Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, (4) results from the IDNR-county voluntary beach monitoring program, and (5) results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 2003.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially supported” due to poor water transparency and nuisance algae blooms that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions as well as violations of the state’s pH criterion. The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” due to violations of the state’s criterion for pH and excessive nutrient loading to the water column, nuisance blooms of algae, and re-suspension of sediment. Fish consumption uses are assessed (monitored) as “fully supported” based on results of fish contaminant monitoring in 2003. Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, (4) results from the IDNR-county voluntary beach monitoring program, and (5) results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 2003.
Note: A TMDL for algae and turbidity at Swan Lake was prepared by IDNR and approved by EPA in 2004; thus, this waterbody was placed into IR Category 4a (TMDL approved) for the 2004 assessment/listing cycle. Because all Section 303(d) impairments identified for the 2008 assessment/listing cycle (algal growth, turbidity, and pH) are addressed by the TMDL, this waterbody remains in IR Category 4a.
EXPLANATION: Results of IDNR county beach monitoring from 2004 through 2006 suggest that the Class A1 uses are assessed (monitored) as "fully supported." Levels of indicator bacteria at Swan Lake beach were monitored approximately once per week during the primary contact recreation seasons (May through August) of 2004 (13 samples), 2005 (14 samples), and 2006 (13 samples) as part of the IDNR county 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 Swan Lake beach, the geometric means of all 27 thirty-day periods during the summer recreation seasons of 2004, 2005 and 2006 were below the Iowa water quality standard of 126 E. coli orgs/100 ml. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was also less than 10% in 2004: 8%, and 2005: 7%. In 2006 15% of the samples exceeded the single-sample maximum criterion, however based on IDNR’s assessment methodology these violations were not significantly greater than 10% of the samples. According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest full support of the Class A1 (primary contact recreation) uses.
Results from the ISU and UHL lake surveys suggest that the Class A1 uses at Swan Lake are “partially supported” due to frequent violations of the state criterion for pH, poor water transparency, and nuisance algae blooms. 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 67, 66, and 85 respectively for Swan Lake. The values for Secchi depth and chlorophyll a show a significant improvement from the 2006 assessment listing cycle (Secchi depth: 87, chlorophyll a: 73). These improvements are likely due to a renovation and re-stocking of the fish populations at Swan Lake conducted in 2004, as well as the addition of a new wetland at the upper end of the lake. While the chlorophyll a and Secchi depth scores improved significantly, the total phosphorus score remains extremely high. According to Carlson (1977) the Secchi depth and chlorophyll a values places Swan Lake in between the eutrophic and the hypereutrophic categories, while the total phosphorus value places Swan Lake in the hypereutrophic category. 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 level of inorganic suspended solids is moderately high at Swan Lake and suggests that non-algal turbidity may contribute to the impairment. The median inorganic suspended solids concentration at Swan Lake was 5.0 mg/L, which was the 54th highest of the 132 monitored lakes.
Data from the 2002-2006 ISU and UHL surveys suggest a large population of cyanobacteria exists at Swan Lake, which contributes to impairment at this lake. These data show that cyanobacteria comprised 81% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (58.5 mg/L) was the 19th highest of the 132 lakes sampled. This median is in the worst 25% of the 132 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 from 2002-2006. 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 “partially supported” due to violations of the Class A1,B(LW) criterion for pH. Using the ISU and UHL lake surveys data during 2002-2006 there were no violations of the Class B(LW) criterion for ammonia in 21 samples, or dissolved oxygen in 27 samples. There were, however, 13 violations of the pH criterion in 27 samples (48%). Based on IDNR’s assessment methodology these violations are significantly greater than 10% of the samples and therefore suggest impairment (partial support/monitored) of the Class B(LW) uses of Swan Lake. The pH violations at Swan Lake likely reflect the excessive primary productivity at the lake and do not reflect the input of pollutants into this lake.
Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at Swan Lake in 2003. The composite samples of fillets from channel catfish and largemouth bass had low levels of contaminants. Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: 0.028 ppm; total PCBs: 0.0.09 ppm; and technical chlordane: <0.03 ppm. Levels of primary contaminants in the composite sample of largemouth bass fillets were as follows: mercury: 0.102 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 Swan 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.
