Assessment Comments
Assessment is based on: (1) the results of the IDNR-UHL beach monitoring program in summers of 2004, 2005, and 2006, (2) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, and (5) results of U.S. EPA/IDNR fish tissue monitoring in 1996 and 2006.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially supported” due to high levels of pH that exceed the state water quality criterion. Bacteria levels that exceed the state's water quality criteria also contribute to the impairment at this lake. The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” due to high levels of pH that exceed the state water quality criterion. The Class C (drinking water) uses remain “not assessed” due to a lack of information upon which to base an assessment. Fish consumption uses are assessed (monitored) as “partially supported.” Sources of data for this assessment include (1) the results of the IDNR-UHL beach monitoring program in summers of 2004, 2005, and 2006, (2) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, and (5) results of U.S. EPA/IDNR fish tissue monitoring in 1996 and 2006.
EXPLANATION: Results of IDNR beach monitoring at Lake Geode from 2004 through 2006 suggest that the Class A1 uses should be assessed (monitored) as “fully supported.” Levels of indicator bacteria were monitored once per week during the primary contact recreation seasons (May through September) of 2004 (22 samples), 2005 (23 samples), and 2006 (28 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) all thirty-day geometric means for 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 5-sample, 30-day geometric mean exceeds the state criterion of 126 orgs/100 ml during the three-year assessment period, the Class A1 uses should be assessed as “not supported.” Also, if 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 Lake Geode beach, the geometric means of all 61 thirty-day periods during the summer recreation seasons of 2004, 2005 and 2006 were below the Iowa water quality standard of 126 E. coli orgs/100 ml. These results suggest generally low levels of indicator bacteria at this lake. Also, the number of samples exceeding the one-time maximum criteria of 235 orgs/100 ml was below 10% for all years (2004: 9%, 2005: 0%, 2006: 0%). Thus, these results do not suggest an impairment of the Class A1 uses of Lake Geode.
Lake Geode was assessed as “not supporting” the Class A1 uses in the 2006 assessment/listing cycle due to violations of the geometric mean criterion for indicator bacteria in 2003. Based on IDNR's assessment methodology, two consecutive assessment/listing cycles are necessary to suggest de-listing of the bacteria impairment. Therefore, the Class A1 uses will remain assessed as "partially supported" for the 2008 assessment/listing cycle.
Results from the ISU lake survey and the UHL ambient lake monitoring program suggest that the Class A1 (primary contact recreation) uses for Lake Geode be assessed (monitored) as “partially supported” for the 2008 reporting cycle due to high levels of pH. Other than high pH, the ISU and UHL data show exceptional water quality for Lake Geode. Using the median values from these surveys from 2002 through 2006 (approximately 23 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 46, 55, and 53 respectively for Lake Geode. According to Carlson (1977) the Secchi depth value places Lake Geode in the mesotrophic category, while the chlorophyll a and total phosphorus values place Lake Geode in the eutrophic category. These values suggest low levels of chlorophyll a and suspended algae in the water, exceptional water transparency, and low levels of phosphorus in the water column.
The level of inorganic suspended solids was low at Lake Geode and does not suggest an impairment due to non-algal turbidity. Based on data from the ISU and UHL surveys in 2002 through 2006, the median concentration of inorganic suspended solids at Lake Geode was 1.3 mg/L, which was the 6th lowest concentration of the 132 lakes monitored by these programs.
Data from the 2002-2006 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at Lake Geode. These data show that cyanobacteria comprised 84% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (16.6 mg/L) was the 71st highest of the 132 lakes sampled. These moderate levels of cyanobacteria at Lake Geode do not suggest an impairment of the Class A1 uses due to nuisance algae blooms, but may contribute to the pH impairment at this lake.
The Class B(LW) (aquatic life) uses are assessed as “partially supported” based on results from the ISU and UHL lake surveys, and results of physical/chemical monitoring associated with IDNR’s beach monitoring program. The ISU and UHL lake surveys data from 2002-2006 show no violations of the Class B(LW) criteria for ammonia in 16 samples or dissolved oxygen in 21 samples. The data, however, show 6 violations of the criterion for pH in 22 samples (27%). Based on IDNR’s assessment methodology, these violations are significantly greater than 10% of the samples and therefore suggest an impairment (partial support/monitored) of the Class A1,B(LW) uses at this lake. Results of physical/chemical monitoring as part of IDNR’s beach monitoring program also suggest that Lake Geode be assessed as “partially supported.” Data from the beach monitoring program from 2004 through 2006 show 2 violations of the Class B(LW) criterion for dissolved oxygen in 69 samples (3%). These two violations are less than 10% of the samples and therefore do not constitute an impairment of the Class B(LW) uses of Lake Geode. The pH data from the beach monitoring program (2004-2006) show 13 violations of Iowa’s criterion for pH in 69 samples (19%). These violations are significantly greater than 10% of the samples and therefore support the impairment (partial support/monitored) of the Class A1,B(LW) uses of Lake Geode.
The Class C (drinking water) uses remain "not assessed" due to the lack of water quality information. The only parameter collected as part of the ISU and UHL lake surveys relevant to support of Class C (drinking water) uses is nitrate. While the results of these surveys from 2002-06 show that nitrate levels at this lake are low relative to the drinking water MCL (10 mg/l) (maximum sample value = 4.2 mg/l; median = 1.6 mg/l), these data are not sufficient for developing a valid assessment of the Class C uses.
Fish consumption uses were assessed (monitored) as “partially supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Lake Geode in 1996 and 2006. 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 Lake Geode show that levels of contaminant are sufficiently high for concern and thus justify follow-up monitoring. Results from the 1996 sampling show that the level of mercury in the sample of largemouth bass fillets (0.348 ppm) exceeds 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 warrants issuance of an advisory nor indicates 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 Lake Geode and the fish consumption uses are assessed as “partially supported.”
