Assessment Comments
Assessment is based on: (1) results of the IDNR-UHL beach monitoring program in the summers of 2004, 2005, and 2006 (2) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 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 violations of the state water quality criteria for indicator bacteria and 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 2004, 2005, and 2006 (2) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, and (5) results of EPA/DNR fish contaminant (RAFT) monitoring in 2003.
EXPLANATION: Results of IDNR beach monitoring from 2004 through 2006 suggest that the Class A1 uses are "not supported." Levels of indicator bacteria at Blackhawk Lake beach were monitored once per week during the primary contact recreation seasons (May through September) of 2004 (16 samples), 2005 (23 samples), and 2006 (28 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) all thirty-day geometric means for 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 5-sample, 30-day 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).
At Blackhawk Lake beach, the geometric means of 2 thirty-day periods during the summer recreation season of 2005 exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml. No geometric means violated this criterion in 2004 or 2006. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was not significantly greater than 10% in any of the years (2004: 0%, 2005: 13%, 2006: 11%). According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest impairment (nonsupport) of the Class A1 (primary contact recreation) uses.
Blackhawk Lake was sampled as part of IDNR’s Safe Lakes Program, which aims to identify sources of bacteria to selected beaches where bacteria levels have consistently violated the state water quality criteria. The Safe Lakes Program found human contamination in a tile about 200 meters east of the beach. This tile had very high concentrations of detergents present and blood worms where the tile was discharging. The tile line was reported to the IDNR Field Office who could not find it when they went to investigate in the summer of 2006. During follow-up sampling in 2007 the IDNR Safe Lakes Program also could not find the tile. This tile was gone, capped off, or underwater as the lake water level was higher in 2007. This tile was a likely source of contamination to Blackhawk Lake beach. Continued follow-up monitoring including investigation for this tile will occur in 2008.
Results of the ISU lake survey and UHL ambient lake monitoring program also suggest that the Class A1 uses are “not supported” at Blackhawk Lake due to poor water transparency due to algal and non-algal turbidity. Using the median values from these surveys from 2002 through 2006 (approximately 27 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 75, 70, and 74 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 high levels of chlorophyll a and suspended algae in the water, very poor water transparency, and very 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 2002-2006 was 18.0 mg/L, which was the 10th highest concentration of the 132 lakes monitored by these programs.
Data from the 2002-2006 ISU and UHL surveys suggest a moderate population of cyanobacteria exists at Blackhawk Lake, which does not contribute to impairment at this lake. These data show that cyanobacteria comprised only 48% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (12.3 mg/L) was also the 44th lowest of the 132 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed as “fully supported” based on information from the IDNR Fisheries Bureau, results from the ISU and UHL lake surveys, and results of physical and chemical monitoring associated with IDNR’s beach monitoring program. 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 2002-2006 there were no violations of the Class B(LW) criterion for dissolved oxygen (27 samples) or pH (27 samples). There was one violation in 21 samples of the Class B(LW) criterion for ammonia. Based on IDNR’s assessment methodology, the one violation of the ammonia criterion does not constitute an impairment of water quality at Blackhawk Lake. The physical/chemical data associated with the beach monitoring data from 2004 through 2006 show 1 violation of the Class B(LW) criteria for dissolved oxygen in 64 samples (1%) and 1 violation of the Class B(LW) criterion for pH in 64 samples (1%). According to IDNR’s assessment methodology these results suggest full support of the Class B(LW) uses at Blackhawk Lake.
Fish consumption uses were assessed (monitored) 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. 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.
