Black Hawk Lake IA 04-RAC-1134

Results of DNR beach monitoring from 2014 through 2016 suggest that the Class A1 uses are "partially supported." Levels of indicator bacteria were measured at multiple beaches located at Black Hawk Lake as part of the DNR beach monitoring program. Denison Beach was monitored once per week during the primary contact recreation seasons (May through September) of 2014 (15 samples), 2015 (20 samples) and 2016 (16 samples). Black Hawk Campground Beach was monitored once per week during the primary contact recreation seasons (May through September) of 2014 (17 samples), 2015 (18 samples) and 2016 (15 samples). According to DNR’s assessment methodology two conditions need to be met for results of beach monitoring to indicate “full support” of the Class A1 (primary contact recreation) uses: (1) the geometric mean of the samples from each recreation season of the three-year assessment period are less than the state's geometric mean criterion of 126 E. coli orgs/100 ml and (2) not more than 10% of the samples during any one recreation season exceeds the state's single-sample maximum value of 235 E. coli orgs/100 ml. If a sampling season geometric mean exceeds the state criterion of 1000 orgs/100 ml during the three-year assessment period, the Class A1 uses should be assessed as “not supported.” Also, if a sampling season geometric mean exceeds the state criterion of 126 orgs/100 ml during the three-year assessment period and/or if significantly more than 10% of the samples in any one of the three recreation seasons exceed Iowa's single-sample maximum value of 235 E. coli orgs/100 ml, the Class A1 uses should be assessed as “partially supported.” This assessment approach is based on U.S. EPA guidelines (see pgs 3-33 to 3-35 of U.S. EPA 1997b).

At Denison Beach, the geometric means from 2014 and 2016 were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean from 2015, however, was greater than the Iowa water quality standard of 126 E. coli orgs/100ml and therefore suggests impairment of the Class A1 uses. The geometric mean was 62 E. coli orgs/100 ml in 2014, 210 E. coli orgs/100 ml in 2015 and 114 E. coli orgs/100 ml in 2016. The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E. coli orgs/100 ml) was 13% in 2014, 35% in 2015 and 38% in 2016. The number of samples exceeding the single-sample maximum criterion was significantly greater than 10% in 2015 and 2016. According to DNR's assessment methodology and U.S. EPA guidelines, these results suggest "partially supported" of the Class A1 uses.

At Black Hawk Campground Beach, the geometric means from 2014, 2015 and 2016 were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 75 E. coli orgs/100 ml in 2014, 35 E. coli orgs/100 ml in 2015 and 30 E. coli orgs/100 ml in 2016. The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E. coli orgs/100 ml) was 24% in 2014, 17% in 2015 and 7% in 2016. The number of samples exceeding the single-sample maximum criterion was significantly greater than 10% in 2014. According to DNR's assessment methodology and U.S. EPA guidelines, these results suggest "partially supported" of the Class A1 uses.

For the 2018 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Black Hawk Lake are assessed (monitored) as "partially supported" due to poor water transparency and 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 62, 68, and 77 respectively for Black Hawk Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Black Hawk 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 3 violations of the Class A1 criterion for pH in 15 samples (20%).Although the index value for Secchi is below the impairment trigger of 65 for this assessment cycle, Black Hawk Lake was listed as partially supporting its Class A1 uses due to aesthetically objectionable conditions. Based on DNR's methodology, the median TSI value for Secchi must be 63 or less for two consecutive assessment/listing cycles before a lake can be removed from the state's Section 303(d) list (IR Category 4a). Therefore, Black Hawk Lake will remain listed as "not supported" for the 2018 assessment/listing cycle.

Data from the 2012-2016 ISU lake survey suggest a moderate population of cyanobacteria exists at Black Hawk Lake. These data show that cyanobacteria comprised 50% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (7.7 mg/L) was ranked 37th of the 138 lakes sampled.

The Class B(LW) (aquatic life) uses are assessed (monitored) as "fully supported" based on information from the DNR Fisheries Bureau and results from the ISU lake surveys. Water clarity has improved during April, May, June, part of July. Nuisance blooms of algae and poor water clarity are still a problem in late summer. Results of the ISU lake survey from 2012-2016 show there were no violations of the criterion for ammonia in 15 samples(0%), no violations of the criterion for dissolved oxygen in 15 samples(0%), and 3 violations of the criterion for pH in 15 samples(20%). 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 Black Hawk Lake.

Note 2: A fishery renovation and lake draw down was completed in 2012 to remove rough fish from Black Hawk Lake.