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" conducted by Syngenta Co. from 2008 through 2012, 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 “partially supported” (IR 5a) due to violations of the criterion for dissolved oxygen. [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" (IR 2a) primarily based on results of Syngenta-sponsored monitoring. Fish consumption uses remain assessed (monitored) as “fully supported” (IR 2a) 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 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 Syngenta-sponsored monitoring for atrazine from 2010 through 2012, 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 “partially supported” due to violations of the Class B(LW) criterion for dissolved oxygen. Results of monitoirng to measure water clarity, however, suggest relatively good water quality conditions at this lake. Using the median values from monitoring surveys from 2006 through 2010 (approximately 22 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 62, 60, and 57 respectively for Three Mile Lake. According to Carlson (1977) the total phosphorus value places Three Mile Lake in the eutrophic category, while the chlorophyll a and Secchi depth values place Three Mile Lake in between the eutrophic and hypereutrophic categories. These values suggest relatively low levels of chlorophyll a and suspended algae in the water, moderately poor 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 3.1 mg/L, which ranked 60th of the 134 monitored lakes.
Data from the 2008-2012 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 81% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (18.9 mg/L) and ranked 70th of the 134 lakes sampled.
The Class B(LW) (aquatic life uses) are assessed (monitored) as "partially supported." Results from the ISU and UHL lake surveys conducted in 2008-2012 show no violations of the Class B(LW) criterion for ammonia in 14 samples, and no violations for pH in 14 samples. There were, however, four violations of the dissolved oxygen criterion in 14 samples (29%). Based on IDNR’s assessment methodology, the violation frequency for dissolved oxygen is significantly greater than 10%, and therefore suggests an impairment of the Class B(LW) uses at Three Mile Lake. Information from the IDNR Fisheries Bureau 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 -sponsored monitoring for atrazine from 2010 through 2012. This monitoring showed that only two of the 102 samples collected (2%) exceeded the Class C criterion of 3 ug/l. Also, none of the 102 moving annual averages for atrazine for the years 2010 through 2012 at Three Mile Lake exceeded the MCL (maximum annual average = 1.8 ug/l). Based on IDNR's assessment methodology, if the average levels in source water are 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 remain 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 was conducted in 2011 and suggests "full support" of fish consumption uses at Threemile Lake. The composite samples of fillets from channel catfish had low levels of walleye fillets were as follows: mercury: 0.120 ppm, which is below the advisory threshold.
