Assessment Comments
Assessment is based on results of (1) water quality (including fish tissue) monitoring conducted as part of the UI/ACOE "Coralville Water Quality Study from 2000-02, (2) ISU statewide survey of lakes from 2000-02, (3) the ISU study on plankton communities in 2000, and (4) information from the IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: Class A (primary contact recreation) uses of the reservoir were assessed (monitored) as "fully supported / threatened” due to poor water transparency caused by high levels of inorganic suspended solids in this reservoir. The primary contact recreation uses at two of the reservoir's three beaches, however, remain assessed (monitored) as "not supported" due to violations of Class A criteria for indicator bacteria in summer 2000. Based on these violations, the overall assessment of Class A uses for the reservoir is thus "not supported" (monitored). The Class B(WW) aquatic life uses remain assessed (evaluated) as "fully supported" although occasional violations of state criteria for dissolved oxygen suggest a potential water quality problem. Fish consumption uses were assessed as "fully supported" based on ACOE-sponsored fish contaminant monitoring from 2000-02. The sources of data for these assessments include (1) the results of water quality monitoring conducted from 2000 through 2002 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, (2) results of the statewide survey of Iowa lakes conducted from 2000 through 2002 by Iowa State University (ISU), (3) information from the IDNR Fisheries Bureau, and (4) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002).
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 28 samples collected in summers of 2000, 2001, and 2002 was 22 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 during recreational seasons of the 2000-02 period. None of the 16 thirty-day geometric means (N=5) for summers 2000, 2001, and 2002 exceeded 200 orgs/100 ml; the maximum thirty-day geometric mean was 111 orgs/100 ml. At Sugar Bottom Beach, however, one of six thirty-day geometric means from summer 2000 exceeded the state water quality standard of 200 orgs/100 ml (248 orgs/100 ml). The five thirty-day geometric means for summers of 2001 and 2002 at this beach were much lower with a maximum geometric mean of 65 orgs/100 ml. Similarly, at Sandy Beach, however, one of six thirty-day geometric means from summer 2000 exceeded the state water quality standard of 200 orgs/100 ml (273 orgs/100 ml). The five thirty-day geometric means for summers of 2001 and 2002 at this beach were much lower with a maximum geometric mean of 130 orgs/100 ml. Based on results of beach monitoring, the violations of the Iowa’s Class A water quality standard for indicator bacteria (fecal coliforms) at Sugar Bottom and Sandy beaches in 2000 suggest that the Class A uses of Coralville Reservoir are "not supported." The absence, however, of any violations during the recreational seasons of 2001 and 2002 at these beaches suggests that bacterial levels at the Coralville Reservoir beaches are, in general, very low.
Results from the ISU statewide survey of Iowa lakes from 2000-02 also suggest impairments to Class A uses of Coralville Reservoir. Data from this study suggest that high levels of non-algal turbidity may affect the Class A uses of this lake. Using the median values from this survey from 2000 through 2002 (approximately nine samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 82, 51, and 66, respectively. According to Carlson (1977), the index value for total phosphorus places this reservoir in the upper range of hyper-eutrophic lakes; the index value for chlorophyll-a is in the lower range of eutrophic lakes, and the value for secchi depth is in the middle range between eutrophic and hyper-eutrophic lakes. These index values suggest extremely high levels of phosphorus in the water column, much less than expected production of suspended algae, and moderately poor water transparency. According to Carlson (1991), the occurrence of a low chlorophyll-a TSI value relative to those for total phosphorus and secchi depth indicate non-algal particles or color dominate light attenuation. The ISU lake data suggest that non-algal particles do likely limit algal production at Coralville Reservoir. The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2000 through 2002 was 4.8 mg/l. Of 131 lakes sampled, Coralville Reservoir eighth highest median level of inorganic suspended solids (21.8 mg/l), thus suggesting that non-algal turbidity limits the production of algae and is the primary cause of the moderately poor water transparency. These conditions suggest an suggest impairments to the Class A (primary contact) uses primarily due to presence high levels of inorganic turbidity that reduce water transparency such that Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions is violated. The level of chlorophyll-a at Coralville Reservoir is very low (24th lowest of the 131 lakes sampled) and does not appear to contribute to impairments at this lakes. The IDNR Fisheries Bureau concurs that turbidity-related impairments exist at this lake. Based on median values from ISU sampling from 2000-02, the ratio of total nitrogen to total phosphorus for this lake is 42; this ratio suggests that algal production at this lake is not limited by the availability of nitrogen. The 2000-02 median concentration of total nitrogen at this reservoir (9.1 mg/l) was the seventh highest of the 131 lakes sampled. Data from Downing et al. (2002) show relatively small zooplankton populations at this lake, including a relatively small population of species known to be algal grazers; the median summer mass of zooplankton grazers at this lake in 2000 (3.6 mg/l) was the 27th lowest of the 131 lakes sampled. The extremely high level of inorganic suspended solids at this lake, however, remains the primary non-phosphorus limitation to algal production at this lake. The application of the TSI assessment methodology developed to Iowa lakes to Iowa’s four federal flood control reservoirs may not be appropriate. That is, these waterbodies, by design, have extremely high watershed-to-surface area ratios. Thus, the occurrence of higher inorganic turbidities in these reservoirs is not unexpected. Nonetheless, the high levels of inorganic suspended solids in this reservoir, and the poor water transparency that results, suggests a water quality concern.
Blooms of bluegreen algae do not appear to be a problem at Coralville Reservoir. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise a very small portion (approximately 4%) of the summertime phytoplankton community of this lake. The average mass of bluegreen algae in summer 2000 at this lake (0.07 mg/l) was the second lowest of the 131 lakes sampled. These conditions do not suggest any impairments due to presence of nuisance (=noxious) aquatic (e.g., algal) species.
The Class B(WW) aquatic life uses of the reservoir were assessed as "fully supported." None of the approximately 60 samples collected during the 2000-2002 period violated Class B(WW) criteria for pH or ammonia-nitrogen. One of 60 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). The single violation during the 2000-2002 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 from 2000 through 2002 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. Analysis of fish tissue for this program does not include analysis for mercury.
