Hannen Lake IA 02-IOW-704

For the 2016 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 2010-2014 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 63, 66, and 71 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 very high levels of phosphorus in the water column. The data show one violation of the Class A1 criterion for pH in 15 samples. 

Based on IDNR’s assessment methodology 2 consecutive assessment/listing cycles without significantly greater than 10% of the samples violating the single-sample maximum criterion are necessary to propose delisting based on pH violations.  There were less than 10% of the samples exceeding the single-sample maximum criterion.   Therefore, the impairment (4a) for pH is suggested for delisting for the 2016 assessment/listing cycle.

While the Secchi 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 "not supported" for algae and turbidity.  

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.

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 (2.5 mg/L) was ranked 39th among the 138 lakes by the ISU lake survey.

Data from the 2010-2014 ISU lake survey suggest a moderately large population of cyanobacteria exists at Hannen Lake. These data show that cyanobacteria comprised 87% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (25.3 mg/L) was ranked 81st 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 2010-2014 show there were one violation of the criterion for ammonia in 15 samples(7%), 3 violations of the criterion for dissolved oxygen in 15 samples(20%), and one violation of the criterion for pH in 15 samples(7%). Based on IDNR'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. Hannen Lake was assessed as (4a)"partially supported" for the 2014 assessment/listing cycle.   Based on IDNR’s assessment methodology 2 consecutive assessment/listing cycles without significantly greater than 10% of the samples violating the single-sample maximum criterion are necessary to propose delisting based on pH violations.  There were less than 10% of the samples exceeding the single-sample maximum criterion.   Therefore, the impairment (4a) for pH is suggested for delisting for the 2016 assessment/listing cycle.