Assessment Comments
Assessment is based on: (1) results of the IDNR-UHL beach monitoring program from 2010-2012, (2) results of the statewide survey of Iowa lakes conducted from 2009-2012 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted in 2008 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, (5) results of a fish kill investigation in May 2004, and (6) 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 “not supported” due to high levels chlorophyll a (algae) and non-algal turbidity that create aesthetically objectionable conditions. Large populations of cyanobacteria also cause a possible impairment due to nuisance aquatic life. In addition, a violation of the state's criterion for indicator bacteria also suggests an impairment at this lake. 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 2010-2012, (2) results of the statewide survey of Iowa lakes conducted from 2009-2012 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted in 2008 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, (5) results of a fish kill investigation in May 2004, and (6) 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 and turbidity levels however have increased since the 2008 assessment/listing cycle and suggested impairment (partial support) of the Class A1 uses for the 2010 and 2012 assessment/listing cycles. Therefore for the 2010 assessment/listing cycle, North Twin Lake returned to Category 4a (TMDL approved). Due to a new impairment for indicator bacteria (pathogens) for the 2012 assessment/listing cycle and remains there for the current assessment/listing cycle, North Twin Lake was moved from Category 4a (TMDL approved) to Category 5a (impaired; TMDL needed).
EXPLANATION: Results of IDNR beach monitoring from 2010-2012 suggest that the Class A1 uses are “partially supported." Levels of indicator bacteria at the North Twin Lake West beach were monitored once per week during the primary contact recreation seasons (May through September) of 2010 (30 samples), 2011 (30 samples), and 2012 (30 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) 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 2014 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 North Twin Lake West beach, the geometric means from 2010-2012 were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 49 E. coli orgs/100 ml in 2010, 24 E. coli orgs/100 ml in 2011, and 12 E. coli orgs/100 ml in 2012. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 27% in 2010, 3% in 2011 and 0% in 2012. The percentage of samples that exceeded the single-sample maximum was significantly greater than 10% in 2010 and therefore suggests impairment of the Class A1 uses. According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest “partial support” of the Class A1 uses.
Note: On multiple dates in 2010, rainfall events occurred on Sunday or Monday immediately prior to regularly scheduled sample collections on Monday. North Twin Lake has generally shown very low levels of indicator bacteria at the beach.
Results from the ISU and UHL lake surveys suggest that the Class A1 uses at North Twin Lake are assessed (monitored) as “not supported” due to high levels of algae and non-algal turbidity 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 2008-2012 (approximately 14 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 72, 66, and 67 respectively for North Twin Lake. According to Carlson (1977) the chlorophyll a and total phosphorus values place North Twin Lake in between the eutrophic and hypereutrophic categories, while the Secchi depth value places North Twin Lake in the hypereutrophic category. These values suggest high levels of chlorophyll a and suspended algae in the water, very poor water transparency, and high levels of phosphorus in the water column.
The level of inorganic suspended solids is very high at this lake and suggests that non-algal turbidity contributes to the impairments at this lake. The median inorganic suspended solids concentration at North Twin Lake was 13.5 mg/L, and ranked 126th of the 134 monitored lakes.
Data from the 2008-2012 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 81% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (48.5 mg/L) ane ranked 118th 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 2008-2012. 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 2012 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 (2009-2012) 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 2014 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 2008-2012 there were no violations of the Class B(LW) criterion for ammonia in 14 samples, or for dissolved oxygen in 14 samples, and no violations of the Class A1,B(LW) criterion for pH in 14 samples. These results 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.
