Iowa River IA 02-IOW-623

Assessment Comments

Assessment is based on results of IDNR ambient monthly monitoring at Columbus Junction.

Basis for Assessment

SUMMARY:  Class A (primary contact recreation) uses are assessed as "not 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 1998-1999 biennial period at the DNR monthly station on the Iowa River at Columbus Junction show that the Class A uses are "not supported."  The geometric mean levels of fecal coliform bacteria in the 14 non-runoff-affected samples collected during summers of 1998 and 1999 (211 orgs/100 ml) exceeded (slightly) the Iowa water quality criterion of 200 orgs/100 ml.   Four of the 14 samples (29%) 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 "not supported" if the geometric mean for fecal coliforms is greater than 200 organisms/100 ml.   Despite the indications of impairment for Class A uses, levels of indicator bacteria at the Columbus Junction monitoring station have been relatively low during recent biennial periods.   Typically, the assessments of impairment have been decided by only one or two samples.   The only violation of Class B(WW) criteria for conventional parameters was in one of the 24 samples analyzed for pH during the biennial period.   The sample collected on December 1, 1997, had a pH level of 9.1 units; this level violated the Class B(WW) (and Class A) criterion of 9.0 units.   According to U.S.  EPA guidelines for Section 305(b) water quality assessments (U.S.  EPA 1997b, page 3-17), the percentage of violations for pH at this station (4%) does not suggest a water quality impairment (the EPA guidelines allow up to 10% violations of these conventional parameters before impairment of water quality is indicated).   Levels of dissolved oxygen and ammonia-nitrogen did not violate the respective Class B(WW) criteria in the 24 samples analyzed during the biennial period, and no violations of Class B(WW) chronic criteria for toxic metals occurred in the two samples analyzed during this period.   Thus, the Class B(WW) aquatic life uses were assessed as "fully supported."  Fish consumption uses remain "not assessed" due to lack of fish tissue monitoring in this river reach.