Iowa River IA 02-IOW-628

Assessment Comments

Assessment is based on results of routine water quality & fish tissue monitoring as part of the UI/ACOE Coralville Water Quality Study.

Basis for Assessment

SUMMARY:  The Class A (primary contact recreation) uses, the Class B(WW) aquatic life uses, and the Class C (drinking water) uses are assessed as "fully supported."  Fish consumption uses remain assessed as "fully supported."  EXPLANATION:  The assessments of support of the beneficial uses are based on results of water quality monitoring conducted by the University of Iowa (under contract with the U.S.  Army Corps of Engineers) as part of the Coralville Reservoir Water Quality Study (see Johnson and McDonald 1999, 2000).   Results of water quality at the long-term station near the University of Iowa water treatment plant suggest that the Class A uses should be assessed as "fully supporting / threatened."  Of the 19 samples analyzed for indicator bacteria (fecal coliforms) during summers of 1998 and 1999, 13 were collected during conditions of high river flow.   These high flows resulted in monitoring at river discharges that exceeded the long-term monthly average flow plus one standard deviation of this average (flow statistics from Fischer et al.  1990).   For purposes of Section 305(b) assessments, DNR uses the long-term average monthly flow plus one standard deviation of this average to identify river flows that are materially affected by surface runoff.   According to the Iowa Water Quality Standards (IAC 1990:8), the water quality criterion for fecal coliform bacteria (200 orgs/100 ml) does not apply "when the waters are materially affected by surface runoff."  The geometric mean of fecal coliform bacteria in the six non-runoff-affected samples was 77 orgs/100 ml; one of the six samples (17%) exceeded the EPA-recommended single-sample maximum value of 400 orgs/100 ml.   The geometric mean (77 orgs/100 ml) is well below the state water quality criterion of 200 orgs/100 ml.   According to U.S.  EPA guidelines for Section 305(b) reporting, if more than 10% of samples exceed the single-sample maximum value of 400 orgs/100 ml, the primary contact recreation uses are only "partially supported" (see pgs 3-33 to 3-35of U.S.  EPA 1997b)  Due, however, to (1) the lack of sufficient data points for developing a "monitored" assessment (according to DNR's 305(b) assessment methodology, at least 10 non-runoff-affected samples), and (2)  the relatively low geometric mean, the Class A uses of this river reach were assessed as "fully supported."  Class B(WW) aquatic life uses were also assessed as "fully supported / threatened."  Results of monitoring at the Iowa/ACOE station showed no violations of Class B(WW) criteria for pH or dissolved oxygen in the 44 samples collected during the 1998-1999 biennial period.   One of 43 samples exceeded the Class B(WW) chronic water quality criterion for ammonia-nitrogen; this sample was collected on February 12, 1998, and contained 5.6 mg/l of ammonia-nitrogen (chronic criterion = 1.25 mg/l) (a similar level (5.1 mg/l) was reported for this date at the South Amana station (330615) in this monitoring network).   This level of ammonia-nitrogen far exceeds levels typical for this station and other river monitoring stations in the state.   According to U.S.  EPA guidelines for Section 305(b) reporting (U.S.  EPA 1997b, page 3-18), one violation of water quality criterion for a toxic contaminant does not suggest an impairment of the aquatic life uses.   According to DNR's methodology for Section 305(b) reporting, the occurrence of this violation indicates that the Class B(WW) uses should be assessed as "fully supporting / threatened."  The Class C drinking water uses are assessed "fully supported / threatened" due to violations of the Class C criterion (MCL) for nitrate+nitrite.   Four of 49 samples (8%) exceeded the 10 mg/l MCL; three of these violations occurred in March and April of 1998; the maximum violation was 11.7 mg/l.   The average nitrate level was 5.8 mg/l (standard deviation = 3.0 mg/l).   Based on DNR's Section 305(b) assessment methodology, if less than 15% of samples collected monthly or more frequently exceed the MCL for nitrate, drinking water uses should be assessed as "fully supporting / threatened."  Fish consumption uses remain assessed as "fully supporting."  Fish contaminant monitoring conducted down river from Coralville Reservoir in 1997 and 1998 as part of the Coralville Reservoir Water Quality Study showed that levels of organochlorine contaminants (chlordane, dieldrin, and heptachlor epoxide) in composite samples of whole-fish carp were well below ½ of the respective FDA action levels (see Johnson and McDonald (1999, 2000) for more information).