Iowa DNR
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Water Quality Assessments

Impaired Waters List

Cedar River IA 02-CED-449

mouth (S20 T75N R4W Louisa Co.) to confluence with Sugar Cr. in S17 T78N R2W Muscatine Co.

Assessment Cycle
2018
Release Status
Final
Data Collection Period
Overall IR Category
5 - Water is impaired or threatened and a TMDL is needed.
Trend
Unknown
Created
5/20/2019 1:40:55 PM
Updated
7/11/2019 7:30:49 AM
Assessment conducted in accordance with Iowa's 2018 IR methodology
Use Support
Class A1
Partially Supported
Bacteria: Indicator Bacteria- E. coli
Support Level
Partially Supported
Impairment Code
5a - Pollutant-caused impairment. TMDL needed.
Cause Magnitude
Slight
Status
Continuing
Source
Unknown: Source Unknown
Source Confidence
N/A
Cycle Added
2010
Impairment Rationale
Geometric mean criterion exceeded
Data Source
Ambient monitoring: Iowa DNR-rivers
TMDL Priority
Tier III
Class BWW1
WINOFI
Biological: low aquatic macroinvertebrate IBI
Support Level
Water in Need of Further Investigation (WINOFI)
Impairment Code
3b-u - Use potentially biologically impaired based on uncalibrated IBI metrics.
Cause Magnitude
Moderate
Status
New
Source
Unknown: Source Unknown
Source Confidence
N/A
Cycle Added
2018
Impairment Rationale
Low Biotic Index
Data Source
Biological monitoring: Iowa DNR WQMA
Class HH
Not Assessed
General Use
Not Assessed
Impairment Delistings
Cycle Added Class Cause Data Source Rationale
2014 Class A1 pH Ambient monitoring: Iowa DNR-rivers New data: WQ improvement (chemical / physical / bacterial)
2014 Class BWW1 pH Ambient monitoring: Iowa DNR-rivers New data: WQ improvement (chemical / physical / bacterial)
Documentation
Assessment Summary

The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially supported” based on results of ambient monitoring for indicator bacteria.  The Class A1 uses are also assessed (monitored) as “partially supporting” due to violations of the Class A1 criterion for pH.  The Class B(WW1) aquatic life uses are assessed (monitored) as "partially supporting” due to violations of the Class B(WW1) criterion for pH.  The Class B(WW1) aquatic life uses are assessed (evaluated) as "not supporting” due to low benthic macroinvertebrate IBI scores.  Fish consumption uses remain “not assessed” due to the lack of recent fish contaminant monitoring in this assessment segment.  The sources of data for these assessments are the results of monthly monitoring from 2014 through 2016 at the DNR ambient station located at the County Road G28 bridge NE of Conesville (IDNR station 10700001) and DNR/SHL biological sampling in 2012 and 2013. 

Assessment Explanation

[Note: A TMDL for indicator bacteria impairments in nine segments of Cedar River was prepared and approved by EPA in February 2010. Because this segment (IA 02-CED-0010_0) had not been previously impaired due to indicator bacteria, this segment was not included in the EPA TMDL. Thus, this impairment is not covered by the TMDL and is considered appropriate for Category 5a of Iowa's Integrated Report.]

The Class A1 (primary contact recreation) uses are assessed (monitored) as "partially supported" based on levels of indicator bacteria that exceeded state water quality criteria. The geometric means of indicator bacteria (E. coli) in the 24 samples collected during the recreational seasons of 2014 through 2016 at station 10700001 were as follows: the 2014 geometric mean was 217 orgs/100 ml, the 2015 geometric mean was 52 orgs/100 ml, and the 2016 geometric mean was 209 orgs/100 ml. Two of the three recreation season geometric means exceeded the Class A1 geometric mean criterion of 126 orgs/100 ml. Six of the combined 24 samples (25%) exceeded Iowa’s Class A1 single-sample maximum criterion of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting and Iowa DNR’s assessment/listing methodology, if the geometric mean is greater than 126 orgs/100 ml., the primary contact recreation uses should be assessed as "impaired" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). Thus, because at least one recreation season geometric mean exceeded criteria for Class A1 uses, these uses are assessed as "partially supported."

Results of chemical/physical water quality monitoring at station 10700001 suggest “full support” of the aquatic life uses. Monitoring showedno violations of Class B(WW1) water quality criteria for 36 Ammonia samples (maximum = 1.3 mg/L), 36 Dissolved Oxygen samples (minimum = 6.6 mg/L), 36 pH samples (range = 6.8 to 9), 36 Temperature samples (maximum = 25.8°C), 36 Chloride samples (maximum = 100 mg/L), or 36 Sulfate samples (maximum = 74 mg/L) occurred during monitoring from January 2014 to December 2016.According to U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-17), the EPA guidelines allow up to 10% violations of these conventional parameters before impairment of water quality is indicated. Thus, these results thus suggest “full support” of the Class B(WW1) aquatic life uses.

Cedar River IA 02-CED-449 was assessed as "partially supported" for the 2014 assessment/listing cycle. Based on DNR’s assessment methodology 2 consecutive assessment/listing cycles without significantly greater than 10% of the samples violating the single-sample maximum criterion are necessary to propose delisting based on pH violations. There were less than 10% of the samples exceeding the single-sample maximum criterion for the 2016 or current 2018 assessment listing cycle. Therefore, the Class A1 and B(WW1) impairments for pH are suggested for delisting for the 2018 assessment/listing cycle.

This evaluated biological assessment was based on data collected in 2011, 2012 and 2013 as part of the DNR/SHL stream nutrient sampling project. A series of biological metrics that reflect stream water quality and habitat integrity were calculated from the biological sampling data. The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa collected in the stream sampling reach. The biological metrics were combined a benthic macroinvertebrate index (BMIBI). The index rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 2012 BMIBI score was 29 (poor) and the 2013 BMIBI score was 15 (poor). The aquatic life use support was assessed (evaluated) as not supporting (=NS), based on a comparison of the BMIBI scores with biological impairment criteria (BIC) established from a statistical analysis of biological data collected at stream ecoregion reference sites from 1994-2008. The BMIBI BIC for this ecoregion is 51. This assessment is considered evaluated because the drainage area (7789 mi2) above this sampling site was greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria. Even though this site failed the BMIBI BIC (0/3), it is uncertain as to whether or not this segment is meeting the aquatic life criteria because the site used for the assessment doesn’t fall in the calibrated watershed size. According to DNR’s assessment/listing methodology, impairments based on “evaluated” assessments are of lesser confidence and are thus not appropriate for Section 303(d) listing (Category 5 of the Integrated Report). DNR does, however, consider these impairments as appropriate for listing under either Category 2b or 3b of the Integrated Report (waters potentially impaired and in need of further investigation).

Fish consumption uses are not assessed due to the lack of recent fish contaminant monitoring in this river segment.

Monitoring and Methods
Assessment Key Dates
8/29/2012 Biological Monitoring
1/9/2014 Fixed Monitoring Start Date
12/1/2016 Fixed Monitoring End Date
10/7/2013 Biological Monitoring
Methods
230 Fixed station physical/chemical (conventional plus toxic pollutants)
315 Regional reference site approach
320 Benthic macroinvertebrate surveys
360 HABITAT ASSESSMENT
420 Indicator bacteria monitoring