Assessment Comments

Assessment is based on: (1) the results from the IDNR/UHL ambient monthly monitoring station downstream from Sac City in Sac County (STORET station 10810001 (formerly station 423014)) located approximately 5 miles south of Sac City from 2006-2008 and (2) results of IDNR/UHL biological monitoring in 1999-2002 and 2008.

Basis for Assessment

[Note:  This assessment is also used, in part (except for the assessment of fish consumption uses and aquatic life using biological monitoring data) for the adjacent downstream segment (IA 04-RAC-0040-5).]

SUMMARY:  The Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" due to levels of indicator bacteria (E.   coli) that violate state water quality standards.   The Class B(WW1) aquatic life uses remain assessed (evaluated) as "partially supporting" based on results of IDNR/UHL stream REMAP biological monitoring in 2005.   The fish consumption uses are assessed as “fully supporting” based on fish contaminant monitoring in 2006.   The sources of data for this assessment include (1) the results from the IDNR/UHL ambient monthly monitoring station downstream from Sac City in Sac County (STORET station 10810001 (formerly station 423014)) located approximately 5 miles south of Sac City from 2006-2008, (2) results of IDNR/UHL biological monitoring in 1999-2002 and 2008, and (3) results of 2006 fish contaminant monitoring conducted as part of the IDNR/UHL REMAP biological monitoring project (REMAP Site 164).  

Note:  A TMDL for pathogen indicator (E.   coli) impairments in the Raccoon River basin was prepared by IDNR and approved by EPA in June 2008.   Because this TMDL covers the primary Section 303(d) impairment identified for the 2006 assessment/listing cycle, this waterbody was moved from IR Category 5a from the 2006 assessment/listing cycle to IR Category 4a (impaired; TMDL approved) for the 2008 cycle.   This waterbody remains in IR Category 4a for the current (2010) cycle.  

EXPLANATION:  The Class A1 uses are assessed (monitored) as "not supported" due to levels of indicator bacteria (E.   coli) that exceed state water quality criteria.   The geometric mean level of indicator bacteria (E.   coli) in the 21 samples collected during the recreational seasons of 2006, 2007, and 2008 (201 orgs/100ml) exceeded the Iowa Class A1 water quality criterion of 126 orgs/100ml.   In addition, seven of the 21 samples (33%) exceeded Iowa’s single-sample maximum criterion 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 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).  

The Class B(WW1) aquatic life uses remain assessed (evaluated) as “partially supporting” based on biological data collected in 1999-2002 and 2008 by the IDNR/UHL stream biological monitoring project.   A series of biological metrics which 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 that were 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 1999-2002 and 2008 BMIBI scores were 61 (good), 50, 53 and 53 (all fair).   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 1994-2004.   The BMIBI BIC for this ecoregion is 62.   This segment passed the BMIBI BIC 0/4 times in the last 10 years.   This assessment is considered evaluated because the drainage area (835 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 the FIBI BIC and failed the BMIBI BIC, 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 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 between 2004 and 2008 to be considered “monitored”.   This segment had multiple samples collected in the previous 10 years (1999-2008); however, the samples were not collected during 2004-2008.   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).  

Results of IDNR/UHL ambient chemical/physical water quality monitoring at the Sac City station during the 2006-2008 assessment period, however, suggest relatively good water quality in this river segment.   Results of this monitoring show no violations of Class B(WW1) (aquatic life) water quality criteria in the 33 samples analyzed for dissolved oxygen, ammonia, and pH or in the two samples analyzed for toxic metals  

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 was conducted in 1993 as part of the U.S.  EPA/IDNR RAFT program.   These data are too old to accurately characterize current contaminant levels.