Corydon Reservoir IA 05-CHA-1331

Wayne County S24T69NR22W at Corydon.

Open Data Collection Period Modal

For purposes of developing stream/river water quality assessments for integrated reporting, three years of water quality data are typically used for both conventional pollutant parameters (e.g., indicator bacteria) and the less frequently monitored toxic parameters (e.g., toxic metals). Since the 2004 IR cycle, DNR has used a three-year data gathering period for streams, rivers, and beaches. For most assessments, the use of three years of data increases the number of samples upon which the decision on use support is based and helps address the problem of weather-related year-to-year fluctuations in water quality.

Due to the lower sampling frequency in Iowa’s ambient lake, biological, fish kill, and fish tissue monitoring programs, five years of data are used for developing IR assessments and for identifying Section 303(d) listings.

Data Type	Data Timeframe
Beach Sampling Data	CY 2018 - 2020
Stream/River Water Quality Data	CY 2018 - 2020
Lake Water Quality Data	CY 2016 - 2020
Biological Sampling Data	CY 2016 - 2020*
Fish Kill Data	CY 2016 - 2020
Fish Tissue Sampling Data	CY 2016 - 2020
Continuous Dissolved Oxygen Sampling Data	CY 2016 - 2020
*Biological sites sampled in both 2015 and 2019 were also assessed.

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Cycle: 2022
Release Status: Final
Overall IR: 4 - Water is impaired or threatened and a TMDL has been completed or is not needed.
Trophic: Unknown
Trend: Unknown
Created: 1/6/2022 2:19:44 PM
Updated: 2/2/2022 2:21:31 PM

Use Support

Class A1

Recreation - Primary contact

Not Assessed

Class BLW

Aquatic Life - Lakes and wetlands

Not Assessed

Class C

Drinking Water -

Not Supported

Pesticides: Atrazine

Support Level

Not Supported

Impairment Code

4a - Pollutant-caused impairment. TMDL has been completed.

Cause Magnitude

High

Status

Continuing

Source

Agriculture

Source Confidence

High

Cycle Added

1998

Impairment Rationale

Average > MCL

Data Source

Volunteer monitoring

Class HH

Human Health -

Not Assessed

Impairment Delistings

No delistings for this assessment cycle.

Documentation

Assessment Summary

The Class C use was assessed (monitored) as "not supported" due to levels of atrazine at this lake that exceed the Class C Iowa water quality criterion. All other designated uses were “not assessed.” The sources of data for this assessment include the results of Syngenta-sponsored monitoring for atrazine conducted from 2009 through 2012. (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 DNR and approved by EPA in 2000. Because all Section 303(d) impairments identified for the current assessment/listing cycle (atrazine) were addressed by the TMDL, this waterbody was placed in IR Category 4a.

Assessment Explanation

Class A1 – No Data

Class BLW – No Data

Class C - Toxic Parameters: Syngenta “Iowa Voluntary Atrazine Monitoring Program” from 2009 through 2012

Parameter Name	# Samples / # Years	Maximum Value	Mean Value	# of Violations	% Violations	Mean Value > Criterion or MCL	Assessment Type	Support Level
Atrazine	84 / 4	30.2	17.6	17	20%	Yes	Monitored	Not
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 DNR'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."

Class HH - Fish Tissue Sampling: No Data

Monitoring and Methods

Assessment Key Dates

1/12/2004

Fixed Monitoring Start Date

12/17/2012

Fixed Monitoring End Date

12/11/2000

TMDL Completed

Methods

270

PWS chemical monitoring (ambient water)