Twelve Mile Creek Lake IA 05-GRA-1367

For the 2018 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Twelve Mile Lake are assessed (monitored) as "fully supported" based on information from the ISU lake survey. Using the median values from these surveys from 2012-2016 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 62, 60, and 73 respectively for Twelve Mile Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Twelve Mile Lake in between the Eutrophic and the Hypereutrophic categories. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and very high levels of phosphorus in the water column. The data show one violation of the Class A1 criterion for pH in 22 samples (5%).

The level of inorganic suspended solids was relatively low at Twelve Mile Lake, and does not suggest water quality problems due to non-algal turbidity. The median level of inorganic suspended solids in Twelve Mile Lake (2.7 mg/L) was ranked 57th among the 138 lakes by the ISU lake survey.

Data from the 2012-2016 ISU lake survey suggest a moderate population of cyanobacteria exists at Twelve Mile Lake. These data show that cyanobacteria comprised 90% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (15.8 mg/L) was ranked 64th of the 138 lakes sampled.

The Class B(LW) (aquatic life) uses are assessed (monitored) as "not supported" due to violations of the Class A1,B(LW) criterion for DO. Results of the ISU lake survey from 2012-2016 show there were no violations of the criterion for ammonia in 20 samples(0%), 6 violations of the criterion for dissolved oxygen in 22 samples(27%), and one violation of the criterion for pH in 22 samples(5%). Based on DNR's assessment methodology these violations are significantly greater than 10% of the samples and therefore suggest impairment (not supported/monitored) of the Class B(LW) uses of Twelve Mile Lake.

The Class C (drinking water) uses are assessed (evaluated) as "fully supported" based on results of the "Iowa Voluntary Atrazine Monitoring Program" in 2004 and 2005. Because only one and half years of data were collected due to the lake drawdown at Twelve Mile Lake, the assessment is considered "evaluated" (lower confidence) rather than "monitored" (higher confidence). Data from this program showed that none of the 20 moving annual averages for 2004 and 2005 exceeded the MCL of 3.0 ug/L (maximum = 1.5 ug/L). The mean and median atrazine level over this two year period (N=53) was 1.4 ug/L. The maximum value for this period was 2.9 ug/L. Based on DNR's assessment methodology, if the average contaminant level in source water is less than the MCL, the Class C uses of the source water should be assessed as "fully supported."In addition,the results of the ISU surveys from 2012-2016 show that nitrate levels are extremely low at this lake (maximum value = 1.7 mg/l; median = 0.1 mg/l).

Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S. EPA/DNR fish contaminant (RAFT) monitoring at Twelve Mile Lake in 2007. The composite samples of fillets from channel catfish and largemouth bass had low levels of contaminants.Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: 0.088 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm. Levels of primary contaminants in the composite sample of largemouth bass fillets were as follows: mercury: 0.249 ppm. 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. The fish contaminant data generated from the 2007 RAFT sampling conducted at this lake show that the levels of contaminants do not exceed any of the advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.