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
2016
Release Status
Final
Release Period
2012 - 2014
Created
8/3/2016 2:29:10 PM
Overall IR Category
5 - Water is impaired or threatened and a TMDL is needed.
Trend
Unknown
Last Updated
9/23/2016 2:08:33 PM
Use Support
Class A1
Recreation
Primary contact
Partially Supporting
Bacteria: Indicator Bacteria, E. coli
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
Partially Supporting
pH
Impairment Code
5a - Pollutant-caused impairment. TMDL needed.
Cause Magnitude
Slight
Status
Continuing
Source
Unknown: Source Unknown
Source Confidence
N/A
Cycle Added
2014
Impairment Rationale
Significantly > 10% of samples fail to meet criterion
Data Source
Ambient monitoring: Iowa DNR-rivers
TMDL Priority
Tier IV
Class B(WW-1)
Aquatic Life
Warm Water Type 1
Partially Supporting
pH
Impairment Code
5a - Pollutant-caused impairment. TMDL needed.
Cause Magnitude
Slight
Status
Continuing
Source
Unknown: Source Unknown
Source Confidence
N/A
Cycle Added
2014
Impairment Rationale
Significantly > 10% of samples fail to meet criterion
Data Source
Ambient monitoring: Iowa DNR-rivers
TMDL Priority
Tier IV
Class HH
Human Health
Not Assessed
General Use
General Use water
Not Assessed
Impairment Delistings
No delistings for this assessment cycle.
Documentation
Assessment Summary

The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially supported” (IR 5a) 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” (IR 5a) due to violations of the Class B(WW1) criterion for pH.  The Class B(WW1) aquatic life uses are assessed (evaluated) as "not supporting” (IR 3b-u) 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 2012 through 2014 at the IDNR ambient station located at the County Road G28 bridge NE of Conesville (IDNR station 10700001) and IDNR/SHL biological sampling in 2011, 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 uses will are assessed (monitored) as "partially supported" based on results of IDNR ambient monitoring near Conesville from 2012 to 2014.  The geometric means of indicator bacteria (E. coli) in the 24 samples collected during the recreational seasons of 2012 through 2014 at station 10700001 were as follows:  the 2012 geometric mean was 20 orgs/100 ml, the 2013 geometric mean was 25 orgs/100 ml, and the 2014 geometric mean was 217 orgs/100 ml.  Only the 2014 geometric mean exceeded—and then only very slightly—the Class A1 geometric mean criterion of 126 orgs/100 ml.  Four of the 24 samples (17%) exceeded Iowa’s single-sample maximum criterion of 235 orgs/100 ml.  Only the 2014 geometric mean suggests impairment of the Class A1 uses.  Of the monitoring results from this station over the last seven years, only the 2008 and 2014 recreation season geometric means (519 and 217 orgs/100 ml, respectively) exceeded the Class A1 criterion.  These monitoring results indicate exceptionally low levels of indicator bacteria in this segment of the Cedar River.  

In addition, the Class A1 uses are assessed (monitored) as “partially supporting” based on results of monitoring for pH during the 2010-2012 period.  Levels of pH during the 2012-2014 monitoring period, however, do not suggest impairment.  During the previous (2010-2012) monitoring period, eight of the 36 monthly samples (29%) violated the pH criterion of 9.0 units.  According to the IDNR assessment/listing methodology, the percentage of samples exceeding the pH criterion was significantly greater than 10%.  According to U.S. EPA assessment guidelines, if significantly more than 10% of samples exceed state criteria for pH, the primary contact (Class A1) uses should be assessed as "impaired" (see pgs 3-17 of U.S. EPA 1997b).  Thus, the Class A1 uses were assessed as "partially supported" for the 2014 IR cycle due to violations of the pH criterion.  Levels of pH during the current monitoring period (2012-2014), however, do not suggest impairment of these uses.  Five of 36 samples (14%) collected during the 2012-2014 period exceeded the pH criterion of 9.0 pH units.  According to the IDNR assessment/listing methodology, the percentage of samples exceeding the pH criterion is not significantly greater than 10% and thus does not indicate impairment.  Because, however, the IDNR assessment/listing methodology requires that, before a pH impairment can be de-listed, pH levels must show “full support” for two consecutive listing cycles (i.e., five consecutive years), the pH impairment of the Class A1 uses will remain in effect.  This impairment also applies to the Class B(WW1) aquatic life uses.  Violations of pH in ambient waters tend to reflect high levels of primary productivity and typically do not reflect the addition of pollutants to surface waters.

This evaluated biological assessment was based on data collected in 2011, 2012 and 2013 as part of the IDNR/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 2011 BMIBI score was 24 (poor), 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 IDNR’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).  IDNR 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.  Previous assessments were based on results of fish contaminant monitoring conducted by USGS in 1995 as part of the NAWQA project.  These data are now considered too old (greater than 10 years) to accurately characterize current water quality conditions.

Monitoring and Methods
Assessment Key Dates
9/8/2011 Biological Monitoring
8/29/2012 Biological Monitoring
1/4/2012 Fixed Monitoring Start Date
12/2/2014 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
420 Water column surveys (e.g. fecal coliform)
360 HABITAT ASSESSMENT