Lake Darling IA 03-SKU-924

Results of IDNR beach monitoring from 2014 suggest that the Class A1 uses are "Fully 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), 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 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 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.   The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E.  coli orgs/100 ml) was 33% in 2014.   None of these are significantly greater than 10% of the samples and therefore do not suggest impairment of the Class A1 uses.   According to IDNR's assessment methodology and U.S.  EPA guidelines, these results suggest "Fully Supported" of the Class A1 uses.

For the 2016 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Lake Darling are assessed (evaluated) as "not supported" due to 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. However, due to the sampling size, this data can only be used for evaluation and not impairment according to the 2016 assessment methodology Using the median values from these surveys from 2014 (approximately 2 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 60, 69, and 88 respectively for Lake Darling. According to IDNR methodology, 9 samples during the assessment period are required for a "monitored" assessment.  Because of the restoration activities, annual monitoring was suspended. 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, moderately poor water transparency, and extremely high levels of phosphorus in the water column. The data show 1 violations of the Class A1 criterion for pH in 2 samples (50%). 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 IDNR and approved by EPA in 2003.   Because all Section 303(d) impairments identified for the 2016 assessment/listing cycle (siltation and nutrients) are addressed by the TMDL, this waterbody is 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 (2.025 mg/L) was ranked 34th among the 138 lakes by the ISU lake survey.

Data from the 2014-2014 ISU lake survey suggest a moderately large population of cyanobacteria exists at Lake Darling. These data show that cyanobacteria comprised 68% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (33.7 mg/L) was ranked 101st 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 2014-2014 show there were no violations of the criterion for ammonia in 2 samples(0%), no violations of the criterion for dissolved oxygen in 2 samples(0%), and one violation of the criterion for pH in 2 samples(50%). Based on IDNR'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.  Lake Darling was listed as "partially supporting" its Class B(LW) uses in 2012 for pH.  Given the lack of data for the current assessment period, the Class B(LW) uses will remain assessed as "partially supported" for the current assessment period.

The Class C (drinking water) uses are not assessed due to the lack of recent information upon which to base an assessment. The only parameter collected as part of the ISU lake surveys relevant to support of Class C (drinking water) uses is nitrate. While the results of the ISU surveys from 2014-2014 show that nitrate levels are low at this lake (maximum value = 0.3 mg/l; median = 0.2 mg/l), these data are not sufficient for developing a valid assessment of support of the Class C uses.

Fish consumption uses were “not assessed” due to a lack of recent fish contaminant monitoring at this lake.    The most recent fish tissue monitoring was conducted in 1997.    While these results suggest that levels of contaminants were low at Lake Darling, they are now too old (greater than 10 years) to be used for an assessment.

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).