Assessment Comments

Assessment is based on results of IDNR monthly ambient water quality monitoring near the mouth of the Rock River in 2000 & 2001.

Basis for Assessment

SUMMARY:  The Class B(WW) aquatic life uses are assessed as "fully supported / threatened."  Fish consumption uses remain "not assessed."  This assessment is based on results of IDNR monthly ambient monitoring conducted near the mouth of the Rock River during the 2000-2001 biennial period at the County Road B-40 bridge north of Hawarden (station 10840001 (formerly station 975005)).   EXPLANATION:  Monitoring at the IDNR station near Hawarden showed no violations of Class B(WW) water quality criteria for dissolved oxygen (minimum value = 5.2 mg/l) or pH (range = 7.5 to 8.4) in the 24 samples analyzed during the 2000-2001 biennial period.   In addition, levels of toxic metals in the four samples analyzed, and levels of pesticides in the six samples analyzed, were all below the analytical level of detection.   One of the 24 samples collected, however, violated the Class B(WW) criterion for ammonia-nitrogen.   The sample collected on April 3, 2001 contained 0.70 mg/l of ammonia-nitrogen; this level slightly exceeded the Class B(WW) temperature/pH-dependent chronic criterion of 0.69 mg/l.   According to U.S.  EPA guidelines for Section 305(b) water quality assessments (U.S.  EPA 1997b, page 3-18), however, one violation of a water quality criterion for a toxic pollutant in an abundant data set (at least 10 samples over at three-year period) set does not indicate an impairment of aquatic life uses.   Based on DNR's assessment methodology for Section 305(b) reporting, this violation suggests that the Class B(WW) aquatic life uses should be assessed as "fully supported/threatened."  Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this river segment.