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Federal Register / Vol. 86, No. 165 / Monday, August 30, 2021 / Rules and Regulations
d. Drinking Water Refinements EPA has defined four assessment tiers for drinking water assessments. Lower tiered assessments are more conservative based on the defaults or upper bound assumptions and may compound conservatisms, while higher tiers integrate more available data and provide more realistic estimates of environmental pesticide concentrations.
These four tiers are generally based on the level of effort, the amount of data considered, the spatial scale, and the certainty in the estimated pesticide concentration. Tier 1 requires the least amount of effort and the least amount of data, whereas Tier 4 is resource intensive, considers a wide range of sources and types of data, and is spatially explicit, resulting in high confidence in the reported pesticide concentration. Each successive tier integrates more focused pesticide, spatial, temporal, agronomic, and cropspecific information. The order in which refinements are considered i.e., the order in which the assessment is refined is pesticide-specific and depends on the nature and quality of the available data used to support the refinement. Additional information on the conduct of drinking water assessments can be found in the Framework for Conducting Pesticide Drinking Water Assessment for Surface Water USEPA, 2020.
As discussed in the Framework document, EPA can incorporate several refinements in higher tiered modeling.
Two such refinements are the percent cropped area PCA and the percent crop treated PCT. These are described in the recently completed document titled Integrating a Distributional Approach to Using Percent Crop Area PCA and Percent Crop Treated PCT
into Drinking Water Assessment Ref.
27 The PCA refers to the amount of area in a particular community water system that is planted with the crop of interest e.g., the default assumption is that the entire watershed is planted with a crop of interest. The PCT refers to the amount of the cropped area that is treated with the pesticide of interest e.g., the default is that the entire cropped area is treated with the pesticide of interest. With additional use and usage data, EPA can refine assumptions about the application rate and PCT for use in modeling to generate estimated drinking water concentrations EDWCs that are appropriate for human health risk assessment and more accurately account for the contribution from individual use patterns in the estimation of drinking water concentrations.

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2. Drinking Water Assessment for Chlorpyrifos.
For the chlorpyrifos drinking water assessment, the metabolite chlorpyrifos oxon, which forms because of drinking water treatment and is more toxic than chlorpyrifos, was chosen as the residue of concern. Ref. 28 and 29 The range of conversion from parent to oxon depends upon the type of water treatment and other conditions. Based on available information regarding the potential effects of certain water treatments e.g., chlorination appears to hasten transformation of chlorpyrifos to chlorpyrifos oxon, EPA assumed that all chlorpyrifos in source water is converted to chlorpyrifos oxon upon treatment.
The Agency used a DWLOC approach for assessing aggregate risk from chlorpyrifos. As such, EPA calculated DWLOCs for different age groups for both the acute aggregate assessment and the steady-state aggregate assessment, taking into consideration the food and residential contributions to the risk cup.
These numbers were provided as a benchmark for evaluating drinking water contributions from uses of chlorpyrifos across the United States, and whether such concentrations would result in aggregate exposures to chlorpyrifos that exceeded the Agencys levels of concern. The lowest acute DWLOC calculated was for exposure to chlorpyrifos oxon to infants <1 year old at 23 ppb; the lowest steady state DWLOC calculated was also for exposure to chlorpyrifos oxon to infants <1 year old at 4.0 ppb. Ref. 9 at 45
45. In other words, EDWCs of chlorpyrifos oxon greater than 4.0 ppb for a 21-day average would exceed EPAs DWLOC and present a risk that exceeds the Agencys level of concern.
In its 2014 drinking water assessment, EPA concluded that there were multiple uses of chlorpyrifos that could lead to exposures to chlorpyrifos oxon in drinking water that exceed the DWLOC
identified at that time. Ref. 29. This assessment provided the basis for the Agencys proposal to revoke tolerances in 2015. Ref. 30. In 2016, EPA
conducted a refined drinking water assessment that estimated drinking water concentrations based on modeling of all registered uses, as well as all available surface water monitoring data.
That assessment considered several refinement strategies in a two-step process to derive exposure estimates for chlorpyrifos and chlorpyrifos oxon across the country. The first step was an assessment of potential exposure based on the current maximum label rates at
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a national level. This indicated that the EDWCs could be above the DWLOC.
Because estimated concentrations at the national level exceeded the DWLOC, the Agency conducted a more refined assessment of uses on a regional level.
Ref. 28 at 7386. This more refined analysis derived EDWCs using the PWC
modeling for maximum labeled rates and 1 pound per acre by region for each use. The analysis indicated that approved uses of chlorpyrifos in certain vulnerable watersheds in every region of the country would result in EDWCs that exceed the DWLOC. For example, Table 25 of EPAs 2016 DWA, which provides the range of estimated concentrations of chlorpyrifos in drinking water from uses on golf courses and agricultural or production crops, shows EDWCs that exceed the DWLOC in vulnerable watersheds in every region in the country. While the lower end of some of the ranges provided in that table are below the DWLOC, those lower numbers reflect a single use i.e., single crop and do not reflect potential exposure from other uses where applications occur at higher rates, more frequently, or in more locations made more vulnerable due to soil type, weather, or agronomic practices. The relevant estimated concentration for risk assessment purposes is the highest concentration across all uses because it reflects concentrations that may occur in vulnerable sources of drinking water Ref. 28 at 7374.
In addition, a robust quantitative analysis of the monitoring data was conducted resulting in concentrations consistent with model-estimated concentrations above the DWLOC. Ref.
28 at 90121. Considering both monitoring data and modeling estimates together supports the conclusion that drinking water concentrations in regions across the country will exceed the DWLOC. Ref. 28 at 121123.
After the EPAs 2016 DWA showed that the DWLOC exceedances are possible from several uses, EPA
developed refinement strategies to examine those estimated regional/
watershed drinking water concentrations to pinpoint community drinking water systems where exposure to chlorpyrifos oxon as a result of chlorpyrifos applications may pose an exposure concern. At that time, EPA
was anticipating that a more refined drinking water assessment might allow EPA to better identify where at-risk watersheds are located throughout the country to support more targeted risk mitigation through the registration review process. The refinements better account for variability in the use area treated within a watershed that may
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