Assessment Comments

Assessment is based on results of DNR monthly water quality monitoring NNW of Cambridge downstream from outfall of Ames WWTF.

Basis for Assessment

SUMMARY:  Assessed support of the Class B(LR) aquatic life uses as "fully supported / threatened."  Fish consumption remain "not assessed."  The source of data for this assessment is the results of IDNR monthly ambient monitoring conducted on the South Skunk River in 2000 and 2001 at the Co.Rd.  E554 bridge approximately 4 miles upstream from Cambridge (station 10850002; formerly station 390566).   EXPLANATION:  The assessment of support of the Class B(LR) uses was based on results of monthly monitoring during the 2000-2001 biennial period at the Cambridge station; this station is located approximately 1/3 mile downstream from the outfall of the Ames wastewater treatment plant.   Results of this monitoring show no violations of Class B(WW) criteria for pH, dissolved oxygen, or ammonia nitrogen (maximum value = 0.05 mg/l) in the 24 samples analyzed during the biennial period and no violations of Class B(LR) chronic criteria for pesticides and other toxic organic compounds in the six samples analyzed during the biennial period.   One of the 16 samples analyzed for toxic metals during the biennial period, however, exceeded the Class B(LR) chronic criterion for copper:  the sample collected on July 18, 2000 contained 160 ug/l, thus far exceeding the Class B(LR) criterion of 55 ug/l.   According to U.S.  EPA guidelines for Section 305(b) reporting (U.S.  EPA 1997b, page 3-18), the single violation of chronic criterion for copper does not suggest an impairment of aquatic life uses.   Based on DNR's assessment methodology for Section 305(b) reporting, however, this violation suggests that the Class B(LR) aquatic life uses should be assessed as "fully supported / threatened."