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 2009-2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 2008 by University Hygienic Laboratory (UHL), (3) results of the IDNR-UHL beach monitoring program in 2010-2012, (4) information from the IDNR Fisheries Bureau, and (5) results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 2004 and turtle contaminant monitoring in 2009.

Basis for Assessment

[Note:  For the 2006, 2008, 2010, 2012, and 2014 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 "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 2009-2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 2008 by University Hygienic Laboratory (UHL), (3) results of the IDNR-UHL beach monitoring program in 2010-2012, (4) information from the IDNR Fisheries Bureau, and (5) results of U.S.  EPA/IDNR fish contaminant (RAFT) monitoring in 2004 and turtle contaminant monitoring in 2009.

EXPLANATION: Results of IDNR beach monitoring from 2010-2012 suggest that the Class A1 uses are “fully."  Levels of indicator bacteria at Pikes Point beach were monitored once per week during the primary contact recreation seasons (May through September) of 2010 (15 samples), 2011 (16 samples), and 2012 (14 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 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).    

NOTE:  Based on consultation with EPA Region 7 staff in 2011, IDNR’s methodology for assessing impairments based on the geometric mean water quality criterion was changed.   Prior to the 2012 listing cycle, IDNR calculated geometric means for lakes based on a 30-day periods within the recreational season.   Any violation of one of these 30-day periods within 3 years resulted in an impairment of the Class A1 uses of that lake.   Because water quality standards do not identify a 30 day period but instead a recreational season, Region 7 concurred that the approach used for rivers and streams with less frequent bacteria data (seasonal geometric means) would be appropriate for identifying §303(d) impairments at lake beaches.   Thus, for the 2014 listing cycle, IDNR identified primary contact recreation impairments for lakes when the geometric mean of all samples from the recreation season of a given year exceeded the geometric mean criterion.  This does not impact the way IDNR assesses beaches for closure to protect the recreating public in the short term.  

At Pikes Point beach, the geometric means from 2010-2012 were all below the Iowa water quality standard of 126 E.  coli orgs/100 ml.   The geometric mean was 19 E.  coli orgs/100 ml in 2010, 31 E.  coli orgs/100 ml in 2011, and 13 E.  coli orgs/100 ml in 2012.   The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E.  coli orgs/100 ml) was 20% in 2010, 13% in 2011 and 7% in 2012.   None of these are significantly greater than 10% of the samples and therefore do not suggest impairment of the Class A1 uses.    According to IDNR’s assessment methodology and U.S.  EPA guidelines, these results suggest “full support” of the Class A1 uses.  

Results of IDNR beach monitoring at Triboji Beach on West Okoboji Lake from 2010-2012 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 2010 (15 samples), 2011 (16 samples), and 2012 (15 samples) as part of the IDNR beach monitoring program.   At Triboji beach, the geometric means from 2010-2012 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 2010, 27 E.  coli orgs/100 ml in 2011, and 12 E.  coli orgs/100 ml in 2012.   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 (2010: 7%, 2011: 6%, 2012: 7%).   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, however, suggest that the Class A1 (primary contact recreation) uses at West Okoboji Lake are “fully supported.”  Using the median values from these surveys from 2008-2012(approximately 14 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 37, 40, and 46 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 among the lowest of the 134 monitored lakes.

Data from the 2008-2012 ISU and UHL surveys suggest a small population of cyanobacteria exists at West Okoboji Lake.   These data show that cyanobacteria comprised 95% of the phytoplankton wet mass at this lake.   The median cyanobacteria wet mass (3.9 mg/L) and ranked 6th of the 134 lakes sampled.  

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 2008-2012 there were no violations of the Class B(LW) criterion for ammonia in 14 samples, dissolved oxygen in 14 samples, or pH in 14 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 2006-10 show that nitrate levels are very low at this lake (maximum value = 0.25 mg/l; median = 0.2 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 and turtle contaminant monitoring in 2009.   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.

In addition to the fish contaminant data, turtle contaminants were also assessed in 2009.   Samples of shoulder tissue from snapping turtle showed low levels of contaminants.   Levels of primary contaminants in the shoulder tissue samples of snapping turtle were as follows: mercury: 0.153 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm.   These results also suggest the fish consumption uses should be assessed as "fully supported."