined and modeled, and estimated exposure concentrations of each compound are summed to create a total residue distribution. This is a different approach compared to the TTR approach and can result in much more favorable risk assessments.

Exponent has substantial experience using both methods and has successfully applied the RS method to help solve risk exceedance issues for a number of pesticides initially found unacceptable using EPA’s TTR method. In most cases, the modeled acute and chronic exposure concentrations using the RS method were several-fold smaller than those EPA obtained using the TTR method. Although the RS method may require additional data for degradates for exposure modeling, these data are often experimentally available. In the absence of data, EPA has tools available for estimating the missing data for exposure modeling. Thus, Exponent’s knowledge and experience with exposure and risk assessments, including the understanding of either the TTR or the RS method, can often result in better pesticide risk outcomes.

How Can Exponent Help?

Exponent is a multi-disciplinary engineering and scientific consulting firm that brings together more than 90 different disciplines to solve important engineering, science, regulatory, and business issues facing our clients. In particular, the Center for Chemical Regulation and Food Safety has extensive expertise in all aspects of pesticide registrations and associated exposure and risk assessments.

Examples of how we can assist you in preparing pesticide registrations and re-registrations related to exposure and risk assessments include:

  • Data gap analysis
  • Estimates of missing data using various EPA tools such as EPI Suite™
  • Risk screening to identify data most critical to your product registrations before generating the data
  • Exposure and risk assessments using various EPA methods including the TTR and RS methods described above
  • Risk mitigation analysis including weight of the evidence-based analysis and public interest document (PID) using AgDRIFT®, AGDISP™, SWAT, PERFUM and VFSMOD-W models
  • Development of risk refinement strategies using such higher-tier models as SWAT, BASINS, RZWQM and LEACHP. 

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