Coralville Reservoir IA 02-IOW-630

Results of ACOE beach monitoring from 2012 through 2014 suggest that the Class A1 uses are "Fully Supported." Levels of indicator bacteria were measured at multiple beaches located at Coralville Lake as part of the ACOE beach monitoring program. Sandy Beach were monitored once per week during the primary contact recreation seasons (May through September) of 2012 (18 samples), 2013 (11 samples) and 2014 (13 samples), as part of the ACOE beach monitoring program. Sugar Bottom Beach were monitored once per week during the primary contact recreation seasons (May through September) of 2012 (19 samples), 2013 (14 samples) and 2014 (16 samples), as part of the ACOE beach monitoring program. West Overlook Beach were monitored once per week during the primary contact recreation seasons (May through September) of 2012 (19 samples), 2013 (16 samples) and 2014 (16 samples), as part of the ACOE beach monitoring program. High water levels at Coralville Lake during the 2013 and 2014 sampling seasons led to flooding and closures of the beaches. Beaches were not sampled during flooded conditions. According to IDNR’s assessment methodology two conditions need to be met for results of beach monitoring to indicate “full support” of the Class A1 (primary contact recreation) uses: (1) the geometric mean of the samples from each recreation season of the three-year assessment period are less than the state's geometric mean criterion of 126 E.  coli orgs/100 ml and (2) not more than 10% of the samples during any one recreation season exceeds the state's single-sample maximum value of 235 E.  coli orgs/100 ml. If a sampling season geometric mean exceeds the state criterion of 1000 orgs/100 ml during the three-year assessment period, the Class A1 uses should be assessed as “not supported.” Also, if a sampling season geometric mean exceeds the state criterion of 126 orgs/100 ml during the three-year assessment period and/or if significantly more than 10% of the samples in any one of the three recreation seasons exceed Iowa's single-sample maximum value of 235 E.  coli orgs/100 ml, the Class A1 uses should be assessed as “partially supported.” This assessment approach is based on U.S.   EPA guidelines (see pgs 3-33 to 3-35 of U.S.   EPA 1997b).

At Sandy Beach, the geometric means from 2012, 2013 and 2014 were all below the Iowa water quality standard of 126 E.  coli orgs/100 ml.   The geometric mean was 27 E.  coli orgs/100 ml in 2012, 16 E.  coli orgs/100 ml in 2013 and 40 E.  coli orgs/100 ml in 2014.   The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E.  coli orgs/100 ml) was 6% in 2012, 0% in 2013 and 8% in 2014.   None of these are significantly greater than 10% of the samples and therefore do not suggest impairment of the Class A1 uses.   According to IDNR's assessment methodology and U.S.  EPA guidelines, these results suggest "Fully Supported" of the Class A1 uses.

At Sugar Bottom Beach, the geometric means from 2012, 2013 and 2014 were all below the Iowa water quality standard of 126 E.  coli orgs/100 ml.   The geometric mean was 23 E.  coli orgs/100 ml in 2012, 16 E.  coli orgs/100 ml in 2013 and 31 E.  coli orgs/100 ml in 2014.   The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E.  coli orgs/100 ml) was 0% in 2012, 0% in 2013 and 13% in 2014.   None of these are significantly greater than 10% of the samples and therefore do not suggest impairment of the Class A1 uses.   According to IDNR's assessment methodology and U.S.  EPA guidelines, these results suggest "Fully Supported" of the Class A1 uses.

At West Overlook Beach, the geometric means from 2012, 2013 and 2014 were all below the Iowa water quality standard of 126 E.  coli orgs/100 ml.   The geometric mean was 15 E.  coli orgs/100 ml in 2012, 11 E.  coli orgs/100 ml in 2013 and 28 E.  coli orgs/100 ml in 2014.   The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E.  coli orgs/100 ml) was 5% in 2012, 0% in 2013 and 13% in 2014.   None of these are significantly greater than 10% of the samples and therefore do not suggest impairment of the Class A1 uses.   According to IDNR's assessment methodology and U.S.  EPA guidelines, these results suggest "Fully Supported" of the Class A1 uses.

Results from the ISU statewide surveys of Iowa lakes from 2010-2014 and the UI/QCOE monitoring for 2010-2014, however, suggest that the Class A1 uses be assessed (monitored) as “partially supported” due to high levels of inorganic turbidity.   (NOTE: Secchi depth measurements are not collected as part of the ACOE monitoring at Coralville Reservoir.) 2010-2014, Carlson 's (1977) trophic state indices for Secchi depth (15 samples), chlorophyll a (74 samples), and total phosphorus (101 samples) were 68, 62, and 80 respectively for Coralville Reservoir. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Coralville Reservoir in between the Eutrophic and the Hypereutrophic categories. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, poor water transparency, and extremely high levels of phosphorus in the water column. The data show no violations of the Class A1 criterion for pH in 108 samples.

The level of inorganic suspended solids was high at Coralville Reservoir, and does suggest that non-algal turbidity contributes to the impairment at this lake. The median level of inorganic suspended solids in Coralville Reservoir (11 mg/L) was ranked 123rd among the 138 lakes by the ISU lake survey.

Data from the 2010-2014 ISU lake survey suggest a small population of cyanobacteria exists at Coralville Reservoir. These data show that cyanobacteria comprised 96% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (7 mg/L) was ranked 20th of the 138 lakes sampled.

The Class B(WW-1) (aquatic life) uses of Coralville Reservoir are assessed (monitored) as "fully supported" based on results from the UI/COE water quality study, results from the ISU lake surveys, and information from the IDNR Fisheries Bureau. The Class B(LW) (aquatic life) uses are assessed (monitored) as "fully supported." Results of the ISU lake survey from 2010-2014 show there were 2 violations of the criterion for ammonia in 99 samples(2%), 13 violations of the criterion for dissolved oxygen in 107 samples(12%), and no violations of the criterion for pH in 108 samples(0%). Based on IDNR's assessment methodology these violations are not significantly greater than 10% of the samples and therefore suggest (fully supported/monitored) of the Class B(WW-1) uses of Coralville Reservoir.

Fish consumption uses are assessed (monitored) as "fully supporting."  Fish contaminant monitoring conducted in Coralville Reservoir in 2010 and 2012 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 very low.   The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses.  The fish contaminant data generated from the 2010 and 2012 ACOE fish contaminant monitoring conducted at Coralville Reservoir show that the levels of contaminants do not exceed any of the advisory trigger levels, thus suggesting no justification for issuance of a consumption advisory for this waterbody.  Monitoring also showed low levels of mercury and PCBs.