Diamond Lake IA 03-NSK-861

Assessment Comments

Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2009-2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 2008 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, (4) results of the Iowa Voluntary Atrazine Monitoring Program from 2010-2012 by Syngenta Crop Protection, Inc, and (5) IDNR RAFT fish contaminant monitoring.

Basis for Assessment

SUMMARY:  The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported".   The Class C (drinking water) uses are assessed as “fully supported.”   Fish consumption uses are asssessed as "fully supporting".   Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2009-2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 2008 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results of the Iowa Voluntary Atrazine Monitoring Program from 2010-2012 by Syngenta Crop Protection, Inc.  

EXPLANATION: For the 2014 reporting cycle, the Class B(LW) (aquatic life) uses for Diamond Lake are assessed as “fully supported” based on results from the ISU statewide survey of lakes and the UHL ambient lake monitoring program.   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 67, 62, and 70 respectively for Diamond Lake.   According to Carlson (1977) the index values for Secchi depth, chlorophyll a, and total phosphorus all place Diamond Lake in between the eutrophic and hypereutrophic categories.   These values suggest moderately high levels of chlorophyll a and suspended algae in the water, poor water transparency, and high levels of phosphorus in the water column.   Althogh a Secchi depth index value 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 levels of inorganic suspended solids at this lake were moderately high but do not suggest impairment of the aquatic life uses.   The median level of inorganic suspended solids in Diamond Lake (5.8 mg/L) and ranked 86th of the 134 lakes sampled by the ISU and UHL surveys.  

Data from the ISU and UHL surveys suggest that cyanobacteria populations were low at Diamond Lake and do not suggest problems with nuisance algal blooms.   These data show that cyanobacteria comprised 78% of the phytoplankton wet mass at this lake.   The median cyanobacteria wet mass (14.8 mg/L) and ranked 50th of the 134 lakes sampled.  

Data from the ISU and UHL surveys from 2008-2012 suggest relatively good chemical water quality at Diamond Lake.   Results from these surveys show no violations of the Class B(LW) criterion for ammonia in 14 samples or pH in 14 samples.   Results show one violation for dissolved oxygen in 14 samples (7%).  According to IDNR methodology, significantly greater than 10% of the samples must violate the criteria for pH to consititute an impairment.  Thus, these results suggest "full support" of the Class B(LW) uses at 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 and UHL lake surveys relevant to support of Class C uses is nitrate.   The results of these surveys from 2008-2010 show that nitrate levels are very low at this lake (maximum value = 2.4 mg/l; median = 0.4 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.