West Okoboji Lake - Main Basin North IA 06-LSR-2069

Assessment Comments

Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (3) results of the IDNR-UHL beach monitoring program in 2004, 2005, and 2006, (4) information from the IDNR Fisheries Bureau, (5) results of the 1998 "Iowa Voluntary Atrazine Monitoring Program", and (6) results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 2004.

Basis for Assessment

[Note:  For the 2006 and 2008 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 “fully supported.”  The Class B(LW) aquatic life uses are assessed (monitored) as “fully supported.”  The Class C (drinking water) uses are assessed (evaluated) as “fully supported.”  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 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (3) results of the IDNR-UHL beach monitoring program in 2004, 2005, and 2006, (4) information from the IDNR Fisheries Bureau, (5) results of the 1998 "Iowa Voluntary Atrazine Monitoring Program", and (6) results of U.S.  EPA/IDNR fish contaminant (RAFT) monitoring in 2004.

EXPLANATION: Results of IDNR beach monitoring at Pikes Point Beach on West Okoboji Lake 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 (17 samples), and 2006 (16 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 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 Pikes Point beach, the geometric means of all 43 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.   None of the 55 samples collected during recreational seasons of 2004, 2005 and 2006 exceeded Iowa’s single-sample maximum criterion of 235 orgs/100 ml.   Thus, these results do not suggest an impairment of the Class A1 uses of West Okoboji Lake.  

Results of IDNR beach monitoring at Triboji Beach on West Okoboji Lake from 2004 through 2006 also 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 (16 samples), 2005 (17 samples), and 2006 (16 samples) as part of the IDNR beach monitoring program.   At Triboji beach, the geometric means of all 37 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 percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E.  coli orgs/100 ml) was not significantly greater than 10% in any of the years (2004: 6%, 2005: 0%, 2006: 13%).   Based on IDNR’s assessment methodology these results suggest “full support” of the Class A1 uses at West Okoboji Lake.

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 2002 through 2006 (approximately 23 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 36, 40, and 47 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 1.0 mg/L, which was the 4th lowest of the 132 monitored lakes.

Data from the 2002-2006 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at West Okoboji Lake.   These data show that cyanobacteria comprised 99% of the phytoplankton wet mass at this lake.   The median cyanobacteria wet mass (21.7 mg/L) was the 59th highest 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, results of the ISU and UHL lake surveys, and results of physical/chemical monitoring associated with the IDNR beach monitoring program.   Data from the ISU and UHL lake surveys show that from 2002-2006 there were no violations of the Class B(LW) criterion for ammonia in 17 samples, dissolved oxygen in 23 samples, or pH in 23 samples.   Based on IDNR’s assessment methodology these results suggest full support of the Class B(LW) uses at West Okoboji Lake.

Results of physical/chemical monitoring from 2004-2006 associated with the IDNR beach monitoring program at Pikes Point and Triboji beaches also suggest “full support” of the Class B(LW) uses of West Okoboji Lake.   These data show 1 violation of the Class B(LW) criterion for dissolved oxygen in 103 samples (<1%) and no violations of the Class A1,B(LW) criterion in for pH in 88 samples.   Based on IDNR’s assessment methodology the dissolved oxygen violation is not significantly greater than 10% of the samples and therefore does not suggest an impairment of the Class B(LW) uses at West Okoboji Lake.

Drinking water uses are remain assessed (evaluated) as "fully supported" based on results of monitoring conducted in 1998 for the "Iowa Voluntary Atrazine Monitoring Program” (VMP) (this lake has not been part of the Iowa VMP since 1998).   However, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is changed from "monitored" (indicating an assessment with relatively high confidence) to "evaluated" (indicating an assessment with relatively low confidence).   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 2002-06 show that nitrate levels are very low at this lake (maximum value = 0.2 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.