Hannen Lake IA 02-IOW-704
Benton County S34T82NR11W 4 mi SW of Blairstown.
- Cycle
- 2018
- Release Status
- Final
- Overall IR
- 4 - Water is impaired or threatened and a TMDL has been completed or is not needed.
- Trophic
- Eutrophic
- Trend
- Stable
- Created
- 4/3/2019 12:56:46 PM
- Updated
- 4/3/2019 2:14:33 PM
The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to aesthetically objectionable conditions caused by algae. The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.” 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 2012 through 2016 by Iowa State University (ISU), and (2) information from the IDNR Fisheries Bureau.
For the 2018 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Hannen Lake are assessed (monitored) as "partially supported" due to aesthetically objectionable conditions caused by algae blooms based on information from the ISU lake survey. Using the median values from these surveys from 2012-2016 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 63, 69, and 75 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 the Hypereutrophic categories. These values suggest high levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and extremely high levels of phosphorus in the water column. The data show one violation of the Class A1 criterion for pH in 15 samples (7%). 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 2016 assessment/listing cycle. DNR's assessment methodology indicates that at least two assessment/listing cycles with a TSI value less than or equal to 63 are necessary to suggest de-listing of an impairment. The Secchi depth TSI value for the 2010 assessment listing cycle was 65, which based on DNR's assessment/listing methodology indicated an impairment based on the narrative criteria protecting against aesthetically objectionable conditions.Because the TSI value for the 2016 assessment/listing cycle was less than the trigger of 65 (2016 listing for Secchi depth a was 63), and because the TSI value for the current listing cycle is also 63, based on DNR's methodology, these values suggest "full support" of the Class A1 uses protecting against aesthetically objectionable conditions. The level of inorganic suspended solids was relatively low at Hannen Lake, and does not suggest water quality problems due to non-algal turbidity. The median level of inorganic suspended solids in Hannen Lake (1.8 mg/L) was ranked 40th among the 138 lakes by the ISU lake survey. Data from the 2012-2016 ISU lake survey suggest a moderate population of cyanobacteria exists at Hannen Lake. These data show that cyanobacteria comprised 78% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (10.5 mg/L) was ranked 52nd of the 138 lakes sampled. The Class B(LW) (aquatic life) uses are assessed (monitored) as "fully supported." Results of the ISU lake survey from 2012-2016 show there were no violations of the criterion for ammonia in 15 samples(0%), 2 violations of the criterion for dissolved oxygen in 15 samples(13%), and one violation of the criterion for pH in 15 samples(7%). Based on DNR's assessment methodology these violations are not significantly greater than 10% of the samples and therefore suggest (fully supported/monitored) of the Class B(LW) uses of Hannen Lake.