Cedar Lake IA 04-LDM-1101

Assessment Comments

Assessment is based on (1) information from the DNR Water Quality Bureau / Water Supply Section and (2) Syngenta's "Iowa Voluntary Atrazine Monitoring Program." See attached document for details.

Basis for Assessment

SUMMARY:  Class C (drinking water) uses were assessed as "partially supported."  The Class B(LW) aquatic life uses and fish consumption uses remained "not assessed."  EXPLANATION:  The Class B(LW) remained "not assessed" due to the lack of recent information on the status of aquatic life at this lake.   Information from the the DNR Water Supply Section was used to assess support of the Class C (drinking water) uses as "partially supported" (=PS) due to issuance by the Winterset Municipal Water Works of three notices of MCL violations for nitrate in 1999 (April 22, May 12, and June 4).   According to EPA and DNR methods for assessing support of Class C (drinking water) uses, one or more drinking water advisories lasting 30 days or less per year suggest that the Class C use is only partially supported (see pages 3-38 to 3-44 of U.S.  EPA 1997b and the DNR assessment methodology for Section 305(b) reporting).   Although levels of nitrate suggest a water quality impairment, levels of atrazine were relatively low and suggest only a "threat" to support of drinking water uses of this waterbody.   The 1998 results of the Novartis "Iowa Voluntary Atrazine Monitoring Program" showed that the time-weighted mean level of atrazine in samples collected from the Winterset raw water source from January to December 1998 (1.2 ug/l, N=29, maximum=7.9 ug/l) was below the MCL of 3.0 ug/l.   Based on DNR's Section 305(b) assessment methodology, if the average contaminant level in source water is less than the MCL, but a level in one or more samples is above the MCL, the Class C (drinking water) uses of the source water should be assessed as "fully supported / threatened."  Due, however, to the three notices of MCL violations for nitrate in 1999, the Class C uses were assessed as "partially supported."  Fish consumption used remained "not assessed" due to lack of fish contaminant monitoring at this lake.