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), and (3) information from the IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to aesthetically objectionable conditions caused by algae and turbidity and violations of the Class A1 pH criterion. The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” due to violations of the Class B(LW) criterion for pH. Violations of the Class B(LW) chronic criteria for ammonia suggest an additional potential impairment of the aquatic life uses. Fish consumption uses remain “not assessed” due to a lack of 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), and (3) information from the IDNR Fisheries Bureau.
EXPLANATION: For the 2012 reporting cycle, the Class A1 (primary contact recreation) uses for Hannen Lake are assessed (monitored) as “partially supported” based on results from the ISU statewide survey of lakes and the UHL ambient lake monitoring program. Using the median values from these surveys from 2006 through 2010 (approximately 23 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 65, 62, and 69 respectively for Hannen Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Hannen Lake 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.
While the chlorophyll a trophic state index value for Hannen Lake is below the impairment trigger of 65, Hannen Lake was assessed as "not supporting" its Class A1 uses for the 2010 assessment/listing cycle due to high levels of algae. Based on DNR's assessment methodology, two consecutive assessment listing cycles where the trophic state index value is less than or equal to 63 are necessary to suggest de-listing of the algae impairment. Therefore for the 2012 assessment/listing cycle the Class A1 uses are assessed as "partially supported" for algae and turbidity.
The levels of inorganic suspended solids at this lake were moderately high and suggest that sediment may contribute to poor water clarity and the turbidity impairment at this lake. The median level of inorganic suspended solids in Hannen Lake (3.0 mg/L) was the 53rd lowest median of the 134 lakes sampled by the ISU and UHL programs.
Data from the 2006-2010 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at Hannen Lake, which contributes to the aesthetically objectionable conditions due to algal turbidity at this lake. These data show that cyanobacteria comprised 100% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (32.2 mg/L) was the 45th highest of the 134 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” based on results from the ISU and UHL lake surveys. The dissolved oxygen data show 2 violations of the Class B(LW) criterion in 23 samples (9%). Based on IDNR’s assessment methodology, these violations are not significantly greater than 10% of the samples and therefore do not suggest impairment of the Class B(LW) uses. The pH data, however, show 7 violations of the Class A1,B(LW) criterion for pH in 23 samples (30%). Based on IDNR’s assessment methodology these violations are significantly greater than 10% of the samples and therefore suggest impairment of the Class A1,B(LW) uses at Hannen Lake. These pH violations, however, likely reflect the excessive primary productivity at Hannen Lake and do not reflect the input of pollutants into this lake.
Results from the ISU and UHL lake surveys from 2006-2010 show two violations of the Class B(LW) criteria for ammonia in 23 samples. Based on IDNR’s 2012 Section 303(d) listing methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s chronic criterion for ammonia and thus do not suggest an impairment of the Class B(LW) aquatic life uses. Because, however, more than one violation of the chronic criterion for ammonia occurred, this potential impairment will be added to Iowa's list of waters in need of further investigation. Note: Based on consultation with other Region 7 states and U.S. EPA Region 7 staff in 2007, IDNR's methodology for assessing impairments due to violations of chronic criteria for toxic parameters (e.g., ammonia and toxic metals) was changed. Prior to the 2008 listing cycle, IDNR followed a U.S. EPA recommendation (U.S. EPA 1997b, page 3-18) that more than one violation of a water quality criterion for a toxic pollutant in an abundant data set indicates an impairment of aquatic life uses. Because no state in Region 7 collects ambient data with the frequency necessary to accurately identify compliance with a chronic criterion (i.e., to allow calculation of a short-term (4-day or 30-day averages)), Region 7 states concluded that another assessment approach (e.g., the 10% rule) was more appropriate. Thus, for the 2008, 2010, and current (2012) listing cycle, IDNR identified aquatic life impairments for toxic parameters when significantly more than 10% of the samples exceeded a chronic criterion.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
