Iowa River IA 02-IOW-623

Results of water quality monitoring during the 2004-2006 assessment period for indicator bacteria (E.  coli) at the IDNR ambient monthly station on the Iowa River at Columbus Junction show that the Class A (primary contact recreation) uses should be assessed as "partially supported".  The geometric mean level of E. coli bacteria in the 21 samples collected during recreational seasons of 2004, 2005, and 2006 (93 orgs/100 ml) is below the Iowa water quality criterion of 126 orgs/100 ml for protection of primary contact recreation uses.  Six of the 21 samples (29%), however, had levels of E. coli greater than the Iowa 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 the geometric mean of E.  coli is less than the applicable state criteria, the contact recreation uses should be assessed as "fully supported" (see pgs 3-33 to 3-35of U.S. EPA 1997b).  Thus, these data suggest that the Class A1 uses should be assessed as “fully supported.”  These EPA guidelines, however, also suggest that if more than 10% of samples exceed the single-sample maximum criterion for indicator bacteria, the primary contact recreation uses should be assessed as "partially supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b).  According to IDNR’s assessment/listing methodology, the monitoring data from 2004-06 suggest that significantly greater than 10% of the samples exceed IDNR’s single-sample maximum criterion, thus suggesting that the Class A1 uses should be assessed as “partially supported/impaired”. 

The results of IDNR ambient monthly chemical/physical water quality monitoring suggest that the Class B(WW1) aquatic life uses should be assessed (monitored) as "fully supported".  Results from DNR ambient monthly monitoring at the Columbus Junction station during the 2004-2006 assessment period showed no violations of Class B(WW1) water quality criteria in the 32 samples analyzed for dissolved oxygen or ammonia-nitrogen.  One of the 32 samples collected, however, exceeded the state standard for pH; the sample collected on September 1, 2005 contained a pH of 9.4, thus exceeding the Class B(WW1)/A1 criterion of 9.0 pH units.  According to U.S. EPA guidelines (U.S. EPA 1997b, page 3-17), a violation frequency of less than 10 % for conventional parameters such as pH nonetheless suggest "full support" of aquatic life uses.  Thus, the percentage of violations of the pH criterion at this station (3%) does not suggest an impairment of aquatic life uses in this stream segment.   

Results of biological (benthic macroinvertebrate) monitoring in 2001, 2002 and 2008, however, suggest that the aquatic life uses should remain assessed (evaluated) as partially supporting.  This evaluated biological assessment was based on data collected in 2001, 2002 and 2008 as part of the IDNR/SHL stream ambient station benthic macroinvertebrate sampling project.  A series of biological metrics that reflect stream water quality and habitat integrity were calculated from the biocriteria 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 to benthic macroinvertebrate index of biotic integrity (BMIBI).  The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).  The 2001, 2002 and 2008 BMIBI scores were 24 (poor), 36 (fair) and 60 (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 data collected at stream ecoregion reference sites from 1994-2008.  The BMIBI BIC for this ecoregion is 51.  This segment passed the BMIBI BIC 1/3 times in the last 14 years.  This assessment is considered evaluated because the drainage area (12,257 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, 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. 

This aquatic life assessment is now also considered "evaluated" based on a change in the 2010 IDNR assessment methodology.  IDNR now requires a segment have two or more biological samples collected from the segment in multiple years over a recent five-year period to be considered “monitored”.  This segment had multiple samples collected in the previous 14 years (2001-2014); however, the samples were not collected during the recent five-year period.  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 lack of fish tissue monitoring in this river segment.