Twelve Mile Creek Lake IA 05-GRA-1367

Assessment Comments

Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2007 through 2010 by Iowa State University (ISU), (2) results of the UHL ambient lake monitoring program in 2007 and 2008, (3) results of the "Iowa Voluntary Atrazine Monitoring Program" done by Syngenta Co. in 2004 and 2005, (4) information from the IDNR Fisheries Bureau, and (5) results of U.S. EPA/IDNR fish tissue monitoring in 2007.

Basis for Assessment

SUMMARY:  The Class A1 (primary contact recreation) uses are assessed (monitored) as “fully supported.”  The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.”  The Class C (drinking water) uses are assessed (evaluated) as "fully supported."  Fish consumption uses are assessed (monitored) as "fully supported."  Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2007 through 2010 by Iowa State University (ISU), (2) results of the UHL ambient lake monitoring program in 2007 and 2008, (3) results of the "Iowa Voluntary Atrazine Monitoring Program" done by Syngenta Co.  in 2004 and 2005, (4) information from the IDNR Fisheries Bureau, and (5) results of U.S.  EPA/IDNR fish tissue monitoring in 2007.

EXPLANATION:  The Class A1 (primary contact recreation) uses are assessed (monitored) as “fully supported.”  Results from the ISU and UHL lake surveys show generally good water quality at Twelve Mile Lake.   Using the median values from this survey from 2007 through 2010 (approximately 11 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 57, 61, and 68 respectively for Twelve Mile Lake.   According to Carlson (1977) the Secchi depth value places Twelve Mile Lake in the eutrophic category, while the chlorophyll a and total phosphorus values place Twelve Mile Lake in between the eutrophic and hypereutrophic categories.   These values suggest moderately high levels of chlorophyll a and suspended algae in the water, relatively good water transparency, and high levels of phosphorus in the water column.  

The level of inorganic suspended solids is relatively low at Twelve Mile Lake and suggests that non-algal turbidity does not cause water quality problems at this lake.   The median inorganic suspended solids concentration at Twelve Mile Lake was 2.0 mg/L, which was the 42nd lowest of the 134 monitored lakes.

Data from the 2006-2010 ISU and UHL surveys suggest a moderately small population of cyanobacteria exists at Twelve Mile Lake, which does not suggest an impairment due to nuisance aquatic life.   These data show that cyanobacteria comprised 87% of the phytoplankton wet mass at this lake.   However, the median cyanobacteria wet mass (34.6 mg/L) was the 40th highest of the 134 lakes sampled.  

Results from the ISU and UHL lake surveys show good chemical water quality at Twelve Mile Lake.   During 2007-2010 there were no violations of the Class B(LW) criterion for ammonia in 11 samples, the Class B(LW) criterion for dissolved oxygen in 11 samples, or the Class A1,B(LW) criterion for pH in 11 samples.   Based on IDNR’s assessment methodology these results suggest "full support" of the Class B(LW) uses of Twelve Mile Lake.   Information from the IDNR Fisheries Bureau, however, suggests that algae blooms are a concern at this 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 IDNR'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, results of the ISU and UHL lake surveys from 2007-10 show that nitrate levels are low at this lake (maximum value = 2.6 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 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.