Iowa DNR

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
Release Status
Result Period
2006 - 2008
Class A1 Class B(WW-1) Class HH
Assessment Type
Integrated Report
Category 5a
Legacy ADBCode
IA 02-CED-0010_0
Overall Use Support
Not supporting
Aquatic Life Use Support
Fish Consumption
Not assessed
Primary Contact Recreation
Not supporting
Assessment Comments

Assessment is based on results of monthly monitoring from 2006 through 2008 at the IDNR ambient station located at the County Road G28 bridge NE of Conesville (IDNR station 10700001).

Basis for Assessment

SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” based on results of ambient monitoring for indicator bacteria.   This is a new impairment for this segment of the Cedar River and represents this segment’s first bacterial impairment.   The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported” based on results of ambient chemical/physical water quality monitoring.   Fish consumption uses remain “not assessed” due to the lack of recent fish contaminant monitoring in this assessment segment.   The source of data for this assessment is the results of monthly monitoring from 2006 through 2008 at the IDNR ambient station located at the County Road G28 bridge NE of Conesville (IDNR station 10700001).  

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) has not been previously impaired due to indicator bacteria, this segment was not included in the EPA TMDL.   Thus, this impairment is considered appropriate for Category 5a of Iowa's 2010 Integrated Report.    

EXPLANATION: The Class A1 uses were assessed (monitored) as "not supported" based on results of IDNR ambient monitoring near Conesville.   The geometric mean level of indicator bacteria (E.  coli) in the 22 samples collected (190 orgs/100ml) exceeds the Iowa Class A1 water quality criterion of 126 orgs/100ml.   Eight of the 22 samples exceeded Iowa’s single-sample maximum criterion of 235 orgs/100 ml.   According to U.S.  EPA guidelines for Section 305(b) reporting and according to IDNR’s assessment/listing methodology, if the geometric mean level of E.  coli is greater than the state criterion of 126 orgs/100 ml., the primary contact recreation uses should be assessed as "not supported" (see pgs 3-33 to 3-35of U.S.  EPA 1997b).   The bacterial impairment identified for this segment for the current (2010) cycle is related to elevated levels of E.  coli during both the 2007 and 2008 recreation seasons.   Based on results of IDNR/UHL routine ambient monthly monitoring, the maximum annual recreation season geometric mean for the years 2000 through 2006 was 98 orgs/100 ml in 2002.   The geometric mean for 2007, however, was 292 orgs/100 ml and for 2008 was 519 orgs/100 ml.   The geometric mean for the 2009 recreation season (112 orgs/100 ml) is more typical for this river segment and may reflect a return to lower levels of E.  coli following the severe flood events on the Cedar River in summer 2008.  

The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported" based on results of monitoring from the IDNR ambient station near Conesville from 2006 through 2008.   Monitoring at this station showed no violations of Class B(WW1) water quality criteria for dissolved oxygen or ammonia in the 34 samples collected or for pesticides or toxic metals in the two samples analyzed during this assessment period.   During the assessment period, levels of pH occasionally violated the Class B(WW1) criterion of 9.0 pH units.   Five of the 34 samples had pH values greater than 9.0 units, with a maximum value of 9.3 units.   According to U.S.  EPA assessment guidelines, if more than 10% of samples exceed state criteria for pH, the primary contact (Class A) and aquatic life (Class B) uses should be assessed as "impaired" (see pgs 3-17 of U.S.  EPA 1997b).   Based on IDNR’s assessment methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s pH criteria and thus do not suggest an impairment of the Class A1 or Class B(WW1) uses of this river segment.   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.  

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
12/11/2008 Fixed Monitoring End Date
1/5/2006 Fixed Monitoring Start Date
230 Fixed station physical/chemical (conventional plus toxic pollutants)
420 Water column surveys (e.g. fecal coliform)
Monitoring Levels
Biological 0
Habitat 0
Physical Chemistry 3
Toxic 3
Pathogen Indicators 3
Other Health Indicators 0
Other Aquatic Life Indicators 0
# of Bio Sites 0
BioIntegrity N/A
Causes and Sources of Impairment
Causes Use Support Cause Magnitude Sources Source Magnitude
pH Aquatic Life Support Not Impairing
  • Natural Sources
  • Not Impairing
pH Primary Contact Recreation Not Impairing
  • Natural Sources
  • Not Impairing
Pathogens Primary Contact Recreation Slight
  • Source Unknown
  • Moderate