Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 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 2008 through 2010.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially 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 remain not assessed due to the lack of recent fish contaminant monitoring at this lake. Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 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 2008 through 2010.
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 2012 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 city/county beach monitoring from 2008 through 2010 suggest that the Class A1 uses are “fully supported." Levels of indicator bacteria at Lake Cornelia beach were monitored once per week during the primary contact recreation seasons (May through September) of 2008 (15 samples), 2009 (13 samples), and 2010 (13 samples) as part of the IDNR beach monitoring program. 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 126 orgs/100 ml during the three-year assessment period, the Class A1 uses should be assessed as “not supported.” Also, 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).
NOTE: Based on consultation with EPA Region 7 staff in 2011, IDNR’s methodology for assessing impairments based on the geometric mean water quality criterion was changed. Prior to the 2012 listing cycle, IDNR calculated geometric means for lakes based on a 30-day periods within the recreational season. Any violation of one of these 30-day periods within 3 years resulted in an impairment of the Class A1 uses of that lake. Because water quality standards do not identify a 30 day period but instead a recreational season, Region 7 concurred that the approach used for rivers and streams with less frequent bacteria data (seasonal geometric means) would be appropriate for identifying §303(d) impairments at lake beaches. Thus, for the 2012 listing cycle, IDNR identified primary contact recreation impairments for lakes when the geometric mean of all samples from the recreation season of a given year exceeded the geometric mean criterion. This does not impact the way IDNR assesses beaches for closure to protect the recreating public in the short term.
At Lake Cornelia beach, the geometric means from 2008, 2009, and 2010 were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 10 E. coli orgs/100 ml in 2008, 14 E. coli orgs/100 ml in 2009, and 8 E. coli orgs/100 ml in 2010. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 0% in 2008, 0% in 2009 and 0% in 2010. 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 “full support” of the Class A1 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 “partially supported” due to poor water clarity caused by non-algal turbidity and nuisance algal blooms. Using the median values from these surveys from 2006 through 2010 (approximately 22 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 69, 62, and 65 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 hypereutrophic categories. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, 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 2006 through 2010, the median concentration of inorganic suspended solids was 5.8 mg/L, which was the 46th highest concentration of the 134 lakes monitored by these programs.
Data from the 2006-2010 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 71% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (67.4 mg/L) was also the 12th highest of the 134 lakes sampled. This median is in the worst 25% of the 134 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 2006-2010. 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. This potential impairment is appropriate for Category 3b of Iowa's 2012 Integrated Report and will be placed on Iowa's list of waters in need of further investigation.
The Class B(LW) (aquatic life) uses are assessed as “fully supported.” The ISU and UHL lake surveys data from 2006-2010 show no violations of the Class B(LW) criteria for ammonia in 21 samples or dissolved oxygen in 22 samples. The data show one violation of the criterion for pH in 22 samples (5%). 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.
