Assessment Comments
Assessment is based on: (1) the results of the Iowa Voluntary Atrazine Monitoring Program conducted in 2008 and 2009 by Syngenta Crop Protection, Inc., (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, and (3) annual water quality reports from the Winterset, Iowa, Municipal Waterworks.
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 "partially supporting." The sources of data for this assessment include (1) the results of the Iowa Voluntary Atrazine Monitoring Program conducted in 2008 and 2009 by Syngenta Crop Protection, Inc., (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, and (3) annual water quality reports for the Winterset, Iowa, municipal water utility.
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. This impairment remains in IR Category 4a. A TMDL for atrazine at Cedar Lake was prepared and approved by EPA in 2010; thus, this lake was placed in IR Category 4a (TMDL approved) for the 2010 assessment/listing cycle. Because atrazine data over the previous two Integrated Reporting cycles have shown “full support” of the drinking water uses, this assessment is moved to IR Category 2a (at least one use fully supported).
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 "partially supported" due to continuing concern over high levels of nitrate. Regarding atrazine, however, the results of monitoring in 2008-09 showed that none of the 17 time-weighted mean levels of atrazine exceeded the MCL of 3.0 ug/L (maximum = 1.0 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.” Based on IDNR's assessment methodology, two assessment/listing cycles with no moving annual averages violating the atrazine MCL are necessary to remove a previous Class C impairment. Due to two consecutive IR cycles in which atrazine levels have suggested “full support” of the Class C uses (i.e., the 2010 & 2012 IR cycles), this impairment is moved from IR Category 4a (impaired; TMDL not required) to Category 2a (use fully supported). There were no MCL violations of the moving annual average concentration of atrazine in 2006 through 2008 or in 2008 through 2010.
The impairment of the Class C (drinking water) uses due to nitrate, however, continues. The most recent nitrate MCL violation and recent advisory for nitrate from the Winterset Municipal Waterworks was in May 2007; the advisory lasted approximately 30 days before compliance was achieved. According to IDNR’s assessment/listing methodology, one drinking water advisory lasting 30 days or less per year indicates impairment (“partial support”) of the Class C uses. No violations of the nitrate MCL occurred at this utility in calendar years 2008 through 2010. If the trend of lower nitrate levels and no nitrate advisories continues through the next assessment period (2010-2012), this impairment will be moved from IR Category 4a (impaired; TMDL completed) to an IR Category indicating “full support” of these uses (IR 2a).
