Assessment Comments

Assessment is based on: (1) the results of IDNR/UHL ambient monthly monitoring during the 2008-2010 assessment period at STORET station 10910001 (formerly station 200551) at the USGS gauging station approximately 1.5 miles west of Highway 65-69 near Indianola and (2) results of IDNR/UHL biological monitoring conducted in 2002 as part of the REMAP project.

Basis for Assessment

[Note:  Prior to the 2008 Section 305(b) cycle, this river 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 in 2007, 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 remain assessed (evaluated) as "partially supported" based on results of IDNR/UHL biological monitoring in 2002 (IR Category 3b-u).   Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this stream segment.   The sources of data for this assessment include (1) the results of IDNR/UHL ambient monthly monitoring during the 2008-2010 assessment period at STORET station 10910001 (formerly station 200551) at the USGS gauging station approximately 1.5 miles west of Highway 65-69 near Indianola and (2) results of IDNR/UHL biological monitoring conducted in 2002 as part of the REMAP project.  

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 10910001 near Indianola were as follows:  the 2008 geometric mean was 1,359 orgs/100 ml, the 2009 geometric mean was 912 orgs/100 ml and the 2010 geometric mean was 899 orgs/100 ml.   All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml.   Sixteen of the 22 samples (73%) 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 IDNR/UHL biological monitoring in 2002 conducted as part of the REMAP project.   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 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 2002 FIBI scores were 21 (poor) and 38 (fair).   The 2002 BMIBI scores were 39 (fair) and 24 (poor).   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 1/2 times and passed the BMIBI BIC 0/2 times in 2002.   This assessment is considered evaluated because the drainage area (502 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 failed both the FIBI (once) and BMIBI (twice) 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 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 nine years (2002-2010); however, the samples were not collected during a five-year period and were not collected in multiple years.   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).  

Previous assessments of this stream segment have indicated potential impacts due to alteration of aquatic habitats.   Thus, problems with habitat quality may explain the relatively poor IBI values for fish and aquatic macroinvertebrates.  

As part of the REMAP project, diurnal dissolved oxygen/temperature monitoring conducted during July, September, and October 2002 found substantial fluctuation of dissolved oxygen levels and temperature, including nighttime sags in dissolved oxygen (<5 mg/L) and daytime temperatures exceeding 35 C (95 F).   These conditions were associated with very high estimated levels of community respiration and primary production.   Levels of BMIBI metrics that are sensitive to organic enrichment suggest a nutrient enrichment/algal growth-related water quality problem.   Habitat modification has resulted in a wide and shallow stream channel subjected to high solar input.   These physical factors probably contribute to excessive levels of algal growth, and extreme dissolved oxygen and temperature fluctuations.   Additional sampling data are needed to evaluate the extent, causes and sources of this water quality concern.  

Despite results of biological monitoring which suggest impairment, results of IDNR/UHL ambient chemical/physical monitoring do not suggest an impairment.   None of the approximately 30 samples collected during the 2008-2010 assessment period at the IDNR monthly station violated Class B(WW1) water quality criteria for dissolved oxygen, pH, or ammonia-nitrogen.  

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