The US EPA has long provided a prescribed approach to conducting drinking water exposure modeling as part of a screening level human health risk assessment. In situations where the screening level modeling indicated potential human health risks, approaches to refining the drinking water exposure modeling were often left to the registrants to put forth and were implemented on a case-by-case basis. Within the past two years, EPA has formalized a multi-tiered approach to conducting surface drinking water assessments. Refined surface drinking water exposure modeling represents a significant component to the higher tiers of this new framework. At Tier 3 of the framework, the exposure modeling scenarios simulated at Tier 2 are modified by considering typical use information (rates, application dates, application methods), percent cropped area (PCA), and percent crop treated (PCT). The PCA and PCT components of Tier 3 are derived using actual Community Water System (CWS) watershed boundaries, recent years of crop/land use spatial data layers derived from remote sensing data, and state-level pesticide usage information. While the formalization of this tiered approach has been highly valuable, the confidentiality of the CWS watershed boundaries and unavailability of this data outside of government authorities makes directly adopting the Tier 3 modeling approach by registrants and other non-government entities unattainable. An approach for applying the Tier 3 refinement principles with surrogate CWS watersheds was developed and applied to an herbicide with a cotton use pattern to demonstrate the levels of refinement achieved with the Tier 3 surface water modeling methods. This modeling showed that reductions in estimated drinking water concentrations (EDWCs) of more than 90% were likely in moving from a Tier 2 to a Tier 3 exposure assessment. An analogous tiered approach to groundwater exposure modeling does not yet exist, however the US EPA has recently provided guidance on appropriate situations for modifying the depth of subsurface degradation assumption in deriving groundwater EDWCs. Using the same cotton herbicide as an example, we will report on the effects of this new refinement on predicted groundwater concentrations, as well as what these higher tier drinking water refinements mean for human health risk assessment outcomes.

Higher tier refinements to drinking water exposure concentrations: a cotton herbicide case study

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