Assessment Comments
Assessment is based on: (1) the results of the Iowa Voluntary Atrazine Monitoring Program conducted in 2006 through 2008 by Syngenta Crop Protection, Inc. and (2) information from the Iowa DNR's Water Supply Section on violations of 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 2006 through 2008 by Syngenta Crop Protection, Inc. and (2) information from the Iowa DNR's Water Supply Section on violations of 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. A TMDL for atrazine at Cedar Lake was prepared and approved by EPA in 2010; thus, this lake is placed into IR Category 4a (TMDL approved) for the 2010 assessment/listing cycle.
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 2006 through 2008 showed that none of the 65 time-weighted mean levels of atrazine exceeded the MCL of 3.0 ug/L (maximum = 2.4 ug/L). Based on IDNR’s assessment methodology, if none of the 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 “fully supported.” However, the Class C uses at Cedar Lake were assessed as "not supported" due to violations of the atrazine MCL for the 2008 assessment/listing cycle (5 moving annual averages violated the MCL between 2004 and 2006). Based on IDNR's assessment methodology 2 assessment/listing cycles with no moving annual averages violating the MCL are necessary to suggest de-listing of the atrazine impairment. Therefore, Cedar Lake remains assessed as "not supporting" for the 2010 assessment/listing cycle. If atrazine levels remain low at this lake it will be suggested for de-listing in the 2012 report.
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).
