Assessment Comments

Assessment is based on results of IDNR/UHL monthly ambient chemical/physical monitoring conducted during the 2004-2006 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 “partially supported” due to high levels of indicator bacteria (E.  coli).   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 2004-2006 assessment period near Larrabee (STORET station 10180001 (formerly station 911060)) and 2005 IDNR/UHL stream REMAP biological sampling near Larrabee.  

EXPLANATION:  The Class A1 uses were assessed (monitored) as "partially supported" due to high levels of indicator bacteria (E.  coli).   Due to recent changes in Iowa’s Water Quality Standards, Iowa’s assessment methodology for indicator bacteria has changed.   Prior to 2003, the Iowa WQ Standards contained a high-flow exemption for the Class A1 criterion for indicator bacteria (fecal coliforms) designed to protect primary contact recreation uses:  the water quality criterion for fecal coliform bacteria (200 orgs/100 ml) did not apply "when the waters [were] materially affected by surface runoff."  Due to a change in the Standards in July 2003, E.  coli is now the indicator bacterium, and the high flow exemption was eliminated and replaced with language stating that the Class A criteria for E.  coli apply when Class A1, A2, or A3 uses “can reasonably be expected to occur.”  Because the IDNR Technical Advisory Committee on WQ Standards could not agree on what flow conditions would define periods when uses would not be reasonably expected to occur, all monitoring data generated for E.  coli during the assessment period, regardless of flow conditions during sample collection, will be considered for determining support of Class A uses for purposes of Section 305(b) assessments and Section 303(d) listings.  

The geometric mean level of indicator bacteria (E.  coli) in the 24 samples collected during the recreational seasons of 2004 through 2006 (130 orgs/100ml) is slightly greater than the Iowa Class A1 water quality criterion of 126 orgs/100ml.   Eight of the 24 samples (33%) exceeded Iowa’s single-sample maximum value 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).   Also, according to U.S.  EPA guidelines for Section 305(b) reporting, if levels of E.  coli exceed the single-sample maximum value in more than 10% of the samples, the primary contact recreation uses should be assessed as “partially supported” (see pgs 3-33 to 3-35of U.S.  EPA 1997b).   According to IDNR’s assessment/listing methodology, these results suggest that significantly greater than 10% of the samples exceed IDNR’s single-sample maximum value, thus suggesting that the Class A1 uses should be assessed as “partially supported/impaired.”  Despite the impairment, the results of monitoring for E.  coli during the 2002-2004 and 2004-2006 periods suggest that levels of indicator bacteria are relatively low in this river segment.

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 36 samples collected during the 2004-2006 assessment period for dissolved oxygen (minimum value = 5.0 mg/l), pH (range = 7.6 to 9.0 units), or ammonia-nitrogen (maximum value = 0.53 mg/l).   In addition, levels of toxic metals in the ten samples analyzed, and levels of pesticides in the seven samples analyzed, were all below their respective Class B(WW1) criteria.   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 57 (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.  

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