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 2002 through May 2005 by Iowa State University (ISU), (2) results of the "Iowa Voluntary Atrazine Monitoring Program" done by Syngenta Co. in 2004 and 2005, and (3) information from the IDNR Fisheries Bureau.

Basis for Assessment

SUMMARY:  The Class A1 (primary contact recreation) uses are assessed (evaluated) as “fully supported.”  The Class B(LW) (aquatic life) uses are assessed (evaluated) as “fully supported.”  The Class C (drinking water) uses are assessed (evaluated) as "fully supported."  Fish consumption uses are “not assessed” due to a lack of recent information upon which to base an assessment.   Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2002 through May 2005 by Iowa State University (ISU), (2) results of the "Iowa Voluntary Atrazine Monitoring Program" done by Syngenta Co.  in 2004 and 2005, and (3) information from the IDNR Fisheries Bureau.   Note: Twelve Mile Lake was drawn down in 2005 and 2006 to remove yellow bass from the lake and renovate the fishery.

EXPLANATION:  The Class A1 (primary contact recreation) uses are assessed (evaluated) as “fully supported.”  Results from the ISU lake survey show generally good water quality at Twelve Mile Lake.   Using the median values from this survey from 2002 through May 2005 (approximately 10 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 58, 59, and 57 respectively for Twelve Mile Lake.   According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Twelve 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 also 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 2.8 mg/L, which was the 37th lowest of the 132 monitored lakes.

Data from the 2002-2006 ISU survey suggest a relatively small population of cyanobacteria exists at Twelve Mile Lake, which does not suggest an impairment of the uses of the lake.   These data show that cyanobacteria comprised 99% of the phytoplankton wet mass at this lake.   However, the median cyanobacteria wet mass (13.9 mg/L) was the 54th lowest of the 132 lakes sampled.  

Results from the ISU lake survey show good chemical water quality at Twelve Mile Lake.   During 2002-2005 there were no violations of the Class B(LW) criterion for ammonia in 3 samples, or dissolved oxygen in 9 samples, and one violation for pH in 10 samples (10%).   Based on IDNR’s assessment methodology this one violation is not significantly greater than 10% of the samples and therefore does not suggest an impairment of the Class B(LW) uses at 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 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 lake survey from 2002-05 show that nitrate levels are very low at this lake (maximum value = 1.7 mg/l; median = 0.7 mg/l) relative to the MCL (10 mg/L).  

Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.