Diamond Lake IA 03-NSK-861

For the 2016 assessment/listing cycle, the Class B(LW) (aquatic life) uses of Diamond Lake are assessed (monitored) as “fully supported” based on information from the ISU lake survey. Using the median values from these surveys from 2010-2014 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 69, 67, and 71 respectively for Diamond Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Diamond Lake 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 very high levels of phosphorus in the water column. The data show no violations of the Class A1 criterion for pH in 15 samples. Although a Secchi depth and chlorophyll a index values of 65 or greater suggests impairment of Class A (primary contact recreation) uses, this water supply reservoir is not designated for Class A (primary contact uses) in the Iowa Water Quality Standards

The level of inorganic suspended solids was high at Diamond Lake, and does suggest that non-algal turbidity contributes to the impairment at this lake. The median level of inorganic suspended solids in Diamond Lake (8 mg/L) was ranked 107th among the 138 lakes by the ISU lake survey.

Data from the 2010-2014 ISU lake survey suggest a moderate population of cyanobacteria exists at Diamond Lake. These data show that cyanobacteria comprised 68% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (19.4 mg/L) was ranked 61st of the 138 lakes sampled.

The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” based on information from the IDNR Fisheries Bureau and results from the ISU lake surveys.   Based on information from the IDNR Fisheries Bureau, common carp are causing major water quality and fish quality problems in this lake. Results of the ISU lake survey from 2010-2014 however, show there were no violations of the criterion for ammonia in 15 samples(0%), one violation of the criterion for dissolved oxygen in 15 samples(7%), and no violations of the criterion for pH in 15 samples(0%). Based on IDNR'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 Diamond Lake.

The Class C (drinking water) uses are assessed (monitored) as “fully supported” based primarily on the results of the Syngenta "Iowa Voluntary Atrazine Monitoring Program" from 2010-2012.   The mean and median atrazine levels over this three-year period were 1.1 ug/l and 0.7 ug/L, respectively.   The maximum value was 4.5 ug/l.   None of the moving annual averages for atrazine for the years 2010 through 2012 at Diamond Lake exceeded the MCL (maximum average = 1.5 ug/l).   Based on DNR's Section 305(b) assessment methodology, if the average contaminant level in source water is less than the MCL, the Class C (drinking water) uses of the source water should be assessed as "fully supported." The only parameter collected as part of the ISU lake surveys relevant to support of Class C (drinking water) uses is nitrate. The results of the ISU surveys from 2010-2014 show that nitrate levels are low at this lake (maximum value = 2.8 mg/l; median = 0.2 mg/l) and are well-below the MCL of 10 mg/l.  

Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S.   EPA/IDNR fish contaminant (RAFT) monitoring at Diamond Lake in 2013.    The composite samples of fillets from largemouth bass and channel catfish had low levels of contaminants.    Levels of primary contaminants in the composite sample of largemouth bass fillets were as follows: mercury: 0.124 ppm.    Levels of primary contaminants in the composite sample of channel catfish fillets were as follows:  total PCBs: <0.6 ppm; and technical chlordane: <0.2 ppm.    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 2013 RAFT sampling conducted at this 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.