Lake Cornelia IA 04-UDM-1264

Results of DNR beach monitoring from 2014 through 2016 suggest that the Class A1 uses are "Fully Supported." Levels of indicator bacteria at Lake Cornelia Park Beach were monitored once per week during the primary contact recreation seasons (May through September) of 2014 (4 samples), 2015 (9 samples) and 2016 (9 samples), as part of the DNR 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 Lake Cornelia Park 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 11 E. coli orgs/100 ml in 2014, 12 E. coli orgs/100 ml in 2015 and 17 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 0% in 2014, 0% 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 Lake Cornelia are assessed (monitored) as "not supported" due to poor water transparency based on information from the ISU lake survey. Using the median values from these surveys from 2012-2016 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 70, 63, and 63 respectively for Lake Cornelia. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Lake Cornelia in between the Eutrophic and the Hypereutrophic categories. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, very poor water transparency, and moderately high levels of phosphorus in the water column. The data show no violations of the Class A1 criterion for pH in 15 samples.While the chlorophyll a TSI values for Lake Cornelia are below the impairment trigger of 65 for the 2010, 2012, 2014, 2016and 2018 reporting cycle, Lake Cornelia are assessed (evaluated) as"partially supporting" the Class A1 uses for the 2016 reporting cycle due to nuisance aquatic life. Therefore, Lake Cornelia will be assessed (evaluated) as "partially supported" due to high levels of algae for the 2016 assessment cycle.

Note: ATMDL for algae and turbidity at Lake Cornelia was prepared by DNR and approved by EPA in 2006. Because all Section 303(d) impairments identified for the 2016 assessment/listing cycle (algal growth and turbidity) are addressed by the TMDL, thiswaterbodyis placed in IR Category 4a (impaired; TMDL approved).

The level of inorganic suspended solids was moderately high at Lake Cornelia, and does suggest that non-algal turbidity may contribute to the impairment at this lake. The median level of inorganic suspended solids in Lake Cornelia (6 mg/L) was ranked 100th among the 138 lakes by the ISU lake survey.

Data from the 2012-2016 ISU lake survey suggest a large population of cyanobacteria exists at Lake Cornelia, which suggests the potential for an impairment due to nuisance aquatic life These data show that cyanobacteria comprised 95% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (93.9 mg/L) was ranked 131st of the 138 lakes sampled. This median is in the worst 25% of the 138 lakes sampled. The presence of a large population of cyanobacteria at this lake suggests a potential violation of Iowa's narrative water quality standard protecting against the occurrence of nuisance aquatic life. This assessment is based strictly on the distribution of the lake-specific median cyanobacteria values from 2012-2016. Median levels greater than the 75th percentile of this distribution were arbitrarily considered to represent potential impairment. No other criteria exist, however, upon which to base a more accurate identification of impairments due to cyanobacteria. Assessments based on level of cyanobacteria will be considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence) to account for this lower level of confidence.

The Class B(LW) (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 15 samples(0%), no violations of the criterion for dissolved oxygen in 15 samples(0%), and no violations of the criterion for pH in 15 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(LW) uses of Lake Cornelia.