Assessment Comments

Assessment is based on: (1) results of IDNR/UHL biological (REMAP) monitoring in 2002, (2) IDNR/UHL biological (Biocriteria) monitoring in 2001 & 2004, and (3) results of IDNR/UHL ambient monthly water quality monitoring during the 2008-2010 assessment period at STORET station 10910002 (formerly station 426058) at the County Road R27 bridge SE of Norwalk.

Basis for Assessment

[Note:  Prior to the 2008 Section 305(b) cycle, this stream segment was designated only for Class B(WW) aquatic life uses, including fish consumption uses.   Due to changes in Iowa’s surface water classification that were approved by U.S.  EPA in February 2008, and due to the completion of a Use Attainability Analysis, this segment is also now designated for Class A1 (primary contact recreation) uses.   This segment remains designated for warmwater aquatic life use (now termed Class B(WW1) uses), and for fish consumption uses (now termed Class HH (human health/fish consumption uses)).]

SUMMARY:  The Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" due to levels of indicator bacteria that violate state water quality criteria.   The Class B(WW1) aquatic life uses are assessed (evaluated) as "partially supported” (IR Category 5b-t) based on results of biological monitoring in 2002 and 2004.   Fish consumption uses are "not assessed" due to the lack of fish contaminant monitoring in this river segment.   The sources of data for this assessment include (1) results of IDNR/UHL biological (REMAP) monitoring in 2002, (2) IDNR/UHL biological (Biocriteria) monitoring in 1999, 2001 and 2004, and (3) results of IDNR/UHL ambient monthly water quality monitoring during the 2008-2010 assessment period at STORET station 10910002 (formerly station 426058) at the County Road R27 bridge SE of Norwalk.  

EXPLANATION:  The Class A1 uses are assessed (monitored) as "not supported" based on results of ambient monitoring for indicator bacteria (E.  coli).   The geometric means of indicator bacteria (E.  coli) in the 22 samples collected during the recreational seasons of 2008 through 2010 at IDNR station 10910002 near Norwalk were as follows:  the 2008 geometric mean was 1,359 orgs/100 ml, the 2009 geometric mean was 890 orgs/100 ml and the 2010 geometric mean was 1,187 orgs/100 ml.   All three geometric means far exceed the Class A1 criterion of 126 orgs/100 ml.   Seventeen of the 22 samples (77%) 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 supported” based on results of biological monitoring conducted in 2001, 2002 and 2004 as part of IDNR/UHL biocriteria and REMAP projects.   A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the biological 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 2001 BMIBI score was 26 (poor).   The 2002 REMAP FIBI score was 29 (fair) and the BMIBI score was 56 (good).   The 2004 biocriteria FIBI score was 25 (poor) and the BMIBI score was 65 (good).   The aquatic life use support was assessed as partially supporting (=PS), based on a comparison of the FIBI and BMIBI scores with biological impairment criteria (BIC) established for previous Section 305(b) reports.   The biological impairment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2004.   The FIBI BIC for this ecoregion is 36 and the BMIBI BIC for this ecoregion is 51.   This segment passed the FIBI BIC 0/2 times in the last nine years and passed the BMIBI BIC 2/3 times in the last 10 years.   This stream segment continues to exhibit movement into, and out of, the range of impairment of aquatic life uses especially the FIBI scores.  

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 over a five-year period to be considered “monitored”.   This segment had multiple samples collected in the previous 10 years (2001-2010); however, the multiple samples were not collected during the recent five-year period.   Additionally, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).   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).   However, despite this change in assessment methodology and type, this waterbody was previously on Iowa’s Section 303(d) list and thus remains on Iowa’s Section 303(d) list of impaired waters (IR Category 5b).

Despite the results of biological monitoring that show aquatic life uses to be only "partially supported," results of chemical/physical monitoring in this river segment continue to show relatively good water quality.   None of the approximately 30 samples collected during the 2008-2010 assessment period at this IDNR monthly ambient station violated Class B(WW1) water quality criteria for pH, dissolved oxygen, or ammonia-nitrogen.  

Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this stream segment.