Assessment Comments

Assessment is based on results of IDNR/UHL ambient monthly monitoring near Knoxville from 2000-02 and on 1999 Fisheries data: Fish IBI = 22 (poor).

Basis for Assessment

SUMMARY:  Based on results of ambient chemical/physical water quality monitoring and 1999 IDNR Fisheries sampling, the Class B(WW) aquatic life uses remain assessed (monitored) as "fully supported / threatened" with a declining water quality trend.   Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this stream segment.   The source of data for this assessment is the results of IDNR/UHL ambient monthly monitoring during the 2000-2002 assessment period at STORET station 10630001 (formerly station 100818) approximately 3 miles southwest of Knoxville.  

EXPLANATION:  The Class B(WW) aquatic life uses are assessed as "fully supported / threatened" with a potentially declining water quality trend.   None of the 36 samples collected during the 2000-2002 assessment period at the IDNR monthly station violated Class B(WW) water quality criteria for pH or ammonia-nitrogen; no violations occurred in the nine samples analyzed for pesticides and other toxic organic compounds.   However, two of the 36 samples violated the Class B(WW) criterion for dissolved oxygen:  the sample collected on August 1, 2001 contained only 0.7 mg/l, and the sample collected on September 4, 2001, contained only 1.6 mg/l of dissolved oxygen; no violations have been documented since September 2001.   According to U.S.  EPA guidelines for Section 305(b) water quality assessments (U.S.  EPA 1997b, page 3-17), the percentage of violations for dissolved oxygen at this station (6%) does not suggest a water quality impairment (the EPA guidelines allow up to 10% violations of these conventional parameters before impairment of water quality is indicated).   Nonetheless, these readings of dissolved oxygen are atypically low for Iowa streams, and the potential exists for a significant water quality impact, especially given the occurrence of consecutive monthly levels of dissolved oxygen far below the state water quality criterion.   Follow-up monitoring should be conducted to better characterize water quality conditions and status of aquatic life of this stream segment.  

The assessment was partially based on data collected in 1999 as part of a DNR Fisheries stream sampling project: Manchester research station.   A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the Fisheries sampling data.   The biological metrics are based on the numbers and types of fish species that were collected in the stream sampling reach.   The biological metrics were combined to make a fish community index of biotic integrity (F-IBI).   The index ranks the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).   The 1999 evaluated Fish IBI was 22 (poor).   The aquatic life use support was assessed (evaluated) as Not Supporting (=NS), based on a comparison of the F-IBI score with biological assessment criteria established for previous Section 305(b) reports.   The biological assessment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2001.


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