Coralville Reservoir IA 02-IOW-630

Assessment Comments

Assessment is based on water quality monitoring.

Basis for Assessment

SUMMARY:  Class A (primary contact recreation) uses of the reservoir and its swimming beaches were assessed as "fully supported;" the Class B(WW) aquatic life uses were assessed as "partially supported."  Fish consumption uses were 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 (Johnson and McDonald 1999, 2000).   Results of water quality monitoring at the long-term station at the MaHaffey Bridge on the main reservoir suggest that the Class A uses should be assessed as "fully supporting."  The geometric mean of indicator bacteria (fecal coliforms) in the 19 samples collected in summers of 1998 and 1999 was 20 orgs/100 ml; one sample (5%) exceeded the EPA-recommended single sample maximum value of 400 orgs/100 ml.   According to U.S.  EPA guidelines for Section 305(b) reporting, a geometric mean for fecal coliforms less than 200 orgs/100 ml, combined with less than 10% of samples exceeding the 400 orgs/100 ml single sample maximum value, suggest full support of primary contact recreation uses.  (see pgs 3-33 to 3-35of U.S.  EPA 1997b).   Levels of indicator bacteria (fecal coliforms) at all three reservoir beaches (West Overlook, Sugar Bottom, and Sandy Beach) were well-below the Class A water quality criterion of 200 orgs/100 ml in 1997:  None of the samples collected during the nine sampling events from June through August exceeded 200 orgs/100 ml (maximum level was 100 orgs/100 ml).   In 1998, however, all reservoir swimming beaches were closed from late June to early August due to high reservoir levels (Johnson and McDonald 2000).   Levels of fecal coliforms began exceeding the 200 orgs/100 ml criterion in early and mid-June 1998 in response to rainfall events and high inflows to the reservoir.   Although West Overlook Beach never exceeded the 200 orgs/100 ml criterion, both Sugar Bottom beach and Sandy Beach had levels above 200 orgs/100 ml in June.   When the beaches were reopened in mid-August 1998, levels of fecal coliforms were again well-below the 200 orgs/100 ml criterion (maximum value for August 1998 was 90 orgs/100 ml).   Because the levels exceeding 200 orgs/100 ml occurred at the beaches during high rainfall / high runoff periods, these levels are not violations of the Iowa Class A criterion for primary contact recreation.   Thus, the Class A uses of these beaches are assessed as "fully supported."  The Class B(WW) aquatic life uses of the reservoir were assessed as "partially supported" due to violations of the state water quality criterion for dissolved oxygen.   Five of the 43 samples (12%) collected during the 1998-1999 biennial period violated the state criterion of 5.0 mg/l dissolved oxygen.   According to U.S.  EPA guidelines for Section 305(b) reporting (U.S.  EPA 1997b:  page 3-17), a violation frequency for conventional parameters of from 11 to 25% suggests "partial support" of aquatic life uses.   The five violations of dissolved oxygen ranged from 4.5 to 4.9 mg/l and occurred during July and August of both 1998 and 1999.   In addition, one of 42 samples collected during the 1998-1999 biennial period violated the chronic criterion for ammonia nitrogen in Class B(WW) waters:  the sample collected on February 12, 1998 contained 4.4 mg/l of ammonia-nitrogen (chronic criterion = 1.85 mg/l).   According to U.S.  EPA guidelines for Section 305(b) reporting (U.S.  EPA 1997b, page 3-18), this one violation does not suggest an impairment of the aquatic life uses.   Violations of ammonia-nitrogen criteria also occurred on this date at monitoring stations upriver (South Amana) and downriver (Iowa City) from Coralville Reservoir.   Fish consumption uses were assessed as "fully supporting."  Fish contaminant monitoring conducted in 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 respective FDA action levels (see Johnson and McDonald (1999, 2000) for more information).