Assessment Comments

Assessment is based on results of IDNR/UHL monthly ambient chemical/physical monitoring conducted during the 2008-2010 assessment period near Larrabee (STORET station 10180001 (formerly station 911060)) and 2005 IDNR/UHL stream REMAP biological sampling near Larrabee.

Basis for Assessment

SUMMARY:  The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to violations of the state water quality criteria for indicator bacteria.   The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported" based on results of ambient physical/chemical monitoring and assessed as "fully supported" (evaluated) based on the 2005 IDNR/UHL stream REMAP biological sampling near Larrabee.   Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this river segment.   This assessment is based on results of IDNR/UHL monthly ambient chemical/physical monitoring conducted during the 2008-2010 assessment period near Larrabee (STORET station 10180001 (formerly station 911060)) and 2005 IDNR/UHL stream REMAP biological sampling near Larrabee.  

EXPLANATION:  The Class A1 (primary contact recreation) are assessed as "not supported" based on levels of indicator bacteria that exceeded state water quality criteria.   The geometric means of indicator bacteria (E.  coli) in the 21 samples collected during the recreational seasons of 2008 through 2010 at the Little Sioux River near Larrabee were as follows:  the 2008 geometric mean was 77 orgs/100 ml, the 2009 geometric mean was 66 orgs/100 ml and the 2010 geometric mean was 145 orgs/100 ml.   The 2010 geometric mean exceeded the Class A1 criterion of 126 orgs/100 ml.   Five of the 21 samples (24%) 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 remain assessed (monitored) as “fully supported.”  Monitoring at the IDNR/UHL station near Larrabee showed no violations of Class B(WW1) water quality criteria in the 29 samples collected during the 2008-2010 assessment period for dissolved oxygen (minimum value = 7.5 mg/l), pH (range = 7.2 to 8.6 units), or ammonia-nitrogen (maximum value = 0.26 mg/l).   (Note: pesticides and toxic metals were not monitored at this location during the 2008 through 2010 period.)  These results suggest "full support" of the Class B(WW1) aquatic life uses.  

Supporting the WQ results and assessment, the results of the 2005 biological monitoring also indicate "full support" (evaluated) of the Class B(WW1) aquatic life uses.   This evaluated biological assessment was based on data collected in 2005 as part of the DNR/UHL stream REMAP 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 and fish species collected in the stream sampling reach.   The biological metrics were combined to make a fish community index of biotic integrity (FIBI) and a benthic macroinvertebrate index (BMIBI).   The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).   The 2005 FIBI score was 51 (good) and the BMIBI score was 56 (good).  The aquatic life use support was assessed (evaluated) as fully supporting (=FS), based on a comparison of the FIBI and BMIBI scores with biological impairment criteria (BIC) established from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2004.   The FIBI BIC for this ecoregion is 43 and the BMIBI BIC for this ecoregion is 54.   This assessment is considered evaluated because the drainage area (1833 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 passed both the FIBI and BMIBI BICs, 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.

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 to be considered “monitored”.   This segment had a single sample collected in 2005.    

Fish consumption uses remain "not assessed due to the lack of recent fish contaminant monitoring in this river reach.