June 2000 - Questions for the SAP re: Drinking Water
- EFED is recommending the following factors to identify target
pesticides for inclusion in the study: extent of use area, environmental
fate properties, and risk.
- Is this a reasonable strategy for selecting chemicals for the survey design?
- Would some other approach better serve the long-term objective of developing
- EFED is recommending that pesticide use area be the primary
domain. In order to better define exposure levels to specific
human populations and to better understand the processes affecting
pesticide concentrations in surface source drinking water, it
may be useful to use smaller domains. Such domains might be spatial
or based on vulnerability criteria.
- Should domains other than a pesticide's use area be used (regional domains, for example)?
- What are the panel's views on minimally acceptable data quality standards if the study had a larger number of domains.
- Are there any suggestions for optimizing survey design to maximize the number of target pesticides represented while minimizing the total number of sampled CWS's without sacrificing data quality?
- Could preferential selection of CWS's with source watersheds containing uses for multiple pesticides be incorporated into the design?
- EFED is considering stratifying the CWS's by vulnerability.
The estimation of vulnerability would attempt to identify, for
each pesticide and CWS, the likelihood of contamination. EFED
would then randomly select CWS's from the vulnerability strata
for inclusion in the survey.
- How should we combine factors into an estimate of site vulnerability?
- Would additional or different strata increase the usefulness of the data in addressing the objectives of the monitoring study?
- What are the relative merits of using vulnerability-based as opposed to geographically-based stratification methods?
- EFED is proposing to use a GIS watershed characterization tool
for identifying CWS's in the domains and strata.
- Does the SAP agree with the usefulness of this tool for these purposes?
- We are aware of limitations in the accuracy of the data used in the site selection tool. Is the panel aware of better ways of handling the accuracy and precision limitations of this data?
- Is the panel aware of alternative data sources that may not have these limitations?
- EFED recommends collecting raw water and finished water samples
with reactive analysis of the finished water if pesticides are
detected in the raw water. Finished water more directly reflects
drinking water exposure, while raw water better reflects watershed
and usage characteristics allowing better risk management decisions.
- Given the analytical costs for paired raw and finished water samples and the lack of comprehensive information on water treatment effects on the removal and transformation of most pesticides, does the panel have alternative recommendations on assessing raw and finished water?
- Preliminary analyses by EFED indicates pesticide fate properties,
sampling frequency, and hydrologic residence time impact the accuracy
of calculated annual mean concentrations.
- Does the SAP have recommendations on balancing the characteristics of the pesticides and the water body being sampled with the number of samples needed to adequately measure the annual mean at different CWS's?
- Annual mean pesticide concentrations occurring at any CWS vary
from year to year. A multi-year study would help to quantify year-to-year
variability but is more costly.
- Does the SAP have any suggestions for assessing annual variability given the financial constraints of the survey?
- Would drawing out the survey over three years (with the same number of samples per CWS) improve it?
- EFED recognizes that pesticide concentrations in drinking water
are dependent on factors including watershed characteristics,
pesticide use, pesticide fate properties, surface water hydrology,
and water treatment processes. Interpretation of the monitoring
data will be dependent on the collection of such related ancillary
- What types of ancillary information does the SAP believe would assist in the interpretation of the monitoring data, and application of the data to model development and validation?
- We have defined our population as CWS's with pesticide use in their watersheds, based on the assumption that runoff and near field spray drift are the major routes of loading to the water supply.
- Is this a reasonable assumption and should we monitor facilities that do not have pesticide use in the watershed?
- One option proposed in the design framework is a census of
the facilities serving the largest cities. These facilities are
believed to have little agriculture in their watersheds, but reflect
the drinking water for a large percentage of the population.
- Does the SAP believe sampling CWS's serving very large populations is useful even if they are expected to be of low vulnerability?
- Modeling should allow the Agency to reduce the extent of future
drinking water surveys, and help to better identify areas at higher
- Are there any changes to the design which will better allow the survey to support model development and testing?