Assessment Comments

Assessment is based on results of monthly monitoring from 2008 through 2010 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.   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 2008 through 2010 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) 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:  The Class A1 uses are assessed (monitored) as "not supported" based on results of IDNR ambient monitoring near Conesville.   The geometric means of indicator bacteria (E.  coli) in the 22 samples collected during the recreational seasons of 2008 through 2010 at station 10700001 were as follows:  the 2008 geometric mean was 519 orgs/100 ml, the 2009 geometric mean was 122 orgs/100 ml and the 2010 geometric mean was 115 orgs/100 ml.   Only the 2008 geometric mean exceeded the Class A1 criterion of 126 orgs/100 ml.   Nine of the 22 samples (41%) exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml.   According to U.S.  EPA guidelines for Section 305(b) reporting and IDNR’s assessment/listing methodology, if a recreation season geometric mean exceeds the respective water quality criterion, the contact recreation uses are "not supported" (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 “impaired.”  

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 2008 through 2010.   Monitoring at this station showed no violations of Class B(WW1) water quality criteria for dissolved oxygen or ammonia in the 33 samples collected or for toxic metals in the three 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.   One of the 33 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.