Assessment Comments
Assessment is based on water quality monitoring conducted as part of the UI/ACOE "Coralville Water Quality Study in 2000-01.
Basis for Assessment
SUMMARY: Class A (primary contact recreation) uses of the reservoir were assessed as "fully supported;" primary contact recreation uses at two of the reservoir's three beaches, however, were assessed as "not supported." The overall assessement of Class A uses for the reservoir is thus "not supported." The Class B(WW) aquatic life uses were assessed as "fully supported." Fish consumption uses were assessed as "fully supported." The source of data for these assessments is the results of water quality monitoring conducted from October 1999 through September 2001 at Coralville Reservoir by the University of Iowa (under contract with the U.S. Army Corps of Engineers) as part of the Coralville Reservoir Water Quality Study. EXPLANATION: Results of water quality monitoring at the UI/ACOE 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 2000 and 2001 was 25 orgs/100 ml; no samples 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, suggests 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 one of the three reservoir beaches (West Overlook) were well-below the Class A water quality criterion of 200 orgs/100 ml in 1997: None of the six 30-day geometric means (N=5) for summer 2000, and none of the five 30-day geometric means (N=5) for summer 2001, exceeded 200 orgs/100 ml. At Sugar Bottom Beach, however, one of six 30-day geometric means from summer 2000 exceeded the state water quality standard of 200 orgs/100 ml (248 orgs/100 ml). The five 30-day geometric means for summer 2001 at this beach were much lower with a maximum geometric mean of 38 orgs/100 ml. Similarly, at Sandy Beach, however, one of six 30-day geometric means from summer 2000 exceeded the state water quality standard of 200 orgs/100 ml (273 orgs/100 ml). The five 30-day geometric means for summer 2001 at this beach were much lower with a maximum geometric mean of 91 orgs/100 ml. Based on results of beach monitoring, the Class A uses of Coralville Reservoir are assessed as "not supported." The Class B(WW) aquatic life uses of the reservoir were assessed as "fully supported." None of the 42 samples collected during the 2000-2001 biennial period violated Class B(WW) criteria for pH or ammonia-nitrogen. One of 42 samples, however, violated the Class B(WW) criterion for dissolved oxygen: the sample from September 18, 2001 contained 3.9 mg/l of dissolved oxygen and violated the Class B(WW) criterion of 5.0 mg/l. According to U.S. EPA guidelines for Section 305(b) reporting (U.S. EPA 1997b: page 3-17), however, a violation frequency for conventional parameters (including dissolved oxygen) of 10% or less does not suggest an impairment of aquatic life uses. During the 1998-1999 biennial period, five of 43 reservoir samples violated the criterion for dissolved oxygen (see assessment for the 2000 report above). The single violation during the 2000-2001 biennial period may suggest improving water quality conditions or may indicate a continuing water quality problem. Regardless, problems with low levels of dissolved oxygen at this reservoir remain a concern. Fish consumption uses were assessed as "fully supporting." Fish contaminant monitoring conducted in Coralville Reservoir in 2000 and 2001 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.
