Lake Darling IA 03-SKU-924

Results of DNR beach monitoring from 2014 through 2016 suggest that the Class A1 uses are "partially supported." Levels of indicator bacteria at Lake Darling Beach were monitored once per week during the primary contact recreation seasons (May through September) of 2014 (6 samples), 2015 (19 samples) and 2016 (18 samples), as part of the DNR beach monitoring program. 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 Lake Darling Beach, the geometric means from 2014 and 2015 were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean from 2016, however, was greater than the Iowa water quality standard of 126 E. coli orgs/100ml and therefore suggests impairment of the Class A1 uses. The geometric mean was 41 E. coli orgs/100 ml in 2014, 80 E. coli orgs/100 ml in 2015 and 267 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 33% in 2014, 37% in 2015 and 39% in 2016. The number of samples exceeding the single-sample maximum criterion was significantly greater than 10% in 2015 and 2016. According to DNR's assessment methodology and U.S. EPA guidelines, these results suggest "partially supported" of the Class A1 uses.

For the 2018 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Lake Darling are assessed (evaluated) 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 2014-2016 (approximately 8 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 65, 66, and 76 respectively for Lake Darling. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Lake Darling in between the Eutrophic and the Hypereutrophic categories. These values suggest high levels of chlorophyll a and suspended algae in the water, poor water transparency, and extremely high levels of phosphorus in the water column. The data show 3 violations of the Class A1 criterion for pH in 8 samples (38%).Given the lack of data for the current assessment period, the Class A1 (primary contact recreation) uses will remain assessed as "not supported" for the current assessment period.

Note: A TMDL for siltation and nutrients at Lake Darling was prepared by DNR and approved by EPA in 2003. Because all Section 303(d) impairments identified for the 2018 assessment/listing cycle (siltation and nutrients) are addressed by the TMDL, thiswaterbodyis placed in IR Category 4a (impaired; TMDL approved).

The level of inorganic suspended solids was relatively low at Lake Darling, and does not suggest water quality problems due to non-algal turbidity. The median level of inorganic suspended solids in Lake Darling (2.85 mg/L) was ranked 62nd among the 138 lakes by the ISU lake survey.

Data from the 2014-2016 ISU lake survey suggest a moderately large population of cyanobacteria exists at Lake Darling. These data show that cyanobacteria comprised 82% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (20.9 mg/L) was ranked 87th of the 138 lakes sampled.

The Class B(LW) (aquatic life) uses are assessed (evaluated) as "not supported" due to violations of the Class A1,B(LW) criterion for pH. Results of the ISU lake survey from 2014-2016 show there were no violations of the criterion for ammonia in 8 samples(0%), no violations of the criterion for dissolved oxygen in 8 samples(0%), and 3 violations of the criterion for pH in 8 samples(38%). Based on DNR's assessment methodology these violations are significantly greater than 10% of the samples and therefore suggest impairment (not supported/evaluated) of the Class B(LW) uses of Lake Darling.Giventhe lack of data for the current assessment period, the Class B(LW) uses will remain assessed as "partially supported" for the current assessment period.

Note 2: Lake Darling was drawn down in 2009 as part of a restoration project at this lake. It remained drawn down in 2010. Therefore, this will be considered an evaluated assessment, as little data was collected at Lake Darling during the assessment period (2010-2014).