Three Mile Lake IA 05-GRA-1371

Assessment Comments

Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 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 2004, 2005, and 2006, and (5) results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 2000.

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 (evaluated) as “fully supported” based on results of fish contaminant monitoring in 2000.   Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results of U.S.  EPA/IDNR fish contaminant (RAFT) monitoring in 2000.

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 2002 through 2006 (approximately 23 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 57, 58, 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 3.2 mg/L, which was the 53rd lowest of the 132 monitored lakes.

Data from the 2002-2006 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 71% of the phytoplankton wet mass at this lake.   The median cyanobacteria wet mass (17.0 mg/L) was the 69th highest of the 132 lakes sampled.  

Results from the ISU and UHL lake surveys show relatively good chemical water quality at Three Mile Lake.   During 2002-2006 there were no violations of the Class B(LW) criterion for ammonia in 17 samples, and one violation for dissolved oxygen in 22 samples (5%).   There were 3 violations of the pH criterion in 23 samples (13%).   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.  

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 2004 through 2006.   This monitoring showed that the time-weighted mean levels of atrazine in the samples collected in 2004, 2005, and 2006 were below the MCL of 3.0 ug/L.   The mean and median atrazine levels over this three year period (N=100) were both 1.3 ug/L.   Also, none of the 69 moving annual averages for atrazine for the years 2004 through 2006 at Three Mile Lake exceeded the MCL (maximum average = 1.4 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 2002-06 show that nitrate levels are low at this lake (maximum value = 4.3 mg/l; median = 0.2 mg/l) relative to the MCL (10 mg/L).    

Fish consumption uses were assessed (evaluated) as “fully supported” based on results of U.S.  EPA/IDNR fish contaminant (RAFT) monitoring at Three Mile Lake in 2000.   Because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).   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.   Results from the RAFT monitoring showed that the composite samples of fillets from channel catfish and walleye had low levels of contaminants.  The fish contaminant data generated from the 2000 RAFT sampling conducted at Three Mile Lake showed that the levels of contaminants do not exceed any of the advisory trigger levels, thus suggesting no justification for issuance of a consumption advisory for this waterbody.