Black Hawk Lake IA 04-RAC-1134

Results of IDNR beach monitoring from 2012 through 2014 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 IDNR beach monitoring program. Denison Beach was monitored once per week during the primary contact recreation seasons (May through September) of 2013 (17 samples) and 2014 (15 samples). Black Hawk Campground Beach was monitored once per week during the primary contact recreation seasons (May through September) of 2012 (15 samples), 2013 (15 samples) and 2014 (17 samples).   According to IDNR’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 2013 and 2014 were all below the Iowa water quality standard of 126 E.  coli orgs/100 ml.   The geometric mean was 49 E.  coli orgs/100 ml in 2013 and 62 E.  coli orgs/100 ml in 2014.   The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E.  coli orgs/100 ml) was 6% in 2013 and 13% in 2014.   None of these are significantly greater than 10% of the samples and therefore do not suggest impairment of the Class A1 uses.   According to IDNR's assessment methodology and U.S.  EPA guidelines, these results suggest "Fully Supported" of the Class A1 uses.

At Black Hawk Campground Beach, the geometric means from 2012, 2013 and 2014 were all below the Iowa water quality standard of 126 E.  coli orgs/100 ml.   The geometric mean was 23 E.  coli orgs/100 ml in 2012, 23 E.  coli orgs/100 ml in 2013 and 75 E.  coli orgs/100 ml in 2014.   The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E.  coli orgs/100 ml) was 0% in 2012, 0% in 2013 and 24% in 2014.   The number of samples exceeding the single-sample maximum criterion was significantly greater than 10% in 2014.   According to IDNR's assessment methodology and U.S.  EPA guidelines, these results however suggest "partially supported" of the Class A1 uses.

Results for the 2016 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 2010-2014 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 65, 66, 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, poor water transparency, and extremely high levels of phosphorus in the water column. The data show 3o violations of the Class A1 criterion for pH in 15 samples.The level of inorganic suspended solids was moderately high at Black Hawk Lake, and does suggest that non-algal turbidity may contribute to the impairment at this lake. The median level of inorganic suspended solids in Black Hawk Lake (5.5 mg/L) was ranked 90th among the 138 lakes by the ISU lake survey.

Data from the 2010-2014 ISU lake survey suggest a moderate population of cyanobacteria exists at Black Hawk Lake. These data show that cyanobacteria comprised 94% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (16.7 mg/L) was ranked 51st of the 138 lakes sampled.

The Class B(LW) (aquatic life) uses are assessed as “fully supported” based on information from the IDNR 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 lake summer. Results of the ISU lake survey from 2010-2014 show there were one violation of the criterion for ammonia in 15 samples(7%), 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 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 Black Hawk Lake.

Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring that is less than ten years old in this lake. Fish consumption uses was assessed (evaluated) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Black Hawk Lake in 2003. This data is beyond the 10 year criterion for credible data.

Note 1:  The TMDL prepared by Iowa DNR for impairments due to algae and turbidity was approved by U.S.  EPA in February 2012. Not all Section 303(d) impairments identified for the 2016 assessment/listing cycle (indicator bacteria), however, are addressed in the TMDL.   Thus, this waterbody remains in IR Category 5a (impaired; TMDL required) for the 2016 assessment/listing cycle.

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