Geode Lake IA 03-SKU-896

Results of IDNR beach monitoring from 2012 through 2014 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 2012 (18 samples), 2013 (17 samples) and 2014 (17 samples), as part of the IDNR beach monitoring program.   According to IDNR’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 2012, 2013 and 2014 were all below the Iowa water quality standard of 126 E.  coli orgs/100 ml.   The geometric mean was 38 E.  coli orgs/100 ml in 2012, 25 E.  coli orgs/100 ml in 2013 and 100 E.  coli orgs/100 ml in 2014.   The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E.  coli orgs/100 ml) was 17% in 2012, 12% in 2013 and 35% in 2014.   The number of samples exceeding the single-sample maximum criterion was significantly greater than 10% in 2014.   According to IDNR's assessment methodology and U.S.  EPA guidelines, these results suggest "partially supported" of the Class A1 uses.

For the 2016 assessment/listing cycle, however, the Class A1 (primary contact recreation) uses of Geode Lake 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 2010-2014 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 55, 55, and 54 respectively for Geode Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Geode 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 low levels of phosphorus in the water column. The data show 5 violations of the Class A1 criterion for pH in 15 samples (33%).

The level of inorganic suspended solids was relatively low at Geode Lake, and does not suggest water quality problems due to non-algal turbidity. The median level of inorganic suspended solids in Geode Lake (2.5 mg/L) was ranked 39th 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 Geode Lake. These data show that cyanobacteria comprised 92% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (14.9 mg/L) was ranked 44th 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 2010-2014 show there were one violation of the criterion for ammonia in 15 samples(7%), no violations of the criterion for dissolved oxygen in 15 samples(0%), and 5 violations of the criterion for pH in 15 samples(33%). Based on IDNR'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 Geode Lake.

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 2010-2014 show that nitrate levels are relatively low at this lake (maximum value = 5.7 mg/l; median = 1.7 mg/l), these data are not sufficient for developing a valid assessment of support of the Class C uses.

Fish consumption uses remain assessed (monitored) as “partially supported” based on results of U.S.EPA/IDNR fish tissue monitoring in 1996, 2006, 2008 and 2009.   The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of support of fish consumption uses in Iowa’s rivers and lakes.   The fish contaminant data generated from the 1996 RAFT sampling conducted at Geode Lake showed that levels of mercury were sufficiently high for concern and thus justified follow-up monitoring.   Results from the 1996 sampling showed that the level of mercury in the sample of largemouth bass fillets (0.348 ppm) exceeded the 1 meal/week trigger level (0.30 ppm) as defined in Iowa’s revised (2007) fish consumption advisory protocol.    According to IDNR’s assessment methodology, the single occurrence of contaminant above an advisory trigger level neither warranted issuance of an advisory nor indicated impairment of the fish consumption uses: two consecutive samplings that show contaminant levels are above the trigger level in fillet samples are needed to justify issuance of an advisory.   Follow-up sampling conducted in 2006 showed that levels of mercury in largemouth bass (0.51 ppm) again exceeded the 1 meal/week trigger level (0.30 ppm).   Thus a one meal per week fish consumption advisory was issued for Geode Lake, and the fish consumption uses were assessed as “partially supported.”  Follow-up sampling was conducted in 2008.   The level of mercury in the sample of largemouth bass fillets (0.267 ppm) was less than the 1 meal/week advisory trigger level (0.30 ppm).   Because the level of mercury was less than the advisory trigger level in 2008, follow-up sampling was conducted again in 2009.   The level of mercury in the sample of largemouth bass fillets (0.437 ppm) was again above the advisory trigger level.   Therefore the fish consumption advisory remains at Geode Lake, and the lake remains assessed as "partially supported" for the 2016 assessment/listing cycle.

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 IDNR and approved by EPA in 2009.   Because not all of the Section 303(d) impairments identified for the 2016 assessment/listing cycle (mercury) are addressed by the TMDL, this waterbody remains in IR Category 5a (impaired; TMDL required) for the 2016 assessment/listing cycles.