Assessment Comments
Assessment is based on (1) information from the IDNR Water Quality Bureau / Water Supply Section and (2) results of monitoring for Syngenta's "Iowa Voluntary Atrazine Monitoring Program" from 2000-2002.
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 notices of nitrate MCL violations issued in 2001. In addition, levels of atrazine appear to be increasing, with the time-weighted average level in 2002 approaching the MCL of 3 ug/l. The sources of data for this assessment include (1) the results of the Iowa Voluntary Atrazine Monitoring Program conducted in 2000, 2001, and 2002 by Syngenta Crop Protection, Inc. (formerly Novartis Crop Protection, Inc.) and (2) information from the Iowa DNR's Water Supply Section on violation of MCLs in finished water as provided by IDNR staff and as summarized in annual compliance reports of Iowa’s public drinking water program.
EXPLANATION: The Class B(LW) 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 were assessed as “not supported” due to notices of nitrate MCL violations issued for nitrate in 2001. Information from the Iowa DNR's Water Supply Section shows the issuance of notices of 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). Thus, the Class C (drinking water) uses for this lake are assessed as "not supported."
In addition, the apparent increase in levels of atrazine in this lake suggest a threat to full support of drinking water use with a declining water quality trend. This assessment was based on the results of the Syngenta "Iowa Voluntary Atrazine Monitoring Program" in 2000, 2001, and 2002. This monitoring showed that the time-weighted mean levels of atrazine in samples collected from Cedar Lake in these three calendar years were below the MCL of 3.0 ug/l but approached the MCL in 2002. The time-weighted mean of atrazine in 2000 (N=31) was 2.42 ug/l, in 2001 (N=31) was 1.61 ug/l, and in 2002 (N=16) was 3.39 ug/l. The 47 moving annual averages calculated for the three-year period were all below the MCL of 3 ug/l (maximum = 2.5 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." Thus, the occurrence of the combined 12 atrazine MCL violations in 2000, 2001, and 2002 suggest that the Class C uses of Cedar Lake should be assessed as "fully supported / threatened." The increase in atrazine levels observed from 2001 to 2002 suggests an increasing trend in atrazine levels in this water supply reservoir and suggest that this waterbody is appropriate for Section 303(d) listing.
Fish consumption uses were "not assessed" due to the lack of fish contaminant monitoring at this lake.
