Assessment Comments
Assessment is based on: (1) results of the IDNR-UHL beach monitoring program from 2006 through 2008, (2) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, (5) results of a fish kill investigation in May 2004, (6) results of IDNR county beach monitoring from 2006, and (7) results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 2002.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially supported” due to high levels chlorophyll a (algae) that create aesthetically objectionable conditions. Large populations of cyanobacteria also cause a possible impairment due to nuisance aquatic life. The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” due to a fish kill in 2004. Fish consumption uses are assessed (monitored) as “fully supported” based on fish tissue monitoring in 2002. Sources of data for this assessment include (1) results of the IDNR-UHL beach monitoring program from 2006 through 2008, (2) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, (5) results of a fish kill investigation in May 2004, (6) results of IDNR county beach monitoring from 2006, and (7) results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 2002.
Note: A TMDL for nutrients and algae at North Twin Lake was prepared by IDNR and approved by EPA in 2004; thus, this lake was placed into IR Category 4a (TMDL approved) for the 2004 assessment/listing cycle. Apparently due to events not related to the TMDL (carp migration in 2001), the conditions that previously indicated impairments of the Class A1 uses at this lake (high levels of algae and turbidity) did not exist during the 2002-2006 assessment period, thus indicating an improving trend in water transparency. Thus, this waterbody was moved from IR Category 4a to IR Category 2b (potentially impaired; need exists for follow-up monitoring) for the 2006 and 2008 assessment/listing cycles. Algae levels however have increased since the 2008 assessment/listing cycle and now suggest impairment (partial support) of the Class A1 uses for the 2010 assessment/listing cycle. Therefore for the 2010 assessment/listing cycle, North Twin Lake will return to Category 4a (TMDL approved).
EXPLANATION: Results of IDNR beach monitoring at the North Twin Lake East beach from 2006 through 2008 suggest that the Class A1 uses should be assessed (monitored) as “fully supported.” Levels of indicator bacteria were monitored once per week during the primary contact recreation seasons (May through September) of 2006 (28 samples), 2007 (15 samples), and 2008 (13 samples) as part of the IDNR beach monitoring program. According to IDNR’s assessment methodology, all thirty-day geometric means for the three-year assessment period must be less than the state’s geometric mean criterion of 126 E. coli orgs/100 ml for results of beach monitoring to indicate “full support” of the Class A1 (primary contact recreation) uses. 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”. This assessment approach is based on U.S. EPA guidelines (see pgs 3-33 to 3-35 of U.S. EPA 1997b).
At the East Beach on North Twin Lake, the geometric means of all 41 thirty-day periods during the summer recreation seasons of 2006, 2007 and 2008 were below the Iowa water quality standard of 126 E. coli orgs/100 ml. These results suggest generally low levels of indicator bacteria at this lake. Also, the percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 0% in 2006, 13% in 2007 and 8% in 2008. Thus, these results do not suggest an impairment of the Class A1 uses of North Twin Lake.
Results of IDNR beach monitoring at the North Twin Lake West beach from 2006 through 2008 also suggest that the Class A1 uses should be assessed (monitored) as “fully supported.” Levels of indicator bacteria were monitored once per week during the primary contact recreation seasons (May through September) of 2006 (16 samples), 2007 (15 samples), and 2008 (13 samples) as part of the IDNR beach monitoring program. At the West Beach on North Twin Lake, the geometric means of all 29 thirty-day periods during the summer recreation seasons of 2006, 2007, and 2008 were below the Iowa water quality standard of 126 E. coli orgs/100 ml. Also, the percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 0% in 2006, 7% in 2007 and 0% in 2008. Thus, these results do not suggest an impairment of the Class A1 uses of North Twin Lake.
Results of IDNR county beach monitoring from 2006 also suggest that the Class A1 uses are assessed (evaluated) as "fully supported." Levels of indicator bacteria at Treman Park Beach at North Twin Lake were monitored approximately once per week during the primary contact recreation season (May through August) of 2006 (14 samples) as part of the IDNR county beach monitoring program. Monitoring was not conducted at this beach in 2007 or 2008. Thus this assessment is considered evaluated rather than monitored. At Treman Park Beach, the geometric means of all 2 thirty-day periods during the summer recreation season of 2006 were below the Iowa water quality standard of 126 E. coli orgs/100 ml. There were also no violations of Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) in 2006. 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 North Twin Lake are assessed (monitored) as “partially supported” due to high levels of algae that create aesthetically objectionable conditions. Large populations of cyanobacteria that violate the narrative criteria protecting against nuisance aquatic life also contribute to the impairment at this lake. Using the median values from these surveys from 2004 through 2008 (approximately 25 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 64, 66, and 64 respectively for North Twin Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place North Twin Lake in between the eutrophic and hypereutrophic categories. These values suggest high levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and moderately high levels of phosphorus in the water column.
The level of inorganic suspended solids is high at this lake but does not suggest impairment due to high non-algal turbidity. The median inorganic suspended solids concentration at North Twin Lake was 5.8 mg/L, which was the 44th highest of the 132 monitored lakes.
Data from the 2004-2008 ISU and UHL surveys suggest a large population of cyanobacteria exists at North Twin Lake, which contributes to impairment at this lake. These data show that cyanobacteria comprised 91% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (39.0 mg/L) was also the 23rd 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 2004-2008. 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 (evaluated) as “partially supported” due to a fish kill on May 10, 2004. According to the IDNR investigation, approximately 500 dead crappies, all about nine inches long, were observed at this lake. IDNR staff noted that healthy fish were also observed at the time of the kill; thus indicating little long-term impact to the lake’s fishery. The kill was believed due to post-spawning stress and was not believed to be related to water quality problems. No estimate of cost of the kill was provided. This is the same assessment as that developed for the 2008 assessment/listing cycle. The continuance of the IR Category 3b listing is based on IDNR's 2010 assessment methodology that states the following: 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 (2006-2009) 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” and 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 impairment will remain in Category 3b of Iowa’s 2010 Integrated Report.
Information from the IDNR Fisheries Bureau and results from the ISU and UHL lake surveys, however, suggest that the Class B(LW) uses are “fully supported.” The ISU and UHL lake surveys show that during 2004-2008 there were no violations of the Class B(LW) criterion for ammonia in 24 samples, or for dissolved oxygen in 25 samples. There was one violation of the Class A1,B(LW) criterion for pH in 25 samples (4%). Based on IDNR’s assessment methodology these violations are not significantly greater than 10% of the samples and therefore suggest full support of the Class B(LW) uses at North Twin Lake.
Fish consumption uses were assessed (evaluated) as “fully supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at North Twin Lake in 2002. The composite samples of fillets from channel catfish and white crappie had extremely low levels of contaminants, with levels of the primary contaminants (mercury, PCBs, and chlordane) all below their respective levels of detection. Levels of primary contaminants in the composite sample of channel catfish 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 white 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 2002 RAFT sampling conducted at North Twin 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.
