Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2009-2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 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. 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 2009-2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.
NOTE: A TMDL was prepared for and approved by the U.S. EPA in 2013 for pH and algae. Because all of the impairments at this lake are addressed by the TMDL, Hannen Lake will be categorized as category 4a; water impaired but no TMDL needed for the 2014 assessment/listing cycle.
EXPLANATION: For the 2014 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 2008-2012 (approximately 15 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 64, 58, and 68 respectively for Hannen Lake. According to Carlson (1977) the Secchi depth, and total phosphorus values all place Hannen Lake in between the eutrophic and hypereutrophic categories and the chlorophyll a value places Hannen Lake in between the eutrophic and hypereutrophic categories. These values suggest relatively low levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and high levels of phosphorus in the water column.
While the chlorophyll a and 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 2014 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 relatively low. The median level of inorganic suspended solids in Hannen Lake (2.5 mg/L) and ranked 27th of the 134 lakes sampled by the ISU and UHL programs.
Data from the 2008-2012 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at Hannen Lake, which likely contributes to the aesthetically objectionable conditions due to algal turbidity at this lake. These data show that cyanobacteria comprised 99% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (39.0 mg/L) and ranked 109th 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 15 samples (13%). 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 showed no violations of the Class A1,B(LW) criterion for pH in 15 samples. Based on IDNR's methodology, these results suggest "full support" of the aquatic life uses. Hannen Lake was assessed as “partially supporting” in the 2012 assessment/listing cycle due to significant violations of the pH criteria. Based on IDNR’s assessment methodology 2 consecutive assessment/listing cycles without significantly greater than 10% of the samples violating the criterion are necessary to propose delisting based on pH violations. Therefore, for the 2014 assessment/listing cycle, the Class B(LW) uses will remain assessed as "partially supporting." 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 2008-2012 shows one violation of the Class B(LW) criteria for ammonia in 15 samples. Based on IDNR’s 2014 Section 303(d) listing methodology, 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.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
