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 2008-2010 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 remain not assessed.   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 2008-2010 and (2) results of IDNR/UHL stream REMAP biological monitoring in 2005.  

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 (2012) 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 means of indicator bacteria (E.  coli) in the 22 samples collected during the recreational seasons of 2008 through 2010 at IDNR station 10810001 near (downstream from) Sac City were as follows:  the 2008 geometric mean was 210 orgs/100 ml, the 2009 geometric mean was 182 orgs/100 ml, and the 2010 geometric mean was 187 orgs/100 ml.   All three geometric means very slightly exceed the Class A1 criterion of 126 orgs/100 ml.   Eight of the 22 samples (36%) 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 (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), 53, 54 and 55 (all fair).   The 2001 FIBI score was 57 (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 1994-2004.   The non-riffle habitat FIBI BIC for this ecoregion is 32 and the BMIBI BIC is 62.   This segment passed the FIBI BIC 1/1 and passed the BMIBI BIC 0/4 times in the last 12 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.   The results of this sampling thus do not meet IDNR guidelines for developing a “monitored” assessment of support of the aquatic life uses that is appropriate to support addition of this waterbody to Iowa's Section 303(d) list.   The assessment type for these waters will be considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to “monitored" (indicating an assessment with relatively high confidence).   Such waters will be placed in either Category 2b or 3b of the IR and thus will be added to the state’s list of “waters in need of further investigation.”

Results of IDNR/UHL ambient chemical/physical water quality monitoring at the Sac City station during the 2008-2010 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 approximately 30 samples analyzed for dissolved oxygen, ammonia, and pH.

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.