Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 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 “partially supported” due to violations of the state criterion for pH and aesthetically objectionable conditions caused by algal turbidity. 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 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.
EXPLANATION: For the 2008 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 2002 through 2006 (approximately 23 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 63, 67, and 70 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 high levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and very high levels of phosphorus in the water column.
The levels of inorganic suspended solids at this lake were relatively low and do not suggest potential problems related to non-algal turbidity. The median level of inorganic suspended solids in Hannen Lake (2.7 mg/L) was the 34th lowest median of the 132 lakes sampled by the ISU and UHL programs.
Data from the 2002-2006 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at Hannen Lake, which contributes to the impairment at this lake for algal turbidity. These data show that cyanobacteria comprised 73% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (23.9 mg/L) was the 51st highest of the 132 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 8 violations of the Class A1,B(LW) criterion for pH in 23 samples (35%). 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 2002-2006 show two violations of the Class B(LW) criteria for ammonia in 12 samples. Based on IDNR’s 2008 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 calculatiion 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 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.
