Document ID: EPA-HQ-OPP-2007-0934-0005
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2007-11-29T05:00Z

Charge Questions to the SAP

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.

SAP Charge Questions on the Atrazine Monitoring Results

(6)	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.

(7)	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.

(8)	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.

Please comment on whether the Agency should consider the low flow sites
and/or intermittent streams as a part of the population estimates or
treat them separately. 

Please comment on whether the aquatic systems and exposure conditions of
the existing microcosm and mesocosm studies adequately represent these
low flow and/or intermittent stream communities.  If not, how could EPA
determine an LOC for low flow conditions?

(9)	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.

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

(10) 	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?

(11) 	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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