Diamond Lake IA 03-NSK-861

Assessment Comments

Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results of the Iowa Voluntary Atrazine Monitoring Program from 2008 through 2010 by Syngenta Crop Protection, Inc.

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 remain "not assessed" due to lack of fish contaminant monitoring at this lake.   Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results of the Iowa Voluntary Atrazine Monitoring Program from 2008 through 2010 by Syngenta Crop Protection, Inc.  

EXPLANATION: For the 2012 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 2006 through 2010 (approximately 22 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 63, 61, and 67 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, moderately poor water transparency, and high levels of phosphorus in the water column.  

The levels of inorganic suspended solids at this lake were moderately high and do not suggest an impairment due to non-algal turbidity.   The median level of inorganic suspended solids in Diamond Lake (4.0 mg/L) was the 66th lowest median 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 only 49% of the phytoplankton wet mass at this lake.   The median cyanobacteria wet mass (9.2 mg/L) was the 21st lowest of the 134 lakes sampled.  

Data from the ISU and UHL surveys from 2006-2010 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 22 samples, dissolved oxygen in 22 samples, or pH in 22 samples.   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 2008 through 2010.   This monitoring showed that the time-weighted mean levels of atrazine in samples collected from the Montezuma raw water source in calendar years 2008 through 2010 were well-below the MCL of 3.0 ug/l.   The mean and median atrazine levels over this three-year period (N=99) were 1.3 ug/l and 0.5 ug/L, respectively.   The maximum value was 5.5 ug/l.   None of the 67 moving annual averages for atrazine for the years 2008 through 2010 at Diamond Lake exceeded the MCL (maximum average = 2.5 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 2006-10 show that nitrate levels are very low at this lake (maximum value = 2.4 mg/l; median = 0.3 mg/l) and are well-below the MCL of 10 mg/l.  

Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.