Cedar River IA 02-CED-468

[Note:  A TMDL for the bacteria impairment in this segment of Cedar River was prepared and was approved by EPA in February 2010.  Thus, the bacteria impairment of the Class A1 (primary contact recreation) uses for this assessment segment were moved to IR Category 4a (TMDL prepared and approved).]

Despite the continued low levels of indicator bacteria in this segment of the Cedar River during the previous (2010-2012) and current (2012-2014) monitoring periods, the Class A1 uses are assessed (monitored) as "partially supported" based on results of monitoring at the IDNR ambient monitoring station upstream from Cedar Falls (STORET station 10700005).  For the current (2012-2014) monitoring period, all recreation season geometric means met the Class A1 criteria for indicator bacteria (E. coli) (geometric mean of 126 orgs/100 ml.  The geometric means of indicator bacteria (E. coli) in the 22 monthly samples collected during the recreational seasons of 2012 through 2014 at IDNR station 10700005 were as follows:  the 2012 geometric mean was 26 orgs/100 ml, the 2013 geometric mean was 99 orgs/100 ml, and the 2014 geometric mean was 58 orgs/100 ml.  These geometric means indicate very low levels of E. coli in this river segment.  Five of the 22 samples (21%), however, exceeded Iowa’s single-sample maximum criterion of 235 orgs/100 ml.  According to Iowa DNR’s assessment/listing methodology, the percentage of samples exceeding the Class A1 single-sample maximum criterion of 235 orgs/100 ml was significantly greater than 10%, thus suggesting that the Class A1 uses should be assessed as “partially supporting.  Despite this impairment, levels of indicator bacteria in this river segment are very low for an Iowa river.

Based on the results of monitoring from the IDNR ambient station at Cedar Falls from 2012 through 2014, the Class B(WW1) aquatic life uses should be assessed as "fully supported" .  Monitoring at this station showed no violations of Class B(WW1) water quality criteria for dissolved oxygen, ammonia-nitrogen, chloride or sulfate in the 33 samples analyzed.  One of the 33 samples (3%) violated the Class B(WW1) criteria for pH.  According to U.S. EPA guidelines for Section 305(b) reporting, if more than 10% of samples exceed state criteria for conventional parameters such as pH and dissolved oxygen, the aquatic life uses should be assessed as "impaired" (see pgs 3-33 to 3-35of U.S. EPA 1997b).  According to IDNR’s assessment/listing methodology, however, the results from station 10070005 do not indicate that significantly greater than 10% of the samples exceed either the Class B(WW1) criteria for dissolved oxygen or pH.  Thus, these results do not suggest impairment of the Class B(WW1) aquatic life uses.

In contrast to the water quality aquatic life assessment, the aquatic life assessment based on biological sampling suggests the aquatic life uses are "partially supporting".  This evaluated biological assessment was based on data collected in 2012, 2013 and 2014 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 2012 BMIBI scores were 58 (good) and 62 (good).  The 2013 BMIBI scores were 64 (good) and 62 (good).  The 2014 BMIBI scores were 63 (good) and 62 (good). The aquatic life use support was assessed (evaluated) as partially supporting (=PS), 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 artificial substrate BMIBI BIC for this ecoregion is 52 and the natural substrate BMIBI BIC is 70.  This assessment is considered evaluated because the drainage area (4734 mi2) above this sampling site was greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria.  All three natural substrate BMIBI scores (58, 62, 64) failed to meet the natural substrate BMIBI BIC of 70.  All three artificial substrate BMIBI scores (62, 62, 63) met the artificial substrate BMIBI BIC of 52. 

Even though this site failed the BMIBI BIC (3/6), 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 remain “not assessed” due to the lack of recent fish contaminant monitoring in this river segment.  The most recent fish contaminant monitoring in this assessment segment was conducted in 1997 as part of the U.S.EPA/IDNR fish contaminant (RAFT) monitoring program.  Because these data are more than 10 years old, they are now considered too old (greater than five years) to accurately characterize current water quality conditions; thus, the assessment was changed from “fully supported” to “not assessed.”