Assessment Comments
Assessment is based on results of Syngenta's Iowa Voluntary Atrazine Monitoring Program from 2000-04. Data from Syngenta monitoring from 2005 and 2006 were used to supplement this assessment.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses, Class B(LW) aquatic life uses, and the fish consumption uses remain "not assessed" due to the lack of water quality information upon which to base an assessment of these uses. The Class C (drinking water) uses remain assessed (monitored) as "not supported" due to (1) average levels of atrazine that exceeded state water quality standards during the previous (2004) Section 305(b) reporting cycle and (2) recent (2005-06) data that suggest an increasing trend in atrazine levels at this reservoir. 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. Snygenta data from 2005 and 2006 were used to better define any trends in atrazine levels at this reservoir. (Note: Corydon Reservoir is not currently used as a source of drinking. Nonetheless, because this waterbody was formerly used as a drinking water source--and remains designated for this beneficial use in the Iowa Water Quality Standards--Corydon Reservoir must be assessed for support of this use if relevant water quality information is available).
Note: A TMDL for atrazine at Corydon Reservoir was prepared by IDNR and approved by EPA in 2000; thus, this waterbody was placed into IR Category 4a (TMDL approved) for the 2004 assessment/listing cycle. Because all Section 303(d) impairments identified for the 2006 assessment/listing cycle (atrazine) are addressed by the TMDL, this waterbody remains in IR Category 4a.
EXPLANATION: The Class A and Class B(LW) uses remain not assessed due to the lack of water quality information needed to assess these uses.
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. Compared to the 2000-2002 period, results of monitoring from 2002 through 2004 suggest a decline in atrazine levels at this reservoir.
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 Corydon Reservoir in 2001 (7.6 ug/l) was above the MCL of 3.0 ug/l. In addition, 37 of the 46 moving annual averages calculated for the 2000-02 period were above the MCL of 3 ug/l (range: 3.6 to 10.9 ug/l). According to IDNR's Section 305(b) assessment methodology, if the average contaminant levels 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 2.9 ug/l, in 2003 (N=25) was 1.7 ug/l, and in 2004 (N=32) was 1.3 ug/l. The overall mean (1.9 ug/l) and median (1.4 ug/l) atrazine levels over this three-year period (N=73) were both less than the MCL of 3 ug/l. The maximum sample value for this three-year period was 20.1 ug/l on May 20, 2002. . None of the 63 moving annual averages for atrazine for the years 2002 through 2004 at Corydon 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 exceed the MCL for atrazine.] Based on DNR's assessment/listing methodology, if the average contaminant level 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 Corydon Reservoir will remain assessed (monitored) 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 Corydon Reservoir did not exceed the atrazine MCL, average levels did approach the MCL (6 of the 63 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”.
In order to better define the apparent improving trend in atrazine levels at Corydon Reservoir, data from Syngenta monitoring from 2005 and 2006 were used to supplement this assessment. These data show relatively high levels of atrazine in Corydon Reservoir in June and July of 2005 and in May and June of 2006. Twelve individual samples exceeded the atrazine MCL in each of these summers, with maximum values of 7.4 ppb in 2005 and 8.5 ppb in 2006. Although no averages (time-weighted means or moving annual averages) exceeded the atrazine MCL during the 2005-06 period, the moving annual averages from early May through October of 2006 ranged from 3.1 to 3.4 ppb of atrazine. While these results do not indicate “impairment” of the Class C uses, they do suggest that (1) atrazine levels in this lake continue, at times, to be well-above the MCL, (2) atrazine levels appear to be trending upward, and (3) average levels of atrazine will likely exceed the MCL in the future. Thus, the supplemental data serve as further justification to continue to assess the Class C (drinking water) uses of Corydon Reservoir as “impaired” due to high levels atrazine.
Fish consumption uses were "not assessed" due to the lack of fish contaminant monitoring at this lake.
