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
2014
Release Status
Final
Result Period
2010 - 2012
Designations
Class A1 Class B(WW-1) Class HH
Assessment Type
Monitored
Integrated Report
Category 5a
Legacy ADBCode
IA 02-CED-0010_0
Overall Use Support
Not supporting
Aquatic Life Use Support
Not supporting
Fish Consumption
Not assessed
Primary Contact Recreation
Partial
Documentation
Assessment Comments

Assessment is based on results of monthly monitoring from 2010 through 2012 at the IDNR ambient station located at the County Road G28 bridge NE of Conesville (IDNR station 10700001) and on IDNR/SHL biological sampling conducted in 2011 and 2012.

Basis for Assessment

SUMMARY:  Although all recreation season bacteria geometric means were below (met) the Class A1 (primary contact recreation) criterion during the 2010-12 period, the Class A1 (primary contact recreation) uses remain assessed (monitored) as “partially supported” (IR 5a) based on results of ambient monitoring from 2008-10 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.   The pH impairments for the Class A1 and Class B(WW1) uses are new impairments for this assessment segment.   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 2010 through 2012 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 and 2012.  

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.  

EXPLANATION:  Although results of bacteria monitoring from 2009 through 2012 suggest “full support”, the Class A1 uses will remain assessed (monitored) as "partially supported" for the 2014 IR cycle based on results of IDNR ambient monitoring near Conesville in 2008.   The geometric means of indicator bacteria (E.  coli) in the 24 samples collected during the recreational seasons of 2010 through 2012 at station 10700001 were as follows:  the 2010 geometric mean was 115 orgs/100 ml, the 2011 geometric mean was 79 orgs/100 ml and the 2012 geometric mean was 20 orgs/100 ml.   All three geometric means are below—and the 2011 and 2012 geometric means are far below—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.   These results do not suggest impairment of the Class A1 uses:  all geometric means meet the Class A1 criterion, and, according to IDNR’s assessment/listing methodology, the percentage of samples exceeding the single-sample maximum criterion is not significantly greater than 10%.   Because, however, the IDNR assessment/listing methodology requires that, before a bacterial impairment can be de-listed, geometric mean levels of E.  coli must all be less than the applicable state water quality criteria for two consecutive listing cycles (i.e., five consecutive years), the impairment of the Class A1 uses will remain in effect.   Of the monitoring results from this station over the last five years, only the 2008 recreation season geometric mean (519 orgs/100 ml) exceeded the Class A1 criterion.   If sufficient additional data are collected during the 2013-2014 period that show a continued decline in bacteria levels and compliance with the Class A1 water quality criterion, this impairment may be removed.  

In addition, the Class A1 uses are assessed (monitored) as “partially supporting” based on results of monitoring for pH during the 2010-2012 period.   Eight of the 36 monthly samples collected (29%) violated the pH criterion of 9.0 units.   According to the IDNR assessment/listing methodology, the percentage of samples exceeding the pH criterion is 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 are assessed as "partially supported" due to violations of the pH criterion.

The Class B(WW1) aquatic life uses are also assessed (monitored) as "partially supporting" due to violations of the Class B(WW1) criterion for pH.   Results of monitoring from the IDNR ambient station near Conesville from 2010 through 2012 show that eight of the 36 monthly samples collected (29%) violated the pH criterion of 9.0 units.   According to the IDNR assessment/listing methodology, the percentage of samples exceeding the pH criterion is significantly greater than 10%.   According to U.S.  EPA assessment guidelines, if significantly more than 10% of samples exceed state criteria for pH, the aquatic life (Class B(WW1)) uses should be assessed as "impaired" (see pgs 3-17 of U.S.  EPA 1997b).   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 and 2012 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) and the 2012 BMIBI score was 29 (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 (twice), it is uncertain as to whether or not this segment is meeting the aquatic life criteria because it 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
12/4/2012 Fixed Monitoring End Date
8/29/2012 Biological Monitoring
9/8/2011 Biological Monitoring
1/4/2010 Fixed Monitoring Start Date
Methods
230 Fixed station physical/chemical (conventional plus toxic pollutants)
420 Water column surveys (e.g. fecal coliform)
320 Benthic macroinvertebrate surveys
315 Regional reference site approach
Monitoring Levels
Biological 3
Habitat 2
Physical Chemistry 3
Toxic 3
Pathogen Indicators 3
Other Health Indicators 0
Other Aquatic Life Indicators 0
# of Bio Sites 1
BioIntegrity Poor
Causes and Sources of Impairment
Causes Use Support Cause Magnitude Sources Source Magnitude
pH Aquatic Life Support Slight
  • Natural Sources
  • Slight
pH Primary Contact Recreation Slight
  • Natural Sources
  • Slight
Pathogens Primary Contact Recreation Slight
  • Source Unknown
  • Moderate
Cause Unknown Aquatic Life Support Moderate
  • Source Unknown
  • Moderate