Three Mile Lake IA 05-GRA-1371

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, (4) results of the "Iowa Voluntary Atrazine Monitoring Program" conducted by Syngenta Co. in 2008, 2009, and 2010 and (5) results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 2010.

Basis for Assessment

SUMMARY:  The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.” [Note: this lake is not designated for Class A1 (primary contact recreation) uses in the Iowa Water Quality Standards.]  The Class C (drinking water) uses are assessed (monitored) as "fully supported" primarily based on results of the Syngenta Iowa Voluntary Atrazine Monitoring Program.   Fish consumption uses are assessed (monitored) as “fully supported” based on results of fish contaminant monitoring in 2010.   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, (4) results of the "Iowa Voluntary Atrazine Monitoring Program" conducted by Syngenta Co.  in 2008, 2009, and 2010 and (5) results of U.S.  EPA/IDNR fish contaminant (RAFT) monitoring in 2010.

EXPLANATION:  The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.”  Results from the ISU and UHL lake surveys show continued good water quality at Three Mile Lake.   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 60, 59, and 57 respectively for Three Mile Lake.   According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Three Mile Lake in the eutrophic category.   These values suggest relatively low levels of chlorophyll a and suspended algae in the water, relatively good water transparency, and relatively low levels of phosphorus in the water column.  

The level of inorganic suspended solids is moderately high at Three Mile Lake and suggests that non-algal turbidity does not cause water quality problems at this lake.   The median inorganic suspended solids concentration at Three Mile Lake was 3.0 mg/L, which was the 58th lowest of the 134 monitored lakes.

Data from the 2006-2010 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at Three Mile Lake, which does not suggest an impairment of the uses of the lake.   These data show that cyanobacteria comprised 61% of the phytoplankton wet mass at this lake.   The median cyanobacteria wet mass (21.6 mg/L) was the 62nd highest of the 134 lakes sampled.  

The Class B(LW) (aquatic life uses) are assessed (monitored) as "fully supported."  Results from the ISU and UHL lake surveys show relatively good chemical water quality at Three Mile Lake.   During 2006-2010 there were no violations of the Class B(LW) criterion for ammonia in 22 samples, and one violation for pH in 22 samples (5%).   There were 4 violations of the dissolved oxygen criterion in 22 samples (18%).   Based on IDNR’s assessment methodology these violations are not significantly greater than 10% of the samples and therefore do not suggest an impairment of the Class B(LW) uses at Three Mile Lake.   Information from the IDNR Fisheries Bureau, however, suggests that algae blooms are becoming more severe and rooted aquatic plants are decreasing likely due to common carp.  

The Class C (drinking water) uses are assessed (monitored) as "fully supported" primarily based 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 the samples collected in 2008, 2009, and 2010 were below the MCL of 3.0 ug/L.   The mean and median atrazine levels over this three year period (N=97) were 1.2 and 1.0 ug/L, respectively.   Also, none of the 63 moving annual averages for atrazine for the years 2006 through 2008 at Three Mile Lake exceeded the MCL (maximum average = 1.3 ug/L).   Based on IDNR's assessment methodology, if the average contaminant level in source water is less than the MCL, the Class C (drinking water) uses should be assessed as "fully supported."  In addition, results of the ISU and UHL surveys from 2006-10 show that nitrate levels are low at this lake (maximum value = 4.3 mg/l; median = 0.3 mg/l) relative to the MCL (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 Threemile Lake in 2010.   The composite samples of fillets from channel catfish had low levels of contaminants.   Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: 0.189 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm.   Levels of mercury in the composite sample of walleye fillets (0.653 ppm), however, exceeded the advisory trigger level.   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.    Based on
IDNR/IDPH protocol, two samples with mercury levels greater than the advisory level are necessary to suggest the issuance of a fish consumption advisory.   The fish contaminant data generated from the 2010 RAFT sampling suggest that mercury levels are high and justify follow-up sampling at this lake to determine if an advisory is justified.   Follow-up sampling will be conducted in 2011.