Assessment Comments
Assessment is based on: (1) results of the IDNR-UHL beach monitoring program in the summers of 2008, 2009, and 2010 (2) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, and (5) results of EPA/DNR fish contaminant (RAFT) monitoring in 2003.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to poor water clarity caused by algal and non-algal turbidity. The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.” Fish consumption uses are assessed (monitored) as “fully supported.” Sources of data for this assessment include (1) results of the IDNR-UHL beach monitoring program in the summers of 2008, 2009, and 2010 (2) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, and (5) results of EPA/DNR fish contaminant (RAFT) monitoring in 2003.
Note 1: The TMDL prepared by Iowa DNR for impairments due to algae and turbidity was approved by U.S. EPA in February 2012. Because this TMDL addresses all of the impairments identified for this lake, this lake is moved from IR Category 5a (impaired; TMDL required) to IR Category 4a (impaired; TMDL approved).
EXPLANATION: Results of IDNR beach monitoring from 2008 through 2010 suggest that the Class A1 uses are “fully supported." Levels of indicator bacteria at Black Hawk Lake beach were monitored once per week during the primary contact recreation seasons (May through September) of 2008 (26 samples), 2009 (26 samples), and 2010 (17 samples) as part of the IDNR beach monitoring program. 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 126 orgs/100 ml during the three-year assessment period, the Class A1 uses should be assessed as “not supported.” Also, 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).
NOTE: Based on consultation with EPA Region 7 staff in 2011, IDNR’s methodology for assessing impairments based on the geometric mean water quality criterion was changed. Prior to the 2012 listing cycle, IDNR calculated geometric means for lakes based on a 30-day periods within the recreational season. Any violation of one of these 30-day periods within 3 years resulted in an impairment of the Class A1 uses of that lake. Because water quality standards do not identify a 30 day period but instead a recreational season, Region 7 concurred that the approach used for rivers and streams with less frequent bacteria data (seasonal geometric means) would be appropriate for identifying §303(d) impairments at lake beaches. Thus, for the 2012 listing cycle, IDNR identified primary contact recreation impairments for lakes when the geometric mean of all samples from the recreation season of a given year exceeded the geometric mean criterion. This does not impact the way IDNR assesses beaches for closure to protect the recreating public in the short term.
At Black Hawk Lake beach, the geometric means from 2008, 2009, and 2010 were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 21 E. coli orgs/100 ml in 2008, 41 E. coli orgs/100 ml in 2009, and 53 E. coli orgs/100 ml in 2010. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 8% in 2008, 19% in 2009 and 18% in 2010. 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 “full support” of the Class A1 uses. NOTE: Black Hawk Lake was impaired for indicator bacteria in the 2010 assessment. Based on the new methodology and results from 2008 through 2010, the bacteria impairment at Black Hawk Lake is suggested for de-listing for the 2012 assessment/listing cycle.
Results of the ISU lake survey and UHL ambient lake monitoring program however suggest that the Class A1 uses are “not supported” at Blackhawk Lake due to poor water transparency caused by algal and non-algal turbidity. Using the median values from these surveys from 2006 through 2010 (approximately 28 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 75, 71, and 78 respectively for Blackhawk Lake. According to Carlson (1977) the index values for Secchi depth, chlorophyll a, and total phosphorus all place Blackhawk Lake in the hypereutrophic category. These values suggest very high levels of chlorophyll a and suspended algae in the water, very poor water transparency, and extremely high levels of phosphorus in the water column.
The median concentration of inorganic suspended solids is very high and contributes to the impairment at Blackhawk Lake. Results from the ISU and UHL lake surveys show that the median level of inorganic suspended solids in Blackhawk Lake from 2006-2010 was 14.8 mg/L, which was the 7th highest concentration of the 134 lakes monitored by these programs.
Data from the 2006-2010 ISU and UHL surveys suggest a large population of cyanobacteria exists at Blackhawk Lake, which also contributes to an impairment for nuisance aquatic life at this lake. These data show that cyanobacteria comprised 84% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (53.6 mg/L) was also the 20th highest of the 132 lakes sampled. This median is in the worst 25% of the 134 lakes sampled. The presence of a large population of cyanobacteria at this lake suggests a potential violation of Iowa’s narrative water quality standard protecting against the occurrence of nuisance aquatic life. This assessment is based strictly on the distribution of the lake-specific median cyanobacteria values for the 2006-2010 period. Median levels greater than the 75th percentile of this distribution were arbitrarily considered to represent potential impairment. No other criteria exist, however, upon which to base a more accurate identification of impairments due to cyanobacteria. The assessment category for assessments based on level of cyanobacteria will be considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence) to account for this lower level of confidence.
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 and UHL lake surveys. The following factors, however, remain concerns at this lake: nuisance blooms of algae, re-suspension of sediment; the increasing population of common carp, and their tendency to increase levels of turbidity through re-suspension of sediment and algal nutrients. The ISU and UHL lake survey results show good chemical water quality at Blackhawk Lake. During 2006-2010 there was one violation of the Class B(LW) criterion for ammonia in 28 samples. Based on IDNR’s assessment methodology, the one violation of the ammonia criterion does not constitute an impairment of water quality at Blackhawk Lake. There was also one violation of the Class B(LW) criterion for dissolved oxygen in 28 samples. 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. There were 2 violations of the Class A1,B(LW) criterion for pH in 28 samples (7%). Based on IDNR's assessment methodology these violations do not suggest an impairment.
Fish consumption uses were assessed (evaluated) as “fully supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at Black Hawk Lake in 2003. The composite samples of fillets from common carp and black crappie had low levels of contaminants. Levels of primary contaminants in the composite sample of common carp fillets were as follows: mercury: <0.0181 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm. Levels of primary contaminants in the composite sample of black crappie fillets were as follows: mercury: <0.0181 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm. Because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses. The fish contaminant data generated from the 2003 RAFT sampling conducted at this lake show that the levels of contaminants do not exceed any of the advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.
