Iowa River IA 02-IOW-641

Assessment Comments

Assessment is based on results of IDNR ambient monthly monitoring downstream from Marshalltown.

Basis for Assessment

Used results from the DNR quarterly WQ monitoring station on the Iowa R.  near the Marshalltown Water Works for developing assessments of support of designated uses.  Support of Class B(WW) aquatic life uses assessed as FST due to lack of violations for Class B(WW) WQ criteria for conventional and toxic parameters in the 8 samples collected over the 1996-97 period.   Assess support of the Class A primary contact uses as PS due to high levels of fecal coliform bacteria in the 5 non-flow affected samples collected during summers of 1996 and 1997:  geometric mean of 184 orgs/100 ml < 200 orgs/100 ml WQS, but > 10% of samples (40%) exceeded 400 orgs/100 ml =PS (see page 3-34 of Supplement to the EPA Guidelines for preparation of the 1998 Section 305(b) reports).   Addendum:  In summer 1997, reports of a reddish/brown coloration in the Iowa River between Eldora and Tama was reported by the public.   Investigations by DNR Field Offices 2 (Mason City) and 5 (Des Moines) indicated that the cause of the unusual coloration was a bloom of algae (diatoms, possibly Stephanodiscus or Cyclotella sp.).   Water quality sampling showed to apparent negative impact to levels of DO or ammonia in the river; no fish kills or other water quality problems observed.