Swan Lake IA 04-RAC-1199

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).     

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.    

For the 2016 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Swan Lake are assessed (monitored) as "not supported" due to poor water transparency, aesthetically objectionable conditions caused by algae blooms and due to frequent violations of the state criterion for pH 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 70, 68, and 87 respectively for Swan Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Swan Lake in between the Eutrophic and the Hypereutrophic categories. These values suggest 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 data show 5 violations of the Class A1 criterion for pH in 15 samples (33%).

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 2016 assessment/listing cycle (algal growth, turbidity, and pH) are addressed by the TMDL, this waterbody remains in IR Category 4a.  

The level of inorganic suspended solids was high at Swan Lake, and does suggest that non-algal turbidity contributes to the impairment at this lake. The median level of inorganic suspended solids in Swan Lake (9.1 mg/L) was ranked 114th among the 138 lakes by the ISU lake survey.

Data from the 2010-2014 ISU lake survey suggest a moderately large population of cyanobacteria exists at Swan Lake. These data show that cyanobacteria comprised 86% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (30.5 mg/L) was ranked 95th of the 138 lakes sampled.

The Class B(LW) (aquatic life) uses are assessed (monitored) as "not supported" due to violations of the Class A1,B(LW) criterion for pH and results of a fish kill investigation in 2015. Results of the ISU lake survey from 2010-2014 show there were no violations of the criterion for ammonia in 15 samples(0%), 2 violations of the criterion for dissolved oxygen in 15 samples(13%), and 5 violations of the criterion for pH in 15 samples(33%). Based on IDNR's assessment methodology these violations are significantly greater than 10% of the samples and therefore suggest impairment (not supported/monitored) of the Class B(LW) uses of Swan Lake.

Although attributed to "natural causes", the occurrence of fish kill in this lake in October 2015 is consistent with the assessment of aquatic life uses as “not supported.”  The kill occurred on or before October 23,2015.   The cause of the kill was identified as dissolved oxygen that dropped to lethal levels.   An estimated 1,000 fish were killed.   No estimate of the value of the fish killed was provided.  According to IDNR’s assessment/listing methodology, the occurrence of a single pollutant-caused fish kill, or a fish kill of unknown origin, on a waterbody or waterbody reach during the most recent assessment period (2012-2014) indicates a severe stress to the aquatic community and suggests that the aquatic life uses should be assessed as “impaired.”  If a cause of the kill was not identified during the IDNR investigation, or if the kill was attributed to non-pollutant causes (e.g., winterkill), the assessment type will be considered “evaluated.”  Such assessments, although suitable for Section 305(b) reporting, lack the degree of confidence to support addition to the state Section 303(d) list of impaired waters (IR Category 5).   Waterbodies affected by such fish kills will be placed in IR subcategories 2b or 3b and will be added to the state list of waters in need of further investigation.   Thus, this assessment segment will be placed in Category 3b of Iowa’s 2016 Integrated Report. 

The level of support of the Class HH-fish consumption uses is changed from “fully supporting” to “not assessed” due to the age of the data upon which the previous assessment was based.  The previous assessment was based on results of U.S. EPA / IDNR fish tissue (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.