Assessment Comments
Assessment based on the results of Syngenta's Iowa Voluntary Atrazine Monitoring Program from 2000-04.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses and Class B(LW) aquatic life uses remain "not assessed" due to lack of water quality information upon which to base an assessment. The Class C (drinking water) uses remain assessed (monitored) as "not supported" due to average levels of atrazine that exceed state water quality standards during the previous (2004) Section 305(b) reporting cycle. Data from the current (2006) reporting cycle, however, indicate declining levels of atrazine at this lake. The source of data for this assessment is the results of the Iowa Voluntary Atrazine Monitoring Program conducted from 2000, through 2004 by Syngenta Crop Protection, Inc.
EXPLANATION: The Class A and Class B(LW) uses remain "not assessed" due to the lack of recent water quality information upon which to base an assessments. This lake is not part of the ongoing statewide survey of Iowa lakes conducted by Iowa State University.
The Class C (drinking water) uses remain assessed (monitored) as "not supported " based on results of sampling conducted as part of the Iowa Voluntary Atrazine Monitoring Program conducted in 2000, 2001, and 2002 by Syngenta Crop Protection, Inc. Results of more recent (2002-2004), monitoring, however, show a decline in atrazine levels at this lake.
Results of the Syngenta "Iowa Voluntary Atrazine Monitoring Program" from 2000 through 2002 showed that the time-weighted mean level of atrazine in samples collected from Centerville Reservoir in 2001 (4.11 ug/l) was above the MCL of 3.0 ug/l. In addition, thirty-two of the 48 moving annual averages calculated for the 2000-02 period were above the MCL of 3 ug/l (range: 3.5 to 4.9 ug/l). According to IDNR's Section 305(b) assessment methodology, if the average contaminant level in source water (either a time-weighted annual average or a moving annual average) is greater than the MCL, the Class C (drinking water) uses of the source water should be assessed as "not supported." Thus, the Class C uses of this lake were assessed as impaired for the 2004 Section 305(b) reporting cycle.
Results of the Syngenta "Iowa Voluntary Atrazine Monitoring Program" from 2002 through 2004, however, show that the time-weighted mean levels of atrazine in samples collected in calendar years 2002 through 2004 were below the MCL of 3 ug/l, thus suggesting a decline (improvement) in atrazine levels at this lake. The time-weighted mean of atrazine in 2002 (N=16) was 1.7 ug/l, in 2003 (N=28) was 2.9 ug/l, and in 2004 (N=33) was 1.2 ug/l. The overall mean (2.0 ug/l) and median (1.7 ug/l) atrazine levels over this three-year period (N=77) were both less than the MCL of 3 ug/l. The maximum sample value for this three-year period was 8.9 ug/l on August 4, 2003. None of the 66 moving annual averages for atrazine for the years 2002 through 2004 at Centerville Reservoir exceeded the MCL (maximum moving annual average = 3.2 ug/l). Note: rounding is required when comparing levels of atrazine to its MCL. Because the MCL, as published by U.S. EPA, is 0.003 mg/l, any values compared to the MCL must be in the proper MCL units and compared at the same number of significant figures. Thus, the maximum moving annual average (0.0032 mg/l) rounds down to 0.003 mg/l; this average value does not exceeds the MCL for atrazine. Based on DNR's Section 305(b) assessment methodology, if the average contaminant levels in source water is less than the MCL, the Class C (drinking water) uses of the source water should be assessed as "fully supported."
For the following reasons, however, the Class C (drinking water) uses of Centerville Reservoir will remain assessed as “not supported” for the current (2006) Section 305(b) reporting cycle: (1) the Class C uses of this lake were assessed as “not supported” for the 2004 Section 305(b) reporting cycle due to average levels of atrazine that exceeded its MCL; (2) levels of atrazine tend to fluctuate significantly in some Iowa lakes, and (3) while average levels of atrazine over the 2002-2004 period at Centerville Reservoir did not exceed the atrazine MCL, average levels did approach the MCL (16 of the 66 moving annual averages over the 2002-04 period were between 3.0 and 3.2 ug/l). Thus, to ensure that the improving trend in atrazine levels at this lake is real and sustained, the Class C uses will remain assessed as “not supported”.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
