Assessment Comments

Assessment is based on results of IDNR monthly ambient monitoring near Knoxville.

Basis for Assessment

SUMMARY:  The Class B(WW) aquatic life are assessed (monitored) as "fully supported / threatened" with a declining water quality trend.   Fish consumption uses remain "not assessed."  The source of data for this assessment is the results of IDNR ambient monthly monitoring during the 2000-2001 biennial period at 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 24 samples collected during the 2000-2001 biennial period at the IDNR monthly station violated Class B(WW) water quality criteria for pH or ammonia-nitrogen; no violations occurred in the four samples analyzed for toxic metals or in the 6 samples analyzed for pesticides and other toxic organic compounds.   However, two of the 24 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.   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 (8.3%) 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.   Follow-up monitoring should be conducted to better characterize water quality conditions and status of aquatic life of this stream segment.   Fish consumption uses were "not assessed" due to the lack of recent fish contaminant monitoring in this stream segment.