Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, (4) results from the IDNR-county voluntary beach monitoring program in 2008, 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 “not 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 (monitored) 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 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, (4) results from the IDNR-county voluntary beach monitoring program in 2008, 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 2012 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 2008 suggest that the Class A1 uses should be assessed (evaluated) as "not supported." Levels of indicator bacteria at Swan Lake beach were monitored approximately once per week during the primary contact recreation season (May through August) of 2008 (7 samples) as part of the IDNR county beach monitoring program. Because a limited number of samples were collected in 2008 these data are considered not sufficient to accurately characterize current water quality conditions, therefore the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). 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 Swan Lake beach, the geometric mean from 2008 exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 177 E. coli orgs/100 ml in 2008. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 43% in 2008. These results are significantly greater than 10% of the samples and therefore suggest impairment of the Class A1 uses. According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest “nonsupport” of the Class A1 uses. However, only 7 samples were collected. Based on IDNR's assessment methodology a minimum of 10 samples over the three-year assessment period are necessary to suggest impairment of the Class A1 uses due to indicator bacteria. Therefore Swan Lake will be placed on the WINOFI list for indicator bacteria.
Results from the ISU and UHL lake surveys suggest that the Class A1 uses at Swan Lake are assessed (monitored) as “not 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 2006 through 2010 (approximately 23 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 71, 73, and 95 respectively for Swan Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Swan 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 level of inorganic suspended solids is high at Swan Lake and suggests that non-algal turbidity contributes to the impairment. The median inorganic suspended solids concentration at Swan Lake was 7.4 mg/L, which was the 28th highest of the 134 monitored lakes.
Data from the 2006-2010 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 50% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (123.4 mg/L) was the 2nd highest of the 134 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 from 2006-2010. 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 (monitored) as “partially supported” due to violations of the Class A1,B(LW) criterion for pH. Information from the IDNR Fisheries Bureau also suggests that algae blooms remain a concern at this lake. Using the ISU and UHL lake surveys data during 2006-2010 there were no violations of the Class B(LW) criterion for ammonia in 23 samples and no violations of the dissolved oxygen criterion in 23 samples. There were, however, 13 violations of the pH criterion in 23 samples (57%). 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 A1 and 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 (evaluated) 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. 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 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.
