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I. Charge Questions to the SAP

  1. SAP Charge Questions on the use of the CASM Model

    1. Please comment on the use of a community simulation model for assessing the relative effects of different exposure time series. Please provide any recommendations for a community response model other than, or along with, CASM that could be used for assessing the effects of atrazine. What are the strengths and weaknesses associated with the other model(s). Please comment on approaches that do not require an aquatic community response model and discuss the advantages and disadvantages of any alternative non-modeling approaches for extrapolating the effects seen in micro/mesocosm data to the effects resulting from field exposure.

    2. The general methodology employed in this analysis consists of (a) correlating model outputs to micro/mesocosm data to determine a model LOC and (b) applying the model to chemographs of interest to determine whether the LOC is exceeded.  Please comment on the scientific strengths and limitations of this approach.

    3. Please comment on the reasonableness of the general CASM_Atrazine model formulation and parameterization, and the various options selected for the base model configuration.

    4. Please comment on whether the described sensitivity analyses are suitable for characterizing uncertainties associated with the choice of options for configuration of the base model and the input variables.  What additional sources of uncertainty alternatives should be examined in this analysis?  Please comment on whether the sensitivity of results to the slope of the toxicity curve, as well as the EC50, should be examined to address possible effects on responses to short pulses.

    5. During its review of the CASM_Atrazine model, the Agency found that the model appears to overestimate the effects of low, chronic concentrations possibly due to the way the model simulates population levels and decline of macrophytes early in the year.

      • The Agency sees two approaches for addressing this issue: (1) exclude early season atrazine exposures from the chemograph inputs, or (2) modify the model to better account for the impacts of early-season exposures. Please comment on the strengths and weaknesses of the Agency’s approaches and provide recommendations for any alternatives.
      • Given that the Agency identified this issue during the exposure evaluation, please provide recommendations on additional steps the Agency could take for quality assurance for the model and methodology.

  2. SAP Charge Questions on the Atrazine Monitoring Results

    1. The monitoring program used a tool (WARP) designed to assess the vulnerability of watersheds and stream segments to (1) identify watersheds within the corn/sorghum growing region that are likely to be most vulnerable to atrazine exposure and, (2) select sampling sites within the watersheds that are likely to be more susceptible to atrazine runoff. 

      • Please comment on the use of WARP predictions for hydrologic units (HUC 10/11) to restrict the survey design to those HUCs in the upper 20th percentile and then (1) to stratify by WARP predictions between 80th – 95th percentiles and above 95th percentile and (2) to select HUCs with probability proportional to higher atrazine use rates.

      • Comment on the use of survey design population estimation approach for estimating the number (and %) of HUCs that may have LOC exceedances.

    2. Once the vulnerable HUC 10/11 watersheds were selected for monitoring, specific monitoring sites were selected within each watershed using criteria that were designed to maximize the potential for selecting the streams most vulnerable to atrazine exposure.  However, with only a single point monitored per watershed, estimates of within-HUC variability for detections of atrazine could not be calculated. The resulting population estimates reflect variability across watersheds but not within the monitored watersheds. Please comment on this approach and identify and discuss any alternative approaches to extend the results of the monitoring sites.

    3. Three monitoring sites in NE experienced low- or no-flow conditions that precluded sampling. While Hampton et al. (2007a) suggest that these sites with intermittent or low flow are already stressed by other factors, Meyer et al. (2007) indicate that such aquatic communities are rich in diversity. The Agency has generated statistics for these three sites as a separate stratum, however the meaning of these separate population estimates is uncertain.
    1. The monitoring study sampled for atrazine concentrations at 4-day intervals to characterize the atrazine chemograph in these low-order Midwestern streams. The CASM_Atrazine model used these chemographs with a stair-step interpolation between samples dates to relate atrazine exposures in the streams to microcosm/mesocosm studies in order to determine whether the exposures triggered LOC thresholds.

      • What other approaches for interpolation should be considered?  Given the concentration-duration endpoint, how frequently must sampling occur to appropriately capture the magnitude and durations of exposure associated with atrazine?

      • Sensitivity analysis of CASM_Atrazine model inputs suggests that some uncertainty bound on model results is appropriate.  The Agency used a 2x multiplication factor from the model sensitivity analysis  to estimate uncertainty in model output.  The sample frequency analysis indicates that there is uncertainty associated with monitoring data that may not be accounted for by the model uncertainty factor of 2x.  Given the importance of sample frequency and interpolation, please comment on whether consideration should be given to placing additional uncertainty bounds on monitoring data to account for uncertainty in the ability of the sampling strategy to capture the magnitude and duration of atrazine exposures. Please provide any suggestions for how to proceed with this approach.


  1. SAP Charge Questions Relating to Identifying Where Atrazine Exceedances Are Likely to Occur

    1. While the monitoring study was based on a watershed vulnerability assessment, the ultimate value is in identifying water bodies where atrazine concentrations exceed the LOC. One approach is to use the updated version of the National Hydrography Database (NHDPlus) and apply the criteria used to select the monitoring locations to identify streams that appear to have the potential to exceed the LOC.

      • Please comment on the strengths and weaknesses of the Agency’s proposed approach for identifying streams within watersheds that exceeded the LOC.

      • In what ways can the preliminary approach be improved?

      • Please recommend alternative approaches, if any, that may be better suited to apply the watershed-based assessment to streams?

    2. In order to identify areas beyond the 40 study sites where higher atrazine exposures are likely to occur, the Agency must determine whether the watersheds that exceeded the LOC in multiple years are randomly distributed within the 1,172 vulnerable watersheds or represent a unique subset of conditions. If the latter and the conditions can be identified, monitoring could be focused only in watersheds where those conditions exist.The Agency has proposed evaluating WARP parameters and other sub-watershed soil and hydrologic properties to determine the extent to which the monitoring results can be used to identify other water bodies exceeding the LOC.

      • To what extent can WARP be used to identify other watersheds of concern? Given the influence of atrazine use on vulnerability and exposure, please comment on whether the extrapolation should be limited to the original 1,172 watersheds or include a broader atrazine use area?

      • Please comment on the soil and hydrology parameters the Agency is evaluating for extrapolation to vulnerable watersheds. What additional soil and hydrologic parameters should the Agency consider?

      • What additional approaches to the identification of watersheds that may have atrazine levels that exceed the LOC should the Agency consider?

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