Assessment Comments
Assessment is based on results of the Iowa Voluntary Atrazine Monitoring Program.
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." The Class C (drinking water) uses are assessed (monitored) as "not supported" (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). The source of data for this assessment is the results of the Iowa Voluntary Atrazine Monitoring Program conducted in 2000 and 2001 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 and 2001. This monitoring showed that the time-weighted mean level of atrazine in samples collected from Corydon Reservoir in calendar year 2000 was below the MCL of 3.0 ug/l but was well-above the MCL in calendary year 2001. The time-weighted mean of atrazine in 2000 (N=31) was 1.84 ug/l and in 2001 (N=31) was 7.61 ug/l. Atrazine levels were relativey low in 2000, with all 26 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. Examples of atrazine levels in summer 2001 are as follows (all ug/l): 62, 52, 38, 26, 17, and 18. 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.
