Assessment Comments
Assessment is based on: (1) results of the Iowa Voluntary Atrazine Monitoring Program conducted from 2000 through 2012 by Syngenta Crop Protection, Inc, and (2) results of U.S. EPA/IDNR fish tissue monitoring in 2007.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses, Class B(LW) aquatic life uses remain "not assessed" (IR 3a) due to the lack of information upon which to base an assessment of these uses. The Class C (drinking water) uses are assessed (monitored) as "not supported" (IR 4a) due to levels of atrazine at this lake that exceed the Class C Iowa water quality criterion. The fish consumption uses remain assessed (monitored) as "fully supported" (IR 2a) based on results of fish tissue monitoring in 2007. The sources of data for this assessment include: (1) results of Syngenta-sponsored monitoring for atrazine conducted from 2009 through 2012 and (2) results of U.S. EPA/IDNR fish tissue monitoring in 2007. (Note: Corydon Reservoir is not currently used as a source of drinking and has not been used for this purpose for decades. Nonetheless, because this waterbody was formerly used as a drinking water source--and thus 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 2002, 2004, 2006, and 2008 assessment/listing cycle (atrazine) were addressed by the TMDL, this waterbody was placed in IR Category 4a. Because atrazine levels in recent years had decreased below the MCL, the drinking water uses were assessed as "fully supported" and therefore this waterbody was moved to IR Category 2a for the 2010 assessment/listing cycle where it remained for the 2012 assessment/listing cycle. Due, however, to a return of impairment-level concentrations of atrazine during the 2010-2012 monitoring period, this drinking water assessment for this lake will returned to IR Category 4a for the current (2014) assessment cycle.
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 are assessed (monitored) as "not supported" based on results of Syngenta-sponsored sampling for atrazine from 2009 through 2012. Although results of the Syngenta-sponsored monitoring from 2006 through 2008 showed a decline in atrazine levels at this reservoir and suggested “full support” of drinking water uses, results from 2009-12 again show levels of atrazine that exceed the Class C water quality criterion. Of the 84 samples collected from 2009 through 2012, 17 samples (20%) exceeded the Class C criterion of 3 ug/ (maximum of 30.2 ug/l on June 7, 2010). Eighteen of the 60 moving annual averages for this dataset also exceeded this Class C criterion. Levels of atrazine were exceptionally high from mid-May to early June of 2010. During this four-week period, atrazine levels averaged 17.6 ug/l. These exceptionally high levels caused the moving annual averages to exceed the Class C criterion for atrazine of 3 ug/l. According to IDNR's Section 305(b) assessment methodology, if a moving annual average level of atrazine in source water is greater than the Class C criterion for atrazine, the Class C (drinking water) uses of the source water should be assessed as "not supported." Thus, the Class C uses of Corydon Reservoir are assessed (monitored) as "not supported". This assessment represents a return of this assessment to Category 4a of Iowa’s Integrated Report.
Fish consumption uses remain assessed (monitored) as “fully supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at Corydon Reservoir in 2007. The composite samples of fillets from common carp and largemouth bass had low levels of contaminants. Levels of primary contaminants in the composite sample of common carp fillets were as follows: mercury: 0.107 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm. Levels of primary contaminants in the composite sample of largemouth bass fillets were as follows: mercury: 0.216 ppm. The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses. The fish contaminant data generated from the 2007 RAFT sampling conducted at this lake show that the levels of contaminants do not exceed any of the advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.
