Three Mile Lake IA 05-GRA-1371

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. Using the median values from these surveys from 2010-2014 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 62, 61, and 56 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 moderately high 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 data show no violations of the Class A1 criterion for pH in 15 samples.

The level of inorganic suspended solids was moderately high at Three Mile Lake, and does suggest that non-algal turbidity may contribute to the impairment at this lake. The median level of inorganic suspended solids in Three Mile Lake (3 mg/L) was ranked 71st among the 138 lakes by the ISU lake survey.

Data from the 2010-2014 ISU lake survey suggest a moderate population of cyanobacteria exists at Three Mile Lake. These data show that cyanobacteria comprised 87% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (20 mg/L) was ranked 65th of the 138 lakes sampled.

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. Information from the IDNR Fisheries Bureau suggests that water quality has declined since the lake was constructed and the lake has increasing populations of common carp and yellow bass.  The lake is in the beginning stages of a restoration project.  Fish renovation is planned for the fall of 2016 to eradicate the current fish population containing common carp and yellow bass.  The sediment retention area is near capacity in the upper end of the lake and will need to be addressed in the future. Results of the ISU lake survey from 2010-2014 show there were no violations of the criterion for ammonia in 15 samples(0%), 3 violations of the criterion for dissolved oxygen in 15 samples(20%), and no violations of the criterion for pH in 15 samples(0%). Based on IDNR's assessment methodology these violations are not significantly greater than 10% of the samples and therefore suggest (fully supported/monitored) of the Class B(LW) uses of Three Mile Lake. Three Mile Lake was listed as "partially supporting" its Class B(LW) uses due to violations of the state's water quality criteria for dissolved oxygen in the 2014 assessment cycle.  According to IDNR's methodology, a lake must meet water quality standards for two consecutive cycles to suggest de-listing.  Therefore, Three Mile Lake will remain listed for the 2016 cycle as "partially supporting" its Class B(LW) uses due to low dissolved oxygen.   

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 surveys from 2010-2014 show that nitrate levels are relatively low at this lake (maximum value = 2 mg/l; median = 0.2 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.