South Skunk River IA 03-SSK-931

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."  EXPLANATION:  The assessment of support of the Class B(LR) uses was based on results of monthly water quality monitoring during the 1998-1999 biennial period at the DNR station on the South Skunk River near Cambridge (station 390566); 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 in the 22 samples analyzed during the biennial period; no violations of Class B(LR) chronic criteria for toxic metals occurred in the two samples analyzed during the biennial period.   Thus, the Class B(LR) aquatic life uses were assessed as "fully supported."  Although not designated for Class C (drinking water) uses, this river reach tends to have high levels of nitrate, with levels in 9 of the 22 samples collected during the biennial period (41%) exceeding the 10 mg/l MCL (mean=9.8 mg/l; max.=17.0 mg/l; SE=0.9).   The results of the DNR stream habitat assessment conducted in August 1991-upon which the previous assessment of the Class B(LR) uses was primarily based ("partially supported," see above)-are now considered too old (greater than five years) to be useful for assessing current water quality conditions.   Thus, the assessment for the 2000 report was changed from "partially supporting" to "fully supporting" based only on results of the chemical water quality monitoring at the DNR monthly station during the 1998-1999 biennial period.   Follow-up monitoring is needed to determine the status of aquatic habitat and aquatic communities in this river reach.