Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results of the IDNR-county voluntary beach monitoring program from 2006 through 2008.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to aesthetically objectionable conditions caused by poor water clarity and nuisance algae (cyanobacteria). The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.” Fish consumption uses remain assessed (evaluated) as “fully supported.” Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results of the IDNR-county voluntary beach monitoring program from 2006 through 2008.
Note: A TMDL for algae and turbidity at Lake Cornelia was prepared by IDNR and approved by EPA in 2006. Because all Section 303(d) impairments identified for the 2010 assessment/listing cycle (algal growth and turbidity) are addressed by the TMDL, this waterbody is placed in IR Category 4a (impaired; TMDL approved).
EXPLANATION: Results of IDNR-county beach monitoring from 2006 through 2008 suggest that the Class A1 uses are assessed (monitored) as "fully supported." Levels of indicator bacteria at Lake Cornelia beach were monitored approximately once per week during the primary contact recreation seasons (May through August) of 2006 (16 samples), 2007 (15 samples), and 2008 (15 samples) as part of the IDNR county beach monitoring program. According to IDNR’s assessment methodology, all thirty-day geometric means for the three-year assessment period must be less than the state’s geometric mean criterion of 126 E. coli orgs/100 ml for results of beach monitoring to indicate “full support” of the Class A1 (primary contact recreation) uses. If a 5-sample, 30-day geometric mean exceeds the state criterion of 126 orgs/100 ml during the three-year assessment period, the Class A1 uses should be assessed as “not 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 beach, the geometric means of all 34 thirty-day periods during the summer recreation seasons of 2006, 2007 and 2008 were below the Iowa water quality standard of 126 E. coli orgs/100 ml. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 6% in 2006, and 0% in 2007 and 2008. According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest full support of the Class A1 (primary contact recreation) uses.
However, based on results from the ISU statewide survey of lakes and the UHL ambient lake monitoring program the Class A1 uses for Lake Cornelia are assessed as “not supported” due to poor water clarity caused by non-algal turbidity and nuisance algal blooms. Using the median values from these surveys from 2004 through 2008 (approximately 25 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 70, 62, and 65 respectively for Lake Cornelia. According to Carlson (1977) the Secchi depth value places Lake Cornelia in the hypereutrophic category, while the chlorophyll a and total phosphorus values place Lake Cornelia in between the eutrophic and hypereutrophic categories. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, very poor water transparency, and high levels of phosphorus in the water column.
The level of inorganic suspended solids was very high at this lake and suggests that non-algal turbidity contributes to the impairment at this lake. Based on data from the ISU and UHL surveys in 2004 through 2008, the median concentration of inorganic suspended solids was 8.0 mg/L, which was the 27th highest concentration of the 132 lakes monitored by these programs.
Data from the 2004-2008 ISU and UHL surveys suggest a large population of cyanobacteria exists at Lake Cornelia, which contributes to the impairment at this lake. These data show that cyanobacteria comprised 99% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (68.7 mg/L) was also the 7th highest of the 132 lakes sampled. This median is in the worst 25% of the 132 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 2004-2008. 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. The assessment category for 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 as “fully supported.” The ISU and UHL lake surveys data from 2004-2008 show no violations of the Class B(LW) criteria for ammonia in 24 samples or dissolved oxygen in 25 samples. The data show one violation of the criterion for pH in 25 samples (4%). Based on IDNR’s assessment methodology, this violation is not significantly greater than 10% of the samples and therefore does not suggest an impairment of the aquatic life uses at this lake.
Fish consumption uses were “not assessed” due to a lack of recent fish contaminant monitoring at this lake. The most recent fish tissue monitoring was conducted in 1997. While these results suggest that levels of contaminants were low at Lake Cornelia, they are now too old (greater than 10 years) to be used for an assessment.
