No Effect Concentrations (NOECs), Exposure-Response Functions, and Toxic
Effects Factors in the CASM_Atrazine: In Charge question #5, EPA notes
that the CASM_Atrazine version they are using appears to overestimate
the effects of low-level, chronic atrazine concentrations.  The enclosed
document details the reasons for this effect, and concludes that it
results from the optional parameters selected by EPA during their
research into the sensitivity of the model, specifically, 1) assigned or
implied NOECs that are significantly less than actual effects observed
within the micro/mesocosm studies and 2) the accumulation of small
biomass deviations due to artificially high macrophyte biomass values
present in the reference simulation during the early part of the year. 
Selection of more realistic NOECs and removing of the artificially high
macrophyte biomass at the beginning of the year are options for
addressing this issue.

