Assessment Comments
Assessment is based on: (1) the results of the Iowa Voluntary Atrazine Monitoring Program conducted in 2004 through 2006 by Syngenta Crop Protection, Inc. and (2) information from the Iowa DNR's Water Supply Section on violationsof MCLs in finished water as provided by IDNR staff and as summarized in annual compliance reports of Iowa’s public drinking water program.
Basis for Assessment
SUMMARY: The Class B(LW) aquatic life uses and fish consumption uses remain “not assessed” due to lack of water quality information upon which to base an assessment. The Class C (drinking water) uses are assessed (monitored) as "not supported" due to average levels of atrazine that exceeded the MCL. Notices of nitrate MCL violations issued in 2001 also contribute to the impairment of Class C uses. The sources of data for this assessment include (1) the results of the Iowa Voluntary Atrazine Monitoring Program conducted in 2004 through 2006 by Syngenta Crop Protection, Inc. and (2) information from the Iowa DNR's Water Supply Section on violationsof MCLs in finished water as provided by IDNR staff and as summarized in annual compliance reports of Iowa’s public drinking water program.
Note: A TMDL for nitrate at Cedar Lake was prepared by IDNR and approved by EPA in 2005; thus, this lake was placed into IR Category 4a (TMDL approved) for the 2004 assessment/listing cycle. Because the Section 303(d) impairment due to atrazine was not addressed in the TMDL, this waterbody was place in IR Category 5a (impaired; TMDL required) of Iowa's 2006 Integrated Report and remains in Category 5a of Iowa's 2008 Integrated Report.
EXPLANATION: The Class B(LW) (aquatic life) uses and fish consumption uses remain not assessed due to the lack of water quality information needed to assess these uses.
The Class C (drinking water) uses are assessed (monitored) as “not supported” due to levels of atrazine that exceed the MCL. Results of the Syngenta “Iowa Voluntary Atrazine Monitoring Program” from 2004 through 2006 showed that 5 of the 70 time-weighted mean levels of atrazine exceeded the MCL of 3.0 ug/L (maximum = 3.5 ug/L). Based on IDNR’s assessment methodology, if a moving annual mean contaminant level in source water is greater than the MCL, the Class C uses of the source water should be assessed as “not supported.”
In addition, notices of nitrate MCL violations issued for nitrate in 2001 contribute to the impairment at this lake. Information from the Iowa DNR's Water Supply Section shows the issuance of notices of MCL violations for nitrate during April, May, and June of 2001; this advisory period lasted for 109 days (a previous advisory period in 1999 lasted 34 days from April 20 to May 23, 1999). No notices were issued in 2000 or 2002. According to EPA and DNR methods for assessing support of Class C (drinking water) uses, one or more drinking water advisory lasting 30 days or more per year suggests that the Class C use is "not supported" (see pages 3-38 to 3-44 of U.S. EPA 1997b and the DNR assessment methodology for Section 305(b) reporting).
