Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), (3) results of the IDNR-UHL beach monitoring program in 2006, 2007, and 2008, (4) information from the IDNR Fisheries Bureau, and (5) results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 2004.
Basis for Assessment
[Note: For the 2006, 2008, and 2010 Section 305(b)/303(d) cycle, West Okoboji Lake was subdivided into the following segments for purposes of developing water quality assessments: main lake basin south, Emersons Bay, Smiths Bay, Millers Bay, and main lake basin north. These segments were created to better use the water quality data collected as part of the IDNR beach monitoring program which includes monitoring at four of this lake’s beaches. Developing separate assessments for the main lake basin as well as for the Lake’s bays represents a more accurate characterization of water quality conditions than did past assessments which developed only one assessment for the entire lake.]
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" due to levels of indicator bacteria that violate the state’s water quality standard for indicator bacteria. The Class B(LW) aquatic life uses are assessed (monitored) as “fully supported.” The Class C (drinking water) uses "not assessed" due to a lack of recent data upon which to base an assessment. Fish consumption uses are assessed (monitored) as “fully supported.” Sources of data for this assessment include: (1) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), (3) results of the IDNR-UHL beach monitoring program in 2006, 2007, and 2008, (4) information from the IDNR Fisheries Bureau, and (5) results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 2004.
EXPLANATION: Results of IDNR beach monitoring from 2006 through 2008 suggest that the Class A1 uses are "not supported." Levels of indicator bacteria at Emerson Bay beach on West Okoboji Lake were monitored once per week during the primary contact recreation seasons (May through September) of 2006 (28 samples), 2007 (15 samples), and 2008 (27 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 Emerson Bay beach, the geometric means of 3 thirty-day periods during the summer recreation season of 2006 exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 11% in 2006, 7% in 2007 and 15% in 2008. According to IDNR’s assessment methodology and U.S. EPA guidelines, the exceedences of the geometric mean criterion suggest impairment (nonsupport) of the Class A1 (primary contact recreation) uses.
West Okoboji Lake near Emerson Bay Beach was sampled as part of IDNR’s Safe Lakes Program, which aims to identify sources of bacteria to selected beaches where bacteria levels have consistently violated the state water quality criteria. The Safe Lakes Program identified the likely source of bacteria to be human waste from a mobile home park. The stream that carried this waste was diverted in the fall of 2006 in an attempt to keep bacteria from reaching the beach. This is expected to continue to decrease bacteria levels at this beach for future assessment/listing cycles.
Results from the ISU and UHL lake surveys suggest that the Class A1 (primary contact recreation) uses at West Okoboji Lake are “fully supported.” 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 38, 42, and 48 respectively for West Okoboji Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place West Okoboji Lake in the mesotrophic category. These values suggest extremely low levels of chlorophyll a and suspended algae in the water, exceptional water transparency, and extremely low levels of phosphorus in the water column.
The level of inorganic suspended solids is also extremely low at West Okoboji Lake and suggests that non-algal turbidity does not cause water quality problems at this lake. The median inorganic suspended solids concentration at West Okoboji Lake was 0.5 mg/L, which was the lowest of the 132 monitored lakes.
Data from the 2004-2008 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at West Okoboji Lake. These data show that cyanobacteria comprised 98% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (13.0 mg/L) was the 62nd lowest of the 132 lakes sampled. However, these results do not suggest impairment of the Class A1 uses at West Okoboji Lake.
The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported” based on information from the IDNR Fisheries Bureau and results of the ISU and UHL lake surveys. Data from the ISU and UHL lake surveys show that from 2004-2008 there were no violations of the Class B(LW) criterion for ammonia in 25 samples, dissolved oxygen in 24 samples, or pH in 25 samples. Based on IDNR’s assessment methodology these results suggest full support of the Class B(LW) uses at West Okoboji Lake.
The Class C (drinking water) uses are "not assessed" based on a lack of recent data upon which to base an assessment. The most recent atrazine was collected in 1998 as part of the Syngenta Iowa Voluntary Atrazine Monitoring Program. While these data suggested "full support" of the Class C uses, they are now too old (>10 years) to base an assessment upon. 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 the ISU and UHL surveys from 2004-08 show that nitrate levels are very low at this lake (maximum value = 0.3 mg/l; median = 0.1 mg/l) relative to the MCL (10 mg/L), these data are not sufficient for developing a valid assessment of support of the Class C uses.
Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at West Lake Okoboji in 2004. The composite samples of fillets from common carp and yellow perch had low levels of contaminants. Levels of primary contaminants in the composite sample of common carp fillets were as follows: mercury: 0.039 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm. Levels of primary contaminants in the composite sample of yellow perch fillets were as follows: mercury: 0.068 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm. The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses. The fish contaminant data generated from the 2004 RAFT sampling conducted at West Lake Okoboji show that the levels of contaminants do not exceed any of the advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.
