Assessment Comments
Assessment is based on results of Syngenta's Iowa Voluntary Atrazine Monitoring Program from 2000-02.
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. The Class C (drinking water) uses are assessed (monitored) as "not supported" due to average levels of atrazine that exceed state water quality standards and the U.S. EPA MCL (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). The source of data for this assessment is 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.).
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 assessment of support of the Class C (drinking water) uses 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 level of atrazine in samples collected from Corydon Reservoir in calendar years 2000 and 2002 were below the MCL of 3.0 ug/l but was well-above the MCL in calendar year 2001; thus suggesting “non-support” of the Class C uses. The time-weighted mean of atrazine in 2000 (N=31) was 1.84 ug/l, in 2001 (N=31) was 7.61 ug/l, and in 2002 (N=16) was 2.08 ug/l. Atrazine levels were relativey low in 2000, with all 31 samples below the MCL of 3 ug/l. In early summer of 2001, however, the atrazine concentration increased dramatically: the sample collected on May 29, 2001, contained 0.9 ug/l of atrazine; the sample collected one week later on June 4, contained 52 ug/l. Levels continued to exceed the MCL until October 2001. Levels remained low until May 2002 when two consecutive samples spiked on May 6 (12.3 ug/l) and May 20 (20.1 ug/l); levels returned to below the MCL for the remainder of 2002. Examples of atrazine levels in summer 2001 are as follows (all ug/l): 62, 52, 38, 26, 17, and 18. Thirty-seven of the 46 moving annual averages calculated for the 2000-2002 period were above the MCL of 3 ug/l (range: 3.6 to 10.9 ug/l). Based on DNR's Section 305(b) assessment methodology, if the average contaminant level in source water is greater than the MCL, the Class C (drinking water) uses of the source water should be assessed as "not supported."
Fish consumption uses were "not assessed" due to the lack of fish contaminant monitoring at this lake.
