Assessment Comments

Assessment is based on: (1) the results of IDNR monthly ambient monitoring from January 2004 through December 2006 at the county road T75 bridge approximately 3.5 miles northwest of Cedar Falls (STORET station 10070001) and (2) results of IDNR/UHL REMAP biological monitoring in 2003.

Basis for Assessment

[Note:  Prior to the current (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 (see http://www.iowadnr.com/water/standards/files/06mar_swc.pdf), this segment is also now presumptively 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 presumptive 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 (monitored) as “fully supported” based on results of ambient water quality monitoring and on IDNR/UHL REMAP data.   Fish consumption uses remained "not assessed" due to the lack of fish contaminant monitoring in this stream segment.   The sources of data for this assessment are (1) the results of IDNR monthly ambient monitoring from January 2004 through December 2006 at the county road T75 bridge approximately 3.5 miles northwest of Cedar Falls (STORET station 10070001) and (2) the results of IDNR/UHL REMAP biological monitoring in 2003.

EXPLANATION:  The presumptive Class A1 uses are assessed as "not supported" based on results of ambient monitoring for 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 A 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 31 samples collected (464 orgs/100ml) exceeds the Iowa Class A1 water quality criterion of 126 orgs/100ml.   Twenty-one of the 31 samples (68%) 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 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).  

The Class B(WW1) aquatic life uses are assessed as “fully supported” based on results of both chemical/physical and biological monitoring.   Results of IDNR ambient water quality monitoring station NW of Cedar Falls during the 2004-2006 assessment period showed no violations of Class B(WW) water quality criteria for dissolved oxygen or ammonia-nitrogen in the 39 samples collected.   The level of pH in one of the 40 samples collected (9.2 pH units) violated the Class B(WW1) criterion of 9.0 pH units.   Because this violation is more likely related to natural conditions than to a pollutant, the occurrence of the high level of pH in this river segment is not seen as a water quality impairment.   In addition, the frequency of pH violations does not suggest impairment:  according to U.S.  EPA guidelines (U.S.  EPA 1997b, page 3-17), a violation frequency of less than 10 % for conventional parameters such as pH suggest "full support" of aquatic life uses.   Thus, the percentage of violations of the pH criterion at this monitoring station (2%) does not suggest an impairment of aquatic life uses.   All levels of toxic metals were below Class B(WW1) criteria in the 10 samples analyzed, and levels of pesticides were below Class B(WW1) criteria in the 7 samples analyzed.  

This assessment remains partially based on results of biological monitoring conducted in 2003 by IDNR/UHL 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 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 2003 FIBI score was 45 (fair) and the 2003 BMIBI score was 77 (excellent).   The aquatic life use support was assessed as Fully Supporting (=FS), 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 non-riffle FIBI BIC for this ecoregion is 44 and the artificial substrate BMIBI BIC for this ecoregion is 52.

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