Assessment Comments
Assessment is based on: results of the Iowa Voluntary Atrazine Monitoring Program conducted from 2000 through 2006 by Syngenta Crop Protection, Inc.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses, Class B(LW) aquatic life uses, and the fish consumption uses remain "not assessed" due to the lack of 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 data that suggest continued elevated atrazine levels at this lake. The source of data for this assessment is the results of the Iowa Voluntary Atrazine Monitoring Program conducted from 2000 through 2006 by Syngenta Crop Protection, Inc. (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. Because all Section 303(d) impairments identified for the 2008 assessment/listing cycle (atrazine) are addressed by the TMDL, this waterbody remains in IR Category 4a.
EXPLANATION: The Class A1 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 2006 suggest a decline in atrazine levels at this reservoir. However, periods of elevated atrazine levels suggest that the Class C uses should remain assessed as “not supported” for the 2008 assessment/listing cycle.
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 2006, however, show that all of the 70 moving annual averages of atrazine in samples collected in calendar years 2002 through 2006 were below the MCL of 3 ug/l (maximum = 3.3 ug/L), thus suggesting a decline (improvement) in atrazine levels at this lake. [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 (0.0033 mg/l) rounds down to 0.003 mg/l; this average value does not exceed the MCL for atrazine.]
The mean and median atrazine levels over the three-year period from 2004 through 2006 were 2.3 ug/L and 1.7 ug/L, respectively. The maximum single-sample atrazine concentration for this three year period was 8.6 ug/L. Twenty-eight of the 102 single sample concentrations exceeded the MCL of 3.0 ug/L. While, based on IDNR’s assessment methodology, this does not indicate an impairment of the Class C uses, it is cause for continued concern over elevated atrazine levels at this lake.
For the following reasons, however, the Class C (drinking water) uses of Corydon Reservoir will remain assessed (monitored) as “not supported” for the current (2008) 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 2004-2006 period at Corydon Reservoir did not exceed the atrazine MCL, average levels did approach the MCL (28 of the 70 moving annual averages over the 2004-06 period were between 3.0 and 3.3 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 were "not assessed" due to the lack of fish contaminant monitoring at this lake.
