Geode Lake IA 03-SKU-896

Results of DNR beach monitoring from 2014 through 2016 suggest that the Class A1 uses are "partially supported." Levels of indicator bacteria at Geode Lake Beach were monitored once per week during the primary contact recreation seasons (May through September) of 2014 (17 samples), 2015 (19 samples) and 2016 (15 samples), as part of the DNR beach monitoring program. According to DNR’s assessment methodology two conditions need to be met for results of beach monitoring to indicate “full support” of the Class A1 (primary contact recreation) uses: (1) the geometric mean of the samples from each recreation season of the three-year assessment period are less than the state's geometric mean criterion of 126 E. coli orgs/100 ml and (2) not more than 10% of the samples during any one recreation season exceeds the state's single-sample maximum value of 235 E. coli orgs/100 ml. If a sampling season geometric mean exceeds the state criterion of 1000 orgs/100 ml during the three-year assessment period, the Class A1 uses should be assessed as “not supported.” Also, if a sampling season geometric mean exceeds the state criterion of 126 orgs/100 ml during the three-year assessment period and/or if significantly more than 10% of the samples in any one of the three recreation seasons exceed Iowa's single-sample maximum value of 235 E. coli orgs/100 ml, the Class A1 uses should be assessed as “partially supported.” This assessment approach is based on U.S. EPA guidelines (see pgs 3-33 to 3-35 of U.S. EPA 1997b).

At Geode Lake Beach, the geometric means from 2014 and 2016 were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean from 2015, however, was greater than the Iowa water quality standard of 126 E. coli orgs/100ml and therefore suggests impairment of the Class A1 uses. The geometric mean was 100 E. coli orgs/100 ml in 2014, 300 E. coli orgs/100 ml in 2015 and 42 E. coli orgs/100 ml in 2016. The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E. coli orgs/100 ml) was 35% in 2014, 58% in 2015 and 20% in 2016. The number of samples exceeding the single-sample maximum criterion was significantly greater than 10% in 2014 and 2015. According to DNR's assessment methodology and U.S. EPA guidelines, these results suggest "partially supported" of the Class A1 uses.

For the 2018 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Lake Geode are assessed (monitored) as "not supported" due to frequent violations of the state criterion for pH 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 55, 53, and 55 respectively for Lake Geode. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Lake Geode in the Eutrophic category. These values suggest 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 data show 4 violations of the Class A1 criterion for pH in 15 samples (27%).

The level of inorganic suspended solids was relatively low at Lake Geode, and does not suggest water quality problems due to non-algal turbidity. The median level of inorganic suspended solids in Lake Geode (2 mg/L) was ranked 45th 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 Lake Geode. These data show that cyanobacteria comprised 77% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (10.1 mg/L) was ranked 46th 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 pH. Results of the ISU lake survey from 2012-2016 show there were no violations of the criterion for ammonia in 15 samples(0%), no violations of the criterion for dissolved oxygen in 15 samples(0%), and 4 violations of the criterion for pH in 15 samples(27%). 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 Lake Geode.

The Class C (drinking water) uses are not assessed due to the lack of recent information upon which to base an assessment. The only parameter collected as part of the ISU lake surveys relevant to support of Class C (drinking water) uses is nitrate. While the results of the ISU surveys from 2012-2016 show that nitrate levels are moderately high at this lake (maximum value = 5.7 mg/l; median = 2.1 mg/l), these data are not sufficient for developing a valid assessment of support of the Class C uses.

In addition to fish tissue, turtle tissue from Geode Lake was also analyzed for levels of primary contaminants. The composite samples of shoulder tissue from snapping turtle had low levels of contaminants. Levels of primary contaminants in the composite sample of snapping turtle shoulder tissue were as follows: mercury: 0.284 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm.

Note: A TMDL for bacteria and pH at Geode Lake was prepared by DNR and approved by EPA in 2009. Because not all of the Section 303(d) impairments identified for the 2018 assessment/listing cycle (mercury) are addressed by the TMDL, this waterbody remains in IR Category 5a (impaired; TMDL required) for the 2018 assessment/listing cycles.