North Twin Lake IA 04-RAC-1167

Results of DNR beach monitoring from 2014 through 2016 suggest that the Class A1 uses are "partially supported." Levels of indicator bacteria were measured at multiple beaches located at North Twin Lake as part of the DNR beach monitoring program. North Twin Lake West Beach was monitored once per week during the primary contact recreation seasons (May through September) of 2014 (15 samples), 2015 (18 samples) and 2016 (15 samples).North Twin Lake East Beach was monitored once per week during the primary contact recreation seasons (May through September) of 2014 (15 samples), 2015 (18 samples) and 2016 (15 samples).Treman Park Beach wasmonitored once per week during the primary contact recreation seasons (May through September) of 2014 (15 samples), 2015 (16 samples) and 2016 (14 samples). According to DNR’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 1000 orgs/100 ml during the three-year assessment period, the Class A1 uses should be assessed as “not supported.” Also, if a sampling season geometric mean exceeds the state criterion of 126 orgs/100 ml during the three-year assessment period and/or 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 North Twin Lake West Beach, the geometric means from 2014, 2015 and 2016 were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 20 E. coli orgs/100 ml in 2014, 40 E. coli orgs/100 ml in 2015 and 13 E. coli orgs/100 ml in 2016. The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E. coli orgs/100 ml) was 7% in 2014, 17% in 2015 and 0% in 2016. None of these are significantly greater than 10% of the samples and therefore do not suggest impairment of the Class A1 uses. According to DNR's assessment methodology and U.S. EPA guidelines, these results suggest "Fully Supported" of the Class A1 uses.

At North Twin Lake East Beach, the geometric means from 2014, 2015 and 2016 were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 18 E. coli orgs/100 ml in 2014, 28 E. coli orgs/100 ml in 2015 and 9 E. coli orgs/100 ml in 2016. The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E. coli orgs/100 ml) was 0% in 2014, 17% in 2015 and 0% in 2016. None of these are significantly greater than 10% of the samples and therefore do not suggest impairment of the Class A1 uses. According to DNR's assessment methodology and U.S. EPA guidelines, these results suggest "Fully Supported" of the Class A1 uses.

At Treman Park Beach, the geometric means from 2014, 2015 and 2016 were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 41 E. coli orgs/100 ml in 2014, 41 E. coli orgs/100 ml in 2015 and 65 E. coli orgs/100 ml in 2016. The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E. coli orgs/100 ml) was 13% in 2014, 13% in 2015 and 21% in 2016. None of these are significantly greater than 10% of the samples and therefore do not suggest impairment of the Class A1 uses. According to DNR's assessment methodology and U.S. EPA guidelines, these results suggest "Fully Supported" of the Class A1 uses.

The 2016 assessment/listing cycle, the Class A1 uses forLevels of indicator bacteria were listed as "partially supported" due to geometric mean or single sample maximum exceedances. According to DNR's methodology, lakes must meet water quality standards for two consecutive assessment/listing cycles to suggest de-listing. Therefore, the Class A1 uses will remain listed as "partially supported" for the 2018 cycle.

For the 2018 assessment/listing cycle, the Class A1 (primary contact recreation) uses of North Twin Lake are assessed (monitored) as "not supported" due to poor water transparency and aesthetically objectionable conditions caused by algae blooms based on information from the ISU lake survey. Using the median values from these surveys from 2012-2016 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 77, 72, and 68 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 the Hypereutrophic categories. These values suggest very high levels of chlorophyll a and suspended algae in the water, extremely poor water transparency, and high levels of phosphorus in the water column. The data show no violations of the Class A1 criterion for pH in 15 samples.

Note: A TMDL for nutrients and algae at North Twin Lake was prepared by DNR 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).

The level of inorganic suspended solids was high at North Twin Lake, and does suggest that non-algal turbidity contributes to the impairment at this lake. The median level of inorganic suspended solids in North Twin Lake (10 mg/L) was ranked 120th among the 138 lakes by the ISU lake survey.

Data from the 2012-2016 ISU lake survey suggest a large population of cyanobacteria exists at North Twin Lake, which suggests the potential for an impairment due to nuisance aquatic life These data show that cyanobacteria comprised 90% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (95.1 mg/L) was ranked 130th of the 138 lakes sampled. This median is in the worst 25% of the 138 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 2012-2016. 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. 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 DNR investigation, approximately 500 dead crappies, all about nine inches long, were observed at this lake. DNR 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 DNR'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 DNR 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 2016 Integrated Report.

Information from the DNR Fisheries Bureau and results from the ISU lake survey, however, suggest that the Class B(LW) uses are “fully supported.”Results of the ISU lake survey from 2012-2016 show there were no violations of the criterion for ammonia in 15 samples(0%), no violations of the criterion for dissolved oxygen in 15 samples(0%), and no violations of the criterion for pH in 15 samples(0%). Based on DNR's assessment methodology these violations are not significantly greater than 10% of the samples and therefore suggest (fully supported/monitored) of the Class B(LW) uses of North Twin Lake.