Assessment Comments
Assessment is based on: (1) the results of the IDNR-UHL beach monitoring program in summers of 2006, 2007, and 2008, (2) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 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, 2006, and 2008.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to high levels of bacteria that exceed the Class A1 standard. High levels of pH that exceed the state water quality criterion also contribute to an 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” due to the issuance of a fish consumption advisory at this lake. Sources of data for this assessment include (1) the results of the IDNR-UHL beach monitoring program in summers of 2006, 2007, and 2008, (2) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 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, 2006, and 2008.
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 2010 assessment/listing cycle (mercury) are addressed by the TMDL, this waterbody remains in IR Category 5a (impaired; TMDL required) for the 2010 assessment/listing cycles.
EXPLANATION: Results of IDNR beach monitoring at Lake Geode from 2006 through 2008 suggest that the Class A1 uses should be assessed (monitored) as “not supported.” Levels of indicator bacteria were monitored once per week during the primary contact recreation seasons (May through September) of 2006 (28 samples), 2007 (19 samples), and 2008 (25 samples) as part of the IDNR beach monitoring program. According to IDNR’s assessment methodology, all thirty-day geometric means for the three-year assessment period must be less than the state’s geometric mean criterion of 126 E. coli orgs/100 ml for results of beach monitoring to indicate “full support” of the Class A1 (primary contact recreation) uses. 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”. 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 mean of one of the 60 thirty-day periods during the summer recreation seasons of 2006, 2007 and 2008 exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml. The percentage of samples exceeding the one-time maximum criteria of 235 orgs/100 ml was 0% in 2006, 21% in 2007, and 24% in 2008. The violation of the geometric mean criterion suggests impairment (nonsupport/monitored) of the Class A1 uses of Lake Geode.
The impairment for indicator bacteria was suggested for delisting in 2008 following 4 years with no violations of the geometric mean criterion, however, recent violations suggest it should be impaired for bacteria in 2010.
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 2010 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 2004 through 2008 (approximately 25 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 48, 56, and 59 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 relatively low levels of chlorophyll a and suspended algae in the water, exceptional water transparency, and relatively 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 2004 through 2008, the median concentration of inorganic suspended solids at Lake Geode was 2.0 mg/L, which was the 26th lowest concentration of the 132 lakes monitored by these programs.
Data from the 2004-2008 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at Lake Geode. These data show that cyanobacteria comprised 90% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (13.5 mg/L) was the 65th lowest 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. The ISU and UHL lake surveys data from 2004-2008 show no violations of the Class B(LW) criteria for ammonia in 25 samples or dissolved oxygen in 25 samples. The data, however, show 11 violations of the criterion for pH in 25 samples (44%). 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. Information from the IDNR Fisheries Bureau suggests that nutrients and short-lived algae blooms are a concern at this lake, but are being addressed by a watershed and lake restoration project.
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 2004-08 show that nitrate levels at this lake are low relative to the drinking water MCL (10 mg/l) (maximum sample value = 4.3 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, 2006, and 2008. 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 mercury are sufficiently high for concern and thus justify 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 Lake Geode and the fish consumption uses were assessed as “partially supported.” Follow-up fish tissue monitoring is now conducted every other year at Lake Geode. 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). According to IDNR's assessment methodology, two consecutive samples that are less than the 1 meal/week advisory trigger level are necessary to suggest removal of a fish consumption advisory and therefore suggest "full support" of the fish consumption uses. Follow-up monitoring will be conducted in 2010 to confirm that levels of mercury are below the advisory trigger level.
