Iowa River IA 02-IOW-623

Assessment Comments

Assessment is based on results of IDNR ambient monthly monitoring at Columbus Junction in 2000-01.

Basis for Assessment

SUMMARY:  Class A (primary contact recreation) uses are assessed as "fully supported."  Class B(WW) aquatic life uses are assessed as "fully supported."  Fish consumption uses remain "not assessed."  EXPLANATION:  Results of water quality monitoring during the 2000-2001 biennial period at the IDNR ambient monthly station on the Iowa River at Columbus Junction show that the Class A uses are "fully supported."  The geometric mean level of fecal coliform bacteria in the 12 non-runoff-affected samples collected during recreational seasons of 2000 and 2001 (57 orgs/100 ml) is well below the Iowa water quality criterion of 200 orgs/100 ml for protection of primary contact recreation uses.   None of the 12 samples had levels of fecal coliforms greater than the EPA-recommended single sample maximum value of 400 orgs/100 ml.   According to U.S.  EPA guidelines for Section 305(b) reporting (pgs 3-33 to 3-35of U.S.  EPA 1997b), primary contact uses are "fully supported" if the geometric mean for fecal coliforms is less than 200 organisms/100 ml and less than 10% of samples exceed U.S.  EPA's recommended single-sample maximum value of 400 orgs/100 ml.   Monitoring results for indicator bacteria during the 2000-2001 biennial period are consistent with the relatively low levels of fecal coliforms at the Columbus Junction station during recent biennial periods.   The 2000-2001 geometric mean (57 orgs/100 ml), however, is considerably lower than recent geometric means (1998-99:  211 orgs/100 ml; 1996-97:  104 orgs/100 ml; 1994-95:  238 orgs/100 ml.).   Previous assessments showing impairment of the primary contact recreation uses for this river segment (e.g., for the 2000 and 1998 reporting cycles) were decided by only one or two samples.   The Class B(WW) aquatic life uses were assessed as "fully supported."  Results from DNR ambient monthly monitoring at this station during the 2000-2001 biennial period show no violations of Class B(WW) water quality criteria in the 24 samples analyzed for conventional parameters (e.g., dissolved oxygen, pH, and ammonia-nitrogen), in the 4 samples analyzed for toxic metals, or in the six samples analyzed for pesticides.   Fish consumption uses remain "not assessed" due to lack of fish tissue monitoring in this river segment.