Corydon Reservoir IA 05-CHA-1331

Assessment Comments

Assessment is based on: (1) results of the Iowa Voluntary Atrazine Monitoring Program conducted from 2000 through 2008 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" 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 "fully supported" due to decreasing levels of atrazine at this lake.    The fish consumption uses are assessed (monitored) as "fully supported" based on results of fish tissue monitoring in 2007.   The sources of data for this assessment include: (1) results of the Iowa Voluntary Atrazine Monitoring Program conducted from 2000 through 2008 by Syngenta Crop Protection, Inc, 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.   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 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 have decreased below the MCL the drinking water uses are assessed as "fully supported" and therefore this waterbody is moved to IR Category 2a for the 2010 assessment/listing 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 "fully supported" based on results of sampling conducted as part of the Iowa Voluntary Atrazine Monitoring Program from 2004 through 2008 by Syngenta Crop Protection, Inc.    Compared to the 2000-2002 period, results of monitoring from 2002 through 2008 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 all of the 70 moving annual averages of atrazine in samples collected in calendar years 2002 through 2004 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 was cause for continued concern over elevated atrazine levels at this lake.  

Results of the Syngenta "Iowa Voluntary Atrazine Monitoring Program" from 2006 through 2008, however, continued to show that all moving annual averages were below the MCL of 3 ug/L (maximum = 2.8 ug/L), thus suggesting continued decline (improvment) in atrazine levels at this lake.  The mean and median atrazine levels over the most recent three-year period from 2006 through 2008 were 2.1 ug/L and 0.8 ug/L, respectively.   The maximum single-sample atrazine concentration for this two year period was 22.6 ug/L.  Nineteen of the 100 single sample concentrations exceeded the MCL of 3.0 ug/L.   Based on IDNR's assessment methodology, this does not indicate an impairment of the Class C uses.   Because there have been two consecutive assessment/listing cycles with no moving annual averages exceeding the atrazine MCL, the Class C (drinking water) uses of Corydon Reservoir are now assessed as "fully supported" and the atrazine impairment will be removed at this lake.  

Fish consumption uses were 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.