Coralville Reservoir IA 02-IOW-630

Results of ACOE beach monitoring from 2014 through 2016 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 was monitored once per week during the primary contact recreation seasons (May through September) of 2014 (13 samples), 2015 (13 samples) and 2016 (16 samples), as part of the ACOE beach monitoring program.Sugar Bottom Beach was monitored once per week during the primary contact recreation seasons (May through September) of 2014 (16 samples), 2015 (20 samples) and 2016 (18 samples), as part of the ACOE beach monitoring program.West Overlook Beach was monitored once per week during the primary contact recreation seasons (May through September) of 2014 (16 samples), 2015 (18 samples) and 2016 (18 samples), as part of the ACOE beach monitoring program.According to DNR’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 2014, 2015 and 2016 were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 40 E. coli orgs/100 ml in 2014, 126 E. coli orgs/100 ml in 2015 and 56 E. coli orgs/100 ml in 2016. The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E. coli orgs/100 ml) was 8% in 2014, 23% in 2015 and 0% in 2016. None of these are significantly greater than 10% of the samples and therefore do not suggest impairment of the Class A1 uses. According to DNR'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 2014, 2015 and 2016were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 31 E. coli orgs/100 ml in 2014, 61 E. coli orgs/100 ml in 2015 and 20 E. coli orgs/100 ml in 2016. The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E. coli orgs/100 ml) was 13% in 2014, 20% in 2015 and 0% in 2016. None of these are significantly greater than 10% of the samples and therefore do not suggest impairment of the Class A1 uses. According to DNR'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 2014, 2015 and 2016were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 28 E. coli orgs/100 ml in 2014, 30 E. coli orgs/100 ml in 2015 and 31 E. coli orgs/100 ml in 2016. The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E. coli orgs/100 ml) was 13% in 2014, 6% in 2015 and 0% in 2016. None of these are significantly greater than 10% of the samples and therefore do not suggest impairment of the Class A1 uses. According to DNR's assessment methodology and U.S. EPA guidelines, these results suggest "Fully Supported" of the Class A1 uses.

For the 2018 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Coralville Reservoir are assessed (monitored) as "not supported" due to poor water transparency based on information from the ISU lake survey and based on information collected as part of the UI/ACOE "Coralville Water Quality Study." Using the median values from theISU lake survey from 2012-2016 (approximately 20 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 70, 56, and 77 respectively for Coralville Reservoir. Additionally, using the median values from theUI/ACOE "Coralville Water Quality Study" from 2012-2016 (approximately 37 Secchi depth, 57 chlorophyll a, and 87 total phosphorus samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 80, 60, and 81 respectively. 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 relatively low levels of chlorophyll a and suspended algae in the water, very poor water transparency, and extremely high levels of phosphorus in the water column. The data show one violation of the Class A1 criterion for pH in 20 samples (5%)based on information from the ISU lake survey andno violations of the Class A1 criterion for pH in 96 samplesUI/ACOE "Coralville Water Quality Study."

The level of inorganic suspended solids was very 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 (14.6 mg/L) was ranked 129th among the 138 lakes by the ISU lake survey.

Data from the 2012-2016 ISU lake survey suggest a small population of cyanobacteria exists at Coralville Reservoir. These data show that cyanobacteria comprised 7% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (0.9 mg/L) was ranked 10th of the 138 lakes sampled.

The B(WW-1) (aquatic life)uses are assessed (monitored) as "fully supported." Results of the ISU lake survey from 2012-2016 show there were no violations of the criterion for ammonia in 20 samples(0%), 2 violations of the criterion for dissolved oxygen in 20 samples(10%), and one violation of the criterion for pH in 20 samples(5%). Additionally, results of the UI/ACOE "Coralville Water Quality Study" from 2012-2016 show there were no violations of the criterion for ammonia in 85 samples(0%), 12 violations of the criterion for dissolved oxygen in 96 samples(12%), and no violations of the criterion for pH in 96 samples(0%). Based on DNR'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.